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MSci Project
10th December 2001
Ions in Intense Femtosecond
Laser Fields
Jarlath McKenna
Supervisor: Prof. Ian Williams
Outline
• Introduction / Background
– Strong Field Ionisation
– Sequential and Non-sequential Ionisation
• Experimental Apparatus / Techniques
– Z-scan
– Intensity scan
• Results and Analysis
Introduction
• Study of the ionisation dynamics of positively charged
atomic ions in intense femtosecond Laser fields
• Experiments carried out in collaboration with a group
from UCL
– February 2002 at RAL using the ASTRA laser
• Analysis and interpretation of results
– familarisation with experimental apparatus
Why?
• Why study strong field ionisation of positive
atomic ions?
• All previous experiments have used neutral
targets
• First study using a beam of positive atomic ions
– allows study of wider range of species e.g. O+, N+
– compare with results from neutral target
• What happens in high intensity Laser
interactions?
– Low intensity:
Single Photon Ionisation
– Higher intensity:
Multiphoton Ionisation
– Very high intensity: Field Ionisation
• Single Photon Ionisation
– Ionisation energy of
valence electron is
supplied by one photon
Ionisation level
Ground state
• What happens in high intensity Laser
interactions?
– Low intensity:
Single Photon Ionisation
– Higher intensity:
Multiphoton Ionisation
– Very high intensity: Field Ionisation
• Multiphoton Ionisation
– Ionisation energy of
valence electron is supplied
by a number of photons
Ionisation level
Real excited state
Virtual excited state
Ground state
• What happens in high intensity Laser
interactions?
– Low intensity:
Single Photon Ionisation
– Higher intensity:
Multiphoton Ionisation
– Very high intensity: Field Ionisation
ATI
• Multiphoton Ionisation
– Ionisation energy of
valence electron is supplied
by a number of photons
– Above Threshold Ionisation
may take place
Ionisation level
Real excited state
Virtual excited state
Ground state
Field Ionisation
• Electric field  distorts the atomic potential well
Atomic Potential well V0(x)
– this lowers the potential barrier seen by an electron in
the atom/ion
Potential well
Electric field 
e-
x0
Potential range (x)
Field Ionisation
• Electric field  distorts the atomic potential well
• Tunnelling Regime
As barrier is lowered,
it’s width decreases.
Increased probability of
electron tunnelling
Atomic Potential well V0(x)
– this lowers the potential barrier seen by an electron in
the atom/ion
Potential well
Electric field 
e-
x0
Potential range (x)
e-
Field Ionisation
• Electric field  distorts the atomic potential well
• Over-the-barrier
Regime
Potential barrier is lower
than electronic state
Electron is free to
escape the atom
Atomic Potential well V0(x)
– this lowers the potential barrier seen by an electron in
the atom/ion
Potential well
Electric field 
e-
x0
Potential range (x)
Field Ionisation
• Electric field  distorts the atomic potential well
• Dynamic Stark Shift
Energy states of electrons
are Stark shifted up
towards the continuum
Dynamic or ac Stark shift
because of oscillating
E-field of laser
Atomic Potential well V0(x)
– this lowers the potential barrier seen by an electron in
the atom/ion
Potential well
Electric field 
e-
x0
Potential range (x)
Sequential and Non-Sequential
Ionisation
• Sequential:
– Ionisation takes place in a series of steps
A+
A
A2+
• Non-Sequential:
– Ionisation takes place in a single step
A
A2+
Recollision Model

A
CORE
Electric field
strength from
laser pulse expels
an electron
Atomic core
with outer shell
of electrons
Recollision Model

A+
CORE
During oscillatory
motion of E-field,
the electron may
make multiple
returns to the atomic
core
Electron may
collide with a
valence
electron
Recollision Model

Collision with
another electron
may directly
remove the
electron or
excite it to a
higher energy
state in which it
then tunnels its
way through the
remaining
barrier
A2+
CORE
Apparatus
Primary
Beam
Collector
Charged
Fragment
Detector
Extraction and Focussing Lenses
Einzel
lens
45o Parallel Plate
deflectors
Neutral Fragment
Detector
-ions are accelerated to 1-2 keV
Deflection
Plates
Laser
Beam
Ion Source
-ions produced via
discharge
Interaction
Region
Selection Magnet
Intensity Selective Scanning
or Z-scan
• Laser Intensity is
• Scan with a
0.5mm aperture
r
z
Radius (mm)
– Lorentzian along z
direction
– Gaussian in radial r
direction
Laser beam
Slit
Z Value (mm)
Intensity Selective Scanning
or Z-scan
• Laser Intensity is
• Scan with a
0.5mm aperture
r
z
Radius (mm)
– Lorentzian along z
direction
– Gaussian in radial r
direction
Laser beam
Slit
Z Value (mm)
Intensity Selective Scanning
or Z-scan
• Laser Intensity is
• Scan with a
0.5mm aperture
r
z
Radius (mm)
– Lorentzian along z
direction
– Gaussian in radial r
direction
Laser beam
Slit
Z Value (mm)
Intensity Selective Scanning
or Z-scan
• Laser Intensity is
• Scan with a
0.5mm aperture
r
z
Radius (mm)
– Lorentzian along z
direction
– Gaussian in radial r
direction
Laser beam
Slit
Z Value (mm)
Intensity Selective Scanning
or Z-scan
• Laser Intensity is
• Scan with a
0.5mm aperture
r
z
Radius (mm)
– Lorentzian along z
direction
– Gaussian in radial r
direction
Laser beam
Slit
Z Value (mm)
Intensity Scan
• Uses a half-wave plate energy selector technique
Fast
/2

