Water assisted multi-photon ionization of N2 by KrF lasers

Effects of water vapor on multiphoton ionization of N2 by a narrowband tunable KrF laser were compared to ionization of dry N2. In dry N2, ionization is non-resonant and requires 3 + 1 photon absorptions. In the presence of water, ionization is through a resonant two-photon excitation of H2O, followed by collisional energy exchange with N2 molecules and two single-photon absorption steps. Excited N2+ ions were detected by emission in the B2Σu+(υ′ = 0) → X2Σg+(υ″ = 0) band. Owing to the resonant enhancement of H2O excitation, this emission is bright and may be used to visualize N2H2O mixing flows.