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'P-3(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'P-3(0)) atoms than for Ar(4s(3)P(2)) atoms. The chemiluminescence is assigned to the ArH(B(2)Pi-X(2)Sigma(+)) and ArD(B(2)Pi-X(2)Sigma(+)) 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'P-3(0)) atoms, respectively. Both the chemiluminescence and dark product channels of these reactions probably proceed by chemical reaction rather than by electronic energy transfer.