Reaction of Rb(5²D, 7²S) with H₂

Abstract

The nascent quantum state distributions of the RbH product resulting from the reaction of Rb(5 2 D 3/2,5/2, 7 2 S 1/2 ) with H 2 are determined using a laser pump−probe technique. For the three investigated reactions, the nascent RbH product molecules are found to populate the lowest three vibrational ( v = 0, 1, and 2) levels of the ground electronic state. The relative vibrational populations are determined to be (0.42, 0.31, 0.27) for the Rb(5 2 D 3/2 ) + H 2 reaction, (0.42, 0.33, 0.25) for the Rb(5 2 D 5/2 ) + H 2 reaction, and (0.45, 0.32, 0.23) for the Rb(7 2 S 1/2 ) + H 2 reaction, each corresponding to a high vibrational temperature. The nascent RbH rotational temperatures are found to be slightly below the cell temperature. By comparing the spectral intensities of the RbH action spectra with those of pertinent Rb atomic fluorescence excitation spectra, the relative reactivity with H 2 for the three studied atoms is in an order of Rb(7 2 S 1/2 ) > Rb(5 2 D 3/2 ) > Rb(5 2 D 5/2 ). The relative fractions (〈 f v 〉, 〈 f R 〉, 〈 f T 〉) of average energy disposal are derived as (0.17, 0.04, 0.79) for the Rb(5 2 D 3/2 ) case, (0.17, 0.04, 0.79) for the Rb(5 2 D 5/2 ) case, and (0.14, 0.03, 0.83) for the Rb(7 2 S 1/2 ) case, all having a major translational energy release and a minor rotational energy release. All of the above results support the assumption that the Rb*−H 2 reaction occurs primarily in a collinear C ∞ v collision geometry by a harpoon mechanism, in which the crossing between the ionic Rb + H 2 - energy surface and the neutral Rb*−H 2 energy surfaces plays a very crucial role. A further comparison with two previous results reveals that the average vibrational disposal 〈 f V 〉 in MH changes dramatically as the excited alkali atom M* is varied from K* to Rb* and to Cs*. The 〈 f V 〉 value for the (K* + H 2 ) system is close to the prior distribution limit, but it becomes smaller and smaller for the (Rb* + H 2 ) system and for the (Cs* + H 2 ) system.