Stark-modulation spectroscopy of molecules using diode lasers

Abstract

Infrared spectrometers with dipole laser sources have become widely used in molecular spectroscopy in recent years. These sources are sufficiently monochromatic (~i0 -4 cm -I) to reproduce the Doppler contours of the lines of vibrational--rotational transitions of molecules, while the noise characteristics and the stability of the radiation parameters ensure that the power level is recorded with an error of less than 1%, and a modulation depth (the ratio of the variable component of the emission power to the constant component) on the order of 10-5-10 -6 . The uses of these spectrometers in traditional methods of IR spectroscopy are well kno~ (see, for example, [1-3]). In this paper we investigate the possibility of the Stark modulation of the lines of vibrationallrotational transitions to study the spectra and to determine molecular constants. The advantages of the Stark-modulation method are well known from experience in microwave spectroscopy [4], so we ~Jill merely note some features related to its use in IR spectroscopy. Firstly, in the IR band one can modulate absorption lines of both polar and nonpolar molecules, whereas in the microwave band the latter are inactive. Hence, the method can be applied to all molecules, apart from homonuclear diatomic molecules. Secondly, the line intensities in the IR band are considerably greater than in the microwave band, which enables one to operate with low partial pressures of the materials being investigated. Thirdly, the splitting of the lines is determined by the differences between the shifts of the Stark sublevels of the upper and lower states of the transition, and in vibrational--rotational spectra the difference between them is much greater than in the microwave band. Fourthly, the Doppler width of the absorption lines in the IR spectra are approximately a hundred times greater than in the microwave band, so that in the spectra of multiatomic molecules, where the intervals between the lines are usually on the order of 0.01 cm -I, resolution of the Stark structure of the lines is only possible by using methods of nonlinear intra-Doppler spectroscopy [5].