RELATIVE INTENSITY MEASUREMENTS AND THE HERMAN-WALLIS EFFECT FOR HCN IN THE 2100 cm$-1$ REGION

Abstract

High-resolution FTS measurements have been used with a White cell with a pathlength of 96 to 240 m and pressures from 0.1 to 5.0 mbar to obtain the relative intensities of a number of transitions of HCN in the region from 1920 to $2750 cm^{-1}$. Particular attention has been given to a study of the Herman-Wallis effect for the very weak CN stretching vibration. It is found that the intensity of this band is primarily due to borrowing of intensity from the pure rotational transitions (or from the constant term in the dipole moment expansion). The transition moment of the CN vibrational fundamental is very small because of a near cancellation of the contribution from the CN stretching motion and the CH stretching motion. This near cancellation is very sensitive to changes in the masses of the atoms and a large change in the relative intensities is observed in going from $H^{12}C^{14}N$ to $H^{13}C^{14}N$ to $H^{12}C^{15}N$. The latter two species are observed in natural abundance. Relative intensity measurements have also been made for the $03^{1}0-00^{0}0$ transitions as well as for other vibrational transitions. The wavenumbers of many hot bands and isotopic transitions have been observed for HCN from 1920 to 6500 cm$-1$ and are also under study.