Partial Integral Cross Sections Research Articles

Coupled-channel investigation of rotationally and vibrationally inelastic collisions between He and H2

The coupled-channel formalism has been employed for He + H2 collisions including rotational (j = 0, 2) and vibrational (n = 0, 1) levels and making use of the Kraus-Mies potential energy surface. Integral and differential cross sections are obtained at the total energies 0.3884, 0.9, and 1.4116 eV for rotational and vibrational transitions separately, and for the combined processes. Considerable use has been made of the microscopic reversibility principle to extract cross sections from the S matrix for de-excitations as well as for excitations. Cross sections for vibrational excitation are found to decrease when accompanied by rotational excitation. For vibrational de-excitation, the effect of rotational excitation is to increase the cross sections. Defining a most probable impact parameter bmax from the maximum in the partial integral cross sections, vibration excitation is found to occur at low bmax, while rotational excitation is observed at comparatively higher values. For simultaneous rotational and transitions, bmax decreases with increasing energy transfer. A deflection angle ©(cl is defined in terms of the impact parameters and the turning point radii for the initial and final channels, and a correlation between this angle and the maxima in the computed angular distributions is found. The transition probabilities for the rotational excitation j = 0 2 with simultaneous vibrational excitation and their physical interpretation are also discussed within the total angular momentum coupling scheme.

An analytic model of rotationally inelastic collisions of polar molecules in electric fields

We present an analytic model of thermal state-to-state rotationally inelastic collisions of polar molecules in electric fields. The model is based on the Fraunhofer scattering of matter waves and requires Legendre moments characterizing the "shape" of the target in the body-fixed frame as its input. The electric field orients the target in the space-fixed frame and thereby effects a striking alteration of the dynamical observables: both the phase and amplitude of the oscillations in the partial differential cross sections undergo characteristic field-dependent changes that transgress into the partial integral cross sections. As the cross sections can be evaluated for a field applied parallel or perpendicular to the relative velocity, the model also offers predictions about steric asymmetry. We exemplify the field-dependent quantum collision dynamics with the behavior of the Ne-OCS((1)Sigma) and Ar-NO((2)Pi) systems. A comparison with the close-coupling calculations available for the latter system [Chem. Phys. Lett. 313, 491 (1999)] demonstrates the model's ability to qualitatively explain the field dependence of all the scattering features observed.

Photodissociation of HBr. 1. Electronic Structure, Photodissociation Dynamics, and Vector Correlation Coefficients

Ab initio potential energy curves, transition dipole moments, and spin-orbit coupling matrix elements are computed for HBr. These are then used, within the framework of time-dependent quantum-mechanical wave-packet calculations, to study the photodissociation dynamics of the molecule. Total and partial integral cross sections, the branching fraction for the formation of excited-state bromine atoms Br(2P(1/2)), and the lowest order anisotropy parameters, beta, for both ground and excited-state bromine are calculated as a function of photolysis energy and compared to experimental and theoretical data determined previously. Higher order anisotropy parameters are computed for the first time for HBr and compared to recent experimental measurements. A new expression for the Re[a1(3) (parallel, perpendicular)] parameter describing coherent parallel and perpendicular production of ground-state bromine in terms of the dynamical functions is given. Although good agreement is obtained between the theoretical predictions and the experimental measurements, the discrepancies are analyzed to establish how improvements might be achieved. Insight is obtained into the nonadiabatic dynamics by comparing the results of diabatic and fully adiabatic calculations.

Interaction anisotropy and quantum dynamics for vibrationally inelastic collisions of LiH( 1Σ) with He( 1S)

The ab initio potential energy surface (PES) already computed for the title system is employed here to study the behaviour of the collisional energy transfer processes over a broad range of energies. The quantum evaluation of rotationally and vibrationally inelastic partial integral cross sections is carried out under different angular momentum coupling conditions and the results are closely analysed in terms of the specific features of the coupling PES. State-to-state rate constants for excitation and relaxation processes are obtained in a broad range of temperatures and the overall efficiency of the collisional energy transfer is analysed. The cooperative effect between the rotational anisotropy and the vibrational excitations is clearly shown and discussed.

