Resonance-enhanced multiphoton ionization of N2 at 193 and 248 nm detected by N2+ fluorescence

Comparison study of atomic and molecular single ionization in the multiphoton ionization regime

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N2+ to Ar+, a higher ion signal is seen in N2+ compared to Ar+ for linear polarization but the difference vanishes in circularly polarized light. In comparing O2+ to Xe+, we observe a suppression in O2+ compared to Xe+ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O2+ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O2+ in the multiphoton ionization regime.

Multiphoton excitation ionization of N(2) following collisional energy transfer with H(2)O: a potential measure of molecular mixing

A multiphoton ionization excitation of N(2) following collisional energy exchange with optically excited H(2) O was demonstrated, and its potential for measuring H(2) O-N(2) mixing at the molecular level was evaluated. In this process, N(2) is sensitized by a collisional energy exchange with H(2)O molecules excited by a tunable KrF laser. The sensitized N(2) molecules are further excited and ionized by two additional photons of the same laser pulse. Independent images of sensitized N2(+) emission at 391.4 nm and by OH at 308 nm formed by the dissociation of excited H(2)O were obtained along a laser beam traversing slow dry-air and N(2) jets entering room air. Although effects of O(2) and N(2) collisional quenching were noted, such images can potentially be used to measure H(2)O-N(2) molecular mixing and the concentration of H(2)O independently. The detection of H(2)O nucleation by this technique suggests that imaging of H(2)O droplet evaporation or visualizing and monitoring H(2)O condensation may also be possible.

Temperature measurements from first-negative N(2)(+) spectra produced by laser-induced multiphoton ionization and optical breakdown of nitrogen

A 248-nm excimer laser was used to produce ionized nitrogen by the process of multiphoton excitation in gaseous nitrogen at room temperature. First-negative N(2)(+) emission spectra were analyzed to yield rotational temperatures of typically 600 to 1200 K. Rotational Raman scattering of H(2) in gaseous mixtures of N(2) and H(2) was used to determine if laser heating of the gas produced the observed increase in temperature, but the room temperature value of 295 K was inferred from the H(2) Raman data. Therefore the use of N(2)(+) spectra produced by multiphoton excitation at 248 nm does not appear to be acceptable for air-temperature diagnostics. N(2)(+) emission spectra were also recorded subsequent to optical breakdown in air induced by Nd:YAG 1064-nm radiation, and temperatures were determined to be greater than 5000 K in the decaying plasma.