Process and arrangement for photothermal spectroscopy

Abstract

A process and arrangements for photothermal spectroscopy (thermal wave analysis) by the single-beam method with double modulation technique. A single-beam method is developed making use of the advantages of double modulation technique in detecting the photothermally generated difference frequency without requiring partial beams and while achieving extensive absence of intermodulation, the intensity of the laser beam is modulated before striking the object in such a way that the modulation spectrum substantially contains a carrier frequency (f.sub.1) and two sideband frequencies (f.sub.1 .+-.F.sub.2), wherein f.sub.2 is the base clock frequency of the modulation, a regulating detector and a control loop intervening in the modulation process suppress that component of the base clock frequency (f.sub.2) in the same phase with the mixed frequency of the carrier frequency and sideband frequencies. After interaction with the object the optical response of the object is measured by means of a measurement detector and frequency-selective and phase-selective device as the amplitude of that component of the base clock frequency (f.sub.2) which, as the photothermal mixed product, has the same phase as the mixed frequency of the carrier frequency (f.sub.1) and sideband frequency (f.sub.1 .+-.f.sub.2). Use for nondestructive and noncontact analysis of the material parameters of areas of solid bodies close to the surface is described.