Download TDR XFEL workshop series Atomic, molecular and cluster physics

TDR XFEL workshop series
Atomic, molecular and cluster physics
applications
edited by : Th. Möller, B. Sonntag
February 20, 2001
This report collects contributions submitted before, at and after the
workshop. The complete workshop program and copies of
transparencies can be found in the DESY report TESLA-FEL 2000-23,
available from DESY. For a list of workshop participants see next page.
Affiliation of editors and workshop participants are listed in the TESLA
Technical Design Report Part V.
Participants of the workshop
J. Bauer
U. Becker
G. Bertsch
T. Brabec
M. Brewczyk
J. Castello
E. Dörner
W. Eberhardt
J. Feldhaus
R. Freeman
M. Gavrila
C. Gerth
C.H. Keitel
E. Kennedy
J. Krzywinski
T. Laarmann
A. Maquet
G. Materlik
K.-H. Meiwes-Broer
M. Meyer
T. Möller
R. Moshammer
J.-P. Mosnier
H.G. Muller
M. Neeb
S. Novikov
J.H. Parks
R.M. Potvliege
G. Prümper
M. Richter
J.-M. Rost
J.-E. Rubensson
J.R. Schneider
A. Scrinzi
B. Sonntag
K. Starke
K. Taylor
Th. Tschentscher A. Yashishita
Contents
1
Multiple charged ions in intense laser fields
3
2
Two photon excitation/ionization of atomic inner shells : the 1sshell of the argon atom
4
3
Dynamics of atomic and molecular autoionization states studied by
two-photon excitation
15
4
XUV nonlinear optics: pulse shaping and applications
17
5
X-ray fluorescence experiments on atoms and molecules using
FEL radiation
20
6
Multi-ionization of metal clusters by strong FEL pulses
25
7
Trapped Ion X-Ray Diffraction
27
8
Absolute photoionization experiments
30
9
Coherence effects in atomic and molecular photoionization
32
10
Multiphoton Multiple-Ionization of Atoms and Molecules using
Reaction-Microscopes
34
Resonant Single and Multi-Photon Excitation and Ionisation of
Highly Charged Ions by FEL-Radiation
36
Velocity Map Imaging of Photoelectrons from Free OrientedMolecules
38
11
12
-2-
Multiple charged ions in intense laser fields
Christoph H. Keitel
Fakultät Physik der Universität, Theoretische Quantendynamik,
Hermann-Herder-Str.3, D-79104 Freiburg i. Br., Germany
Multiple charged ions are appropriate to study bound dynamics in extremely
intense laser fields, as ionization can be very small with a high charge state. The
energy separation increases strongly with increasing charge state so that high
frequency and high intensity laser fields are very attractive to study highfrequency resonant dynamics, possibly even in the weakly relativistic regime.
Recent calculations in our group [1] have shown weakly relativistic results in ions
with laser intensities comparable with those envisaged for the FEL in Hamburg
under construction. The magnetic component of the laser fields induces a strong
Lorentz force in particular in the vicinity of the ionic core where the velocity of
the wavepacket is very high. Then a reduced likelyhood of the electronic
wavepacket near the nucleus was predicted along with an enhanced angular
momentum. The strong angular momentum induces an enhanced spin orbit
coupling due to the intense laser fields with novel spin signatures in the
wavepacket dynamics and the radiation spectrum.
We propose to expand those calculations to the high frequency regime where ions
with higher charge states can be considered and where novel resonant and
possibly nonresonant effects are likely both with respect to fundamental aspects
and as a source of high harmonic generation.
[1] S.X. Hu and C. H. Keitel, Phys. Rev. Lett 83, 4709 (1999) and Eur. Phys.
Lett. 47, 318 (1999).
- 3-
Two-photon excitation/ionization of atomic inner shells:
the 1s-shell of the argon atom
Sergey NOVIKOV
Chair of Mathematics-1, Rostov State University of Transport
Communication,
Narodnogo Opolcheniya Square 2, Rostov-on-Don, 344038 , Russia
E-mail: [email protected], [email protected]
Abstract
The absolute values and the shape of the two-photon one-electron
excitation/ionization cross section of the 1s-shell of the argon atom with inclusion
the effects of relaxation of the atomic residue in the field of the creating vacancies
are calculated.
The results of calculation have the character of prediction.
Introduction
Recently, the interest for experimental and theoretical investigations of interaction
processes of x-ray radiation with atom has increased [1,2]. Similar investigations
are very important for modern fundamental and applied physics. So, the
explorations of non-linear processes of the x-ray photons interaction with atom,
particularly, have direct relation to solving the problem of x-ray laser design [3].
In this paper we consider the problem of theoretical description of multiphoton
excitation/ionization of inner shell of free atom.
The cross sections of two- and three- photons absorption by many-electron
atoms’ outer shells are calculated in refs [4-7]. The analogous investigations in
the region of absorption thresholds in the x-ray range of photon energy are absent
now. The aim of this work is to calculate the cross section of two-photon
absorption by an atomic inner 1s-shell of a simple system with the 1 S 0 ground
state term – the argon atom.
It is well known, that at the theoretical description of the cross section of the onephoton absorption by inner and outer shells it is necessary to take into account
various types of many-electron correlations [8]. The most important manyelectron effect in photoabsorption by inner shell is monopole rearrangement of
electron shells of the atomic residue within the field of an inner shell vacancy
[9,10]. In this work we present the results of theoretical study of the absolute
-4-
values and the shape of the two-photon one-electron excitation/ionization cross
section of the 1s-shell of argon atom with inclusion the effects of relaxation of the
atomic residue in the field of the creating vacancies.
Theory
The cross section of the two-photon absorption of atom is the probability of the
process divided by the squared density of the flux of photons incident on atom
(the dimension of the two-photon absorption cross section is cm 4 s ). In the case
of absorption by 1s-shell of the argon atom we have:
σ 1(s2) (ω ) = 8π 3 Dα 2 a 04 (Dω ) 2 S (ω ) ,
2
(1)
where
S (ω )
2
å
A ms (ω )
å
A md (ω )
=
m> f
+
= S sL (ω )
m> f
2
2
+ S dL (ω )
[2 D ω + E
2
[2 D ω + E
2
=
Γ1 s / 2π
0
− E (1s − 1 ms )
Γ1 s / 2π
0
]
− E (1s − 1 md )
2
]
2
+ Aε s = 2 D ω + E
+ Γ12s / 4
+ Γ12s / 4
0
+ Aε d = 2 D ω + E
− E (1 s
0
−1
(ω )
)
− E (1 s − 1 )
2
(ω )
+
2
(2)
for linearly (L) polarized photons,
and
S (ω )
polarized photons. It is possible to show that
In (2)
Aì m ü ì s ü
2
= S dC (ω )
2
2
S dC (ω ) =
for circularly (C)
2
3 L
S d (ω ) .
2
is the amplitude of the process calculated in the second-order
í ýí ý
î ε þî d þ
perturbation theory with the use of the methods of the theory of non-orthogonal
orbitals [11,12]. The expression for the amplitude has the form:
Aì m ü ì s ü (ω ) =
í ýí ý
î ε þ îd þ
∞
å
β
+ ò dε ν
0
ìm üì s ü
1s −1 í ýí ý D0 β
î ε þîd þ
Dω + E 0 − E ( β ) + i
β D0 0
+
Γβ
2
ìmüì s ü
ìm ü
1s −1 í ýí ý D0 ν ν D0 0
1s −1 í ýs 0
N e0
î ε þîd þ
îε þ
−
.
