DIFFERENCE FREQUENCY LASER SPECTROSCOPY OF MOLECULAR IONS

Abstract

Address of Nakanaga and Amano: Herzberg Institute of Astrophysics, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6.Author Institution:A hollow cathode discharge cell has proven to be an excellent source of positive ions. In the past few years, we have attained a significant improvement in sensitivity of the system which enables us to search for weaker lines of more complicated ions. In this talk, we will present our recent results on three symmetric top ions of astrophysical interest, $NH_{3}D^{+}, CH_{3}CNH^{+}$, and $SH^{+}_{3}$, detected in absorption in a modulated discharge through the mixtures of $NH_{3}$ and $D_{2}. CH_{3}CN$ and $H_{2}$, and $H_{2}S$ and $H_{2}$, respectively. This is the first high-resolution spectroscopic detection of $CH_{3}CNH^{+}$ and $SH_{3}^{+}$. Both the $\nu_{1}$ band of $CH_{3}CNH^{+}(\nu_{0}-3527.288 cm^{-1})$ and the $\nu_{3}$ band of $SH_{3}^{+}(\nu_{0}=2525.78 cm^{-1})$ exhibit a clear sign of perturbations. In $SH^{+}_{3}$, the perturbation is more conspicuous, and an extra band yet to be identified is also obsered. The rotational constants for the ground state have been determined for both $SH^{+}_{3}$ and $CH_{3}CNH^{+}$ through a least squares fit by using the combination differences. The lines of $SH_{3}^{+}$ show no sign of the inversion doubling, and the preliminary molecular constants obtained agree well with ab initio predictions. The rotational transition frequencies have been calculated to assist in future search for these ions in interstellar space. $NH_{3}D^{+}$ possesses a permanent dipole moment of about 0.26 D, and it will be possible to detect the rotational transitions in the millimeter and sub-millimeter-wave region in interstellar space and in the laboratory. Our analysis of the $\nu_{4}$ fundamental band $(\nu_{0}=3341.0764 cm^{-1})$ provides a prediction of the rotational transition frequencies