Predissociation threshold

Photon absorption provides another route for obtaining specific rate constants from highly excited molecules. Making some assumptions about collisional energy transfer, one may use the collision frequency as a clock in the measurement of the competition between photoreaction and collisional stabilization. Alternatively, one may make some model calculations for the specific rate constants for photoreaction and for collisional energy transfer. From these one may compute experimental quantities like photolysis quantum yields as a function of pressure and test them for consistency with experiments. This procedure has been used in a detailed study of NO2 photolysis at wavelengths between 313 and 416 nm (predissociation threshold at 397.9nm) and N2 pressures between 0 and 1000atm. The... 

Xie et al. [62] presented (transition) dipole moments for the X A /C B2 system of SO2 calculated at the MR-AQCC/ANO level of theory. The DMSs and TDMSs were used to simulate spectra in semi-quantitative agreement with available experimental data theoretical studies of the absorption spectrum of SO2 and the resonance emission spectra from vibrational levels in the C 52 electronic state below the predissociation threshold. A strong non-Condon effect is found for the emission spectra. The TDM surfaces were plotted and analysed but the analytical representation was not given. 

M. S. Child I would like to ask to T. Softley what limitations apply to vibrational states of H2 + that can be prepared by the mass-analyzed threshold ionization (MATI) technique. In particular it would be interesting to known whether the states observed in Carrington s H3+ predissociation excitation experiment [1] could be more selectively prepared by the reaction... 

D. M. Neumark We make no effort to produce vibrationally cold O2, since the B < — X transitions to predissociating upper state levels are rotationally resolved and completely understood. In the case of CH3O, we detach the CH3O- just above the detachment threshold so that we do not produce vibrationally excited CH3O. 

Okabe (773) has derived the electronic energy E0(C2H)<4.11 + 0.05 eV from the threshold incident wavelength for the production of C2H, which is predissociated from the electronically excited C2H2. The C2H has a lifetime of 6 //see (81). 

The width of the bands suggests that each of the excited states is strongly predissociated. Most of the recent work has been aimed at determining the branching ratio for the various possible primary processes. These primary processes are listed in Table 1 with their threshold wavelengths (124). 

R. Cote, E. I. Dashevskaya, E. E. Nikitin, and J.Troe, Quantum enhaneement of vibratonal predissociation near the dissociation threshold, Phys.Rev.A 68, 06327xx (2003)... 

Vibrational predissociation (VP) of a van der Waals triatomic complex A..BC is an example of a unimolecular reaction the rate of which is controlled by the intramolecular vibrational energy redistribution (TVR) [1]. Within a rigorous quantum mechanical approach, the VP dynamics is completely characterized by the complex-valued energies E = - /T / 2 that lie above the dissociation threshold of A..BC into an atom A and... 

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. 

Vibrational predissociation is of course intimately related to vibrationally and rotationally inelastic collisions. Both processes involve coupling between the same initial and final state channels, but they differ in Chat the inelastic processes must occur at energies above the appropriate internal motion excitation threshold and are observed in collision experiments, while predissociation (usually) occurs at energies below this threshold and is observed spectroscopically. While the present paper focusses most attention on the phenomenon of predissociation, the nature of the information contained in these two types of experiments will be compared. 

The results in the first two columns of Table III imply that H2(v=1,j 2)-Ar complexes will predissociate almost 3 times as rapidly as H2(v 0,j=2)-Ar. However, within a first-order treatment, rotational inelasticity depends on the same type of squared matrix element of V2(v,j v j r) as does the level width, except that the (isoenergetic) wavefunctions being coupled are both continuum functions lying above the rotational threshold. In tenns of Figure 1, they would be continuum eigenfunctions of V (R) and V2(R) at... 

Another interesting point is Che similarity between Che type of information contained in vibrational predissociation data and that associated with the corresponding (above threshold) inelastic cross sections, and the fact that these phenomena are sensitive Co Che potential energy surface in a different region than are the discrete transition frequencies. Moreover, the present predictions that inelastic rotational and vibrational cross sections for molecular hydrogen will vary inversely as with the size of inert gas partner, decreasing from Ne to Ar to Kr to Xe, and that they will increase rapidly with the degree of internal excitation of the diatom, are of considerable practical importance. 

By forming ion-rare gas atom (Rg) complexes, the dissociation threshold of the system is lowered, generally below the photon energy and these predissociation spectra directly reflect the linear absorption spectrum. This technique has also been used to great effect in anion spectroscopy experiments [13]. The multiphoton dissociation approach remains attractive for systems in which the perturbation of the messenger atom carmot be neglected, or in instruments where rare gas attachment is difficult. 

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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