Experimental Investigation of Mechanisms of Forming RbH by Irradiating a Rb+H2 Mixture with Laser Light
SHEN Xiao-yan1,LIU Jing2,DAI Kang2,SHEN Yi-fan2*
1. Department of Chemistry, East China University of Science & Technology, Shanghai 200237, China 2. Department of Physics, Xinjiang University, Urumqi 830046, China
Abstract：The radiation of a laser photoexcited Rb atoms from the ground state to the 5P3/2 level in a mixture of Rb vapor and hydrogen. The energy-pooling collision 5P3/2+5P3/2→5S1/2+5D producted 5D state. The Rb(5P3/2)density and spatial distribution were mapped by monitoring the absorption of a counter-propagating laser beam, tuned to the 5P3/2→7S1/2 transition, which could be translated parallel to the pump beam. In the presence of radiation trapping, the spontaneous radiation rate is multiplied by the transmission factor T5P3/2→5S1/2, which describes the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed. The T5P3/2→5S1/2 is related to the frequency dependent absorption cross section and the density and spatial distribution of atoms in the level of the transition. The effective radiative rates of the Rb D2 line as a function of the H2 pressure were obtained. These quantities were combined with the measured excited atom density and fluorescence ratio to yield absolute energy-pooling rate coefficient. The quenching collision Rb(5P3/2)+H2(v=0)→Rb(5S)+H2(v=2) producted state H2(v=2). This process is at least 16 times faster than the Rb(5P3/2) radiative decay rate. The reverse process of this process is relatively unlikely due to their large translational energy defect. The cross section for the process H2(v=2)+H2(v=0)→H2(v=1)+H2(v=0)+3 920.2 cm-1 is 7.7×10-19 cm2. Hence the relaxation rate of this vibrational level is relatively slow and the nuclear spin statistics is conserved. The H2(v=2) density was determined by using the cross section for Rb(5P3/2)-H2 quenching. RbH was fromed by the Rb(5D)+H2 and Rb(5P3/2)+H2(v=2) reactions and observed by laser absorption. The ratio of 5D→5P3/2 to 5P3/2→5S1/2 fluorescence was measured as a function of the H2 density. The absorption of the laser beam tuned to the X1Σ+→A1Σ+ line of RbH was also measured as a function of the H2 density. From these measurements we obtained the cross section of 4.02×10-17 cm2 for the process Rb(5D)+H2→RbH+H. The cross section for Rb(5P3/2)+H2(v=2) →RbH+H is 1.00×10-18 cm2. This experiment showed that under our experimental conditions, RbH molecules are not created by a direct interaction of Rb(5P) with H2 but through a two-step reaction.
Key words：Laser chemistry; Collisional energy transfer; Fluorescence; Optical absorption; RbH
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