Two-photon excitation and excited-state absorption cross sections forH2E,FΣg1(v=6): Measurement and calculations

The absolute two-photon excitation cross section for the ${\mathrm{H}}_2$ E,F ${}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}$ (v’=6)←${\mathrm{X}}^1$ ${\mathrm{\Sigma }}_{\mathit{g}}$ (v’ ’=0) Q(1) transition at 193 nm has been measured by observing the E,F ${}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ (v’=6)->B ${}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ (v’ ’=0) fluorescence at ∼750 nm. The measured integrated two-photon excitation cross section, (2.0±0.9)×${10}^{\mathrm{-}36}$ ${\mathrm{cm}}^4$, is in good agreement with the theoretical value of 2.8×${10}^{\mathrm{-}36}$ ${\mathrm{cm}}^4$, which is obtained from previously published calculations [Huo and Jaffe, Chem. Phys. Lett. 101, 463 (1983)]. The absolute cross section for photoabsorption by the E,F state at 355 and 193 nm was also measured by monitoring the depletion of the 750-nm fluorescence caused by a second laser. The measured cross sections for photoabsorption by the E,F (v=6) state are (9.7±2.4)×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 355 nm and (6.4±1.3)×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 193 nm. Comparison with theoretical estimates [Cohn, J. Chem. Phys. 57, 2456 (1972)] of the direct photoionization cross section of this state (8.5×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 355 nm and 3.2×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 193 nm) suggests that other processes may contribute to the photoabsorption.