Dynamics of electronic energy quenching: The reaction of H2(B)+He

摘要

The room temperature rate constants for quenching of the fluorescence of H2, HD, and D2thinsp;B1Sgr;+uby4He have been measured as a function of the initially excited rotational and vibrational levels of the hydrogen molecule. The effective quenching cross sections increase with increasing vibrational energy from about 1 Aring;2up to a maximum of about 6 Aring;2. The effective cross sections for D2(B,thinsp;vrsquo;= 0) were independent of the rotational level excited for 0 Jrsquo;le; 7, and the cross sections for (vrsquo;= 0,thinsp;Jrsquo;= 0) were about 80percnt; of the values for (vrsquo;= 0,thinsp;Jrsquo;gsim; 0) for all three isotopes studied. Quenching occurs via formation of an electronically excited (H2He)ast; collision complex followed by crossing to the repulsive H2(X)ndash;He potential energy surface. The vibrational state dependence of the quenching cross sections fits a vibrationally adiabatic model for complex formation. From the vibrational state dependence of the quenching cross section, the barrier height for the quenching reaction is found to be 250plusmn;40 cmminus;1, and the difference in the Hndash;H stretching frequencies between H2(B) and the H2ndash;He complex at the barrier to reaction is 140plusmn;80 cmminus;1. Both values are substantially smaller than results fromabinitiocalculations. The rotational state dependence of the quenching cross sections suggests that quenching occurs with H2rotating in a plane perpendicular to the relative velocity vector, in qualitative agreement with the rotational anisotropy of the H2(B)ndash;Heabinitioelectronic potential energy surface.