Collisional and photoabsorption properties of electronically excited molecular hydrogen are studied by means of selective excitation of the ${\mathrm{H}}_2$ ($E$,$F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$) double minimum state. The $v=2\mathrm{}$ level of the inner well of the $E$,$F$ state is populated by two-photon absorption of ArF* laser radiation at 193 nm. Intracavity prisms are used to narrow the laser linewidth and tune the laser to excite single rotational levels selectively. Both ${\mathrm{H}}_2$ and HD have been excited in this manner, but the ${\mathrm{D}}_2$ absorption lines are outside the laser tuning range. The population densities of the $E$,$F$ rovibrational levels are measured by monitoring the near-infrared $E$,$F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}\to B{{}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ fluorescent emission. The $E$,$F$-state radiative lifetime, electronic and rotational collisional relaxation rates, and photoionization cross section at 193 nm are measured. The large electronic quenching cross section (∼100 ${\mathrm{\AA }}^2$) observed is compared to a Born approximation calculation of inelastic scattering in the ${\mathrm{H}}_2$ ($E$,$F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$) system and is found to be due to collisional population of the $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state. Observations of vacuum-ultraviolet $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}\to X{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$ emission support this conclusion. The rotational relaxation cross sections are ≲0.2 Å for ${\mathrm{H}}_2$, but are much larger in HD (∼10 ${\mathrm{\AA }}^2$).

• Received 31 July 1979

DOI:https://doi.org/10.1103/PhysRevA.21.607