Observations of Excited Metastable and Radiative States of He2, Ne2, and Ar2 by Neutralized Ion-Beam Spectroscopy

Rare gas dimers formed when a fast beam of X₂+ (X = He, Ne, Ar) is neutralized in the reaction $$ X_2^ + + {K_{(g)}} \to X_2^* + {K^ + } $$ have been studied by neutralized ion beam techniques. From measurements of the kinetic energy released in the dissociation of X₂* and with consideration of available spectroscopic information, it is proposed that the neutral dimers are formed initially in their first excited singlet and triplet electronic states. Estimates of the radiative lifetimes of the 3∑ _u + states of He₂*, Ne₂*, Ar₂* are >3.2, >7.2, and 2.1 µs, respectively. Radiation from the 1∑ _u + → X1∑ _g + transitions is predicted to occur at λ ≈ 83, 80, and 114 nm for He₂, Ne₂, and Ar₂, respectively. Ion beam attenuation cross-sections for He₂+, Ne₂+, and Ar₂+ are 250, 180, and 80 Å2, respectively, indicating that electron transfer from K_(g) is a highly efficient process. The capability of generating relatively intense beams of metastable He₂ and Ne₂ dimers may have applications for other beam experiments.