Major Pathway For Multiphoton Air Ionization At 248 Nm Laser Wavelength

Multiphoton ionization mechanisms and ionization rates of atmospheric air and constituent gases are studied at the 248-nm KrF laser wavelength within a laser pulse intensity range of 10(8)-10(13) W/cm(2) using both long 25-ns and short 160-fs pulses. We have experimentally shown that it is the photoionization of water vapor naturally contained in atmospheric air that acts as the dominant process of air ionization. (2+1) Resonance-Enhanced Multiphoton Ionization (REMPI) occurs through 2-photon resonant excitation of water molecules, which results in a quadratic dependence of electron density on laser intensity at lower laser intensities of 10(8)-10(10) W/cm(2) in the long pulse and in a cubic dependence at higher intensities of 10(10)-10(13) W/cm(2) in the short pulse. Direct 3-photon ionization and (3+1) REMPI take place in pure O-2 and N-2, respectively, and their contributions to air ionization are in the ratio of 5: 3. The total ionization rate of O-2 and N-2 in atmospheric air is about an order of magnitude less than that of water vapor. Relevant ionization coefficients (effective multiphoton ionization cross sections) have been measured and that for the H2O molecule is more than 2-3 orders of magnitude larger than the others. Published by AIP Publishing.