Γν
2
(Dω ) 2
Dω − IPν − ε ν + i
2
- 5-
(3)
Here IPν is the ionization potential of the ν -shell, D0 ≡ D z is z-component of the
operator of interaction in the dipole approximation, Γν is the total width of the ν vacancy decay, N=18 is total number of the electrons of the argon atom,
e0 =
m e4
≡ 1 a.u. = 27.21 eV . In the case of transition into d-symmetry the channel
D2
of the last (contact) term in (3) is absent.
In expressions (1),(2),(3): a0 is the Bohr radius; α is the constant of fine
structure; Dω is the energy of photons incident on atom; E0 , E (1s −1 ), Eçç1s −1m í
æ
è
ì s üö
ý ÷÷
îd þ ø
are the total Hartree-Fock energies of ground state, of single ionization state, and
of excitation states of an atom, respectively; f is the Fermi level (the set of the
quantum numbers of the valence shell of an atom); 0 = 1s 2 2 s 2 2 p 6 3s 2 3 p 6 (1 S 0 )
is initial state of the argon atom.
(2)
-52
4
σ1s , 10 cm s
25
10
linearly polarized photons
18
Ar
15
B
10
C
5
G-K
10
-5
D
A
10
1
10
100
F
E
1000
Photon energyω, eV
Figure 1 The cross section σ 1(s2) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated
taking into account the effect of relaxation of the atomic residue in the
field of the creating vacancies with (solid line) and without accounting
of this effect (dotted line). Notation A,B,C etc. see in the text.
-6-
3. Results of calculation
In the fig. 1 (fig. 2) the results of the calculations of the absolute values of the
cross section of the absorption of two linearly (circularly) polarized photons by
1s-shell of the argon atom are shown. The effects of radial monopole
rearrangement of electron shells within the field of the creating vacancies is taken
into account by the modification of the dipole matrix elements in expression (3)
by methods of the theory of non-orthogonal orbitals [11,12].
The calculations of the cross section of the two-photon absorption are fulfilled
using the following values of the decay widths: Γ1tots = 0.656 eV [13],
−7
−9
Γ2tots = 2.40 eV [14], Γ2totp = 0.13 eV [15], Γ3rad
eV [16], Γ3rad
eV .
p = 10
s = 1.4 ⋅ 10
(2)
σ1s , 10
10
25
10
15
10
5
10
-5
-52
4
cm s
circularly polarized photons
18
Ar
G-K
B
C
D
1
10
F
E
100
1000
Photon energy ω, eV
Figure 2 The same as in Figure 1 for circularly polarized photons.
In
expression
(3)
the
following
channels
ìs ü
ìs ü
β = 1s −1np , 2s −1 np , 2 p −1n í ý , 3s −1 np , 3 p −1 í ý ,
îd þ
îd þ
n=3,4)
for
discrete
- 7-
have
been
used:
n=4,5 (for d-symmetry intermediate
states,
ìsü
ìs ü
ν = 1s −1εp , 2 s −1εp , 2 p −1ε í ý , 3s −1εp , 3 p −1ε í ý for continuous intermediate
îd þ
îd þ
ìmüì s ü
states, and 1s −1 í ýí ý for the final states, m=4,5 (for d-symmetry – m=3,4).
î ε þîd þ
For configurations of intermediate and final states only the quantum numbers of
the vacancy and excited electron are shown.
The separate self-consistent solutions of the Hartree-Fock equations are used for
the initial, intermediate, and final states.
The contribution of contact interaction into the cross section of the process is
non-zero only for linearly polarized photons, it is substantial at small energy of
photon, and it rapidly decreases with the increase of the photon energy (fig.1,
region A). In the case of the contact interaction the 1s-shell simultaneously
absorbs two photons and creates s-electron, after that the virtual 1s-vacancy
decays through Auger channels. The contact interaction appears because of the
fact that the electron shells of the atom are rearranged in the field of the 1svacancy. Without inclusion of the effect of relaxation of the atomic residue in
the field of the 1s-vacancy the amplitude of the contact interaction is equal to
zero (fig.1, region A, dotted line).
In the region B (D) , see fig.1 and fig.2 , the energy of one photon is
approximately equal to the ionization potential of the 3p (2p)-shell of the argon
atom IP3 p = 14.762 eV ( IP2 p = 248.741eV ). In this spectral region the theory
predicts the existence of the structure caused by resonant [ Dω ≅ IP3 p ,( Dω ≅ IP2 p ) ]
excitation of the 3p (2p)-shell by one photon into the final excitation/ionization
(s,d)-states. The second photon is absorbed then by electron of the 1s-shell and
this electron fills the 3p (2p)-vacancy. In the region B (D) of the spectrum the
ì4, 5üì s ü
ýí ý
î3, 4þîd þ
discrete intermediate states β = 3 p −1 í
ì4, 5üì s ü
ýí ý
î3, 4þîd þ
( β = 2 p −1 í
) are
apparent. For circularly polarized photons the transitions into the s-symmetry of
intermediate states are forbidden. The resonant structure in the region B (D)
(fig.1 and fig.2) exists exclusively due to the existence of the virtual 1s-vacancy.
In the region C (E) , see fig.1 and fig.2, the energy of single photon is
approximately equal to the ionization potential of the 3s (2s)-shell of the Ar atom
IP3s = 33.416 eV ( IP2 s = 327.00 eV ). In this spectral region the theory predicts the
existence of the structure caused by resonant [ Dω ≅ IP3s , ( Dω ≅ IP2 s )] excitation of
the 3s (2s)-shell by the photon into the intermediate (p)-states of the discrete
spectrum. The second photon transforms the excited intermediate (p)-states into
final excitation/ionization states of (s,d)-symmetry, and one of the 1s-electrons
fills the 3s (2s)-vacancy. In the region C (E) of the spectrum the discrete
-8-
ì4ü
î5 þ
ì4ü
î5 þ
intermediate states β = 3s −1 í ý p ( β = 2s −1 í ý p ) are apparent. The resonant
structure in the region C (E) (fig.1 and fig2.) exists exclusively due to the
existence of the virtual 1s-vacancy.
In the region F, see fig.3 (fig.4), the energy of one photon is approximately equal
to half of the 1s-shell ionization potential of argon atom IP1s = 3206.982 eV . Now
the energy of two photons is enough to generate of structure of the cross section
deals with resonant ( 2Dω ≅ IP1s ) formation of the 1s-vacancy and the (s,d)-states
of final discrete spectrum. In the region F of the spectrum the discrete final
ì4, 5üì s ü
for linearly polarized photons (fig.3) and discrete final
1s −1 í ýí ý
î3, 4þîd þ
ì3ü
states 1s −1 í ýd for circularly polarized photons (fig.4) are apparent.
î4þ
states
-52
(2)
σ1s , 10
10
-2
4
cm s
F
linearly polarized photons
18
Ar
IP1s/2
1 2
10
-3
10
-4
-1
1 - |1s 4s;4d>
-1
2 - |1s 5s;3d>
MC
1600
1605
1610
1615
1620
Photon energy ω, eV
Figure 3 The cross section σ 1s( 2) (the region F) of the process of
excitation/ionization of the Ar 1s-shell by two linearly polarized
photons calculated taking into account the effect of relaxation of the
atomic residue in the field of the creating vacancies with (solid line)
and without accounting of this effect (dotted line). In the figure the
discrete final states are indicated. MC – minimum of Cooper[17] (the
amplitude of transition 4p εd change sign in this range).
à
- 9-
Perhaps, just the range F is perspective region for future experiments. In the first
place, the total decay widths of these states are more larger.
