Measurement of small absorbances by picosecond pump-probe spectrometry

The absorption sensitivity of picosecond pump-probe ground state recovery spectrometry is examined critically. It is found that the detection sensitivity is limited by the quantum noise present on the probe laser. This ultrahigh sensitivity is attainable only with lasers producing transform limited pulses and a high-frequency triple modulation detection technique. With a detection limit of 96,000 cresyl violet molecules in the sampled volume, it is shown that this technique is comparable to other trace absorption methods for sensitivity and provides the added selectivity of a two-color experiment. 10 references, 3 figures, 2 tables.