Total internal reflectance optoacoustic spectroscopy

Abstract

When radiation undergoes total internal reflection at a solid–gas interface, an exponentially decaying wave, known as the evanescent wave, propagates into the gas. If the incident radiation is amplitude modulated and the gas has an absorption at the wavelength of the radiation, the optoacoustic effect can be produced by absorption of energy from the evanescent wave. Experiments with 10.6 μ radiation internally reflected in an NaCl prism show the amplitude of the optoacoustic signal (in SF6) to be linearly proportional to the product of the square of the electric field amplitude and the penetration depth of the evanescent wave as given by Fresnel’s equations. The amplitude of the optoacoustic signal in a 100-μl detection cell is found to increase linearly with the mole fraction of SF6 in He over the range from 10−5 to 10−2.