Two-photon-induced polarization spectroscopy of molecular nitrogen in the <i>a</i><sup>1</sup>Π<sub><i>g</i></sub>(ν′ =) ← <i>X</i><sup>1</sup>Σ<sub><i>g</i></sub><sup>+</sup> (ν″ =) 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<sub>2</sub><sup>+</sup> ions created in the multiphoton absorption process at the high laser intensities required.
© 2000 Optical Society of America
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(230.5440) Optical devices : Polarization-selective devices
(300.6410) Spectroscopy : Spectroscopy, multiphoton
(300.6420) Spectroscopy : Spectroscopy, nonlinear
Clemens F. Kaminski and Thomas Dreier, "Investigation of Two-Photon-Induced Polarization Spectroscopy of the a-X (1, 0) Transition in Molecular Nitrogen at Elevated Pressures," Appl. Opt. 39, 1042-1048 (2000)