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

The reactions of Ar(4s 3 P 2 ) and Ar(4s‘ 3 P 0 ) metastable atoms with H 2 and D 2 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 H 2 and D 2 at 300 K and for H 2 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 H 2 and D 2 . The branching fractions for chemiluminescence, which range from 0.07 to 0.30, are higher for D 2 than H 2 and higher for Ar(4s‘ 3 P 0 ) atoms than for Ar(4s 3 P 2 ) atoms. The chemiluminescence is assigned to the ArH(B 2 Π X 2 Σ + ) and ArD(B 2 Π X 2 Σ + ) transitions. The total quenching rate constants for H 2 decline by factors of 80 and 30 at 80 K, relative to the room-temperature values, for the reactions of Ar(4s 3 P 2 ) and Ar(4s‘ 3 P 0 ) atoms, respectively. Both the chemiluminescence and dark product channels of these reactions probably proceed by chemical reaction rather than by electronic energy transfer.