Millimeter-wave intracavity-jet OROTRON-spectrometer for investigation of van der Waals complexes

A highly sensitive intracavity millimeter-wave spectrometer was developed f or the investigation of the absorption spectra of van der Waals complexes i n a supersonic jet. The key element of the spectrometer is a tunable oscill ator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of a hi gh quality (Q approximate to 10(4)) Fabry-Perot resonant cavity. This cavit y consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the r est of the resonator by a mica foil is kept under ultrahigh vacuum conditio ns. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuri ng the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10-15 kHz providing the capabil ity of sub-Doppler spectral resolution without phase locking. An increase i n sensitivity of a factor of about 100 in comparison with the usual single pass arrangement was evaluated from the measurements of the absorption line s of the CO rare isotopomers, the Ar-CO and Ne-CO van der Waals complexes. The high sensitivity, wide spectral range, and simple tunability of the spe ctrometer make it a very efficient tool for the searching of weakly absorbi ng species in a jet. (C) 2001 American Institute of Physics.