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

Rainbow scattering in rotationally inelastic collisions of HCl and H2

We examine rotational transitions of HCl in collisions with H₂ by carrying out quantum mechanical close-coupling and quasi-classical trajectory (QCT) calculations on a recently developed globally accurate full-dimensional ab initio potential energy surface for the H₃Cl system. Signatures of rainbow scattering in rotationally inelastic collisions are found in the state resolved integral and differential cross sections as functions of the impact parameter (initial orbital angular momentum) and final rotational quantum number. We show the coexistence of distinct dynamical regimes for the HCl rotational transition driven by the short-range repulsive and long-range attractive forces whose relative importance depends on the collision energy and final rotational state, suggesting that the classification of rainbow scattering into rotational and l-type rainbows is effective for H₂ + HCl collisions. While the QCT method satisfactorily predicts the overall behavior of the rotationally inelastic cross sections, its capability to accurately describe signatures of rainbow scattering appears to be limited for the present system.

Competing Dynamical Mechanisms in Inelastic Collisions of H + HF

The dynamics of inelastic collisions between HF and H has been investigated in detail by means of time-independent quantum mechanical calculations on the LWA-78 potential energy surface ( Li , G. ; et al. J. Chem. Phys. 2007 , 127 , 174302 ). Reaction probabilities, differential cross sections, and three-vector correlations have been calculated and analyzed. Our results show that there are two competing collision mechanisms that correlate with low and high impact parameters and show very different stereodynamical preferences. The mechanism promoted by high impact parameters is the only one present at low collision energies. We also observe the presence of an apparent threshold in the inelastic cross section for relatively high initial HF rotational quantum numbers, which is associated with the larger energy difference between adjacent rotational quantum states with increasing rotation.

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

A natural generalization of the classical deflection function, the functional dependence of the deflection angle on the angular momentum (or the impact parameter), is the joint probability density function of these two quantities, revealing the correlation between them. It provides, at a glance, detailed information about the reaction mechanisms and how changes in the impact parameter affect the product angular distribution. It is also useful to predict the presence of quantum phenomena such as interference. However, the classical angular momentum-scattering angle correlation function has a limited use whenever quantum effects become important. Rigorously speaking, there is not a quantum equivalent of the classical joint distribution, as the differential cross section depends on the coherences between the different values of J caused by the cross terms in the expansion of partial waves. In this article, we present a simple method to calculate a quantum analog of this correlation, a generalized deflection function that can shed light onto the reaction mechanism using just quantum mechanical results. Our results show that there is a very good agreement between the quantum and classical correlation functions as long as quantum effects are not all relevant. When this is not the case, it will also be shown that the quantum correlation function is most useful to observe the extent of quantum effects such as interference among different reaction mechanisms.

Reaction dynamics and mechanism of the Cl + HD(v = 1) reaction: a quantum mechanical study

Time-independent quantum mechanical calculations have been performed in order to characterize the dynamics and stereodynamics of Cl + HD reactive collisions. Calculations have been carried out at two different total energy values and for various initial states using the adiabatic potential energy surface by Bian and Werner [J. Chem. Phys. 2000, 112, 220]. Special attention has been paid to the reaction with HD(v = 1) for which integral and differential cross-sections have been calculated and the effect of vibrational vs translational energy on the reactivity has been examined. In addition, the reactant polarization parameters and polarization-dependent differential cross-sections have been determined. From these results, the spatial preferences of the reaction and the extent of the control of the cross sections achievable through a suitable preparation of the reactants have been also studied. The directional requirements are tighter for the HCl channel than for the DCl one. Formation of the products takes place preferentially when the rotational angular momentum of the HD molecule is perpendicular to the reactants approach direction. Cross-sections and polarization moments computed from the scattering calculations have been compared with experimental results by Kandel et al. [J. Chem. Phys. 2000, 112, 670] for the reaction with HD(v = 1) produced by stimulated Raman pumping. The agreement so obtained is good, and it improves the accordance found in previous calculations with other methodologies and potential energy surfaces.

Orientation effects in Cl + H2 inelastic collisions: characterization of the mechanisms

Based on quantum mechanical scattering (QM) calculations, we have analyzed the polarization of the product hydrogen molecule in Cl + H(2) (v = 0, j = 0) inelastic collisions. The spatial arrangements adopted by the rotational angular momentum and internuclear axis of the departing molecule have been characterized and used to prove that two distinct mechanisms, corresponding to different dynamical regimes, are responsible for the inelastic collisions. Such mechanisms, named as low-b and high-b, correlate with well defined ranges of impact parameter values, add in an essentially incoherent way, and can be clearly differentiated through the quantum mechanical polarization moment that measures the orientation of the products rotational angular momentum with respect to the scattering plane. Other directional effects turn out to fail when it comes to distinguishing the mechanisms. Quasiclassical trajectories (QCT) calculations have been used as a supplement to the purely quantum mechanical analysis. By combining QM and QCT results, which are in very good agreement, we have succeeded in obtaining a clear and meaningful picture of how the two types of collisions take place.