Nonadiabatic transitions in dissociating molecules

Transitions between electronic states are formally equivalent to transitions between different vibrational or rotational states which were amply discussed in Chapters 9 11. Computationally, however, they are much more difficult to handle because they arise from the coupling between electronic and nuclear motions. The rigorous description of electronic transitions in polyatomic molecules is probably the most difficult task in the whole field of molecular dynamics (Siebrand 1976 Tully 1976 Child 1979 Rebentrost 1981 Baer 1983 Koppel, Domcke, and Cederbaum 1984 Whetten, Ezra, and Grant 1985 Desouter-Lecomte et al. 1985 Baer 1985b Lefebvre-Brion and Field 1986 Sidis 1989a,b Coalson 1989). The reasons will become apparent below. The two basic approaches, the adiabatic and the diabatic representations, will be outlined in Sections 15.1 and 15.2, respectively. Two examples, the photodissociation of CH3I and of H2S, will be discussed in Section 15.3. 

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Figure 1. Potential energy curves of the Nal molecule. Two excitation processes are indicated, where a first field E (t) induces a 11) <— 0) electronic transition, and a second field E2(t) triggers an excited-state dissociation with yield B (t). Due to the nonadiabatic coupling in the region around Rc, predissociation can also occur, leading to ground-state atomic fragments (yield B0(t)).

Experimental beam data on some molecule-surface systems [149] show that the probability for dissociative sticking approaches a constant value (sometimes as small as 0.01-0.05) for energies well above the barrier. A possible and plausible explanation of this observation could be that a nonadiabatic transition from the lower adiabatic surface (see Fig. 4.7) to the upper surface, which is nonreactive, takes place. In order to incorporate such an effect in the dynamical calculations, we need to consider two configurations, AB -f surface, AB -f surface, or perhaps... 

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