Ionization of hydrogen and deuterium atoms in thermal energy collisions with metastable He*(2³S) atoms

Abstract

Abstract. We report experimental and theoretical results for the basic autoionization processesHe*(2'S)+H,'D(I~),leadingtoPenning(-He+H',:D'te~(@. &)) andassociative ionization (-HeH+/HeD' (vi, J ') +e-(@, E ) ) . The experimental electron energy spectra. measured at thermal collision energies (&%50 meV), show an angulardependent behaviour which is due to the interplay between the orientation of the collision complex at the moment of autoionization-as referred to the relative velocity direction-and a strongly non-isotropic internal angular distribution for electron emission from the excited quasimolecule. The ab inilio work on He*(Z'S)+H/D includes electron angular momentum dependent complex (resonance-to-continuum) coupling elements. revealing substantial contributions of autoionization into the electronic d wave. Comparison between the anglediRerentia1 experimental electron spectra. measured with an energy resolution of 30 meV, and the corresponding theoretical distributions, obtained in the framework of the local complex potential theory, yields good agreement for both isotopes.

Diatomic potential well depths from analyses of high-resolution electron energy spectra for autoionizing collision complexes

“…Angular momentum exchanging processes lead to some spectral broadening and angle-dependent shifts, as ®rst observed for He*2 3 S Li2 2 S 26. Theoretical calculations for the systems He*2 3 S H, Li showed that electron spectra measured at an angle H 90 (relative to the collision velocity direction) are to a large extent representative for angular-integrated spectra in agreement with experimental observations [6,7,18]. The following discussion exclusively addresses electron spectra measured at H 90 .…”

“…This approximation is equivalent to assuming that the ejected electron does not carry away angular momentum and therefore leaves as an s-wave. Classical estimates [18,26] for the expected angular momentum exchange between the heavy-particle system and the emitted electron show that higher electron partial waves l may in fact be present, but are typically restricted to values l 3 [6,7,18,26]. Angular momentum exchanging processes lead to some spectral broadening and angle-dependent shifts, as ®rst observed for He*2 3 S Li2 2 S 26.…”

“…Combined theoretical and experimental studies have demonstrated that reactions (7a) and (7b) are very well described by local complex potential theory within the Born-Oppenheimer approximation [6,7]. The entrance channel * is well-separated from the ionic curve (i.e., electrons with energies of several eV are emitted) and other excited states are also quite distant.…”

“…Within the electron exchange model for PI [5] the target electron may be viewed to ®ll the Rg* hole with simultaneous emission of the excited electron in Rg*. As demonstrated in great detail in recent experimental and theoretical work on one of the simplest PI/AI systems He*2 3 S 1 H1 2 S 1a2 [6,7], the process can be very well described by local complex potential theory within the framework of the Born-Oppenheimer approximation, as long as the electron energy is suciently large. Within this theory [7±10] the entrance channel Rg* A is described by two (real) local (i.e., only dependent) functions * and C, which form the local complex potential LCP * À iCa2.…”

“…For detailed interpretation of these experimental data close collaboration with the group of Wilfried Meyer proved to be crucial and highly bene®cial. Through the group's continued eorts, most notably by Mladen Movre and Wilfried Meyer himself, accurate ab initio potential curves became available for several basic PI systems: He*2 3 S 1 Y 2 1 S 0 H1 2 S 1a2 , D1 2 S 1a2 , Li2 2 S 1a2 , Na3 2 S 1a2 , He à 2 3 S 1 , and Ca4 1 S 0 , including local autoionization width functions [6,7,15,18,21,25]. Based on these (complex) potentials, scattering calculations for the relevant experimental observables were carried out; through detailed comparison of theoretically predicted with experimentally measured quantities, most notably (angle-dependent) electron energy spectra, substantial insight was gained into the autoionization dynamics in these systems [6,7,15,18,21,22,25,26].…”