VELOCITY MODULATED LASER ABSORPTION SPECTROSCOPY OF MOLECULAR IONS

Abstract

Doppler shifts in the transition frequencies of molecular ions produced in DC glow discharges were first reported by woods and co-$workers^{1}$ for pure rotational spectra in the 3 = region. These Doppler shifts were $\sim 10$ times smaller than the pressure broadened linewidths and were therefore too small to produce well-resolved red- and blue-shifted components, which could provide velocity modulation of absorption signals for lock-in detection. Because pressure broadening is usually negligible at infrated wavelengths, and high ion velocities (drift random) can berealized in light gases such as $H_{2}$ and He, velocity-modulated ion absorption spectroscopy in audio frequency discharges with lock-in detection at the discharge frequency becomes a straight forward and powerful technique for ion absorption spectroscopy when used in conjunction with a narrow bandwidth tunable laser source. Absorptions due to neutral atoms and molecules, which are much more abundant in these discharges than charged species, are suppressed by about a factor of 100. Furthermore, a first derivative line shape is characteristic of ion signals, while a single Gaussian shape is observed for neutrals, providing unambiguous differentiation between charged and neutral spectra. In this paper we describe a velocity modulation absorption spectrometer which consists of a commercial color center laser (Burleigh FCL-20), an 1 cm $\times$ 1 m liquid nitrogen cooled AC discharge cell, an InSb photovoltaic detector, and a lock-in amplifier. Operating characteristics, including sensitivity, lineshapes, and spectral and dynamical information obtained, will be discussed. The extension of this method to absorption spectroscopy with visible dye laser will be $described.^{1}$ R. C. Woods. T. A. Dixon. R. J. Saykally, and P. C. Szanto, Phys. Rev. Lett. 35, 1269 (1975): C. S. Gudeman, N. D. Piltch, and R. C. Woods, 37th Symposium on Molecular Spectroscopy, Columbus, OH, June 1982, paper TB7. $^{*}$Supported by NSF Grant # CHE8207307.