Transient state model of optical saturation of pulsed photothermal signalswith broadband laser excitation

A two-level transient-state model is presented which describes the effect o f optical saturation on the photothermal signals in a flowing medium, valid for conditions where a steady state may not exist. The theory is quite gen eral, as it considers arbitrary pulse shapes, arbitrary spectral profile of excitation, as well as homogeneously and inhomogeneously broadened lines. This model has been used to describe the saturation behavior of the phototh ermal deflection signals obtained with broadband laser excitation. Equation s for the temporally and spatially dependent absorption coefficient are der ived, and time-dependent photothermal deflection signals in a flowing mediu m are calculated. The saturation behaviors of the photothermal signals obta ined by broadband rectangular and Q-switched pulse excitations are compared .