Significant infrared absorption can arise from vapors adsorbed on laser window surfaces during exposure to the atmosphere, during exposure to low-vapor-pressure system components, and during surface passivation or stabilization treatments. The photoacoustic effect has proved to be useful quantitative technique for the measurement of surface optical absorption and, when using a carbon dioxide laser source, provides a sensitivity of a few percent of one monolayer for vapor molecules chemisorbed on highly transparent substrates.

The work reported here involves an application of photoacoustic spectroscopy to surface absorption studies of high power laser window materials. Results are presented for the time dependence of surface absorption during controlled vapor exposure and for the wavelength dependence of surface absorption, in the 9–11 μm range, following atmospheric contamination. The application of these techniques to the determination of adsorption rate constants, activation energies, and the chemical identification of surface impurities is discussed.