			15min:

CAVITY ENHANCED ABSORPTION SPECTROSCOPY OF THE 1.5 µm BAND SYSTEM OF JET COOLED AMMONIA.

GIEL BERDEN, RUDY PEETERS AND GERARD MEIJER, Department of Molecular and Laser Physics, University of Nij\-megen, Toernooiveld 1, NL-6525 ED Nij\-megen, The Netherlands.

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. have assigned 381 rotational-vibrational transitions to the v₁+v₃ band (at 6609.6~cm^-1) and to the 2v₃ 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 in combination with a slit nozzle expansion.