Photothermal Spectroscopy of Weakly Absorbing Samples Using a Thermal Wave Phase Shifter.

The concept of a thermal wave phase shifter for photothermal studies of weakly absorbing samples is described. Both sample and reference are excited simultaneously, but their respective photothermal signals reach the detector subsequently. Depending on the phase shift photothermal absorption or transmission spectra can be recorded readily with the same sample. The versatility of this technique is demonstrated by spectroscopic studies of a neodymium oxide doped polymethyl metacrylate film. At energy fluences in the order of 10 mjsq cm sensitivities of 10 to the 14th power moleculessq cm are achieved with conventional, unstabilized, light sources. The quantum yield for nonradiative decay is determined to be 90 throughout the entire visible spectrum. Author