Theory of chemical reactions of vibronically excited H2(B 1Σ+/sub u/). II. Noble gas dihydrides

The recently discovered chemically bound excited state of HeH2 from large MRD--CI calculations can be explained in terms of the highly ionic character of the H2B 1Σ+/sub u/ excited state at R = 4.0 a.u. and the resulting binding between it and the polarized He via electrostatic and overlap (with the diffuse ''H -'' center) effects. Based on this explanation, a theory for chemical reactions of normally nonreactive systems is proposed. These reactions may occur when one of the reacting molecules is excited vibronically to a state of maximum ionicity. The H2B 1summation+/sub u/ state presents a convenient test case. State-specific, open shell SCF calculations have been carried out for the He+H(2, Ne+H(2, and Ar+H(2 systems. The results for He+H(2 agree with the MRD--CI calculations and demonstrate that electron correlation is reasonably constant over the potential energy surface. NeH(2 and ArH(2 molecules are predicted for the first time to bind chemically in an excited 1A' state at geometries similar to HeH(2 and close to a crossing with the unbound ground state into which they can decay radiatively or via nonadiabatic coupling. Our results suggest that, in accordance with the previous observations by Fink et al. [J. Chem. Phys. 56, 900 (1972)] of quenching of fluorescence of the H2B 1Σ+/sub u/→X 1Σ+/sub g/ transition in a He+H(2 system, quenching should be observed in Ne+H(2 and Ar+H(2 systems, but this time for higher vibrational numbers