Opto-Acoustic Spectroscopy of Condensed Matter

Abstract

The availability of tunable lasers in the visible and infrared regions of electromagnetic spectrum has led to a resurgence of a variety of new application of opto-acoustic detection technique first described¹ a hundred years ago. The combination of a reasonably high power tunable spin-flip Raman laser and opto-acoustic cells using very sensitive acoustic microphones have resulted² in an ability to measure gaseous absorption coefficients as small as ~10^-10 cm^-1 using optical cell lengths of only 10 cm and a total gas volume of ~1 cm³. Extensions of this remarkable ability to measure small absorption coefficients in gases to measurements of small absorption coefficients in condensed media has been slow in coming. Straight-forward adaptation of the gas-phase acoustic microphone technique, as discussed in Ref. 3, to the study of condensed media has not been very successful for measuring small absorption losses. During the last two years we have developed a different opto-acoustic technique which is ideally suited for measuring weak optical absorption spectra of condensed phase materials including liquids and solids. The technique differs from the earlier gas phase studies in respect of both the sources of radiation used in the study as well as the transducer used for detection of the acoustic signals.