A millimeter/submillimeter direct absorption spectrometer has been constructed that employs velocity modulation to selectively detect molecular ions. The instrument consists of a phase-locked Gunn oscillator/Schottky diode multiplier source, a gas absorption cell, and an InSb hot-electron bolometer detector. The gas cell is a single-pass system with two ring-type discharge electrodes at either end, which are connected to an rf power supply. Modulation of the ac discharge at a rate of 50 kHz and phase-sensitive detection at 1f allows for selective observation of molecular ion signals and suppression of absorption from neutral species. The spectrometer can also be used in source-modulated mode, where the signal-to-noise ratio for signals generated in an ac plasma are significantly better than for dc discharges. Combining source modulation with the ac discharge for signal detection and velocity modulation for ion identification provides a powerful technique for molecular ion spectroscopy at millimeter/submillimeter wavelengths. This instrument has been used to measure the pure rotational spectra of C O+, HC O+, and S H+ with better precision than previous studies.
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