Double-Resonance-Modulated Microwave Spectroscopy

Abstract

Microwave spectroscopy has been a very useful tool in the determination of many molecular structures. Most of these structures have been determined using a standard⁽¹⁾ Stark-modulation, or single-resonance, spectrometer. In such a spectrometer, a single frequency source operating in the 8–60-GHz frequency range (0.3–2 cm⁻¹) is used to irradiate a gaseous sample. The characteristic absorption of power from this source as a function of frequency gives information concerning the molecular structure and dipole moment of the molecules in the gas. Ordinarily, the power absorbed can be detected by measuring the change in current through a point-contact diode⁽²⁾ as a function of the frequency of the irradiating microwave source. Such an experimental arrangement, however, gives rise to only a dc voltage change, which is somewhat difficult to amplify and detect. In order to enhance signal-to-noise ratio and increase the ease of detection of microwave signals, the standard⁽¹⁾ spectrometer employs a large-amplitude dc electric field to modulate the molecular-gas sample. This electric field is varied between 0 and approximately 2000 V/cm at a rate of (typically) 100 kHz in square-wave fashion. The change in the electric field causes a shift in the characteristic rotational absorption frequencies of any molecules which possess a permanent dipole moment. Because of the square-wave modulation of the electric field, the shift in the absorption frequencies is also square-wave modulated at 100 kHz. Thus, any power absorbed by the gas from the single microwave-frequency source operating in the 8–60-GHz range will also be modulated at 100 kHz. The first harmonic (100-kHz component) of this modulated signal can readily be amplified and detected using narrowband amplifiers and phase-sensitive detection. A considerable enhancement in signal-to-noise ratio is achieved by this technique over that which could be obtained by measuring the dc power absorption.

This research was supported by the National Aeronautics and Space Administration under Contract NASI-6308.