Abstract

Two-photon-induced polarization spectroscopy of molecular nitrogen in the a ¹Π_g(ν′ = 1) ← X ¹ ${\Sigma }_g^{+}$ (ν″ = 0) 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 ${\mathrm{N}}_2^{+}$ ions created in the multiphoton absorption process at the high laser intensities required.

© 2000 Optical Society of America

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