Knowledge Bank

Improved experimental methods have recently allowed us to analyze the structure of the fundamental bands in both the IR and UV spectrum of benzene with rotational resolution. Deviations from regular behavior could be well accounted for by anharmonic, Coriolis, and rotational l-type interactions. However, the spectra of multiply degenerate states are still incompletely understood. Large splittings of the various vibrational angular momentum components and the observed relative intensities remain to be explained. With sub-Doppler spectroscopy in a collimated beam, rotationally resolved spectra of the two vibronic bands which were previously assigned to the $60\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}(\ell 0\pm 1)(l001)1002(\ell 00)$ and $60\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}(101)1002(ell0)$ transitions (Wilson numbering) were measured in the Munich laboratory. Each band could be separately described in terms of effective rotational and Coriolis $\zeta$-constants. However, the values of these effective band constants would imply unrealistically large deviations from planarity $(C\nu -Bv\prime /2$ of $-5.46\times 10-4$ and $6.72\times 10-4$ respectively for the two bands), and effective $\zeta$'s of -0.0083 and $+0.0656$ instead of the expected values $\mp \zeta 0=\mp 0.578$. We have therefore carried out a simultaneous analysis of the two bands with inclusion of a vibrational l-type interaction of the form in the Hamiltonian. The resulting constants, $Cv\prime -B\prime v$, of $1.6\times 10-5cm-1$, and $\zeta eff$ of -0.5248 and +0.5822, appear very reasonable; the coupling parameter C has the value $1.575cm-1.$ This result, and a similar one for the $60\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}160\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ bands, shows the importance of vibrational $\ell$-type resonances in the multiply degenerate combination states of benzene. The vibrational angular momentum character of such states, which comprise the majority of all vibrational states of benzene at higher excess energies, is therefore not well defined. Important implications for the intramolecular vibrational redistribution and for the nonradiative decay of benzene states are expected to arise.