Absolute nonresonant multiphoton ionization cross section of NO at 532 nm

Abstract

The ion yields of NO molecules under irradiation of focused, high-intensity laser beams at 532 nm were measured by a reflecting time-of-flight mass spectrometer. The intensity distributions of the laser beams at the focal region were measured by direct imaging through a microscopic objective charge-coupled-device camera assembly. NO does not significantly fragment under high-intensity light field when examined using two laser systems with 6-ns and 35-ps pulse widths, the latter delivering a power density up to ${10}^{14}$ W/${\mathrm{cm}}^{2}$ . The ion yield of NO at various laser power densities was measured over three orders of magnitude, ranging from the unsaturated region to the complete saturation regime. A multiphoton ionization model was used based on rate equation and perturbation theory. The four-photon nonresonant multiphoton ionization cross section for NO at 532 nm was determined to be (7.1\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}117}$ ${\mathrm{cm}}^{8}$ ${\mathrm{s}}^{7}$ by fitting experimental data to the model.