Predissociation nonradiative rate

Time-dependent picture. Experimental observations can be discussed in terms of a competition between two processes the radiative process (absorption or emission), characterized by a radiative rate r"1, and a nonradiative process, predissociation, characterized by a nonradiative rate. There are two possible decay pathways for the excited state,... 

Rates for nonradiative spin-forbidden transitions depend on the electronic spin-orbit interaction matrix element as well as on the overlap between the vibrational wave functions of the molecule. Close to intersections between potential energy surfaces of different space or spin symmetries, the overlap requirement is mostly fulfilled, and the intersystem crossing is effective. Interaction with vibrationally unbound states may lead to predissociation. 

Only the total lifetime r of a level can be measured, r is related to the rate of decrease of the number of molecules initially in a given level via both radiative and nonradiative routes. Let kr be the radiative rate constant (the probability per unit time that a molecule will leave the level as a result of emission of a quantum of light) and knr the predissociation rate (the dissociation probability per unit time). Recall that the pressure is assumed to be low enough that the rates are not affected by collisions. The number of molecules leaving the initial state during the time interval dt is given by... 

The mechanism of a predissociation may be characterized by measurements of the lifetime, rv, of each vibrational level or, even better, tvj, of every rotational level, carefully extrapolated to zero pressure. When the two unknown rates that appear in Eq. (7.4.6) have similar magnitudes, it is necessary to partition the observed total decay rate into tt and rnr. If the radiative lifetime is known for a non-predissociated level of the same electronic state, rT can be calculated for predissociated levels assuming an //-independent value for the electronic transition moment. The nonradiative lifetime is then deduced by subtraction of 1 /tt from the experimental 1/t value as follows ... 

When the linewidth exhibits no oscillations, this suggests the occurrence of an inner crossing, but two cases exist where an outer crossing is shown to display no linewidth oscillation. The first example concerns the OD molecule. Below the energy of the curve crossing, the bound free vibrational overlap comes only from the tail of the discrete wavefunction (tunnelling). The nonradiative decay rate is very slow, but it increases smoothly with J [predissociation of the OD A2E+(v = 0-2) levels by the 4E state (Bergeman, et ai, 1981)]. 

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] —... 

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