Band 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.)...

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). 

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

The lifetime can be determined by the measurement of the spectral line broadening produced by the finite lifetime of the initially excited state. A typical fluorescence spectrum of the I2 He excited in the B(v = 12) <— X(v" = 0) band is shown in Figure 24.23 the half-width at halfmaximum intensity converts to a predissociation lifetime of 230 ps. 

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 Line positions were based on published analyses of the well-studied transitions and unpublished spectra by Stark et al.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... 

Table 1 Assignments, frequency (in cm spectra at T = 300 K of the Hj and Dj ), intensity (in a.u.), and predissociation half-widths (in cm using the DFT/B3(H) surface ) for the main bands of the calculated IR ...

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