A multiphoton ionization excitation of N<sub>2</sub> following collisional energy exchange with optically excited H<sub>2</sub> O was demonstrated, and its potential for measuring H<sub>2</sub> O–N<sub>2</sub> mixing at the molecular level was evaluated. In this process, N<sub>2</sub> is sensitized by a collisional energy exchange with H<sub>2</sub>O molecules excited by a tunable KrF laser. The sensitized N<sub>2</sub> molecules are further excited and ionized by two additional photons of the same laser pulse. Independent images of sensitized N2<sup>+</sup> emission at 391.4 nm and by OH at 308 nm formed by the dissociation of excited H<sub>2</sub>O were obtained along a laser beam traversing slow dry-air and N<sub>2</sub> jets entering room air. Although effects of O<sub>2</sub> and N<sub>2</sub> collisional quenching were noted, such images can potentially be used to measure H<sub>2</sub>O–N<sub>2</sub> molecular mixing and the concentration of H<sub>2</sub>O independently. The detection of H<sub>2</sub>O nucleation by this technique suggests that imaging of H<sub>2</sub>O droplet evaporation or visualizing and monitoring H<sub>2</sub>O condensation may also be possible.
© 1997 Optical Society of America
Gabriel Laufer, Anthony S. Lee, and Harsha K. Chelliah, "Multiphoton excitation ionization of N2 following collisional energy transfer with H2O: a potential measure of molecular mixing," Appl. Opt. 36, 3278-3287 (1997)