Predissociation processes

We can exclude a predissociation process [39] responsible for the decrease of the lifetime for three reasons, (i) Dispersed emission spectra did not show any indication of emission from the fragment monomer [40]. Thus no dissociation occurs on the time scale of the fluorescence emission, (ii) The additional excitation of the van der Waals stretching vibration in benzene-Ar does not lead to a further decrease of the lifetime, (iii) The stronger decrease of the lifetime of the 61 state in benzene-Kr would not be expeced for a predissociation process since the benzene-Kr complex is more strongly bound and has only a slightly higher density of states since the frequencies of the three van der Waals modes do not differ very much from that of benzene-Ar [41]. 

After exclusion of a predissociation process responsible for the lifetime shortening in complexes of benzene with noble gases, we consider the external heavy-atom effect on the intersystems crossing rate as the origin of the lifetime shortening [42]. The strong decrease of the lifetime in the... 

There is no experimental evidence of any predissociation process from excited pyridine in the usual range 2600-2800 A. Investigations have not revealed any stable decomposition products from pyridine excited by wavelengths shorter than 2600 A but HCN and acetylene are formed by flash photolysis.88... 

Another important spectral range, between 200 and 175 nm, is related to the 02 Schumann-Runge band system which includes 18 bands, (2-0) to (19-0) subject to the predissociation processes, particularly in the mesosphere. 

The assigment of the parent ion [FB(CD3OD)2] as the precursor of the fluoroanisole product has been clearly demonstrated by an infrared UV doubleresonance experiment. Here, the product ion signal is monitored as the infrared excitation is scanned. When the IR laser is in resonance with the C-O stretching of the methanol dimer within the 1-2 cluster, a decrease of the product signal is observed since the population of the parent ion decreases by the loss of one methanol molecule through the IR predissociation process (Brutschy et al. 1992). 

Some conclusions on the excited state dynamics (such as the existence of nonradiative and, particularly, predissociative processes, the estimation of dissociation limit, etc.) can be derived from a detailed consideration of the rovibronic structure and measurements of the lifetimes. It is noteworthy that the observation of the fine rovibronic structure can be hampered and its analysis can be complicated by the presence of several isotopes of the constituent atoms (e.g. Ge, Sn, Pb, Cl and Br) and also by a significant increase in the principal moments of inertia due to the presence of heavy elements in the molecule. 

CCU- The authors found a faster predissociation process with time constants of 250 fs (vpu = 3225 cm-1) to 900 fs (vPu = 3450 cm For the subsequent reassociation a time constant of 15 ps was measured. From subsequent investigations of the probe transmission with perpendicular polarization, the authors inferred a fast delocalization of the deposited vibrationally energy along the oligomer chain confirming the findings of Ref. 78. 

Effect of Diatom Stretching Dependence. The features of the poten-tial energy surface most central to a discussion of its effect on the predissociation process are not the individual radial strength functions V j((R), but rather the vibrational matrix elements (integrated over the diatom bond length) of the full potential... 

The method was found equally successful for similar processes in more complicated van der Waals systems. Schatz et al.49 studied vibrational predissociation processes of the type... 

Figure 2.9. Potential energy curves for a predissociative process in a diatomic molecule.

Calculation of the photolysis rate of nitric oxide must include the predissociation process which occurs at wavelengths less than 192 nm. In particular, predissociation occurs (Figure 4.43) in the 5 bands, the (3 bands (v > 6) and the e bands (v > 3). Callear and Pilling (1970 a,b) indicate that emission dominates predissociation in the e bands. At zero optical depth, the contribution of the 7 bands is small compared to those of the 5 and / bands. 

The distinction between direct dissociation processes discussed in the present section and indirect dissociation or predissociation processes discussed in Section 7.3 to Section 7.14 is that in a direct process photoexcitation occurs from a bound state (typically v = 0 of the electronic ground state) directly to a repulsive state (or to an energy region above the dissociation asymptote of a bound state) whereas in an indirect process the photoexcitation is to a nominally bound vibration-rotation level of one electronically excited state which in turn is predissociated by perturbative interaction with the continuum of another electronic state. Direct dissociation, often termed a half collision is much faster and dynamically simpler than indirect dissociation. In a direct dissociation process the distance between atoms increases monotonically and the time required for the two atoms to separate is shorter than a typical vibrational or rotational period (Beswick and Jortner, 1990). 

In addition to line broadening, the predissociation process can cause line shifts. Each discrete or diffuse level can be shifted by its interaction with the entire continuum of the predissociating state, but this effect is considerably smaller than level shifts caused by interactions between discrete levels. The orders of magnitude of predissociation-induced level shifts and linewidths are comparable. 

Figure 10.7 Schematic illustration of the potential energy curves and wave functions for an atom-diatom system governing the vibrational predissociation process, q and R are the coordinates for the diatom and the van der Waals bonds, respectively. The angle between the diatom and atom is held fixed. Taken with permission from LeRoy et al. (1991).

In Section IV.D, the crisis is studied in classical mechanics. In this section, we study how it manifests itself in quantum mechanics. There are two phenomena where the outcomes of the crisis can be observed. The first is bifurcation in reaction paths, which directly corresponds to the changes of the connections. The second is multiexponential decay in the predissociation processes, which reflects the existence of multiple dissociation paths with different decay times. We will further discuss the possibility of observing phenomena related to the crisis. 

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