Photodouble Ionization of Molecular Hydrogen T.J. Reddish1

1
Photodouble Ionization of Molecular HydrogenT.J.
Reddish1, D.P. Seccombe1, and A. Huetz21
Physics Department, University of Windsor, 401
Sunset Ave, Windsor, Ontario, Canada, N9B 3P4.2
LIXAM, UMR 8624, Université Paris Sud, Bâtiment
350, Orsay Cedex, FranceEmail
reddish_at_uwindsor.ca Web-Site
http//zeus.uwindsor.ca/courses/physics/reddish/TJ
RWelcome.htm
 
 
Comparison between the (?, 2e) TDCS of He and
D2 at E 25 eV, ?1 0?, S1 1
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 dont 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!
Photon Beam Direction
h? H2 ? H H e- e-
H2/D2 (?,2e) 5C Predictions for selected
molecular orientations at E1 E2 10eV
He / D2 TDCS with E1 E2 10eV, S1 0.67
Polarization (?)
h? He ? He e- e-
R ? paper
Schematic Diagrams of Toroidal Photoelectron
Spectrometers

• 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.
• 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.

Evolution of Similarities and Differences with
E2/E1
(a) (b) similar electron repulsion
Perpendicular Plane Geometry   k? ? ?, k1 and k2
He
D2
(?,2e) D2 5C and He 3C from Walter and Briggs for
R E2/E1 24, 11.5, 4, 2.67, S1 1, ?1 0?.
Coplanar
Reddish et al Rev. Sci. Instrum. 68 (1997) 2685

• Data obtained with identical spectrometer
  conditions.
• Note variations in y-scales

• Velocity gauges arbitrary normalised to data at
  ?2 180?

(d) nuclei suppresses electron repulsion
Coplanar Detection Geometry   k?, ?, k1 and
k2 all coplanar
extra lobes due to higher L components
Photon Energy
Excess Energy E1 E2
Mazeau et al J. Phys. B. 30 (1997) L293
Total Ion Energy 18.8eV
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.
D2 seems to have similar structure. but with
narrower lobes and a filled-in node
(highlighted in ratio plot)
Walter and Briggs J. Phys. B (1999) 32 2487
Binding Energy 31.7eV
He and D2 TDCS in perpendicular plane geometry
with E1 5eV, E2 20eV, S1 0.9 Helium
HRM-SOW Theory ?1 0? (?20?), 10?(?10?),
20?(?10?) and 90? (?7?)
?1 98? 115? 132?
Fitted curves using Feagins He-like model with
?1/2 77?
Data from Seccombe et al J Phys B 35 (2002) 3767
Future Prospects
D2 (?,2e) 5C calculations for E1 E2 10eV
integrated over all molecular orientations
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
electron-ion correlation / dichroism /
interference effects in the ionization/dissociatio
n of light molecules. Experimentally, this
requires helical / linear VUV undulators at
synchrotron sources and/or ultra-fast laser
facilities, together with the continued
development of detector technology.
Double ionisation potential depends upon
internuclear separation - nominally at 51.1eV. (
He ? He 79eV )
Mutual Angle (?12) - Degrees
Acknowledgements

• 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?

He-Like Model  Based on dominant, 96, 1Se ?
1Po character. Explained yield at ?12 ?
Selection rule differences and solid angle
effects.  Atom-like when ? gtgt Re
Publications
S Collins S Cvejanovic C Dawson J Wightman M
Walter J Briggs A Kheifets
LURE LIXAM SRS EPSRC Leverhulme
Trust EU Newcastle University
D. P. Seccombe et al J. Phys. B. (2002) 35
3767 S. A. Collins et al Physical Review A
(2001) 64 062706 J. P. Wightman et al J Phys B.
(1998) 31 1753 T. J. Reddish et al Phys Rev Letts
(1997) 79 2438
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
Collins et al Physical Review A (2001) 64 062706