Dynamical regimes on the Cl + H2 collisions: Inelastic rainbow scattering

Abstract

While Cl + H(2) reactive collisions have been a subject of numerous experimental and theoretical studies, inelastic collisions leading to rotational energy transfer and/or vibrational excitation have been largely ignored. In this work, extensive quantum mechanical calculations covering the 0.5-1.5 eV total energy range and various initial rovibrational states have been carried out and used to perform a joint study of inelastic and reactive Cl + H(2) collisions. Quasiclassical trajectories calculations complement the quantum mechanical results. The analysis of the inelastic transition probabilities has revealed the existence of two distinct dynamical regimes that correlate with low and high impact parameters, b, and are neatly separated by glory scattering. It has been found that while high-b collisions are mainly responsible for |Δj| = 2 transitions which dominate the inelastic scattering, they are very inefficient in promoting higher |Δj| transitions. The effectiveness of this type of collision also drops with rotational excitation of H(2). In contrast, reactive scattering, that competes with |Δj| > 2 inelastic transitions, is exclusively caused by low-b collisions, and it is greatly favored when the reactants get rotationally excited. Previous studies focusing on the reactivity of the Cl + H(2) system established that the van der Waals well located in the entrance channel play a key role in determining the mechanism of the collisions. Our results prove this to be also a case for inelastic processes, where the origin of the double dynamical regime can be traced back to the influence exerted by this well that shapes the topology of the entrance channel of the Cl-H(2) system.

Angular momentum–scattering angle quantum correlation: a generalized deflection function

“…The semi-quantitative agreement between the classical and quantum P r ( J ) and DCSs seems to indicate that quantum effects associated with interference between the two groups of partial waves are not expected to be important. 41 Therefore, the Cl + H 2 ( v = 0, j = 0) inelastic scattering seems to be a good example of a collision system in which the QCT and QCT GDFs would be similar.…”

“…Regarding inelastic collisions, some interesting features emerged from previous studies. 41 , 42 QM and QCT calculations using the BW2 PES 43 showed that at relatively high collision energies ( E coll > 0.6 eV) and for small Δ j values (Δ j = j ′ – j ), the inelastic probabilities, P r ( J ), exhibit two maxima separated by a minimum in the QCT and QM results. This minimum was identified as that corresponding to the glory impact parameter.…”

“…The analysis of the results showed that there are two mechanisms responsible for the inelastic scattering, possibly associated with different regions of the PES and resulting in very different stereodynamical behaviours. 41 , 42 Both dynamical regimes depend primarily on the value of the total (here also orbital) angular momentum: (i) for J s below the glory impact parameter, collisions seem to take place following a sort of “tug-of-war” mechanism, 44 which indicates the stretching of the H–H bond; 42 and (ii) for J ⪆ 40 collisions can be assigned to rainbow scattering in which the attractive part of the PES is sampled. 41 For transitions involving higher Δ j , which require more head-on collisions, the contribution of high impact parameters wanes rapidly, and the second maximum in the P r ( J ) leading to small scattering angles disappears.…”

“… 41 , 42 Both dynamical regimes depend primarily on the value of the total (here also orbital) angular momentum: (i) for J s below the glory impact parameter, collisions seem to take place following a sort of “tug-of-war” mechanism, 44 which indicates the stretching of the H–H bond; 42 and (ii) for J ⪆ 40 collisions can be assigned to rainbow scattering in which the attractive part of the PES is sampled. 41 For transitions involving higher Δ j , which require more head-on collisions, the contribution of high impact parameters wanes rapidly, and the second maximum in the P r ( J ) leading to small scattering angles disappears. The semi-quantitative agreement between the classical and quantum P r ( J ) and DCSs seems to indicate that quantum effects associated with interference between the two groups of partial waves are not expected to be important.…”

Rainbow scattering in rotationally inelastic collisions of HCl and H$_2$

“…Aoiz and cowokers have examined the role of long-range attractive forces in l-type rainbow in a series of studies of rotationally inelastic atom + diatomic molecule collisions: Ar+NO, Cl+H 2 , Ne+NO, Xe+NO, and Kr+NO. [31][32][33][34] We note that there is no definitive criterion to classify the rainbows into the two types in advance. 26,28,30 Recent experimental progress in high-resolution measurements of state-to-state DCSs and stereodynamic control of rotationally inelastic collisions may facilitate observation of rainbows with fewer averaging effects.…”