Two-photon excitation of atomic oxygen at 200.6, 192.5, and 194.2 nm: Absolute cross sections and collisional ionization rate constants.

Abstract

Absolute two-photon absorption cross sections have been measured for the np $^{3}P_{2}$,1,0\ensuremath{\leftarrow}2p $^{3}\mathrm{P}_{2}$ transitions in atomic oxygen (n=4 and 5) using the technique of two-photon--excited fluorescence. For the n=4 transition at 200.6 nm the integrated cross section is ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2) =(4.8\ifmmode\pm\else\textpm\fi{}2.4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}36}$ ${\mathrm{cm}}^{4}$. The corresponding number for the n=5 transition at 192.5 nm is (0.${7}_{\ensuremath{-}0.5}^{+0.7}$)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}36}$ ${\mathrm{cm}}^{4}$. These numbers are in moderately good agreement with ab initio calculations presented previously [Phys. Rev. A 34, 199 (1986)] for n=4 and in this paper for n=5. Calculations are also reported for the 4f $^{3}F_{4}$,3,2\ensuremath{\leftarrow}2p $^{3}\mathrm{P}_{\mathrm{J}\mathcal{'}\mathcal{'}}$ transition at 194.2 nm. Atoms in the np $^{3}P$ excited states produce positive ions and electrons upon collisions with ${\mathrm{O}}_{2}$. In each case, Penning ionization and associative ionization are possible, and cannot be distinguished.Total rate constants for collisional ionization by ${\mathrm{O}}_{2}$ at room temperature are (2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$for n=4 and (4\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=5. Einstein A coefficients are calculated for all allowed fluorescence transitions originating from the 4p $^{3}P$ and 5p $^{3}P$ electronic states. For each state, the sum of the calculated Einstein A coefficients agrees well with the experimental zero-pressure decay rate. Rate constants for collisional removal of population from these states by the background gas (which is mostly ${\mathrm{O}}_{2}$) are 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{3}$${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=4 and 7.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=5. The photoionization cross section for atoms in the 4p $^{3}P$electronic state is (3\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$. For atoms in the 5p $^{3}P$ state the photoionization cross section is less than 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$.