Intracavity millimeter wave spectroscopy of molecules in excited vibrational states

A recently developed highly sensitive intracavity millimeter wave (MMW) spe ctrometer has been applied for spectroscopy of molecules in excited vibrati onal states. The spectrometer utilizes an Orotron oscillator as the tunable , coherent source of radiation from 90 to 160 GHz. A gas cell is placed int o a high quality (Q = 10(4)) Fabry-Perot resonator of Orotron, and the abso rption signal is detected from the variation of the Orotron electron curren t in a collector circuit. The spectrometer combines a wide spectral range w ith Doppler spectral resolution without any phase or frequency lock techniq ues and achieves the sensitivity in absorption of (3-5) x 10(-10) cm(-1). T he measurements are presented for rotational and re-vibrational transitions of SiH4 in the nu(2)/nu(4) vibrational dyad and high-J pure inversion spec trum of ND3 in the nu(2) = 1 vibrational state. Qualitative description of MMW nu(2)/nu(4) dyad spectrum of SiH4 and analysis of available microwave ( MW) data on the pure inversion spectrum of ND3 in nu(2) = 1 state are also given. (C) 2000 Elsevier Science B.V. All rights reserved.