Quantitative measurements of very weak H2O absorption lines by time resolved intracavity laser spectroscopy

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Abstract
Time resolved quasicontinuous intracavity dye laser absorption spectroscopy (ICLAS) experiments have been made without and with absorbing species inside the cavity. In the absence of an absorber, a near Gaussian shape with a half‐width decreasing as the square root of the time has been found for the spectrum. With atmospheric water vapor as the absorber, absorption lines in the 600 nm range appear on the spectrum, and the absorption intensity increases with time. This time evolution leads to a quantitative determination of the absorption coefficient of the lines. We confirm that ICLAS follows the Lambert–Beer law with an equivalent length of the cell given by Leq = ct (c is light velocity and t is the time between the laser turn on and the time of observation). This experiment result is in good agreement with time‐dependent theories.