Knowledge Bank

We report small homogeneous widths of high overtones of mode 1 (C-C ring breathing) in benzene and $benzene-d6$, in contrast with much larger widths of C-H stretch overtones at similar energies. From the uncertainty principle, these widths provide an upper bound to the potential rate of IVR. We used a tunable. nsec UV laser to excite benzene molecules seeded in a supersonic free jet to the $6112$ vibrational level in the $S1$ electronic state. We recorded emission spectra including transitions terminating at various levels up to ca. $10,000cm-1$ in the $S0$ state. With the same upper state, any changes in transition linewidth would be characteristic of the lower state. Our emission spectra consist of several hundred sharp vibrational bands and a congested background which breaks into many additional bands at higher resolution. Our major finding is that the sharp structure persists throughout the spectrum. Most of these bands can be assigned readily: in particular, we have located members of the $6011n2$ progression. A comparison of the band shapes in this progression indicates little or no change in width. Considering our limiting spectral resolution of $13.5cm-1$, we estimate that the homogeneous linewidths of the highest levels we observe are probably less than $5cm-1$ for both benzene and $benzene-d6$. This result is significantly different from that found by Reddy and co-workers for the C-H and C-D stretching overtones (homogeneous widths of $23cm-1$ for $3vCH$ at $8786cm-1$ in benzene and $53cm-1$ for $4\nu cp$ at $8734cm-1$ in $benzene-d6$). [1] Clearly, the couplings of the C-C ring breathing mode to other modes are much different.