Transcript Photodouble Ionization of Molecular Hydrogen T.J. Reddish , D.P. Seccombe

Photodouble Ionization of Molecular Hydrogen
T.J. Reddish1†, D.P. Seccombe1, and A. Huetz2
1
Physics Department, University of Windsor, 401 Sunset Ave, Windsor, Ontario, Canada, N9B 3P4.
2 LIXAM, UMR 8624, Université Paris Sud, Bâtiment 350, Orsay Cedex, France
†Email: [email protected]
Web-Site: http://zeus.uwindsor.ca/courses/physics/reddish/TJRWelcome.htm
What happens when a hydrogen molecule absorbs a photon of sufficient energy to
eject both electrons? In which directions do the electrons go? What happens to the
ions during the Coulomb explosion? Why don’t two equal energy electrons leave in
opposite directions? These are the sorts of fundamental questions that this project
has tried to address. The experiments are difficult, requiring very efficient
coincidence techniques to ensure the electrons come from the same event.
Theoretically, even the simplest molecule creates an unexpected challenge!
h + H2  H+ + H+ + e- + e-
e
e
e
He++
H+
h + He  He++ + e- + e-
He / D2 TDCS with
E1 = E2 = 10eV, S1 = 0.67
H+
e
He

D2
 1 = 115o
Characteristic
two lobes with
node at 12 = .
Total Ion
Energy
~18.8eV
Binding
Energy
31.7eV
E1 = 5 eV
Polarization ()
R  paper
Schematic Diagrams of Toroidal
Photoelectron Spectrometers
(a) & (b)
similar
electron
repulsion
k  , k1 and k2
Reddish et al
Rev. Sci. Instrum.
68 (1997) 2685
(d) nuclei
suppresses
electron
repulsion
3
2
extra lobes
due to
higher L
components
Gas Beam
1
Hemispherical Analyser
Entrance Optics
Toroidal Analyser
Entrance Optics
A Kheifets
EPSRC
Leverhulme Trust
EU
Newcastle University
1
0
0
0
60
120
180
240
300
0
60
120
180
k, , k1 and k2
all coplanar
D2/He
300
360
8
E1 = 5eV, E2 = 20eV
E1 = 7eV, E2 = 18eV
3
Hemispherical
Analyser
240
2
360
2
4
6
D2/He
2
4
Walter and Briggs J. Phys. B (1999) 32 2487
2
Outer Toroid
Mazeau et al
J. Phys. B.
30 (1997) L293
0
0
60
120
180
240
300
360
0
0
60
120
2
Exit
Optics
Z Stack MCP
Position Sensitive Detector
(Resistive Anode Encoder)
He and D2 TDCS in perpendicular plane
geometry with E1 = 5eV, E2 = 20eV, S1 = 0.9
 1:
98
115
132
180
240
300
360
2
Despite large gauge variation in 5C (&3C), plus its
tendency to exaggerate the yield at small mutual angles,
there is nevertheless a remarkable consistency with the
data to evolving shape of the ratio trends at E = 25eV!
The reason for this is not yet understood.
Data from: Seccombe et al J Phys B 35 (2002) 3767
Helium HRM-SOW Theory
1 = 0 (20), 10(10), 20(10) and 90 (7)
D2 (,2e) 5C calculations
for E1 = E2 = 10eV integrated over
all molecular orientations
Future Prospects
The main challenge now is 2-centered systems. Double ionization of H2 is in its
infancy. The main theoretical challenge is to adapt the ab initio methods
developed for helium to 2-centered systems.
Ideally one needs to have a "fixed-in-space” molecular axis, which is technically
possible with suitable equipment. Such studies will be most sensitive to electronion correlation / dichroism / interference effects in the ionization/dissociation of
light molecules.
Mutual Angle (12) - Degrees
M Walter
J Briggs
D2/He
D2/He
( He  He++ : 79eV )
LURE
LIXAM
SRS
E1 = 2eV, E2 = 23eV
E1 = 1eV, E2 = 24eV
2
Double ionisation potential depends upon
internuclear separation - nominally at 51.1eV.
S Collins
S Cvejanovic
C Dawson
J Wightman
• Data obtained with ‘identical’ spectrometer conditions.
• Velocity gauges arbitrary normalised to data at 2 =
180
• Note variations in y-scales
1
D2
He
Acknowledgements
Evolution of Similarities and Differences with E2/E1
(,2e) D2 5C and He 3C from Walter and Briggs
for R = E2/E1 = 24, 11.5, 4, 2.67, S1 = 1, 1 = 0.
Inner Toroid
D2 seems to have similar structure….
but with ‘narrower’ lobes and a ‘filledin’ node (highlighted in ratio plot)
Fitted curves
using Feagin’s
He-like model
with 1/2 = 77
E1 = 7 eV
Perpendicular
Plane Geometry
Coplanar Detection
Geometry
 1 = 132o
Excess Energy:
E1 + E2
E1 = 2 eV
Photon Beam
Direction

 Note: Triple" Differential Cross Section “TDCS” Appropriate
terminology for helium - with electron energies (E1 and E2) and directions
(1 and 2). We can still use "TDCS" for H2 by implying a fixed
equilibrium internuclear separation: Re = 1.4 Å and ignoring any possible
coupling between electronic and nuclear motion during double ionisation.
D2
E1 = 1 eV
Coplanar
1  S1 3 1  S1 3 S 3 3
 3
 3 
x 
y
    3
 1  2 E 1
2
2
2
Photon
Energy
 1 = 98o
He
H2/D2 (,2e) 5C Predictions for selected
molecular orientations at E1 = E2 = 10eV
Note the strong similarity in the TDCSs for He
and D2. This can be summarized using Feagin’s
He-like model with Gaussian parameterisation
(black curves) with different half-widths 1/2 91
(He), 78 (D2)
Why Study Double Ionization?
 Fundamental theoretical interest: Electron-Electron (& Ion) Correlation,
to which angular distributions are sensitive probe.
 Development of sensitive detection techniques (++ ~ 10-20 cm2)
 Accurate test for theory in a ‘simple’ system, which can then be extended
to more complex targets.
Requirement:
 Synchrotron radiation with well defined polarization properties (Stokes
Parameters: S1, S2, S3) and high photon flux.
TDCS 
Comparison between the (, 2e) ‘TDCS’
of He and D2 at E = 25 eV, 1 = 0, S1 = 1
He-Like Model:
 Based on dominant, 96%, 1Se  1Po character.
 Explained yield at 12 = :
Selection rule differences and solid angle effects.
 Atom-like when  >> Re
Experimentally, this requires helical / linear VUV undulators at synchrotron
sources and/or ultra-fast laser facilities, together with the continued development
of detector technology.
Observations
 Even the simple E1 = E2 case is intrinsically more
complex in diatomic molecules than for helium.
 5C provides some justification for observed ‘narrower’
lobes compared to the corresponding He case.
 Extra lobes due to higher L components?
Publications
D. P. Seccombe et al J. Phys. B. (2002) 35 3767
S. A. Collins et al Physical Review A (2001) 64 062706
Wightman et al J. Phys B. 31 (1998) 1753
Feagin (1998) J. Phys. B. 31 L729
Reddish and Feagin (1999) J. Phys. B. 32 2473
Data: Wightman et al J. Phys B. 31 (1998) 1753
Scherer et al J. Phys. B. 31 (1998) L817
Theory: Walter and Briggs J. Phys. B 32 (1999) 2487
J. P. Wightman et al J Phys B. (1998) 31 1753
Collins et al Physical Review A (2001) 64 062706
T. J. Reddish et al Phys Rev Letts (1997) 79 2438