Secondly, the shape of the spectrum in this region strongly depended on the type
of polarization of incident x-ray photons.
There are two type of resonances at the range 2800 eV ≤ Dω ≤ 3300 eV (fig.5,7 and
fig.6,8). The first group of resonances (G: Dω = IP1s − IP2 s ; H: Dω = IP1s − IP2 p ;
I: Dω = IP1s − IP3s ; J: Dω = IP1s − IP3 p ) corresponds to resonant transition of electron
from 1s-shell into 2s,2p,3s,3p-shell, responsible. One of two photons knocks off
an electron of the 2s,2p,3s,3p-shell into the continuous spectrum of final (s,d)states. The second photon is absorbed by an electron of the 1s-shell, after which
the electron fills the 2s,2p,3s,3p-vacancy. The second group (K) corresponds to
resonant ( Dω ≅ IP1s ) excitation of an electron of the 1s-shell into virtual (p)-states
of the discrete spectrum by one of the photons. Absorbing the second photon,
these (p)-states are transformed into the final excitation/ionization (s,d)-states.
(2)
σ1s , 10
10
-2
10
-3
10
-4
10
-5
10
-6
18
-52
Ar
4
cm s
circularly polarized photons
F
IP1s/2
1 2
-1
1 - |1s 4d>
-1
2 - |1s 3d>
MC
1600
1610
1620
Photon energy ω, eV
Figure 4 The same as in Figure 3 for circularly polarized photons.
- 10 -
(2)
-52
σ1s , 10
10
4
cm s
linearly polarized photons
30
18
10
20
10
10
10
0
Ar
I
J
G
H
K
2900
3000
3100
3200
Photon energy ω, eV
Figure 5 The cross section σ 1s( 2) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated
taking into account the effect of relaxation of the atomic residue in the
field of the creating vacancies with (solid line) and without accounting
of this effect (dotted line). Notation G,H,I,J,K see in the text.
Account of the effects of relaxation of the atomic residue in the field of the
creating vacancies (solid line in the figs) changes essentially the theoretical
absolute values of cross section of the process calculated without inclusion of
these effects (dotted line in the figs).
It follows to note, that the problem of completeness of basis set of the
intermediate virtual states in (3) is not considered in the present work.
- 11-
(2)
σ1s , 10
10
-52
4
cm s
30
18
Ar
10
20
10
10
I
J
G
10
circularly polarized photons
H
K
0
2800
3000
3200
Photon energy ω, eV
Figure 6 The same as in Figure 5 for circularly polarized photons
(2)
-52
σ1s , 10
10
-1
10
-4
18
Ar
4
linearly polarized photons
cm s
K
J
12
-1
1 - |1s 4s>
-1
2 - |1s 5s>
-7
103180
3200
3220
Photon energy ω, eV
Figure 7. The cross section σ 1s( 2) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated
taking into account the effect of relaxation of the atomic residue in the
field of the creating vacancies with (solid line) and without accounting
of this effect (dotted line). In the figure the intermediate states are
indicated
- 12 -
(2)
-52
σ1s , 10
18
0
10
4
cm s
Ar
circularly polarized photons
J
K
1 2
-4
10
-1
1 - |1s 4s>
-1
2 - |1s 5s>
-8
10
3190
3200
3210
Photon energy ω, eV
Figure 8 The same as in Figure 7 for circularly polarized photons.
Conclusion
The fulfilled investigation has shown that the process of two-photon
excitation/ionization of the inner atomic shell has essentially the many-electron
character. The second photon realizes resonance structures by many-electron
effects in the regions of absorption spectrum where they are absent for onephoton process. So, the regions A-E (fig.1 and fig.2) deal with the effect of
quantum interference of the excitation/ionization process of the atomic 1s-shell
with the process of Auger decay of the creating virtual 1s-vacancy. Moreover, the
existence of the contact interaction of two photons with inner shell is additionally
due to effect of relaxation of atomic residue in the field of inner vacancy. It
follows to note that the inner atomic shell exposes itself already at the energies of
laser beam photons significantly less than the ionization potential of this shell.
This theoretical prediction can be experimentally investigated using existing
lasers.
- 13-
References
[1] Schmidt V 1992 Rep. Prog. Phys. 55 1483
[2] Pratt R H 1999 18th Int. Conf. X-ray and Inner Shell Processes (Chicago,
IL,USA), Book of abstracts, T12, p 38
[3] Kato Y 1999 18th Int. Conf. X-ray and Inner Shell Processes (Chicago, IL,
USA), Book of abstracts, T54, p 80
[4]
McGuire E J 1981 Phys. Rev. A 24 835
[5]
Moccia R, Rahman N K and Rizzo A 1983 J. Phys. B: At. Mol. Phys. 16
2737
[6]
L’Huillier A, Jönsson L and Wendin G 1986 Phys. Rev. A 33 3938
[7]
L’Huillier A and Wendin G 1987 J. Phys. B: At. Mol. Phys. 20 L37
[8]
Amusia M Ya 1990 Atomic Photoeffect (New York, London: Plenum
Press)
[9]
Amusia M Ya and Cherepkov N A 1975 Case Stud. At. Phys. 5 47
[10] Sukhorukov V L, Demekhin V F, Timoshevskaya V V and Lavrentiev S V
1979 Opt. Spectrosc. 47 228
[11] Jucys A P and Savukinas A J 1973 Mathematical Foundations of the
Theory of Atom (Vilnyus: Mintis)
[12] Åberg T and Howat G 1982 Handbuch der Physik vol 31, ed W
Mehlhorn (Berlin: Springer) pp 469-619
[13] Chen M H, Crasemann B and Mark H 1980 Phys.Rev A 21 436
[14] Papp T, Campbell J L and Varga D 1997 17th Int. Conf. X-ray and Inner
Shell Processes (Hamburg, Germany), AIP Conference Proceeding, No
389, p 431 (Woodbury, New York)
[15] Nakamura M, Sasanuma M, Sato S, Watanabe M, Yamashita H, Iguchi Y,
Ejiri A, Nakai S, Yamaguchi S, Sagawa T, Nakai Y and Oshio T 1968
Phys. Rev. Lett. 21 1303
[16] Lauer S, Liebel H, Vollweiler F, Schmoranzer H, Lagutin B M, Demekhin
Ph V, Petrov I D and Sukhorukov V L 1999 J. Phys. B: At. Mol. Phys. 32
2015
[17] Cooper J W 1962 Phys. Rev. 128 681
- 14 -
Dynamics of atomic and molecular autoionization states
studied by two-photon excitation
M. Meyera, A. Grum-Grzhimailob,
a
L.U.R.E., Centre Universitaire Paris-Sud, 91898 Orsay, France
b
Moscow State University, Moscow 119899, Russia
As an extension of our pump-probe studies on highly excited states [1, 2] some
experiments are proposed, which will benefit in particular from the short temporal
width and the high photon flux available at the TESLA-FEL. With respect to the
former investigations, the FEL will replace the VUV photons from synchrotron
radiation or high-harmonic generation sources. This arrangement will open the
vast field of studies on atomic and molecular autoionization states and their
relaxation dynamics. Out of the huge number of possible experiments three
examples are presented for illustration of the basic ideas:
i) Investigation of autoionization states with same parity as the ground state, e.g.
the Xe* 4d9 5s2 5p6 nd and ms resonances. Due to dipole selection rules, these
states can only be excited either by a direct two-photon one-color excitation or by
a two-photon two-color excitation via an intermediate resonant state, like the
strong Xe* 4d9 6p resonance. The short lifetime of the 4d9 6p resonance (Γ =
0.11eV, τ = 6 fs) necessitates the combination and synchronization of fs-pulses
(VUV+Visible). Resonant Auger spectroscopy will be used to study the
relaxation dynamics and the influence of alignment of the intermediate state when
the polarization of the two photon beams can be changed in a controlled way.
