[Study of the energy transfer between excited Rb2 and H2 using the CARS technique]

Using the CARS (coherent anti-Stokes Raman spectroscopy) detection technique, the authors investigated the electronic-to-rovibrational levels energy transfer between electronically excited Rb2 and H2. In this CARS experiment, the S-branch (delta upsilon = 1, delta J = 2) transition of H2 was excited by two laser pulses, the pump and the Stokes, respectively, centered at 532 and 690 nm. The internal state distribution of collisionally populated H2 was probed. The scanned CARS spectra reveal that during energy transfer processes H2 molecules were produced only at the upsilon = 1, J = 1,2 and upsilon = 2, J = 0,1,2 rovibrational levels. From scanned CARS spectral peaks the population ratios were obtained. The n1/n5, n2/n5, n3/n5 and n4/n5 are 3.57 +/- 0.71, 2.65 +/- 0.53, 3.00 +/- 0.60 and 0.93 +/- 0.17, respectively, where n1, n2, n3, n4 and n5 represent the number densities of H2 at the rovibrational levels (2,0), (2,1), (2,2), (1,1) and (1,2), respectively. The population ratios indicate that the H2 molecules produced by the energy transfer process are 83% populated at the upsilon = 2 vibrational level and 17% at upsilon = 1. The relative fractions ( : = of average energy disposal were derived as (0.48, 0.01, 0.51), with major translational and vibrational energy release. Through semilog plot of the time-resolved CARS profiles under a simple kinetic model under the experimental conditions of T = 573 K and P = 5 x 10(3) Pa, the collisional transfer rate coefficients k12 = (3.1 +/- 0.6) x 10(-14) cm(-3) x s(-1) and k2 = (4.9 +/- 1.0) x 10(-15) cm(-3) x s(-1) have been obtained.