Download Ionic Collision Processes Relevant to Astronomy Studied in the

Ionic Collision Processes Relevant to Astronomy
Studied in the Laboratory
Prof. Adalbert Ding
Institut fuer Technische Physik and Technische Universitaet Berlin, Germany
February 16, 2017
Molecular Ions in Space
Molecular ions play a significant role ion space. A large number of
these ions exists in everyday life, but there are a number of these ions
which would not be stable under normal telluric conditions.
Ionic collisions play a central role in the understanding of processes in
space in particular in the evolution of biological important chain
processes. They might be building blocks for biomolecules or catalysts
for helping to generate these molecules.
The molecular ions I will be talking about are not stable under normal
telluric conditions. The have only been discovered in space one by one
during the last 2 decades
Molecular Ions in Space: Detection Methods
Solar System: Particle Detection(particle detection/mass spectrometry)
and Optical Spectroscopy
Example for particle detection: ULYSSES spacecraft: SWOOPS
Most of the satellites use some sort of optical spectroscopy
Outer Space:
Only optical (rotational, vib-rotational, vibronic) spectroscopy
Solid buckyballs (C60) discovered in space
by Amanda Doyle for ASTRONOMY NOW
Posted: 28 February 2012
NASA Spitzer Telescope Finds Elusive Buckyballs in Space
07.22.10
Molecular Ions in Space
• Basics of Reaction Dynamics
• Ion-Molecule Reactions
• Scattering of Ions to Retrieve the Intermolecular
Forces of Molecular Ions
Molecules in Comets
1864 first spectrum of a comet (Donati, comet Tempel C/1864 N1)
1868 identification of carbon and Swan bands C2 (Huggins)
1881 identification of Na, other emissions of CH, CN, C2, C3 (comet Cruls-Tebbutt 1881III)
1911 indentification of CO+ by de la Baume Pluvinel and Baldet (comet Morehouse 1908III)
1941 idendification of OH (comet Cunningham 1941I)
Interatomic Electrostatic Forces (ionic bond)
• Ion-Ion interaction: Na+ - Cl-
1/r
+
-
• Ion-Dipole interaction: Na+ - H2O
1/r2
+
-
+
• Dipole-Dipole interaction: H2O - H2O
1/r3
-
+
• Ion induced-Dipole interaction: H+ - Ar
1/r4
• Dipole induced-Dipole interaction: H2O - Ar
1/r6
-
• Induced-Dipole induced-Dipole interaction: Ar - Ar
1/r6
-+
-
+ Electron-Correlation Forces (covalent bond)
+
+
+
-+
-+
-+
V(bXY , bYZ )
Reaction Dynamics
X
Y
bXY
Reaction: X+YZ
bXY
Vibrational excitation
3 atom recombination
bXY
Chemical reaction
Complex formation
Z
bYZ
XY + Z
The total energy – potential energy surface –
V(bXY , bYZ )
of the system depends on the type of atoms
and the distances bXY and bYZ. It can be calculated using different approximations.
Basically it consists of 2 valleys connected
through a “mountain” saddle. Sometimes
there are 2 saddles enclosing a “mountain”
lake. The particles enter through one of the
valleys, overcome the saddle and roll further
into the other valley or back. If the outgoing
valley is “deeper” the reaction is exothermic,
if the valley is “higher” it is endothermic.
First paper on reaction dynamics 1931
by Henry Eyring and Michael Polanyi. Later
Corrected by Eugene Wigner (at the time Ph.D.
Student of Michael Polanyi).
They used only linear encounters which could be
proven to be the path of smallest energy, as
suggested by Fritz London. Later improved by Sato:
LEP- and LEPS energy surfaces important in
stellar surroundings
H + H2
H2 + H
H + D2
HD + H
H + HD
H2 + D
H+ + H2
H2+ +H
He++ H2
HeH+ + H
Non-Boltzmann distribution can be used to operate chemical lasers and masers. The former have been used
to build extremely powerful lasers in the mid IR (2…6 um), with very poisonous reaction products;
the latter have been found in space. Chemical laser radiation has not been found yet.
A.D., et al., Far.Disc.Chem.Soc. 55, 252 (1973)
Ion Molecule Reactions
X + + H2
X + H2+
XH+ + H
XH+ + H
X++ HY
X+ HY+
XH+ + Y
XH+ + Y
X: Ar, Kr, Xe, O2, CO, N2, CH4, CO2, D2O, H2S,
Also H2 + + H2
CH+3 +CH4
CH+4 +CH4
Example: Ar+ + D2
Kr+ + D2
H can be exchanged against D
Ion
source
Ion
lens
Wien
filter
(MS)
Ion
lens
H3 + + H
C2H5+ +H2
CH5+ + CH3 and many others
D used instead of H because of
better mass resolution. Bonding
reaction properties are nearly
identical.
collimator
Ion
beam
Energy
filter
Deflection plates
Molecular
beam
ArD+ + D
KrD+ + D
Quadrupole
MS
Ion
detector
X+
Compound Mechanism
(also known in complex mechanism)
v
b
angular momentum: L= m*v*b
Y
b: impact parameter
Z
Ion X+ docks onto molecule YZ and forms
an intermediate complex XYZ+ for several
rotational periods.