The results will extend and complete the conventional studies using one-photon
excitation and will provide new and important basis for the theoretical treatment
of electronic interaction and correlation in atomic multi-electron systems.
ii) Coupling of autoionization resonances by a strong laser field: Similar to the
above example, the FEL and a second synchronized fs-laser are used in a pumpprobe arrangement to excite and to couple to autoionization resonances, like He*
2s2p 1P and 2s 3d 1D. The profile of the resonances, being strongly asymmetric
for He* 2s2p 1P [3], will strongly depend on the intensity of the two laser beams
and on small changes in the relative time delay of the pulses and the photon
energy of the second laser (detuning). As a concrete example, a theoretical
treatment [4] predicts a minimum in the resonance profile for laser intensities of
1GW/cm2, which will be accessible in the planned experimental configuration.
iii) Relaxation of molecular autoionization states: Resonant photoexcitation of
molecules in the VUV leads in most cases to the dissociation of the molecule on
a time scale ranging from femtoseconds (i.e. at the same time as autoionization)
- 15-
to nanoseconds (i.e. after autoionization). Using high-resolution laser
spectroscopy the rotational structure of diatomic fragments (N2+, CO+) which are
produced upon resonant excitation of larger molecules (N2O, CO2, COS,), direct
information on the geometry of the dissociating state can be obtained. Timeresolved spectroscopy on molecular or atomic fragments will give complementary
insight, especially with respect to the dissociation dynamics, e.g. comparison
between fast direct dissociation, stepwise dissociation via intermediate products
or dissociation of metastable states of long lifetime.
[1] M. Gisselbrecht, A. Marquette et M. Meyer, J.Phys.B31, L977 (1998)
[2] M. Gisselbrecht, D.Descamps, C.Lynga, A.L'Huillier, C.G.Wahlström,
M.Meyer, PRL 82, 4607 (1999)
[3] M. Domke et al., Phys.Rev.Lett. 69, 1171 (1992)
[4]
H. Bachau, P. Lambropoulos et Shakeshaft, Phys.Rev.A34, 4785 (1986)
- 16 -
XUV nonlinear optics: pulse shaping and applications
Thomas Brabec and Armin Scrinzi
Institut für Photonik, Technische Unviersität Wien,
Gusshausstraße 27/387, A-1040 Wien, Austria
E-mail: [email protected]; [email protected]
The high intensities of the FEL source at wavelength in the VUV and XUV range
will allow to systematically study multiply excited states of inner shell electrons.
If timing and a sufficient degree of temporal coherence of the pulse are secured,
the decay dynamics of highly correlated multiply excited states may be observed
directly for the first time by pump-probe schemes.
We propose a scheme to control the pulse shape of the FEL source by a laser
induced x-ray switch (LIXS) which has the potential to generate sub-10 fs pulses
and we discuss the application of such pulses as pump in pump-probe
experiments on doubly excited states of Helium.
The idea of the LIXS is as follows: the resonant interaction of an xuv pulse with
an ion gas leads to coherent population transfer between the ground and the
resonant (excited state) causing absorption or amplification of the xuv pulse in
dependence on the phase between electric field and medium polarization (which
is determined by the population difference between the two states). Absorption is
maximum when the phase difference between electric field and polarization is
π/2, amplification is maximum when the phase difference between electric field
and polarization is 3π/2. When the pulse parameters are chosen so that the phase
difference is π/2 at the pulse peak and π at the trailing pulse edge, the xuv pulse
experiences loss over its whole temporal profile, with maximum loss at the pulse
peak, see Fig. 1. The basic idea of LIXS is to change absorption into
amplification by using a short laser pulse copropagating with the the xuv pulse.
The parameters of the laser pulse are chosen so that the laser pulse shifts the
phase (as a result of the Stark effect) between xuv pulse and polarization by 2π,
thus opening a gain window, see Fig. 1. The gain window cuts out s short fraction
of the xuv-pulse resulting in pulse compression.
The question whether LIXS can be realized experimentally depends crucially on
the propagation behavior of the laser and xuv pulse in the ion gas. The xuv pulse
experiences strong changes during propagation, which can significantly change
the gain window and destroy the switching mechanism. To test LIXS under more
realistic conditions including propagation we have solved self consistently 1D
(laser and xuv) pulse propagation in combination with the microscopic atomic
response (3 level system, 1s,2p,2s). The results are encouraging. The gain
- 17-
window is not changed during propagation leading to the generation of a 5fs xuv
pulse. Calculations with modified parameters have shown that the mechanism
works in a broad range of intensity parameters so that it should also work in the
case of 3D pulse propagation.
0.10
Loss [arb.u.]
Loss [arb.u.]
0.10
0.05
0.05
0.00
-0.05
-0.10
-10
-5
0
5
10
Time [fs]
0.00
-0.05
-0.10
-600
-300
0
Time [fs]
300
600
Figure 1 shows the imaginary part of the polarization (loss/gain)
induced by a laser and an xuv pulse in Be3+ gas. The inset shows a
zoom of the loss/gain curve between –10 fs and 10 fs. The parameters
are as follows. The xuv wavelength λxuv = 7.6 nm is chosen to be
resonant with the 1s => 2p transition of Be3+. The xuv peak intensity
is chosen to be I xuv = 2x1010 W/cm2 and the (FWHM) pulse duration
τxuv = 300 fs. The laser wavelength, intensity, and (FWHM) pulse
duration are λ0 =800 nm, I0=2x1015 W/cm2, and τ0 =5 fs, respectively.
The center of the laser pulse coincides with the center of the xuv
pulse.
A significant population of even angular momentum doubly excited states of
Helium by a non-linear two-photon process require intensities beyond 1014
W/cm2. A Floquet calculation of the correlated two-electron system shows that
the competition of single photon ionization is very strong and requires very short
pulses with durations of less than 10 fs. For more highly charged He-like ions
required times and intensities scale like Z2 and Z6, respectively.
- 18 -
Alternatively, even angular momentum states can be populated by combining a
strong XUV pulse with an optical laser pulse, where a long-lived doubly excited
P state is used as a gateway state (see Fig. 2).In this case, timing requirements on
the FEL source are less stringent, since the optical laser provides the crucial time
mark. Floquet calculations and a solution of the time dependent Schrödinger
equation show that populations of the order 10 % can be transfered into doubly
excited states with pulse durations and intensities of the projected FEL source.
Figure 2 XUV-optical pumping of doubly excited states by using a
long-lived double excited state as gateway. With pulse durations of
300 fs and XUV and laser intensities of the order 1013 W/cm2 and 108
W/cm2, respectively, up to 10 % of the population can be transferred
into a selected doubly excited state.
- 19-
X-ray fluorescence experiments on atoms and molecules
using FEL radiation
Jan-Erik Rubensson and Joseph Nordgren
Physics Department
Uppsala University
E-mail: [email protected],
[email protected]
Fundamental atomic physics in few-electron systems
In comparison to measurements of charged particles emitted after exposure to
VUV and soft X-ray radiation, there has been relatively little interest in
measurements of secondary photons. The reasons are the notorious low count
rates due to the low fluorescence yield and the low probability for direct nonresonant scattering. At FEL intensities this will no longer be a limitation. In
contrast to measurements of charged particles photon based experiments are not
affected by plasma potentials and radiation induced changes in the work function
of the analyzing instrument. These problems have to be dealt with today in high
resolution photoelectron spectroscopy [1], and at FEL intensities we anticipate
that they will be much more severe.
