Relocking of intrinsic angular momenta in collisions of diatoms with ions: Capture of H2(j = 0,1) by H2+

Abstract

Rate coefficients for capture of H(j = 0,1) by H are calculated in perturbed rotor approximation, i.e., at collision energies considerably lower than Bhc (where B denotes the rotational constant of H). The results are compared with the results from an axially nonadiabatic channel (ANC) approach, the latter providing a very good approximation from the low-temperature Bethe-Wigner to the high temperature Langevin limit. The classical ANC approximation performs satisfactorily at temperatures above 0.1 K. At 0.1 K, the rate coefficient for j=1 is about 25% higher than that for j = 0 while the latter is close to the Langevin rate coefficient. The Bethe-Wigner limit of the rate coefficient for j = 1 is about twice that for j = 0. The analysis of the relocking of the intrinsic angular momentum of H during the course of the collision illustrates the significance of relocking in capture dynamics in general.

Deviation of the rate of the reaction from Langevin behaviour below 1 K, branching ratios for the and product channels, and product-kinetic-energy distributions

“…The sets of measurements obtained for different ground-state-beam velocities are presented in different colours, each with its own vertical axis offset by an amount proportional to the velocity increments of the ground-state beam. All measurement sets exhibit a clear enhancement of the relative product yield of about 30% highlighted by full lines, which correspond to a numerical simulation of the experiment based on the rate coefficients reported by Dashevskaya et al [26], as explained below. The dashed vertical lines indicate the positions of maximal H + 3 yields, which, in all cases, correspond to zero relative velocities between the two beams.…”

“…They increase in a step-like manner as the collision energy decreases below 1 K, with a half-rise point at an E coll /k B value of ≈500 mK indicated by the red crosses in the figure. This rise of the rate coefficient at low collision energies results from the charge-quadrupole electrostatic interaction between H + 2 and the neutral molecule (H 2 and D 2 have the same quadrupole moment in the Born-Oppenheimer approximation), as demonstrated earlier for the H + 2 + H 2 reaction [24,26,42]. The amplitude of the rise is, however, qualitatively different and amounts to 28(2)% and 10(1)% for the H + 2 + H 2 and H + 2 + D 2 reactions, respectively.…”

“…(ii) The amplitude of the deviation from Langevin behaviour is expected to be approximately the same for the two reactions but to occur over a broader collision-energy range in the collision system having the smaller reduced mass. Dashevskaya et al [24,26] have demonstrated that the collision-energy width of the deviation scales with the reduced mass as μ −2 (see Figure 7 of Dashevskaya et al [24]). Because the collision-energy and the collision-energy resolution themselves are proportional to μ, the integral over the deviation from Langevin behaviour at low collision energies scales as μ −1 .…”

“…The grey data points in this figure were obtained from measurements carried out for numerous combinations of velocities of the ground-state and H 2 -Rydberg beams, such as those depicted in Figure 3. The rate coefficients were normalised to the values determined at E coll /k B values above 3 K, where the reaction rates are known to closely follow classical Langevin behaviour [12,25,26,43,56]. For each of these measurements, the collision energy E coll = 1 2 μ(v Rg − v GS ) 2 was obtained from the velocities v Rg and v GS selected for the Rydberg and ground-state beams, respectively, which we can determine accurately, as demonstrated in Subsection 3.1.…”

“…Examples involving the theoretical description of fast, barrier-free ion-molecule reactions relevant for the present investigation include Refs. [17][18][19][20][21][22][23][24][25][26].…”