MULTIPHOTON IONIZATION OF N2 BY THE 3RD HARMONIC OF A ND-YAG LASER - A NEW AVENUE FOR AIR DIAGNOSTICS

Ionization of N2 by the third harmonic of a Nd:YAG laser is demonstrated and confirmed experimentally from both spectral and ion-collection measurements. Six photons at 355 nm excite the ground-state neutral molecule to the B2 SIGMA-u+(v' = 8) ionized state. Intense radiation at 391.4 nm associated with the B2 SIGMA-u+(v = 0) --> X2 SIGMA-g+(v" = 0) transition was detected. The intensity of this line scales quadratically with the laser power. An ion concentration of 3.25 x 10(13) ions/cm3 was detected after irradiation by a 45-mJ laser pulse focused to a diameter of 17-mu-m. The potential application of this ionization scheme for air velocity and temperature measurements is presented.