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AMPD: EPS AMPD
AMPD 10: Sitzung 10
AMPD 10.4: Vortrag
Freitag, 6. April 2001, 11:45–12:10, H104
Recent results on the theory of dissociative recombination: Threshold effects — •Xavier Urbain — Département de Physique, Université catholique de Louvain, chemin du cyclotron 2, B1348 Louvain-la-Neuve, Belgium
The dissociative recombination (DR) of polyatomic ions has for long been at the centre of an intense activity, both theoretical and experimental. Recent advances opened the field to energy-dependent measurements and branching ratio determination. The present paper wants to address the specific question of the influence of thresholds, that is the opening of new dissociative channels, on the total and partial DR cross sections.
In most cases, all channels are present throughout the energy range spanned by the process, and their closed or opened nature determines the role they play in the reaction. In that sense, the most appropriate description is given by the multichannel quantum defect theory, according to which a global reactance matrix is built, that contains all channels irrespective of their long-range behavior, either ionising (closed for dissociation) or dissociative (closed for ionisation). Recent experiments on HD+ have demonstrated the enhancement of DR by the opening of new dissociation routes not directly coupled to the ionisation continuum. This was originally regarded as inconsistent with the two-step picture of DR, i.e. electron capture followed by dissociation, eventually distributed among secondary channels by non-adiabatic couplings at intermediate internuclear distances. In that picture, the flux is expected to be shared by an increasing number of channels as the energy increases, with no global effect on the probability of the process.
The stepwise growth of the cross section accompanying the opening of additional channels can however be explained by their role of competitors below threshold, as they reinforce back–autoionisation. When becoming open, these channels actually compete with the autoionisation due to the non-local character of the interaction and produce a substantial increase of the total dissociation probability. This threshold effect can be accounted for by an appropriate treatment of the dissociative Rydberg channels, that pertain to the ionisation channels below their dissociation threshold, and to the dissociative channels above it. A multistate curve crossing treatment was successfully applied to the HD+ problem, and recent calculations by wavepacket propagation confirm our results.
Another important and more puzzling threshold effect is the channel switching that occurs in specific systems like HeH+. The empirical propensity rule, according to which the highest accessible channel is receiving all the flux, has not received any satisfactory explanation so far. An important clue is the vibronic character of the capture process for all such systems in the absence of any doubly-excited resonant state in the vicinity of the ionic potential well. This situation does not admit a diabatic representation and hence escapes the traditional treatment of DR. A step in that direction has been made by Guberman and Schneider et al. in their studies of the DR of O2+ and H3+, but the channel switching effect is still to be demonstrated. A possible scheme will be discussed, in which DR is mediated by Rydberg resonances that couple to the dissociative continuum by rovibronic predissociation.