Vibrational Heating Efficiency of LiH Molecules in Collision with He Atoms

The full Born−Oppenheimer (BO) potential energy surface that describes the interaction between a vibrating and rotating LiH molecule and an He atom, obtained from ab initio VB calculations described before (Chem. Phys. 1997, 215, 287), is employed here to evaluate the corresponding partial integral cross sections over a range of energies of astrophysical relevance. The process examined involves rotovibrational heating of the initially “cold” target molecule in a single-collision situation, and it provides specific indications on the process efficiency for modeling global kinetics of primordial clouds. The details of the dynamical coupling produced and some specific features of the potential energy surface are analyzed in relation to the behavior of the inelastic cross sections. The average energy transfer values and the relaxation rates are obtained from quantum dynamics and found to be unexpectedly large for such a weakly interacting system.

Atom–molecule interactions from multiproperty analysis. An integrated study of the dynamics for oxygen–rare-gas systems

The interaction potential between the ground electronic state of the oxygen molecule and the helium atom is obtained through a numerical fit of data from collision experiments and is given via an effective, spherically symmetric analytic form. The latter expression, together with similar ones obtained previously for the O2–Ar and O2–O2 systems, is then used within a quantum-mechanical model to treat vibrationally inelastic processes and to generate various partial integral cross-sections at several energies. A comparison with experimental relaxation data is then carried out for the three cases examined; the results show generally good agreement for the He–O2 and Ar–O2 systems, suggest a greater influence of rotational inelasticity for the O2–O2 case and allow us to assess the importance of Van der Waals complex formation on the relaxation mechanism.

Computed (V, T) energy transfer from realistic interactions. I. The O 2Ar mixture

An experimentally determined potential energy surface (PES) for the O 2Ar gaseous mixture was here employed to evaluate vibrationally inelastic partial integral cross sections, as functions of collision energy, within a coupled quantum treatment of the dynamics. The spherical component of that interaction, determined from a multiproperty analysis of the above system, was thus used to generate the approximate vibrational coupling matrix elements with which the scattering equations were in turn numerically solved. Vibrational relaxation times were also obtained by convoluting computed cross sections over velocity distributions for a wide range of temperatures in the gaseous mixture and were finally compared with available experiments. The very good agreement found with the latter data, for T values varying between 8000 and 300 K, provides here a further, independent test of the high quality of experimental potential given in an accompanying paper.

Vibrational relaxation of HF colliding with helium. A coupled quantum treatment

Vibrationally inelastic, partial integral cross sections are computed for the HeHF system by solving the vibrational close-coupled (CC) equations via a spherical potential already suggested in the literature. Numerically converged results are obtained, at several collision energies, for single- and multiquantum transitions relevant to yield rate constants in the 1000–10 000 K range of temperatures. Very satisfactory agreements is found with the available experimental data; the highly quantum nature of the present system is surmised as causing the failing of more approximate approaches used before.

Investigation of the assumptions of the multiple-scattering method for electron–molecule scattering cross sections

We present a comparative test of three assumptions critical to the applicability of the multiple scattering (MS) method as it has been applied to the scattering of electrons by molecules. The MS method divides space into three regions: (I) atomic spheres, (II) an interatomic region between the atomic spheres and an outer boundary, and (III) an extramolecular region outside that boundary. The assumptions tested are (i) neglect of charge polarization except in region III, (ii) constancy of the potential in region II, and (iii) spherical symmetry of the potential in region III. We consider electron–N2 scattering at impact energies 5–50 eV. We show that the use of assumptions (i) – (iii) results in cross sections that differ by about 30% from those obtained from reasonably well converged close coupling calculations without such assumptions. The differences are up to a factor of two or three for the partial integral cross sections. We have not tested whether the multiple-scattering results are improved by using a more consistent treatment of region II.

Low-energy electron–molecule scattering: Comparison of coupled channel treatments of e−N2 scattering at 13.6 eV using various approximations to the static and exchange potentials and an approximate polarization potential

We consider vibrationally and electronically elastic electron scattering by N2 at 13.6 eV impact energy. We use four combinations (S, SP, SE, and SEP) of the static (S), exchange (E), and polarization (P) potentials, four target molecular orbital wave functions (INDO/1s, INDO/1s with double zeta core, and Cade–Sales–Wahl and Ermler ab initio), and two local exchange approximations (semiclassical and Hara free-electron gas). We also consider the effect of using an accurate wave function for the spherical average and lowest order anisotropy and an INDO/1s wave function for higher order anisotropies, as well as other combinations. The effects of approximations to the various terms in the interaction potential and the target wave function are illustrated by calculations of the partial integral cross sections.