CAVITY ENHANCED ABSORPTION SPECTROSCOPY OF THE $1.5 \mu m$ BAND SYSTEM OF JET COOLED AMMONIA

Abstract

The high resolution absorption spectrum of ammonia in the $6400-6800 cm^{-1}$ region, measured at room temperature, contains about 1700 spectral features. Lundsberg-Nielsen $at al.^{a}$ have assigned 381 rotational-vibrational transitions to the $v_{1}+v_{3}$ band (at $6609.6 cm^{-1}$) and to the $2v_{3}$ band (at $6794 cm^{-1}$). In order to aid in the further analysis, we decided to focus on the rotational states with low rotational quantum numbers by recording the absorption spectrum of ammonia in a supersonic molecular jet. Due to rotational cooling the spectrum is strongly simplified, and an assignment of the lowest rotational-vibrational transitions can be made. The spectrum is recorded with a diode laser (tunable over $6270-6630 cm^{-1}$) using the Cavity Enhanced Absorption $technique^{b}$ in combination with a slit nozzle expansion.