Reaction and reorientation of electronically excited H<sub>2</sub>(B)

Abstract

The room temperature rate (TR) constants for fluorescence quenching fluorescence of H2, HD, and D2 B 1Σ$+\atop{u}$ by 4He were measured as a function of the initially excited rotational and vibrational level of the hydrogen molecule, and the RT rate constants for molecular angular momentum reorientation of H2, HD and D2 (B 1Σ$+\atop{u}$. v'=0, J'=1, MJ=0) in collisions with He, Ne, Ar and H2(X 1Σ$+\atop{g}$ ) were also measured. Vibrational state dependence of the quenching cross sections fits a vibrationally adiabatic model of the quenching process. From the vibrational state dependence of the quenching cross section, the barrier height for the quenching reaction is found to be 250±40 cm-1, and the difference in the H-H stretching frequencies between H2(B) and the H2-He complex at the barrier to reaction is 140±80 cm-1. The effective cross sections for angular momentum reorientation in collisions of H2, HD, D2 with He and Ne were found to be about 30 Å2 and were nearly the same for each isotope and with He and Ne as collision partners. Cross sections forreorientation of HD and D2 in collisions with Ar were 10.6±2.0 and 13.9±3.0 Å2, respectively. Reorientation of D2(B) in collisions with room temperature H2(X) occurs with a 7.6±3.4 Å2 cross section. Calculated cross sections using semiclassical and quantum close coupled methods give cross sections for reorientation of H2(B) and D2(B) in collisions with He that agree quantitatively with experiment. Discrepancy between the calculated and experimental cross sections for HD(B)-HE are likely due to rotational relaxation in HD a Turbo PASCAL version of the data-taking program is included.