Superexcited state decay pathways

As discussed in Section 8.2, superexcited states, AB, can decay by both autoionization and dissociation (more specifically, by predissociation). Decay by spontaneous fluorescence can be neglected for superexcited states because, generally, the predissociation or autoionization rates (l/rnr 1012 to 1014s-1) are much faster than the fluorescence rate (l/rr 108s-1). Only two examples of detected spontaneous fluorescence from superexcited states have been reported (for H2, Glass-Maujean, et ai, 1987, for Li2, Chu and Wu, 1988). The H2 D1 e-symmetry component is predissociated by an L-uncoupling interaction with the B 1B+ state (see Section 7.9 and Fig. 7.27). Since a 4E+ state has no /-symmetry levels, the /-components of the D1 A-doublets cannot interact with the B E+ state and are not predissociated. The v = 8 level of the D1 state, which lies just above the H/ X2E+ v+ = 0 ionization threshold, could in principle be autoionized (both e and / components) by the X2E+ v+ = 0 en continuum. However, the Av = 1 propensity rule for vibrational autoionization implies that the v = 8 level will be only weakly autoionized. Consequently, the nonradiative decay rate, 1 /rnr, is slow only for the /-symmetry component of the D1 v = 8 state. Thus, in the LIF spectrum of the D1] — 

Several examples of competition between autoionization and predissociation, from the spectra of HC1 and H2, are discussed below. 

Superexcited Rydberg states of HC1 converging to the excited HC1+ A2E+ state may be autoionized by the continuum of the X2n state, predissociated by repulsive Rydberg states built on the dissociative HC1+ a4n ion-core state (see Section 7.11.1), and predissociated by the V1E+ ion-pair state that dissociates to H+ and Cl  

The total width, T, is the sum of partial widths, which can be calculated but not observed separately. Only the total width can be observed experimentally. This width does not depend on whether the line is observed in an absorption, photoionization, photodissociation, or emission spectrum because the width (or the lifetime) is characteristic of a given state (or resonance). In contrast, the peak profile can have different line shapes in different channels the line profile, q, is dependent on the excitation and decay mode (see Sections 7.9 and 8.9). For predissociation into H+CT, the transition moment from the X1E+ state to the 3n (or 3E+) predissociating state is zero, consequently q = oo and the lineshape is Lorentzian. In contrast, the ratio of the two transition moments for transitions to the XE+ continuum of the X2n state and to the (A2E+)1E+ Rydberg states leads to q 0 for the autoionized peaks (see Fig. 8.26) (Lefebvre-Brion and 

The cross section for excitation into the H++ Cl- ion pair state is relevant to the line profile for excitation into the 1E+ Rydberg states. The V1E+ state (configuration 7r4crcr ) is the only state that dissociates into H++ Cl-. This state is involved in a a 4scr electrostatic interaction with the A2E+ (Tr4a)4sa 1E+ Rydberg state (See Fig. 8.24). 

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Figure 8.25 Four spectra which sample different decay pathways for superexcited states of HC1 (a) autoionization of HC1+ (A2E+)nZA (Frohlich and Glass-Maujean, 1990), (b) dissociation into H (n = 2)+Cl (Frohlich and Glass-Maujean, 1990), (c) dissociation into H+Cl (3p4(3P)4s) (White, et al, 1987), and (d) dissociation into H+4- Cl- (Yencha, et al, 1993) [from Lefebvre-Brion and Suzor-Weiner, 1994],...

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