Even at conventional synchrotron radiation sources there is still a lot to learn by
simply measuring the integrated fluorescence yield as a function of energy of the
incoming radiation. In this way we could recently demonstrate that, contrary to
common notion, the radiative decay channel dominates for most of the doubly
excited 2lnl' states of helium [2]. Analysis of the behavior close to threshold in
terms of many body quantum defect theory unravels a fascinating dynamics
where the splitting of the N=2 threshold due to relativistic and quantum
electrodynamic effects is essential [3]. The potential for the outer (Rydberg)
electron is influenced by the energy of the inner (N=2) electron, which is
determined by the fundamental interactions. This leads to a beak-down of LScoupling already for the helium atom.
These results directly suggest several experiments with FEL radiation:
• Fluorescence yield measurement as a function of energy and intensity.
The simplest experiment at stage 2 of the TESLA project would be to repeat
the fluorescence yield experiment on the doubly excited states of helium and
to explore the intensity dependence of the cross sections. The energy should
be varied not only in the 60-66 eV region where one photon can excite both
- 20 -
electrons, but also in the 30-33 eV energy region where two- photon
excitation may occur.
• Pump-probe measurements.
The double excitations of helium close to the N=2 threshold have a lifetime of
around 0.1 ns, set by the radiative decay rate. The long lifetime makes these
states ideal for pump-probe experiments. As the energy spacing between the
Rydberg states corresponds to the FEL pulse duration the states are excited
coherently, and electron wavepacket dynamics may be probed by a delayed
FEL pulse. Several interesting pump-probe experiments using fast
conventional laser pulses as probe may be considered.
• Energy-resolved photon detection
Measuring the emission with high-energy resolution gives complementary
information on the electron dynamics. Again, the long lifetime of the doubly
excited states, together with the Rydberg wavepacket dynamics makes them
ideal as a test case for time and energy resolved VUV spectroscopy. Energy
analysis using grazing incidence grating spectrometers constitutes the bulk of
our research activities, and we are presently developing a Fourier transform
interferometer, matching the photon energies reached in stage two of the
TESLA project.
• Doubly excited states of He-like ions.
The doubly excited states of Li+, Be2+, B3+, C4+ etc can be studied in the same
way as for the helium atom. The fundamental interactions will split the N=2
level in a different way for theheavier ions, and thereby influence the Rydberg
series. Highly charged ions at sufficient concentrations can be produced by
ECR type ion sources. One may also use the FEL beam itself to produce the
highly charged ion target. This is a suggestion for the final stage of the TESLA
project.
Resonant inelastic scattering in molecules
Experiments where the energy distribution of the secondary photons in the subkeV region is determined is the heart of the activity in our research group. Gasphase measurements can only be performed at the most powerful beamlines of
third generation sources, and still they are severely restricted by the available
primary intensity.
Our experiments have significantly contributed to the understanding of the corestate excitation-emission scattering process [4]. Obviously, this understanding is
- 21-
restricted to the linear limit, which is accessible at conventional synchrotron
facilities.
The validity of the dipole approximation implies that the symmetry of the
involved states largely determines the scattering spectrum. If, e. g., the molecule
has inversion symmetry, only transitions which change parity from gerade to
ungerade (or ungerade to gerade) are allowed.
At conventional intensities the symmetry selection rules are strictly valid. Due to
vibronic coupling the electronic symmetry may break during the scattering
process, thereby relaxing the selection rules and the appearance of 'forbidden'
peaks in the spectra. Using this symmetry signature we have investigated the
dynamic vibronic coupling. Most interesting, it is found that the vibronic coupling
is quenched when the primary energy is tuned some eV away from the resonance
[5]. We have recently used this phenomenon to characterize the bonding in a
complex transition metal fulleride compound, with implications for the
organometallic bond in general [6]. Of course, excitation away from threshold
requires even more primary intensity.
Based on this experience we suggest the following soft X-ray spectroscopy
experiments using FEL radiation:
• Intensity dependence
A first experiment at the FEL is to repeat the measurements on small
homonoculear diatomics at different intensities. In the absence of vibronic
coupling the appearance of 'forbidden' peaks would immediately indicate a
departure from the conventional description of the scattering process. For
stage two of the FEL project the L shell of the Cl2 molecule is just within
reach for one-photon excitations. Sufficient density of more unusual diatomic
molecules can be created in cluster sources. For two-photon resonant
excitation we note that K shell excitations of the N2 molecule can be reached
already with the energies of stage two of the TESLA project. In the scattering
spectra two-photon excitation can easily be separated from higher-order
excitation due to the complementary selection rules.
• Pump-probe experiments
A laser pulse can be used to pre-excite a symmetric molecule. By excitation to
high vibrational and rotational states, one can appreciably increase the average
bond distance in a homonuclear diatomic. Close to the dissociation limit the
vibrational wavefunction has a large weight at the classical outer turning point.
By performing scattering experiments with initial states which have been preexcited in this way one can examine effects beyond the dipole approximation.
This approximation is expected to break down when the distance between the
- 22 -
atoms becomes longer than the wavelength of the radiation. Via the symmetry
selection rules this can be followed experimentally. The break-down of the
dipole approximation can be directly associated with the localization of the
core hole to one of the atoms.
• Characterization of bonding in complex molecules and clusters
To take advantage of the selection rules in the scattering process it is
necessary to excite a few eV below threshold. FEL intensity is required to
characterize the bonding in larger molecules e. g. of biological interest, both in
gas-phase or dilute liquid form. The same principle can be used to study free
clusters where target density necessarily is small.
• Plasma analysis
At FEL intensities there is a large cross section for multielectron emission,
resulting in a multitude of highly charged ions. Analysis of the soft X-rays will
be a useful tool for characterization of the dynamics initiated by the FEL
pulse. X-ray spectroscopy is already used for plasma diagnosis in fusion
experiments.
Exotic atoms and molecules
In exotic atoms and molecules at least one electron is replaced by a heavier
particle. The simplest system is the muonic hydrogen atom, consisting of a proton
and muon. The binding energy is around 2.5 keV, and the characteristic X-rays
from muonic hydrogen have already been measured [7]. Thus, the fundamental
excitations are all within in reach of the X-ray FEL. The prospect of performing
'classical synchrotron radiation experiments' on such systems would open up an
entirely new field of research, with the prospect of gaining new insights into the
electroweak interaction and the structure of the constituent particles.
We suggest that a feasibility investigation regarding such experiments be started
in the near future. A key issue is to estimate the efforts needed to produce
sufficient concentrations of exotic systems. Theoretical predictions and detailed
formulation of the physical questions that may be addressed are also needed.
For this type of studies there may also be important applications. The formation
of the dµt molecule is essential for muon catalyzed fusion. This molecule is
similar to molecular hydrogen, except that one proton is replaced by a d+, t+, and
a µ- particle. The muon induces the d+t→α+n fusion reaction, and the rate for
this process has recently been measured [8]. The feasibility to perform
- 23-
spectroscopy on such systems using X-ray FEL radiation may have an impact on
our efforts to control the fusion process. Also regarding this question a theoretical
investigation is required.
References
[1] G. Öhrwall, thesis, Uppsala University (1999)
[2] J-E. Rubensson et al. Phys. Rev. Lett. 83, 947 (1999).
