Photoacoustic Raman Spectroscopy with Two Counterprapagating Laser Beams

An improved experiment setup for Stimulated Photoacoustic Raman Spectroscopy（PARS）was reported. The employed two laser beams，unlike general arrangement of propagating in the same direction，counterpropagated with each other. This arrangement can eliminate the limitation that the small Raman shift could not be measured with general arrangement. Therefore，this improved setup was especially useful for measuring the PARS spectrum with small Raman shift，such as the pure rotational Raman spectrum. In the experiment，taking CH4 molecules as a research object，one laser was fixed at 532. 1 nm while another tunable dye laser scanned. The PARS spectrum of CH4 molecules for the v1 ，v2 and v3 bands was recorded in the two wavelength ranges of 625-642 nm and 573-589 nm，and the results agreed well with those obtained by Spontaneous Raman Scattering（ORS）technique，confirming the validity of the improved experimental setup.