Reactions of Metastable Argon Atoms with Molecular Hydrogen at 300 and 80 K:  Origin of the Ultraviolet Chemiluminescence

The reactions of Ar(4s3P2) and Ar(4s‘3P0) metastable atoms with H2 and D2 have been studied at 300 and 80 K by the stationary and flowing-afterglow techniques. Total quenching constants were measured for both metastable atoms for H2 and D2 at 300 K and for H2 at 80 K. Optical pumping in the stationary-afterglow experiment was used to select one or the other metastable atoms, and chemiluminescence spectra were obtained for state-selected metastable atoms reacting with H2 and D2. The branching fractions for chemiluminescence, which range from 0.07 to 0.30, are higher for D2 than H2 and higher for Ar(4s‘3P0) atoms than for Ar(4s3P2) atoms. The chemiluminescence is assigned to the ArH(B2ΠX2Σ+) and ArD(B2ΠX2Σ+) transitions. The total quenching rate constants for H2 decline by factors of 80 and 30 at 80 K, relative to the room-temperature values, for the reactions of Ar(4s3P2) and Ar(4s‘3P0) atoms, respectively. Both the chemiluminescence and dark product channels of these reactions probably proceed by chemical reaction rather than by electronic energy transfer.