We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b Σg-X Σg transition in gaseous molecular oxygen. Absorptions signals of less than 1 part in 10 6 can be detected.

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The recently developed technique of infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) has been employed in the 3.0 μm region to determine the absolute concentrations of water dimers, trimers, tetramers, and pentamers in a pulsed supersonic expansion for the first time. Additional spectral features are reported, one of which we assign to the bound O−H stretching bands of the hexamer. Additionally, by simple variation of the jet stagnation pressure, the collective O−H stretching absorption from all clusters produced in the expansion was observed to change from that of discrete features of small clusters to band profiles of liquid water and finally to amorphous ice.

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