[3] T. W. Gorczyca, et al. Phys. Rev. Lett., 85, 1202 (2000).
[4] see e.g Appl. Phys. A65, (1997).
[5] P. Skytt, et al., Phys. Rev. Lett. 77, 5035 (1996).
[6] L. Qian et al, submitted for publication.
[7 ] B. Lauss et al. Phys. Rev. Lett., 80, 3041 (1998).
[8] M. C. Fujiwara et al., Phys. Rev. Lett 85, 1642 (2000)
- 24 -
Multi-ionization of metal clusters by strong FEL pulses
Josef Tiggesbäumker and Karl-Heinz Meiwes-Broer
Fachbereich Physik, Universität Rostock, 18051 Rostock
http://www.physik.uni-rostock.de/Cluster/
The coupling of intense radiation into matter is an interesting topic in fundamental
and applied sciences. In contrast to bulk surfaces, low-dimensional objects turned
out to be particularly photosensitive. Rare gas, molecular as well as metal clusters
exhibit giant photoabsorption cross sections when compared to the respective
atomic constituents. E.g., in the case of platinum clusters we showed that the
multiply charging process can be explained by the evolution of the plasmon
energy of the metal cluster upon the change in electron density during the
Coulomb explosion [1]. The corresponding temporal evolution of the cluster
density is revealed by molecular dynamics simulation, see Fig. 1 for the example
of Pt 5810+. Then, for each time step the optical response is calculated in RPA.
The resulting dynamics of the absorption is in agreement with the experimentally
determined charging efficiency as function of the light pulse width [2].
Figure 1 Coulomb explosion of Pt58 10+. The cluster density stays
nearly unchanged during the first 100 fs. Then a rapid decrease is
accompanied by an increase in radius.
Similar time dependencies are found in lead clusters. Carbon clusters behave
differently as they show the highest charging efficiency with the shortest light
pulse [2,3]. Whereas details of the light coupling and the dynamics of the charge
flow are still the matter of several theoretical investigations, the interesting
- 25-
question arose about the role of the wavelength of the exciting radiation. With
conventional femtosecond laser systems there is not much room for a variation.
FEL sources, on the other hand, will provide intense fs pulses with peak powers
of 5 GW at photon energies of 60-200 eV. At these photon energies, in some
systems (e.g., alkalis, semiconductors) weakly bound core levels can be ionized.
Will it be possible to coherently excite a manifold of those core electrons?
[1] L. Köller, M. Schumacher, J. Köhn, S. Teuber, J. Tiggesbäumker, K.H.
Meiwes-Broer, Phys. Rev. Lett. 82 (1999) 3783
[2] T. Döppner, S. Teuber, M. Schumacher, J. Tiggesbäumker, K.H. MeiwesBroer, Int. J. Mass Spec. 192 (1999) 387
[3] M. Schumacher, S. Teuber, L. Köller, J. Köhn, J. Tiggesbäumker, K.H.
Meiwes-Broer, Europ. Phys. J. D 9 (1999) 411
- 26 -
Trapped Ion X-Ray Diffraction
Joel H. Parks
Rowland Institute for Science
100 Edwin H. Land Boulevard
Cambridge, MA 02142
Abstract
The recent application [1] of electron diffraction to trapped cluster ions has
provided a measure by which structural phase changes can be observed as a
function of cluster size and temperature. This technique has been applied [2] to
the study of CsI clusters and has identified the appearance of bulk structure
isomers at a cluster size corresponding to a completed atomic shell having
rhombic dodecahedron geometry. The proposed research will investigate the
feasibility of extending these diffraction techniques to utilize the X-Ray FEL
device to study both elastic and inelastic scattering from trapped cluster ions.
The intensity of the FEL device under consideration offers the possibility of
overcoming the significantly smaller cross-section for x-ray elastic scattering, in
particular at larger scattering angles as shown in Fig. 1. The right hand axis
indicates the ratio of x-ray to electron total scattering assuming an incident
electron beam of 2.5x1012 e-/s and an x-ray flux of 4x1018 photons/s. For these
parameters, the gain in large angle scattering is substantial. However, such high
x-ray fluxes require orders of magnitude reduction in background scattering
compared to electrons to achieve this advantage. For this reason, design of the
trapping structure placement of the detector and the x ray beamline through the
UHV system needs to be relatively open and free of apertures to avoid
background scattering. The possibility of strong inelastic x-ray scattering is
another important issue which primarily determines the rate at which the trap has
to be reloaded with the primary cluster ions. The trapped ion density is limited by
electrostatic interactions. Both of these issues suggest that a linear quadrupole
trap may be more appropriate than a standard rf Paul trap because it provides a
more open structure and the stability of trapping ~10 times more clusters.
The technical issues which were central to achieving electron diffraction of
trapped clusters which will determine the feasibility of x-ray diffraction are listed
in Tables 1 and 2. A current projected parameter value and its physical
constraints are included for each item listed. The issues will be discussed in the
presentation and more fully in a detailed proposal.
- 27-
Table 1 : Trapped Ion Electron Diffraction Issues
δI/I0 ~2 x 10–8
1. Background Scattering
- aperatures
- He
N ~ 2 x 104 ions
2. Cluster Number
- source/trapping efficiency
- space charge
~ 3 x 1012 e - /s
3. Incident Flux
- detector saturation
4. Exposure Time
~ 30 s
- inelastic scattering
- detector saturation
5. Data Duty Cycle
~ 30 %
- trap conductance
6. M/Z Range
M/Z ~ 35 kD
- Vrf perturbation escattering
7. Mass Selection/Resolution
M/δM ~ 50
- space charge
Table 2 : Trapped Ion X-Ray Diffraction Issues
<< 10–8
1. Background Scattering
- aperatures
- He
N ≥ 2 x 105 ions
2. Cluster Number
- source/trapping efficiency
- space charge
~ 4 x 1018 hν/s
3. Incident Flux
- detector saturation
- 28 -
4. Exposure Time
1 min
- inelastic scattering
- detector saturation
≥ 50 %
5. Data Duty Cycle
- inelastic scattering
6. M/Z Range
M/Z ~ 50 kD
- trap electronics
δM/M ~ 50
7. Mass Selection/Resolution
- space charge
References
[1] M. Maier-Borst, D. B. Cameron, M. Rokni and J. H. Parks Phys. Rev. A
59, R3162 (19
[2] J. H. Parks, presented at The Third International Symposium on Theory of
Atomic and Molecular Clusters, Humboldt-Universität Berlin, Berlin,
Germany, 1999 (unpublished).
- 29-
Absolute photoionization experiments
M. Richter a, G. Ulm a, S. V. Bobashev b, A. A. Sorokin b
a
PTB Berlin, Germany
b
Ioffe-Institute St. Petersburg, Russia
Experience of the team
The team brings together the experience of PTB, the German national institute of
science and technology, and Ioffe Physico-Technical Institute St. Petersburg in
the fields of photon metrology and photoionization experiments in the spectral
ranges of ultraviolet (UV) radiation, vacuum ultraviolet (VUV) radiation, and x
rays. For many years, both institutes cooperate on the development of absolute
photon-flux detectors as well as on absolute ionization experiments to obtain
cross-section data for photon and electron impact on rare gases. The activities are
embedded in fundamental experience in the physics of electronic and atomic
collisions, UV, VUV, and x ray radiometry and spectroscopy, research with
synchrotron radiation and laser-produced plasmas, electron, ionmass and
photoabsorption spectroscopy, as well as time-of-flight and coincidence
techniques. The experience is demonstrated by numerous refereed articles in
international journals and contributed as well as invited papers at national and
international conferences for more than twenty years up to now.
