CONCENTRATION-MODULATED ABSORPTION-SPECTROSCOPY .5. INFLUENCE OF MOLECULAR-DIFFUSION

The absorption of laser radiation by a chromophore creates a non-equil ibrium distribution of excited-state species whose temporal decay back to the equilibrium distribution can be monitored by the familiar tech niques of pump-probe spectroscopy. When this anisotropic distribution is created in a highly localised region at the focus of a laser beam a nd when the excited-state lifetime is suitably long, molecular diffusi on provides an additional decay route for the recovery of the ground-s tate population in that region of space interrogated by the probe beam . The influence of this spatial decay in space and time on the populat ion difference between the two electronic states coupled by the pump a nd probe laser beams is investigated theoretically and experimentally using the technique of concentration-modulated absorption spectroscopy . A modified form of the 'gain' expression of our earlier studies is p roposed which is shown to account satisfactorily for the form of the ' gain' vs. the logarithm of the modulation frequency curves observed ex perimentally. Although thermal lensing plays a part in influencing the transmission characteristics of the probe laser it is apparent that m olecular diffusion is the dominant process in the experimental arrange ment employed. The potential value of this technique for the study of molecular diffusion in highly localised regions of space is emphasised .