Polaroid
Laser
/2 Plate
Slow
• By rotating the angle of polarisation , the
intensity is given by I = I0cos2
Results and Analysis
• Z-scan and Intensity scan results for ionisation
of positively charged ions: C+, Ne+, He+, Kr+
• Model the results using theoretical approaches
– Volume fit for saturation ionisation
– ADK tunneling model
• Suggest explanations for some of the main
features of the results
Z Scan results for C2+ ion production
• Z scan displays the classic Gaussian volume shape
Production of C2+ ions from a C+ laser beam as a function of focusing lens position (z)
1.6e-9
Integrated Ion Yield (arb.)
1.4e-9
1.2e-9
1.0e-9
Shoulder
feature
8.0e-10
6.0e-10
4.0e-10
2.0e-10
0.0
0
2
4
6
8
10
12
14
Z Position (mm)
• Shoulder feature is indicative of a secondary process at a
lower threshold intensity
Saturated Volume Method
• Determines the ion production volume at saturation
 2   z
V ( z )   0 z 1  
2
  z 0



2
Rayleigh range z0=02/
  I ( z ) 
0

 ln
  I s 
Waist radius 0=2f/D
z -aperture size
Is -Saturation intensity
Laser beam
z
Radius (mm)
• For saturated regime;
Ion yield  Interaction
volume
r
Is
Slit
Z Value (mm)
Theoretical Volume fit to Z-scan of C2+
Volume Curve Fit for Z-Scan of C 2+
2e-12
Volume (arbitrary units)
2e-12
Vol fit C 2+ Groundstate
Vol fit C 2+ Metastable
Vol fit C 3+ Groundstate
Sum of Volume fits
2e-12
1e-12
1e-12
DATA Z-scan
1e-12
8e-13
6e-13
4e-13
2e-13
0
0
2
4
6
8
Z Position (mm)
• Volume method only works well for ‘over-the-barrier’
ionisation
– It doesn’t describe the tunneling ionisation regime at low intensities
Intensity Scan for C2+ Production
Laser Intensity Scan for production of C2+ from C+
• Two distinguishable
regions to the results:
1. Low intensity curve
indicating C2+ production
from the C+ metastable
state.
2. High intensity curve
indicating production
from C+ groundstate.
Integrated Ion Yield (arb. units)
1e-8
1e-9
1e-10
1e-11
1e-12
1e-13
1e+14
1e+15
1e+16
Laser Intensity (Wcm-2)
Intensity Scan for C2+ Production
Laser Intensity Scan for production of C2+ from C+
1e-8
1. ADK is a quasi-static
tunneling method which
models ionisation rate w
2. Provides a probability
of tunnel ionisation as a
function of the intensity
of the alternating E-field
Integrated Ion Yield (arb. units)
• ADK Tunneling Model
1e-9
1e-10
1e-11
C+ GS – C2+ GS
C+ MS – C2+ GS
C+ MS – C2+ MS
1e-12
MS –Metastable
GS -Groundstate
1e-13
1e+14
1e+15
1e+16
Laser Intensity (Wcm-2)
Intensity Scan for C2+ Production
Laser Intensity Scan for production of C2+ from C+
1e-8
1. ADK is a quasi-static
tunneling method which
models ionisation rate w
2. Provides a probability
of tunnel ionisation as a
function of the intensity
of the alternating E-field
Integrated Ion Yield (arb. units)
• ADK Tunneling Model
1e-9
1e-10
1e-11
SUM
1e-12
1e-13
1e+14
1e+15
1e+16
Laser Intensity (Wcm-2)
Intensity Scan for Ne2+ Production
Laser Intensity Scan for production of Ne2+ from Ne+
• Best fit includes
ionisation to states
which require the spin
flip of an electron
Integrated Ion Yield (arb. units)
1e-8
1e-9
Ne+ GS – Ne2+ GS
Ne+ 4P – Ne2+ GS
Ne+ 4P – Ne2+ 1S
Ne+ 4P – Ne2+ 1D
SUM
1e-10
1e-11
1e-12
1e-13
1e+13
1e+14
1e+15
1e+16
Laser Intensity (W/cm2)
Non-Sequential Ionisation in C3+
Laser Intensity Scan for the production of C3+ form C+
• The best physical model
for these non-sequential
processes is the
‘recollision model’
1e-10
Integrated Ion Yield (arb. units)
• At low intensity there is
the apparent onset of
non-sequential ionisation
processes
1e-11
ADK fit
1e-12
1e+14
1e+15
1e+16
Laser Intensity (W/cm2)
Summary
• In an intense Laser field, atoms and ions are
ionised via field ionisation
– distortion of Coulomb potential by E-field of laser
– sequential and non-sequential ionisation processes
• Experimental techniques employed are the z-scan
and intensity scan
• ADK and Saturated Volume models appear to work
well
– suggestion of spin-flips due to magnetic field effects
Future
February 2003, 4 week experimental run at RAL
• Multiply charged positive ions
• Compare results from positive ion target to
neutral target
• Repeat some of the results from previous run
• Limit the interaction volume for the intensity
scan studies
Acknowledgements
Many thanks to….
•
•
•
•
Prof. Ian Williams
(Dr) Gail Johnston
Dr B. Srigengan
Dr Jason Greenwood
…..et al