Research proposal
Previous research at the radiometry laboratory of PTB at the electron-storage ring
BESSY I in Berlin, the two groups have developed a common total ion-yield
experiment that enables to directly compare photon- and electron-impact
ionization of rare gases. The current of the ionising impact particles is measured
using a Faraday cup for electrons and precisely calibrated semiconductor
photodiodes for photons, respectively. Based on available photoionization crosssection data obtained by precise photoabsorption measurements, resent
experiments considerably improved the data base for absolute electron-impact
ionization cross sections [1]. With the help of these data, the experimental setup
now can be used as a so-called quantometer for absolute photon flux
determination. In order to realise true absolute ion-mass spectroscopy from first
principles that is not based on photoabsorption data, the setup is going to be
completed with respect to the detection of ionic fragments on an absolute scale
by the application of time-of-flight and coincidence techniques that have been
developed in a previous work [2]. The experiments will be extended to electronimpact ionization of molecular targets to considerably improve the corresponding
- 30 -
cross-section data bases that may also be used to enable standard-free gas
analysis.
Research proposal
Up to now, the study of non-linear dynamics and multi-photon excitation of
matter is limited to the application of highly intense laser radiation in the optical
region. However, the free-electron laser (FEL) that is under construction at
DESY in Hamburg promises a radiant power density of focalised VUV radiation
that is sufficient to extend the experiments of strong-field physics considerably to
shorter wavelengths for a better understanding of many new and fundamental
phenomena. Because of the suspicion that highly intense FEL radiation will
saturate or even destroy common solid state detectors, it is proposed, in a first
experiment, to use the rare-gas photoionization quantometer developed by Ioffe
Institute and PTB for an absolute radiometric characterisation of the DESY-FEL
out of the focal plane. This gives, in a second step, the opportunity for a
quantitative investigation of non-linear effects upon photoionization in free raregas atoms using the same experimental setup moved into the focal plane. Gas
pressure, electron gun, and ion detection of the setup will be matched to the
requirements of FEL-radiation detection.
The two most significant recent references
[1] A. A. Sorokin, A. Shmaenok, S. V. Bobashev, B. Möbus, and G. Ulm:
Measurements of electron-impact ionization cross sections of neon by
comparison with photoionization. Phys. Rev. A 58, 2900-2910 ( 1998); A.
A. Sorokin, S. V. Bobashev, B. Möbus, M. Richter, and G. Ulm:
Measurements of electron-impast ionization cross sections of argon,
krypton and xenon by comparison with photoionization. PRA 61, 022723
[2] M. Richter: Combined Electron and Ion Spectroscopy with Synchrotron
Radiation of Free Metal Atoms and Ions. J. Electr. Spectrosc. Relat.
Phenom. 76, 21-28 (1995); T. Luhmann, Ch. Gerth, M. Martins, M. Richter,
and P. Zimmermann: Final Ion-Charge Resolving Electron Spectroscopy:
Photoionization Studies on Sm and Eu. Phys. Rev. Lett. 76, 4320-4323
(1996); A. Gottwald, Ch. Gerth, and M. Richter: 4d Photoionization of Free
Singly Charged Xenon Ions. Phys. Rev. Lett. 82, 2068-2070 (1999)
- 31-
Coherence effects in atomic and molecular photoionization
Uwe Becker
Fritz-Haber Institut, Faradayweg 4-6, 14195 Berlin, Germany
Coherence phenomena occur whenever two or more particle waves that have a
fixed phase relation interact. The typical feature of coherence is the occurrence of
interference. Quantum optics is the field where coherence became the key to
fundamentally new insights and a vast number of applications. The progress in
this field promoted coherent atom optics. An outstanding example is the BoseEinstein-condensation. Only in the field of electron emission or more generally
electron beams coherence phenomena emerge only in rare cases. The reason for
this is the difficulty to produce coherent electrons and to transport them without
loss of coherence. One method is to induce coherence on incoherent electron
beams by self-induced coupling, an effect that is used in the free electron laser.
Another method is the use of coherent electron emission from atoms or
molecules. The coherent electron emission from two equivalent centres is being
discussed for many years in connection with localized and delocalized core
electrons in homo-nuclear molecules. It is unanimously assumed that for the
emission of photoelectrons with high kinetic energy the time for the following
Auger-decay is to short to erase the memory of the photo electron about its
origin. The perturbation of the symmetry by the emission process stays present
and causes an incoherent emission of the photoelectron. One open question is
what happens to those photoelectrons that leave the molecule so slowly that the
Auger-decay occurs in a time interval shorter than the time that corresponds to
the symmetry-energy separation. In this case the photo electron's memory of its
origin should be erased and the emission should be coherent. The following
experiment could yield evidence for this behavior. The coincidence measurement
of a 1 s photo electron from N2 and the N+ fragments yields the photo electron
angular distribution in the molecular reference frame. As the ionic fragmentation
produces N+ + N+ the detection of N+ contains no information about the origin of
the photoelectron i.e. the emission should be coherent. When the coincidence is
performed for the photoelectron and the N++ fragment it is much more probable
that the electrons origin is the N++ fragment than its partner the N or N+ ion. So
partial information about the electron's origin is preserved and the emission
should be incoherent. The two angular distributions - the coherent and the
incoherent one - should differ dramatically i.e. depending on the coincidence
partner one should observe distinct angular distributions.
Another process that should produce a pronounced coherence effect in the
angular distribution of photo electrons is the double photoionization of H2 or D2.
- 32 -
If the two photoelectrons are measured in coincidence with the recoil protons, the
two-centre emission of spherical waves should produce interesting interference
effects for the electron intensity in the molecular frame, which should be
contrasted with the incoherent emission from HD.
- 33-
Multiphoton Multiple-Ionization of Atoms and Molecules
using Reaction-Microscopes
R. Moshammer, J. Ullrich
University Freiburg, Germany
Goal of Research
Due to the high photon intensity available at the upcoming FEL sources, unique
mechanisms of double and multiple ionization of atoms, molecules and clusters
can be realised for the first time and shall be explored in kinematically complete
experiments.
Proposal
It is proposed to start with the most simple system, the helium atom, where two
electrons might individually be photo-ionised by subsequently absorbing one
photon each, within the duration of the pulse of about 300 fs. As one competing
process, only one photon might be absorbed by one of the electrons and the
second electron is shaken into the continuum which is the dominant contribution
for single photon absorption at third generation synchrotrons. Third, one electron
might be ionised, be accelerated in the laser field and "rescattered" to its parent
ion causing double ionisation due to an electron-electron collision in an
intermediate Volkov state. This has recently been identified to be the dominating
mechanism in non-sequential double ionisation in strong conventional laser fields.
Finally each of the electrons might absorb several photons in a multi-photon
ionisation reaction. Presently, it is by no means clear which of the mechanism
dominates, whether all of them are possible at all or what other dynamic reactions
might occur.
All of these reactions shall be identified by their characteristic kinematical
signatures, observed in kinematically complete experiments where the vector
momenta of both electrons and of the recoiling target ion are measured in
coincidence. In a further step, it is proposed to perform similar investigations
using first more complicated atomic targets like neon or argon and, second,
simple molecules. Such measurements will for the first time give basic
information on the fundamental mechanisms of energy transfer from exotic
radiation fields atoms and molecules. They will serve as benchmark systems to
develop theoretical models and come to detailed understanding of more
complicated situations where the radiation interacts with biological matter or
solid state targets.
