Line width predissociation

Figure C 1.3.5. Spectra of two different infrared bands of HF dimer, corresponding to excitation of the bound (lower panel) and free (upper panel) HF monomers in the complex. Note the additional line width for the bound HF, caused by vibrational predissociation with a lifetime of about 0.8 ns. (Taken from 1211.)...

In the weak interaction approximation, the Wentzel-Rice formula can be applied to calculate the predissociation line width T and thereby the lifetime for the nonradiative decay t = h/T.187-190... 

Ar-N2 (10). and Ar-HCl (16). In this section, we present the results for the Ar-H2 and Ar-HD systems. The Ar-H2 system Is one of the most extensively studied vdW molecules, and several reliable anisotropic potential energy surfaces derived from experimental data are available. Although rotational predissociation (RP) line widths for Ar-H2 systems have not been measured, McKellar (25) has... 

He-CO and He-HF complexes are very suitable systems to verify how well the theory that goes all the way from ab initio calculations of the in-termolecular potentials to line positions and intensities can reproduce the experimentally observed spectra. For both systems high-resolution infrared spectra have been recorded (67, 68). In addition, for He-HF also the rotational predissociation line widths have been measured (68). 

The correctness of the small odd terms has been checked by computing the rotational predissoriation line widths. Rotational predissociation life-... 

Much work has been done to predict the locations of the zeroes of the line width. Child (1976) has demonstrated that, in the case of intermediate coupling strength, a zero occurs in the linewidth whenever the energies of the diabatic and adiabatic levels coincide (and this conclusion is also valid in the case of weak coupling). However, until the work of Kim, et al., (1994) and recently of Cornett, et al., (1999), it seems that the connection between zeroes of predissociation linewidth and q reversal was not appreciated. 

If the tunnelling mechanism provides the only predissociative pathway, the isotopic sensitivity should be greatest when v = 0, since this would correspond to the maximum barrier width. In fact, the rates of predissociation from levels with v = 0 in HCN and DCN, as reflected by the observed line widths, polarization ratios and relative CN(5) quantum yields, are very similar, and an alternative pathway must still be available when the tunnelling route is too slow to compete i.e. when ui = 0 in HCN and when ui < 2 in DCN,... 

It should therefore be clear that, through careful analysis of the details of the corresponding scattering resonance, we can derive knowledge about the resonant metastable predissociative state and its break-up characteristics. Ashton, Child and Hutson have pioneered this approach in applications to van der Waals predissociation processes. Their paper outlines the full theory needed to extract the line widths Fr and the partial line widths Fr f from the energy dependence of the complex S scattering matrices. 

The depth-dependence of the absorption rate in each line is controlled by self-shielding, by shielding by coincident lines of H and H2, and by dust attenuation. In order to account for the first two effects, van Dishoeck and Black (1988) have simulated the full absorption spectrum of CO at each depth into the cloud. Line positions were based on published analyses of the well-studied transitions and unpublished spectra by Stark et al. (19876). Line widths were estimated from high-resolution spectra on the basis of the diffuseness of the bands. Since the predissociation rates are rapid, ss 10 - 10 s , the lines are intrinsically very broad with line widths of 2-20 km s in Doppler velocity units. Figures 8a and 6 contain two small portions of the absorption that would be produced by CO and by H and H2 at the center of a diffuse cloud such as that toward f Oph. It is clear that some bands like the C-X (1,0) band at 1063 A are coincident with strong H or H2 features, and are effectively blocked by them. Figure 86 also illustrates that the predissociation rates... 

The simplest approach to estimate the probability of predissociation due to spin-orbit coupling is the use of the Fermi-Wentzel-Rice golden rule. The line width for radiationless transition from the bound state level Xv to a level of the continuum at energy e can be approximated by... 

Predissociation of molecules in excited states plays a very important role in atmospheric processes. Details of the competition between the radiative and radiationless transitions for 02 and OH are described in Hess et al. The potential curves for H states of NO are shown in Figure 12 together with the important perturber state a n which causes predissociation via spin-orbit coupling. The calculated data for the predissociation rate k, the lifetime, and the line width F for the low vibrational levels of Fl and two rotational levels N are given in Table 2. The calculations predict the predissociation process even at the v = 0 level to be distinctly faster than the radiative transition (r = 70 ns calculated. 

Table 2 Calculated Predissociation Rates. Lifetimes, and Line Width for the C fl State of NO...

Predissociation rates and line widths for use with the golden rule approximation and the nonadiabatic coupling (equation 4) have been calculated for the first few levels of the and 3 E+ states of ArH and ArD. In this case the repulsive part of the and 3 E+ states couple with the repulsive part of the ground state. The computed line width of... 

Among them, Li-i-HP can be considered a benchmark model system [29, 30] because its low number of electrons makes possible to calculate accurate PES s. Its electronic spectrum has been meassured by Polanyi and coworkers [22], and has been recently very nicely reproduced using purely adiabatic PES s [31]. In the simulation of the spectrum[31], the transition lines were artificially dressed by lorentzians which widths were fitted to better reproduce the experimental envelop. The physical origin of such widths is the decay of the quasibound states of the excited electronic states through electronic predissociation (EP) towards the ground electronic state. This EP process is the result of the non-adiabatic cou-... 

The primary observable result of predissociation is, of course, line broadening. A transition involving a predissociating level has a width at half-height E given... 

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