Reaction and reorientation of electronically excited H[sub 2](B)

Abstract

The room temperature rate (TR) constants for fluorescence quenching fluorescence of H[sub 2], HD, and D[sub 2] B [sup 1][Sigma][sub u][sup +] by [sup 4]He 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 H[sub 2], HD and D[sub 2] (B [sup 1][Sigma][sub u][sup +]. v[prime]=0, J[prime]=1, M[sub J]=0) in collisions with He, Ne, Ar and H[sub 2](X [sup 1][Sigma][sub g][sup +]) 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[plus minus]40 cm[sup [minus]1], and the difference in the H-H stretching frequencies between H[sub 2](B) and the H[sub 2]-He complex at the barrier to reaction is 140[plus minus]80 cm[sup [minus]1]. The effective cross sections for angular momentum reorientation in collisions of H[sub 2], HD, D[sub 2] with He and Ne were found to be about 30 [Angstrom][sup 2] and were nearly the same for each isotope and with He and Ne as collision partners. Cross sections forreorientation of HD and D[sub 2] in collisions with Ar were 10.6[plus minus]2.0 and 13.9[plus minus]3.0 [Angstrom][sup 2], respectively. Reorientation of D[sub 2](B) in collisions with room temperature H[sub 2](X) occurs with a 7.6[plus minus]3.4 [Angstrom][sup 2] cross section. Calculated cross sections using semiclassical and quantum close coupled methods give cross sections for reorientation of H[sub 2](B) and D[sub 2](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.