Intracavity millimeter wave spectroscopy of molecules in excited vibrational states

A recently developed highly sensitive intracavity millimeter wave (MMW) spectrometer has been applied for spectroscopy of molecules in excited vibrational states. The spectrometer utilizes an Orotron oscillator as the tunable, coherent source of radiation from 90 to 160 GHz. A gas cell is placed into a high quality ( Q ≈10 4 ) Fabry–Perot resonator of Orotron, and the absorption signal is detected from the variation of the Orotron electron current in a collector circuit. The spectrometer combines a wide spectral range with Doppler spectral resolution without any phase or frequency lock techniques and achieves the sensitivity in absorption of (3–5)×10 −10 cm −1 . The measurements are presented for rotational and ro-vibrational transitions of SiH 4 in the ν 2 / ν 4 vibrational dyad and high- J pure inversion spectrum of ND 3 in the v 2 =1 vibrational state. Qualitative description of MMW ν 2 / ν 4 dyad spectrum of SiH 4 and analysis of available microwave (MW) data on the pure inversion spectrum of ND 3 in v 2 =1 state are also given.