Optothermal Spectroscopy

Abstract

The energy absorbed in a sample can be thermally detected at the first step of OA signal generation from sample heating. By analogy with the OA method, this technique may be called optothermal (OT), and it is similar to OA in application and the general measuring techniques. Sometimes it is difficult to separate these methods from each other. In OAS of condensed media with indirect recording of signals, for example, in a gas, nonapparent physical features of OT appear such as heating of the gas when heat is transferred to it from the irradiated sample. When sound waves in a solid sample are directly detected with a piezoelectric pickup joined to it, output signals can also arise from thermal expansion of the sample due to the OT effect. In some cases OA and OT can be used together and favorably complement each other. Therefore, it is pertinent to consider here briefly the characteristics and technique of OT spectroscopy (OTS) and compare its analytical potential with OAS. Originally the possibility of detecting absorbed energy thermally in gases was first noted in [4.1] and later OTS was experimentally realized in [4.2,3] with molecules excited by continuous radiation and in [4.4] with pulsed excitation. As applied to solids, OTS, also known as calorimetry, is widely used to measure absorption in crystals with low absorption [4.5] as well as to analyze highly scattering and absorbing samples using noncoherent radiation sources [4.6] analogously to OAS.