Experimental and Theoretical Investigations of Rotational Energy Transfer in HBr + He Collisions

Abstract

Rotational relaxation rates for HBr(v = 1) colliding with helium atoms at room temperature have been measured using a time-resolved optical-optical double resonance technique. Rotational state selective excitation of v = 1 for rotational levels in the range J = 1-9 was achieved by stimulated Raman pumping. The population decay in the prepared states and the transfer of population to nearby rotational states was monitored via 2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy using the g(3)Σ(-)-X(1)Σ(+) (0-1) band. Collision-induced population evolution for transfer events with |ΔJ| ≤ 8 was observed at pressures near 0.7 Torr. The experimental data were analyzed using fitting and scaling functions to generate state-to-state rotational energy transfer rate constant matrices. Total depopulation rate constants were found to be in the range (1.3 to 2.0) × 10(-10) cm(3) s(-1). As a test of current computational methods, state-to-state rotational energy transfer rate constants were calculated using ab initio theory. The total removal rate constants were in good agreement with the measured values, but the transfer probabilities for events with |ΔJ| ≥ 3 were underestimated. Inspection of the anisotropic characteristics of the potential energy surface did not yield an obvious explanation for the discrepancies, but it is most likely that the problem stems from inaccuracies in the potential surface.

Doppler-Resolved Kinetics of Saturation Recovery

“…The background loss rate, k loss , varied between 2 × 10 4 and 7 × 10 4 s –1 , as determined independently from the unbleached transient data recorded in each scan. The functional form of single exponential decay to a final offset is exact for a two-state reversible system returning to equilibrium, as if the bath of all unobserved states were an undifferentiated kinetic species with a single reverse rate to the probed level, and often provides a good, though not exact representation of total RET kinetics. , By restricting the temporal region of interest to the initial stages of relaxation, prior to attainment of full equilibrium, the functional form is flexible enough to represent accurately the type of decelerating decay illustrated in Figure c for times as long as 2 or 3 time constants. The adjustable parameter A ∞ can then be reinterpreted not as the true equilibrium Boltzmann population ratio of the detected state to the sum of all other states, but rather as an empirical deceleration parameter, representing the curvature of the initial log decay.…”

Experimental Investigations of Vibration–Vibration Energy Transfer in HBr(X1Σ+v'' = 5, 6)–H2 Collisions

“…Compared to the theoretical research of modified energy gap(MEG)and statistical power exponential gap (SPEG) [12], the experimental results demonstrated that the theoretical value of the RET rate constant agreed well with the experimental values. The time-resolved two-photon resonance enhancement technique was used to study the rotational relaxation rates of HBr (v = 1, J = 0-9) and He atoms [13]. The total depopulation rate constant was in the range of…”

Experimental investigations on rotation–vibration energy transfer in H2–N2 collisions

“…Heaven et al [18,19] studied the rotational energy transfer between the HBr (v = 1, J) vibrational state and the ground states of HBr and He atoms. The experimental results demonstrate that the theoretical value of the rotational relaxation rate is in good agreement with the experimental value.…”