Knowledge Bank

The absorption spectrum of benzene vapor between 3020 and $3125cm-1$ has been recorded at room temperature and at 204 K with Doppler-limited resolution using a difference-frequency laser spectrometer. Since in some regions of this highly complex spectrum many ‘lines’ observed are still blends of two or more transitions, the resolution in such regions was further enhanced by deconvolution to -$0.0012cm-1$(~1/3 of the Doppler width at 204 K). The allowed fundamental transition in this region is the perpendicular, C-H stretching band $\nu 12$ of symmetry $E1u$, centered near $3047.91cm-1$. However, at least three additional fairly strong bands can be observed in the spectrum: one of these decreases in intensity at the lower temperature and can therefore be interpreted as a hot band, presumably $\nu 12+\nu 20-\nu -20$. The other features can be attributed to the combination $\nu 13+\nu 16$ with components $B1u+B2u+B1u$, of which the $E1u$ component is split by vibrational l-type doubling and its intensity is enhanced by close Fermi and z-type Coriolis resonance with $\nu 12$. The $B1u$ and $B2u$ components are coupled to the $E1u$ component by rotational l-type resonance. Additional perturbations are observed in the spectrum. A partial analysis of the observed spectrum will be reported.