Diffuse vibrational structures

Pack R T 1976 Simple theory of diffuse vibrational structure in continuous UV spectra of polyatomic molecules. I. Collinear photodissociation of symmetric triatomics J. Chem. Phys. 65 4765... 

The NH(c ri) lluorescencc excitation spectrum shows diffuse vibrational structure corresponding to the absorption spectrum below 1450 A, while the spectrum is continuous above 1450 A. The results indicate that the NH(< n) state may be formed from predissociation of the electronically excited HN3 below the incident wavelength 1450 A, while the NHtc ri) is dissociated directly above 1450 A. 

The ground state is planar X1/ , of C2l, symmetry. O0(Ol—CSCl) = 2.75 + 0.02 eV. Very weak absorption bands in the region 5300 to 7000 A correspond to the a3/l2-X1/l1 transition [Moule and Subramaniam (722)]. Weak absorption bands with fine structure in the region 3900 to 5950 A are ascribed to the A /lj-X /l, transition by Brand et al. (127). Strong bands with diffuse vibrational structure in the region 2400 to 2970 A have been assigned to the B1 /41- X1 /4, transition by Farnworth and King (343). The absorption coefficients in the visible and ultraviolet have been measured by Levine et al. (627) and are shown in Fig. VII-10. 

If the partial spectra cr(E, n) are broader than the spacing between them, the total spectrum atot(E) is structureless the upper part of Figure 6.4 provides a typical example. However, if the widths of the cr(E, n) are smaller than the separations, atot(E) exhibits broad, so-called diffuse vibrational structures. The necessary condition for this to happen is that, according to (6.7), the upper-state PES in the FC region is relatively flat along the dissociation coordinate R. The lower part of Figure 6.4 illustrates this case. 

Symmetric triatomic molecules like H2O, 03, and CO2, for example, are good candidates for the discussion of diffuse vibrational structures in absorption spectra. We will first discuss a simple model system and then illustrate the general predictions by realistic examples. 

Schinke, R., Hennig, S., Untch, A., Nonella, M., and Huber, J.R. (1989). Diffuse vibrational structures in photoabsorption spectra A comparison of CH3ONO and CH3SNO using two-dimensional ab initio potential energy surfaces, J. Chem. Phys. 91, 2016-2029. 

Weide, K. and Schinke, R. (1989). Photodissociation dynamics of water in the second absorption band. II. Ab initio calculation of the absorption spectra for H2O and D2O and dynamical interpretation of diffuse vibrational structures, J. Chem. Phys. 90, 7150-7163. 

At higher energies (i.e. in the vacuum ultraviolet region), COCl exhibits six absorption bands (Fig. 17.19) [1207,1552]. There is a broad absorption with a maximum at 64700 cm", the short-wavelength end of which overlaps with a second vibrationally structured band whose maximum is at 66707 cm" (rj = 267 cm" ) [1207]. Vibrational structure was also found on the next two absorptions, which have band onsets of 72538 cm" (rj = 335 cm" ) and 74810 cm" (> 2 = 562 cm" ), respectively. Only two further absorption bands were detected, an intense feature with an onset at 80816 cm" and a broad band with an onset at 88253 cm" and a maximum at 89043 cm" the latter displays a diffuse vibrational structure (>>3 = 290 cm" ). All of the instances of vibrational structure in the vacuum u.v. spectrum of COClj are simple progressions in a, modes. Continuous absorption occurs above... 

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