[Experimental evaluation of cross section for Rb(5Dj)+H2-->rbH+H reaction]

The Rb(5Dj )+H2-->RbH[X 1sigma+(v"==0)]+H photochemical reaction was studied in a cell experiment applying a laser pump-absorption technique. Using two-photon excitation of the Rb5 (2)D atomic level in a Rb-H2 vapor mixture, the resulting fluorescence includes a direct component arising from the optically excited state and a sensitized component due to the collisionally populated fine-structure state. The RbH molecules are formed in three-body reactive collisions between excited Rb5 (2)D atoms and ground state H2 molecules. Near-infrared absorption band RbH X (1)sigma+ (v"==0-->v'==17) near 852 nm by using a diode laser was measured. The absorbed intensity of laser beam through a length L of the RbH vapor is defined as deltaI' and deltaI" where deltaI' and deltaI" are the absorbed intensity of pumping 5D(3/2) and 5D(5/2) levels, respectively. The ratio of deltaI' to deltaI" contains information on reactivity. w5D(3/2) and W5D(3/2) are the production rates of Rb in the 5D(5/2) and 5D(3/2) levels by direct laser excitation from the 5S(1/2) level. Using a second experiment in which pump laser is used to pump the 5D(3/2) and 5D(5/2) states in a pure Rb vapor (T = 290 K), and the i'/i" where i' and i" are measured intensities of the 5D(3/2)-->5P(1/2) and 5D(5/2)-->5P(3/2) transition, respectively, is determined. At low density of Rb atoms, the 5D mixing rate is neglect. The rate of 5D(3/2) and 5D(5/2) fluorescence yields the ratio of 5D(3/2) to 5D(5/2) pump production rate. The rate equations were solved, and the authors estimate the value of the cross section at T=385 K and P(H2) =400 Pa for collisional energy transfer from Rb5D(3/2) to 5D(5/2), from Rb(5D)to Rb states other than Rb(5D)to be 9.8 x 10(-16) cm2 and 2.0 x 10(-16) cm2, respectively. The reaction cross sections [i.e., Rb(5Dj)+H2-->RbH+H] for j being 3/2 and 5/2 are 5.4 x 10(-7) and 2.3 x 10(-17) cm2, respectively. The relative reactivity with H2 for two studied atoms is in an order of Rb(5D(3/2)>Rb(5D(5/2)), and this is consistent with the result obtained from a laser pump-probe technique.