EXPERIMENTAL DETERMINATION OF INFRARED TRANSITION DIPOLE MOMENTS FOR HNC FROM HERMAN-WALLIS EFFECT

Abstract

The transition dipole moments for the fundamental bands of HNC were obtained from analyses of Herman-Wallis effect on the absorption intensities. All the fundamental bands were measured using a Fourier transform infrared spectrometer (Bruker IFS 120HR) at Nobeyama Radio Observatory. A glow discharge in a mixture of $CH_{3}CN$ ($\sim 50$ mTorr), $H_{2}(\sim 150 ; mTorr)$, and Ar($\sim 100$ mTorr) was used for production of HNC The spectra were recorded with resolution of $0.01 cm^{-1}$ and the absorption path length was 24 m. The experimentally determined first order Herman-Wallis coefficients for the $\nu_{1}$ and $\nu_{3}$ bands, combined with the relative value of the transition dipole moments for the $\nu_{1}$ and $\nu_{3}$ bands derived from the relative intensity measurements, yielded the following values for the transition dipole moments (in Debye), using the expressions for Herman-Wallis coefficients given by $Watson^{8}$. [ R_{1} = 0.194(13),\quad R_{2} = -0.886 (13),\quad R_{3} = -0.169 (11). ] These values are considerably larger than the corresponding values for $HCN^{h}$ and are in reasonable agreement with {ab initio} $values^{ij}$.