The energy is partitioned statistically between all particles. Finally the compound
decays into 2 or 3 fragments
chemical bonds
chemical bonds
forming
Therefore the complex XYZ+ is highly
rotationally excited, but the angular
momentum can be carried off by the
products.
Multi-atom systems are more likely to
form intermediate complexes as the
number of degrees of freedom is larger
Y
X+
X+
v
b
Spectator stripping model:
Y
Angular momentum: L= m*v*b
in respect to Y is conserved and
ends up in the product XY. Atom
Z is un-affected spectator!
Z
(also known in nuclear physics)
Ion picks up one atom, does not interact
with the other atom. Bond switches
instanteneously.
Therefore the product XY+ is highly
rotationally excited, as the reaction
cross section of many ion-moleculereactions is large (in the order of
100 Å2 )
Can be explained with charge exchange processes
Y
X+
Product intensity as a function of translational velocity and angle for different initial center-of-mass energies Er.
The contour lines give the intensity of products in that particular velocity and angular range. CM and S are the
center-of-mass and the stripping velocities, respectively.
…A.D., Z.Naturforschung 26a, 932 (1971)
List of Some Ion-Molecule Reactions
Exotic molecular ions act also as intermediates or catalysts
to biologically important molecules by building up organic
compounds from primitive building blocks
Ion molecule reactions usually have an enormous
cross section (several 100 Å2), because of the far
extending ion-induced dipole potential which is
proportional to 1/r4.
Also there are always 2 interacting potential curves,
e.g. Ar+ + H and H+ +Ar both correlating with ArH+ .
Some important products from ion-molecule reactions:
H3+ , ArH+ , HCO+ , N2H+ , CH5+ , C2H5+ ,H3O+
H2O+ , O2H+
C2+, C3+, …, Cn+, C60+, C70+
H3+,H5+,H7+,…
Also there are various stable products which have not
yet been found in space like
HeH+ (charge exchange of He and H; plays an important role in Solar processes)
NeH+ , KrH+ , XeH+
also
ArB+ , ArC+ , ArN+ ,
Intermolecular Forces in Molecular Ions
One method to determine a potential energy surface of a complex multi-atomic system one needs diatomic potentials
which can be used by the method of diatomic in molecules to obtain the full energy surface. The diatomic potentials
can be obtained by scattering ions on atoms.
b
b: impact parameter
Light Waves
Matter Waves
interference
interference
Water droplet
wave packets (matter waves, light )propagating into the same scattering angle
thus exhibiting interference.
trajectories
deflection function
interference pattern
Set-up for elastic ion-atom scattering
Top: reduced interaction potential (Vmin = -1)
Bottom : deflection function
Interference and diffraction of water waves
27.02.2017
Themenkreis 35: Mikroskop, Vergrößerung
23
Classical rainbow
Better resolution
Rainbow Scattering of protons and deuterons on rare gases
𝟏
Deuterons have a matter
wavelength
which
is
smaller than that of Hydrogen
rainbow
√𝟐
…,A.D., Z.Naturforschung 26a,1112 (1971)
red
green
blue
Optical rainbow including several supernumerary rainbows
A.D.: Elastic Scattering of Ions in Interactions between Ions and Molecules, P.Ausloos ed., Plenum Press, New York,(1975)
Example for systems not containing protons
…,A.D. , J.Chem.Phys. 70, 864 (1979)
A + B+
A+ + B
Typical adiabatic potential energy curves for a diatomic molecule with a bound ground state.
Ua is the potential energy of the system, ΔR is the interatomic distance, De is the equilibrium bond energy of the
ground state. The blue and red curves denote the diabatic potential curves. Wave packets progressing on different
curves through transitions at the avoided crossing generate interference pattern, termed Stueckelberg oscillations
(after Ernst C.G. Stueckelberg)
Diabatic
potential
Adiabatic
potential
A: coupling coefficient
v: velocity
ΔF: slope difference
Transition probability
Top:
Diatomic potentials:
levels and wave
functions for the
harmonic and anharmonic oscillator
Bottom:
Different types of
curve crossings
(avoided crossings)
Example of a complicated set of potential curves:
HeH+
1 a.u.
Oscillations stemming from interference of wave packets
travelling on the g- and u-potential curves (g-u-interference)
of identical atoms
Oscillations caused by interference of wave packets due to various quantum effects
Glory
Rainbow
trajectories
deflection function interference pattern
trajectories
deflection function interference pattern
Summary
• Molecular ions are stable in space
• Ion induced dipole forces and charge exchange processes are
responsible for the large formation cross sections of molecular ions
• Molecular ions form highly rotationally (observed in space)
and vibrationally excited states (not yet observed)
• Laboratory experiments can predict unstable ionic species
Thank you for your attention