Detection of the a 1Πg (v′=0, 1)←X 1Σ+g (v″=0) transition in N2 by laser‐induced fluorescence

Repopulation of nitrogen excited triplet state following laser-induced filamentation

Laser-induced filamentation was used to study the dynamics of excited molecular nitrogen decay processes. It is well-known that upper excited nitrogen triplet states can be repopulated at time delays far longer than their fluorescence lifetimes. Examination of the time-resolved emission from several different species indicates that there are two major mechanisms acting to repopulate the N2(C(3)Πu) excited state. The results implicate dissociative electron recombination with the nitrogen cation dimer, N4(+), and energy pooling between two N2(A(3)Σu(+)) triplet states as the main pathways to repopulate the emissive upper triplet state. The densities of N2(A(3)Σu(+)) and free electrons produced during filamentation were measured under atmospheric pressures in nitrogen and estimated to be [N2(A(3)Σu(+))]0 = 3 × 10(15) cm(–3) and [e(–)]0 = 3 × 10(13) cm(–3). The methods outlined in this report could find significant utility in measuring the concentration profiles of these important reactive intermediates within laser-induced filaments produced under different conditions.

Investigation of two-photon-induced polarization spectroscopy of the a-X (1,0) transition in molecular nitrogen at elevated pressures

Two-photon-induced polarization spectroscopy of molecular nitrogen in the alpha 1IIg(nu' =) <-- X 1Sigma(g)+ (nu" =) system near 283 nm was performed, and its signal dependence investigated over the pressure range from 1.2 to 5 bars at 300 K. A significant increase of the signal intensity with pressure beyond the expected square law for a two-photon process was observed for pure nitrogen. Similar behavior was also found for a constant nitrogen partial pressure with increasing partial pressures of argon buffer gas. In both cases the spectral linewidth of the excited transitions increased dramatically with overall pressure. A possible explanation is given for the observed behavior in terms of contributions to the nonlinear susceptibility of the medium from the population of one-photon resonantly absorbing excited-state nitrogen and ground state N(2)(+) ions created in the multiphoton absorption process at the high laser intensities required.

Correction of the quenching effect in two-dimensional laser-induced fluorescence measurement of laser-ablation processes

Laser-induced fluorescence (LIF) and two-dimensional laser-induced fluorescence were used for measurement of the fluorescence lifetime of the A(1)?(+) state of BaO molecules in the laser-ablation process of BaTiO(3) and YBa(2)Cu(3)O(7-x) in the oxygen background gas. The zero-pressure lifetime and the quenching rate were also determined as tau(0)=357ns and k=5.1 x 10(-6)ns(-1)mTorr(-1) , respectively. The spatiotemporal changes of the collisional quenching of BaO molecules were successfully visualized, and the influence of the quenching on the spatial distribution imaging of BaO molecule was corrected.

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.