Collisional and radiative properties of theH2E,FΣg1+state

Abstract

Collisional and photoabsorption properties of electronically excited molecular hydrogen are studied by means of selective excitation of the ${\mathrm{H}}_{2}$ ($E$,$F{^{1}\ensuremath{\Sigma}_{g}}^{+}$) double minimum state. The $v=2$ 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 $E$,$F{^{1}\ensuremath{\Sigma}_{g}}^{+}\ensuremath{\rightarrow}B{^{1}\ensuremath{\Sigma}_{u}}^{+}$ 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 (\ensuremath{\sim}100 ${\mathrm{\AA{}}}^{2}$) observed is compared to a Born approximation calculation of inelastic scattering in the ${\mathrm{H}}_{2}$ ($E$,$F{^{1}\ensuremath{\Sigma}_{g}}^{+}$) system and is found to be due to collisional population of the $C^{1}\ensuremath{\Pi}_{u}$ state. Observations of vacuum-ultraviolet $C^{1}\ensuremath{\Pi}_{u}\ensuremath{\rightarrow}X{^{1}\ensuremath{\Sigma}_{g}}^{+}$ emission support this conclusion. The rotational relaxation cross sections are \ensuremath{\lesssim}0.2 \AA{} for ${\mathrm{H}}_{2}$, but are much larger in HD (\ensuremath{\sim}10 ${\mathrm{\AA{}}}^{2}$).