ACCOUNTING FOR ABSORPTION SATURATION EFFECTS IN PULSED INFRARED LASER-EXCITED PHOTOTHERMAL SPECTROSCOPY

Equations that relate photothermal lens focal lengths and photothermal deflection angles to saturation absorption coefficients are derived. These equations are derived for two-level absorbers with both homogene ously and inhomogeneously broadened transitions. Initial and time-depe ndent photothermal lens signals are calculated. Equations describing t he zero-time signals are exact to within the simplifying assumptions o f the derivation, while the time-dependent signals are approximate. Th e approximation is performed by the use of a finite series of Gaussian functions to model the temperature change profile distorted by nonlin ear absorption. The excitation irradiance-dependent signal behavior fo r rectangular and exponential excitation pulse time profiles for homog eneously and inhomogeneously broadened transitions are compared. Absor bed energies are used to calculate effective absorbances obtained by t he use of conventional and photothermal lensing spectrometry. The conc lusions drawn from these comparisons are that pulsed laser phototherma l spectroscopy is sensitive to the excitation laser's pulse temporal p rofile and the transition broadening mechanism.