Absolute two-photon absorption and three-photon ionization cross sections for atomic oxygen

Abstract

The technique of two-photon excited fluorescence has been used to measure the cross section for the 3p ${\mathrm{}}^{3}$${P}_{2}$,1,0\ensuremath{\leftarrow}2p ${\mathrm{}}^{3}$${\mathrm{P}}_{2}$ transition in atomic oxygen. Excitation was monitored by observing fluorescence at 845 nm from the 3p ${\mathrm{}}^{3}$${P}_{2}$,1,0\ensuremath{\rightarrow}3s ${\mathrm{}}^{3}$${\mathrm{S}}_{1}$ transition. Spontaneous Raman scattering from ${\mathrm{H}}_{2}$ was used to calibrate the fluorescence collection system. The spatial and temporal profiles of the exciting dye-laser pulses were carefully measured in a mildly focused excitation geometry, allowing the integrated two-photon absorption coefficient to be measured absolutely. The integrated absorption coefficient is the product of an atomic quantity, ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2), and the second-order correlation function of the laser field, ${G}^{(2)}$. The experimental value of this coefficient is ${\mathcal{J}}_{\mathrm{J}\mathcal{'}}$${\mathrm{\ensuremath{\sigma}}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2)${\mathrm{G}}^{(2)}$ =(2.66\ifmmode\pm\else\textpm\fi{}0.80)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{4}$. If chaotic photon statistics are assumed (i.e., ${G}^{(2)}$=2), excellent agreement with the calculations presented in the companion paper is obtained. The cross section for photoionization by a third, identical 226-nm photon has been determined by measuring the absolute number of ions produced per laser pulse. The excited-state photoionization cross section is ${\ensuremath{\sigma}}_{\mathrm{pi}=(5.3\ifmmode\pm\else\textpm\fi{}2.0)\ifmmode\times\else\texttimes\fi{}{10}^{\mathrm{\ensuremath{-}}19}}$ ${\mathrm{cm}}^{2}$.