[Rotational and vibrational state distributions of CsH in the reactions of Cs(6 2D5/2) with H2].

The nascent quantum state distributions of the CsH product resulting from the reaction Cs(6D5/2) with Hz were determined using a laser pump-probe technique in a five-arm crossed heat-pipe oven. Cs-H2 mixture was irradiated with pulses of 885.4 nm radiation from a OPO laser, populating 6D5/2 state by two-photon absosption. Laser induced fluorescence was used to detect CsH molecules directly at the collision volume by scanning pulse tunable dye laser over X 1 sigma+ (v", J") --> A 1 sigma+ (v', J' = J" +/- 1) absorption line. The vibration bands (v" = 0, v' = 6) and (v" = 1, v' = 9) were chosen. For the investigated reaction, the nascent CsH product molecules were found to populate the lowest two vibrational (v" = 0, 1) levels of the ground electronic state but could not be detected in any higher vibrational state. Rotational distributions of CsH products obtained for v" = 0 and 1 states appear to be monomodal , peaking in J = 6-8. The rotational population profile is roughly consistent with a statistical distribution at the system temperature. A plot of logarithm of relative population of states J divided by the degeneracy factor (2J + 1) against J (J + 1) was yielded. The linearity of the plot establishes the Boltzmann form for rotational distributions of both the v" = 0 and 1. The rotational temperatures are (458 +/- 20) K and (447 +/- 18) K for v = 0 and 1, respectively. The nascent CsH rotational temperatures were found to be slightly below the cell temperature. The relative vibrational population was determined to be 0.527 and 0.473. The average vibrational and rotational energy release can be computed. The relative fractions (fv), (fR) and (fT) of average energy disposal were derived as 0.25, 0.10 and 0.65 respectively, having a major translation energy release. All of the above results support the assumption that the Cs(6 2D5/2)-H2 reaction occurs primarily in a collinear geometry by a harpoon mechanism but not an insertion.