Knowledge Bank

The $\nu 1+\nu 2$ and $2\nu 1+\nu 9$ combination bands of the HCN dimer (where $\nu 1,\nu 2$ and $\nu 9$ are the frequencies of the outer CH stretch, the inner CH stretch and the bending of the angle between the two monomer units) have been observed. The apparatus used is a high resolution optothermal molecular beam laser spectrometer based on a commercial $1.5\backslash mum$ c.w. color center laser and a 1.5 K bolometer. [Kerstel et al. J. Chem. Phys. 95, 8282 (1991)]. The observation of the weak transitions reported (with a signal to noise of about 10) have been made possible by recent improvements of the spectrometer, namely the addition a $1.5\backslash mum$ resonant power build-up cavity (BUC) and an intracavity electro-optic frequency stabilization system which have been extensively described at last year's conference (talk FB08). Transit time broadening and residual Doppler broadening due to the finite beam divergence give an instrumental linewidth of 5 MHz. Laser frequency jitter gives a negligible contribution because the laser frequency is locked to the transmission peak of the BUC to better than 500 kHz. The $\nu 1+\nu 2$ and $2\nu 1+\nu 9$ bands show comparable intensities (about 10 times less than the $2\nu 1$ overtone) despite the very weak mechanical coupling between the $\nu 1$ and $\nu 2$ modes which was expected to make the $\nu 1+\nu 2$ band much weaker. The $2\nu 1+\nu 9$ band is affected by perturbations which are presently being analyzed.