- 34 -
Experience
Our former group at GSI has developed so-called reaction microscopes that
enable to measure the vector momenta of up to ten electrons and of several
recoiling target ions produced in one single interaction with radiation or charged
particles1. Within the last years we have performed several benchmark
experiments on multiple ionization of helium and neon in attosecond intense
fields of relativistic highly charged ions2, 3, in collisions with electrons4 and, very
recently, in 30 fs intense laser fields5. Members of our group have been involved
in a number of such experiments for single photon impact at photon energies
ranging from a few eV above threshold6 to 100 KeV7.
Researchers
Dr. R. Moshammer, Dr. A. Dorn, Dr. C.D. Schröter, Dr. J. Crespo. Dr. B.
Feuerstein, C. Höhr. D. Fischer, Prof. Dr. J. Ullrich
References
[1] R. Moshammer, et al., NIM B108 (1996)42S
[2] R. Moshammer et al., PRL 79 (1997) 3621: PRL 73 (1994): PRA 56 (1997)
1351
[3] W. Schmitt et al., PRL 81 (1998) 4337: M. Schulz et al., PRL 84 (2000)
863
[4] A. Dorn et al., PRL 82 (1999) 2496
[5] R. Moshammer et al., PRL 84 (2000) 447
[6] R. Dörner et al., PRL 76 (1996) 2654
[7] L. Spielberger et al., PRL 76 (1996) 4685
- 35-
Resonant Single and Multi-Photon Excitation and
Ionisation of Highly Charged Ions by FEL-Radiation
J. Crespo. J. Ullrich
University Freiburg, Germany
Goal of Research
The photon flux at the upcoming FEL sources will, for the first time, allow the
systematic study of the interaction between VUV-photons (X-rays in the second
stage) and highly charged ions (HCI). An electron beam ion trap (EBIT) will
produce and trap ions ranging from medium nuclear charges Z up to the heaviest
systems like Li-like Uranium. In this way, the static structure of heavy fewelectron systems as well as the dynamics of multi-photon processes with
relativistic, bound electrons can be investigated with unprecedented resolution.
Proposal
Up to now, few studies have been reported on the photon-ion interaction at 3rd
generation synchrotrons due to the intrinsic photon flux limitations and even in
those cases only low charged ions have been used. At the FEL sources a broad
spectrum of such investigations will become possible, as for example the resonant
excitation or single and multiple ionisation of ions by single and multiple photon
absorption, experiments on high harmonic generation as well as time-resolved
pump-probe experiments on the lifetime of excited states or on wave-packet
dynamics in ionic systems.
We propose to build a combined electron beam ion trap and source (EBIT/S) to
produce, and subsequently store or extract ions up to Li-like Uranium in a first
step. A later upgrade should extend the maximum charge state to hydrogen-like
ions of all elements, as demonstrated at the Livermore SuperEBIT. Due to the
excellent emittance (≈ 1 π mm mrad) of these type of ion source, slow ion beams
can be efficiently focussed into a scattering chamber equipped with photon
detectors (VUV, visible) as well as with electron and ion spectrometers. When
higher ion-target densities (1011 ions/cm-3) are required, the VUV-light might be
directly guided into the trap.
As one of many examples, we propose to start with the precision spectroscopy of
the 2s1/2 - 2p1/2 transition in Li-like HCls. Currently, the best value for Uranium
(280 eV photon energy) has a resolution of ∆E/E = 3.6·10-4. With an envisaged
resolution of 105, the FEL sources would allow the most sensitive test of QED in
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a three electron system in strong fields, where a perturbation expansion in terms
of Z·α (α: finestructure constant) is not valid and nuclear effects become large.
Experience
The necessary experimental techniques have been already developed in our
group. We have designed and built an EBIT/S [1,2] in Freiburg to produce
hydrogenic ions up to U92+, operational since 1999. Members of the group have
previous experience in high-resolution photon spectroscopy with HCIs from the
visible region up to the hard x-ray dornain [3,4]. Also within the last two years,
we have been performing first fully differential experiments on multiple ionisation
of atoms and molecules in femtosecond intense laser fields [5] of up to 1015
W/cm2; our work with relativistic HCIs at storage rings (GSI) provides us with
experience in accelerator environments.
Researchers
Dr. J. Crespo. A. Werdich, Dr. A. Dorn, Dr. B. Feuerstein. D. Fischer, C. Höhr,
Dr. R. Moshammer, Dr. C.D. Schröter, Prof. Dr. J. Ullrich
References
[1] J. R. Crespo López-Urrutia et al., Physica Scripta T80 (1999) 502;
[2] J. R. Crespo López-Urrutia et al., Hyperfine Interactions (2000) (accepted)
[3] J. R. Crespo López-Urrutia et al., PRL 77 (1996) 826; also PRA 57 ( 1998)
879.
[4] P. Beiersdorfer, A. L. Osterheld, J. H. Scofield, J. R Crespo López-Urrutia
and K. Widmann, PRL 80 ( 1998) 3022; P. Beiersdorfer, J. R. Crespo
López-Urrutia, E. Föster, J. Mahiri, and K. Widmann, Rev. Sci. Instrum. 68
(1997) 1077.
[5] R. Moshammer et al., PRL 84 (2000) 447
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Velocity Map Imaging of Photoelectrons from Free
Oriented-Molecules
A. Yagishita1, M.Takahashiand2, T. Kasai3
1 Photon Factory, Institute of Materials Structure Science, Oho 1 - 1,
Tsuba-shi 305-0801, Japan
2 Research Institute for Scientific Measurements, Tohoku University,
Katahira 2- 1 - 1, Aoba-ku 980-8577, Japan
3 Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Background
One of the authors initiated the measurements on photoelectron angular
distribution (PAD) from fixed-in-space-molecules by angle-resolved
photoelectron-photoion coincidence at 1995 [1]. In contrast to PAD from
randomly oriented molecules, PAD from the fixed-in-space molecules provides us
the most detailed information on the photoionization dynamics of molecules [2].
However, PAD measurements from the fixed-in-space molecules are limited to
the dissociative photoionization channels, because photoelectron-photoion
coincidence technique is applied to realise them.
Proposal
Here we would like to propose a project on PAD from free oriented-molecules as
a general extension of PAD from the fixed-inspace molecules. Because oriented
molecules, which are prepared by the supersonic beam source, the electrostatic
hexapole, and the guiding field [3] (see Fig.1), are very dilute, intense photons
per pulse of FEL sources are inevitable to realise the measurements of PAD from
the oriented molecules. And also timing structure of FEL sources is ideal for
velocity map imaging of photoelectrons (see Fig.1) [4]. Triggered by FEL signals,
one can obtain the velocity map image of photoelectrons, i.e., PAD (see Fig.2).
We think that a FEL is the ideal photon source for the project on PAD from the
oriented molecules. We believe that the perfectly new field of molecular
photoionization dynamics will be opened by this project.
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References
[1] E. Shigemasa, J. Adachi, M. Oura, and A. Yagishita, Phys. Rev. Lett. 74,
359(1995)
[2] for example, A. Yagishita, in Proceedings of the 20th ICPEAC, Viena,
1997, edited by F. Aumayer and Winter (World Scientific, Singapore, 1998)
p.149.
[3] T. Kasai, T. Fukawa, T. Matunami, D.-C. Che, K. Ohashi, Y. Fukunishi, H.
Ohyama, and K. Kuwata, Rev. Sci. Instrum., 64, 1150(1993).
[4]
A.T.J. Eppink and D. H. Parker, Rev. Sci. Instrum., 68, 3477(1997).
Figure 1
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Figure 2
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