Applications of Photoacoustic Raman Spectroscopy

Abstract

Photoacoustic Raman spectroscopy (PARS) is a nonlinear spectroscopic technique which has been developed recently [1]. The first experimental demonstration of the PARS technique was accomplished using low power continuous wave laser excitation of a gaseous sample [2]. Significant improvements in the experimental capability of PARS were realized by the use of pulsed laser excitation [3]. The application of the PARS technique to the problem of trace analysis of gaseous mixtures has resulted in a trace detection capability of about one part per million [4]. Recent improvements [5] in the design of the photoacoustic sample cell should make it possible to achieve even lower detection limits. The PARS technique has been used successfully to study pure rotational Raman transitions in gases [6–7]. One of the characteristic features of pure rotational PARS spectra is the absence of the Rayleigh component at zero frequency shift. In pure rotational Raman spectra recorded by conventional optical techniques, Rayleigh scattering, which is much more intense than Raman scattering, can sometimes interfere with the observation of low lying rotational states. Care must be taken to reduce stray light in the experimental arrangement (sample cell, spectrometer, etc.) as much as possible in order to be able to successfully record a spectrum. Since the PARS technique detects only the energy that is deposited in the gaseous sample, it is not affected by problems with Rayleigh scattering.