Multiphoton ionization of N_2 by the third harmonic of a Nd:YAG laser: a new avenue for air diagnostics

Abstract
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∑u+(υ′=8) ionized state. Intense radiation at 391.4 nm associated with the B2∑u+(υ=0)→X2∑g+(υ″=0) transition was detected. The intensity of this line scales quadratically with the laser power. An ion concentration of 3.25 × 1013 ions/cm3 was detected after irradiation by a 45-mJ laser pulse focused to a diameter of 17 μm. The potential application of this ionization scheme for air velocity and temperature measurements is presented.