Pulsed photoacoustic system calibration for highly excited molecules: II. Influence of the laser beam profile and the excitation energy decay

Abstract

Previously, we developed a simple semi-theoretical calibration procedure for a pulsed photoacoustic set-up. Also, we analysed the simplest, idealized case: top hat spatial profile of the laser beam and an exponential decay of excitation energy. The spatial profile of the laser beam is usually considered to be top hat or Gaussian in the photoacoustic measurements. In reality, there are always small discrepancies. Also, in these measurements the excitation energy decay is usually considered to be an exponential one. This assumption is commonly valid. Still, a non-exponential decay can exist as well. In this paper, we have examined the influence of these discrepancies on the measurement of the vibrational-to-translational relaxation time, as well as on the calibration of the photoacoustic set-up. We have theoretically examined gas mixtures in the case of pulsed excitation (multiphoton regime). Then, we have verified theoretical conclusions in one experimental example. It has been shown that the non-ideal profile and excitation energy decay can significantly influence the measurement of the vibrational-to-translational relaxation time. Also, it has been shown that they do not significantly influence the calibration of the photoacoustic set-up.