FIR FT ABSORPTION SPECTROSCOPY OF SHORT-LIVED SPECIES OF ASTRONOMICAL INTEREST

Abstract

High resolution FT spectroscopy has been applied for the first time to the detection of short- lived species in the far infrared region. It has allowed us to measure the rotational spectrum of OH, SH, and $NH_{2}$, as well as the vibrational spectrum of hydroxylamine $NH_{2}OH$ in the region of the torsion mode $\nu_{9}$ (band center at $386 cm^{-1}$), of which it is the first direct high resolution observation. This technique proved really efficient, and absorption of up to 20% have been obtained using a multiple-pass cell. The large transition moment of the rotational lines in this region compensates for the relatively high noise level. The sensitivity has been estimated to be of the order of $10^{9} mol/cm^{3}$, which is about two orders of magnitude below the most sensitive techniques, such as FIR-LMR. Owing to its wide spectral coverage, FIR-FT spectroscopy is thus suitable for the systematic study of short-lived species.