Effect of optical saturation on pulsed photothermal deflection signals in flowing media

A two-level transient-state model has been developed which describes the ef fect of optical saturation on the photothermal signals, valid for condition s where a steady state may not exist. The theory is quite general, as it co nsiders arbitrary pulse shapes, homogeneously and inhomogeneously broadened lines, and arbitrary spectral profile of the excitation. Equations for the temporally and spatially dependent absorption coefficient are derived, and , for generality, time-dependent photothermal deflection signals in a flowi ng medium are calculated (stationary medium being just a special case). The saturation behaviors of the photothermal signals obtained by monochromatic rectangular and Q-switched pulse excitations for homogeneously and inhomog eneously broadened lines are evaluated and compared. Differences between th e predictions of the steady-state and transient-state models are discussed. Experiments have been performed using NO2 as a sample gas in a flowing med ium. The experimental results are in agreement with the predictions of the transient state model. (C) 2000 American Institute of Physics. [S0021-8979( 00)04822-2].