Predissociation and autoionization

In general, complex features may arise in the absorption spectrum due to competition between the two processes, autoionization and predissociation (see Section 8.12). For example, as will be shown in Section 8.8, the partial width due to electronic autoionization is independent of v for a given nlX Rydberg state, but the partial width due to predissociation can vary considerably from one vibrational level to another because of oscillatory variations of the vibrational part of the interaction with the predissociating state. 

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

In practice, as will be clear from Section III, a majority of the experimental studies of the molecular excitations is based on luminescence measurements. To obtain further information, it is desirable to detect transitions leading to decay channels different from photon emission, e.g., autoionization and predissociation. The description is then quite similar to that developed for photodetection. Denoting by q) the state vector of the final channel, the transition matrix elements <0g-,/c r( ) are replaced by q T E ) (j)g ky and the characteristic detection operator = d> = H qy, with H denoting the interaction energy term responsible for decay of the excited molecular states in channel q. ... 

Here we are chiefly interested in the intrinsic causes of line broadening. We do not include among these dissociation, ionization, predissociation, autoionization, and pieisomerization, since few unambiguous examples of their occurrence have been reported for the low-lying excited electronic states. Attention is devoted to broadening through anharmonicity (vibrational relaxation) and in particular to electronic relaxation. 

Both electronic and vibrational shape resonances arise from a direct process and can be explained by a single potential (McKoy, et al., 1984). Shape resonances (single Vi(r) or Vj(R)) differ from autoionization resonances and predissociation (with the exception of predissociation by rotation), which involve two potentials or two states with different quantum numbers. 

The state AB, often called the superexcited state, is an autoionized or resonance state. Autoionization is called preionization by Herzberg (1950). This can be justified by the analogy between preionization and predissociation. In predissociation, the interaction of a discrete state with the vibrational continuum of the nuclei allows this discrete state a finite probability of dissociation. In preionization, it is the mixing of a discrete state with the electronic continuum that provides a finite ionization probability. 

For NO, the separation between the first two excited states of NO+ is considerably larger than for Nj, giving nm-m > 2. A linewidth of 500 cm-1 has been observed in the ultraviolet spectrum of NO (Takezawa, 1977), for a 3p7r Rydberg state converging to the NO+ b3II excited state. If that width is caused exclusively by electronic autoionization and not by predissociation, it corresponds to a value of about 0.05 for the I parameter. 

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] —... 

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" ... 

A relaxation process will occur when a compound state of the system with large amplitude of a sparse subsystem component evolves so that the continuum component grows with time. We then say that the dynamic component of this state s wave function decays with time. Familiar examples of such relaxation processes are the a decay of nuclei, the radiative decay of atoms, atomic and molecular autoionization processes, and molecular predissociation. In all these cases a compound state of the physical system decays into a true continuum or into a quasicontinuum, the choice of the description of the dissipative subsystem depending solely on what boundary conditions are applied at large distances from the atom or molecule. The general theory of quantum mechanics leads to the conclusion that there is a set of features common to all compound states of a wide class of systems. For example, the shapes of many resonances are nearly the same, and the rates of decay of many different kinds of metastable states are of the same functional form. 

Analyzing the data on molecular gases irradiated by vacuum UV emission,60 Platzman2 has noted that for certain gases the probability of ionization 77 (Eph) is smaller than unity when Eph exceeds Ix by 10 eV or more. This was confirmed in his subsequent study of molecule-noble-gas mixture,61 done in collaboration with Jesse. They have also observed an isotopic effect the substitution of deuterium for hydrogen increases the ionization probability. Platzman thus concluded that in such discrete states with E>lx the predissociation efficiently competes with autoionization. Platzman has named them the superexcitation states (SES). The SES were discussed in a special issue of Radiation Research62 (see also Refs. 25 and 63). 

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