Electrostatic Predissociation

The complete wave function of a molecule will often change in the presence of foreign fields. Foreign fields, either electrostatic or electromagnetic, may under certain conditions change wave functions sufficiently to permit perturbations which would otherwise be unimportant. Perturbations may also be induced by collisions with other molecules, particularly with molecules which are themselves paramagnetic. These effects give rise to what is often called collision-induced predissociation. 

This integral has been evaluated ab initio and found equal to 300 cm-1 (Felen-bok and Lefebvre-Brion, 1966). (The one-center part of this integral is approximately the nonzero atomic integral sp 1/ r 12]dp).) This calculated value is in fair agreement with the semiexperimental value of 450 cm-1 found by a deperturbation procedure (Jungen, 1966). Note that this electrostatic interaction is responsible not only for perturbations between states of identical symmetry but also for predissociation (Section 7.8.1) and auto-ionization (Section 8.8). 

Values are collected in Table 5.4 for the electronic part of the electrostatic interaction. It is clear that this interaction will give rise either to perturbations or to predissociations depending on whether the interaction occurs between discrete states or discrete and continuum states. In the above example, concerning the N2 1IIU states of Worley s third series, it was shown that the electrostatic interaction can also give rise to autoionizations (Chapter 8). 

Figure 7.20 Example of an outer-limb curve crossing an electrostatic predissociation of the N2 C3nu state by the continuum of the C 3nu state. The curves relate the values of level shifts calculated by the coupled equations approach [Eq. (7.12.1)] for energies below the dissociation limit of the C,3ITU state to the level shifts and level widths obtained by the semiclassical method [Eq. (7.6.3) and Eq. (7.6.12)] for energies above the dissociation limit. The points shown on the level shift and width curves correspond to the vibrational energies of the C3nu state (indicated by the turning points on the potential curve). If all of these values had been observed, they would have been insufficient to suggest the actual shape of the SE and T curves. [Courtesy of J.M. Robbe from data of Robbe (1978).]...

Figure 7.25 Example of 2n 2n Rydberg vaJence electrostatic interaction in NO. The v = 0 and 1 levels of the 5pir Q2I1 Rydberg state are perturbed by bound levels (u = 29 and 35) of the B2n valence state, but the v > 2 levels of Q2n are predissociated by the continuum of the same B2II state. (Adapted from Gallusser and Dressier (1982).]...

These /-levels, in addition to being weakly predissociated, are also very weakly autoionized (see also Fig. 8.23). Another example appears in the spectrum of Li2 (Chu and Wu, 1988). In many other cases, decay by predissociation can be very fast, particularly for electrostatic predissociations (see Tables 7.3 and 7.4). For example, in the spectrum of the NO molecule, most 2Il Rydberg states are predissociated by the vibrational continuum of the B2n valence state so rapidly that autoionization cannot compete (Giusti-Suzor and Jungen, 1984). 

Here NaCl(lOO) is the face of a single crystal of salt and the dots represent the physisorbed bond dominated by electrostatic contributions as in the case of van der Waals molecules. The vibrational energy in CO of 2100 cm exceeds the physisorbed bond strength of 1500 cm so predissociation is possible and relaxed CO flies away with kinetic energy AE 600 cm. For the same reason vibrational predissociation is inefficient for N2 -N2 so it is for the relaxation of eq. 12. Indeed attempts to photodesorb CO from NaCl(lOO) have failed and the quantum yield for this relaxation channel is < 10". Theory to account for relaxation channels from excited molecules physisorbed to surfaces is developing but the paucity of experimental data prevents a calibration of the calculations. The selection rule of eq. 5 can be easily extended to qualitatively account for photodesorption and becomes... 

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