Role of water vapour in the absorption of nanosecond 266-nm laser pulses by atmospheric air

The absorption of the Nd : YAG fourth harmonic in air and binary mixtures of water vapour with nitrogen and oxygen at atmospheric pressure has been measured as a function of pulse energy (peak intensity). The mixtures obtained by adding equal amounts of water vapour to dry nitrogen and oxygen have been found to differ significantly in absorption. Preliminary quantitative data have been obtained for two- and three-photon absorption cross sections of water and oxygen molecules: σ⁽²⁾(H₂O) = (4 ± 1) × 10^-49 cm⁴ s and σ⁽³⁾(O₂) = (5.6 ± 1.4) × 10^-78 cm⁶ s². The absorption of 266-nm pulses with peak intensities from 0.05 to 2 GW cm^-2 in the near-surface atmosphere has been shown to be determined by two-photon absorption in water vapour and three-photon absorption in oxygen. In moist air containing 1% water vapour, the absorption coefficient for 266-nm laser pulses exceeds that in dry air by four to five times. There is no absorption in nitrogen. We have developed a technique for photoacoustic measurements of multiphoton absorption cross sections in single-component gases and gas mixtures.