Collisionless Multiple Photon Excitations in SF₆: Thermal or not?

Abstract

Recently, BLACK, KOLODNER, SCHULTZ, YABLONOVITCH, and BLOEMBERGEN (BKSYB) [1] made the claim that IR laser excitation in polyatomic molecules leads to a thermal population distribution. The far reaching implications of this result for laser induced chemistry stimulated several doubting researchers to perform experiments to either prove or disprove this result [2]. The experiments so far are not conclusive. One might then hope that existing theory could resolve this question, but to date theory remains incomplete, and divided into two regimes: detailed low level coherent excitation including first order rotation-vibration structure [3.a] and high level quasi-continuum (qc) absorption typically described by incoherent rate equations [3.b]. Joining the υ₃ low level excitation with the higher lying qc in a smooth continuous manner has been the major stumbling block in forming a complete theoretical model of multiple photon excitation (MPE) and dissociation (MPD). In fact the lack of a complete model has forced researchers to speculate on the effects of the so called υ₃ ladder “bottleneck” [1]. Therefore, the problem of whether laser excitation in polyatomic molecules leads to a thermal population distribution has remained unresolved.

Work performed under the auspices of the U.S.D.O.E.