Improved technique for detection of gas-phase molecular ions by laser absorption spectroscopy

摘要

Abstract: Molecular ions play an important and often dominant role in the gaseous media in which they occur, even when present in very small quantities. Ion densities are typically lower than densities of excited neutrals, often by orders of magnitude. Conventional laser spectroscopy must deal with the strong interfering signal from excited neutrals. One widely used technique for detecting ions is velocity-modulated spectroscopy. A gas-phase discharge is driven by an audiofrequency power supply, impressing a modulation upon the ion velocity distribution. A laser beam traverses the discharge and falls upon a detector, whose signal is demodulated by a lock-in amplifier. The velocity-modulation technique enhances the ion signal and suppresses the interfering signal. However, the technique is plagued by a coherent background signal, arising from electrical pick-up or fluorescence from the discharge. Variation in the background can obscure the desired signal. This paper describes an improvement in the velocity-modulated technique that suppresses the background. A double-modulation technique is used, in which low-frequency amplitude modulation of the laser beam is combined with conventional velocity-modulation spectroscopy. Two lock-in amplifiers are used in series. This suppresses the background by several orders of magnitude. !10