Assignments of Laser Lines in Optically Pumped CH3CN

Abstract

Near-millimeter laser action from optically pumped CH 3 CN was first obtained by Chang and Mc Gee in 1971 [1]. New emissions were then reported by the same authors [2] and by Radford [3]. At the present the methyl cyanide molecule has been used for obtaining more than thirty lines in the wavelength range from 281 μm up to 1814 μm. Only for the seven lines reported first partial assignments were given. The purpose of this work is to assign more FIR lines through the identification of the rotovibrational transitions of CH 3 CN which are in coincidence with CO 2 laser lines. In similar cases the assignment of the transitions on an optically pumped laser has allowed an analysis of the rotovibrational structure of the active molecule, as in [4] for CH 3 Br. When for the assigned IR and FIR transitions precise frequency measurements are available, the inclusion of the experimental values in a simultaneously least-squares fit all the spectroscopic data, can provide new accurate sets of molecular constants, as in [5] for CH 3 F. Methyl cyanide is a symmetric top molecule, with a large electric dipole moment. Many microwave spectroscopy investigations have been performed and high resolution rotational spectra were recently reported for ground an excited states [6,7]. Extensive studies have been made of the molecular infrared spectrum. The most recent investigations [8, 9] have pointed out the importance of Fermi and Coriolis interactions in the overlapping band systems.