Predissociation accidental

From this starting point, the authors develop equations leading to the evaluation of the real symmetric K matrix to specify the scattering process on the repulsive surface and propose its determination by a variational method. Furthermore, they show explicitly the conditions under which their rigorous equations reduce to the half-collision approximation. A noteworthy result of their approach which results because of the exact treatment of interchannel coupling is that only on-the-energy-shell contributions appear in the partial linewidth. Half-collision partial linewidths are found not to be exact unless off-the-shell contributions are accidentally zero or (equivalently) unless the interchannel coupling is zero. The extension of the approach to indirect photodissociation has also been presented. The method has been applied to direct dissociation of HCN, DCN, and TCN and to predissociation of HCN and DCN (21b). 

Predissociation is governed not only by the intersection of the potential energy curves (Franck-Condon principle) but by the selection rules which specify the types of state between which transitions may take place. These are treated fully by Herzberg. Accidental predissociation is said to occur when the dissociation takes place by two radiationless transitions via the intermediacy of a third state. 

Recently Simmons and Tllford (126) have presented spectroscopic evidence for an accidental predissociation of CO at 94,872 cm. This energy is below the 99,650 cm threshold energy for production of 0( D) and just above that for process 3. They observe that the R(30) doublet in the 0,0 band of the E-X system is enhanced in absorption and missing in emission and attribute the predissociation to a perturbing state which correlates with ground state atoms. 

One of the most dramatic manifestations of an interference effect is the vanishing of a line or of an entire band that, on the basis of known Franck-Condon factors and inappropriately simple intensity borrowing ideas, should be quite intense (see Fig. 6.6). This effect can easily be mistaken as an accidental predissociation (Section 7.13). Yoshino, et al, (1979) have studied the valence Rydberg N2 b, E+ cVE+ perturbations. Abrupt decreases in emission intensity for c 4 — X1E+ (v = 1 and 4) and b — X (v = 4) bands had been attributed to weak predissociation rather than perturbation effects (Gaydon, 1944 Lofthus, 1957 Tilford and Wilkinson, 1964 Wilkinson and Houk, 1956). The b (v = 4) C4 (v = 1) and b (v = 13) C4 (v = 4) deperturbation models of Yoshino et al., (1979) provide a predissociation-ffee unified account of both level shift and intensity effects. Weak predissociation effects cannot be ruled out, but are not needed to account for the present experimental observations. 

Figure 7.14 Measured lifetimes of the CH A2A state. The decay of the v = 0 level is purely radiative. The lifetime of the v = 1 level indicates a weak predissociation which has been attributed to the continuum of the X2If state. The shorter lifetimes for the /-levels of the F component probably reflect an X B A accidental predissociation (see Section 7.13) through the B2 - state. [From Elander, et al.( 1979) based on data of Brzozowski, et al.( 1976).]...

When a predissociation is weak, its interpretation is often difficult small first-order effects can be masked by second-order effects. If only a few lines are missing or weakened, it is necessary to consider the possibility of an accidental predissociation, or, in other words, a three-state interaction involving a local perturbation by a weakly predissociated level (See Section 7.13). Predissociation of normally long-lived (metastable) states detected in emission may originate from very small interactions such as spin-spin or hyperfine interaction, as is the case for the I2 B3II0+ state (Broyer, et al., 1976). 

Anomalous isotope effects occur at accidental or indirect predissodations, which are discussed in Section 7.13. The accidentally predissociated v, J-level is perturbed by a v, 7-level that is directly predissociated by a third (unbound) state. The accidentally predissociated level, having acquired an admixture of the perturber s wavefunction, borrows part of the characteristics of the perturber,... 

Indirect or accidental predissociation, which is treated in the following section. 

In Section 7.8 the possibility of predissociation of isolated lines was mentioned. This is usually called accidental predissociation and can be interpreted as perturbation of a nominally bound rotational level by a predissociated level that lies nearby in energy for this value of J. This type of predissociation should more generally be called indirect predissociation, since the predissociation takes place through an intermediate state (or doorway state, see Section 9.2). 

This formula shows how interference between terms in the sum over predissociated perturbers can increase or decrease the accidental width (Lefebvre-Brion and Colin, 1977). 

Figure 7.39 Schematic mechanism of the indirect or accidental predissociation described in Fig. 7.38.

Hi2- However, analysis of the two-level problem (related to accidental predissociation, discussed in Section 7.13, autoionization, discussed in Section 8.4, and Intramolecular Vibrational Redistribution, discussed in Section 9.4.14) provides insights into the unique effects that derive from widths and decay rates of the basis states. 

In the predissociation problem the coincidence can only be accidental since the potentials and the coupling cannot be modified. For a diatomic molecule submitted to an electromagnetic field, fhe wavelength and the intensity of the field are two external parameters which allow one to produce at will such coincidences. This explains the occurrence of ZWRs in laser-induced photodissociation. Such a flexibility can also be exploited to produce the EPs occurring in this context. 

I. Schmidt-Mink and W. Meyer, Predissociation Lifetimes of the b State of Li2 and the Accidental Predissociation of its A Eu" State, Chem. Phys., submitted for publication. 

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