K. J. McCann

New semiclassical treatments of rotational and vibrational transitions in heavy‐particle collisions. I. H–H2 and He–H2 collisions

Two new semiclassical methods—the multistate orbital treatment and the multichannel eikonal treatment—are proposed for the description of rotational and vibrational excitation in heavy‐particle collisions. The first method includes appropriate trajectories determined from a certain optical potential designed to couple the response of the internal structure, which is described by a quantal multistate expansion, to the orbit for the relative motion and vice versa. While this approach is, in general, valid when the quantal imprecision in the classical trajectories is small (as for heavy particles) the second method based on the use of a straight‐line eikonal for the relative motion, of different local momenta in the various channels and of a multistate expansion for the internal motions, is valid for scattering mainly about the forward direction. These procedures are applied to representative rotational transitions in H–H2 and He–H2 collisions at 0.25–1.5 eV and yield angular distributions and integral cross...

Vibrational deactivation of oxygen ions in low velocity 02+(X2Πg,υ=1)+02(X 3Σg−,υ=0) collisions

The deactivation of 02+(X 2Πg, υ=1) ions in collisions with 02(X 3Σg−,υ=0) molecules has been examined using multistate impact parameter eikonal and orbital treatments. Cross sections for the formation of various product states in the charge exchange and direct scattering channels have been computed for ions with 0.5 to 8.0 eV c.m. kinetic energies. The relative probabilities for forming products in given vibrational states at the higher kinetic energies are similar for the eikonal and orbital approaches. At energies below several eV it is necessary to employ the multistate orbital treatment which takes explicit account of the strong ion–molecule scattering. Cross sections for reaction channels leading to de‐excitation and/or excitation of the product 02+(X 2Πg,v=1) ions have been computed for both charge exchange and direct scattering processes. The channels leading to vibrationally deactivated 02+(X 2Πg, v=0) product ions are strongly favored at low velocities over the excitation processes in the charge...

Photoionization of metastable rare‐gas atoms (He*,Ne*,Ar*,Kr*,Xe*)

Cross sections σ for the photoionization of metastable rare‐gas atoms (He*‐Xe*) over a wide range of photon energies are presented. In marked contrast to that found for He* where σ∼10−18 cm2 and decreases monotonically with photon energy, the cross sections σ for Ne*‐Xe* are much smaller ∼10−19 cm2 and exhibit optical windows within the wavelength range 2000–3000 A of current laser interest.

Elastic scattering and rotational excitation in ion‐molecule collisions. II. Li+–H2 and H+–H2 collisions

A general semiclassical treatment of elasticscattering and of rotational excitation in ion–molecule collisions is presented. When the orbits associated with the different channels corresponding to the internal modes do not differ significantly, simplification occurs and the internal degrees of freedom can then be coupled to the relative motion via the introduction of an optical potential (which in turn depends on the transition amplitudes). Total energy is consequently conserved. An expression is derived for the inelastic scattering amplitude which acknowledges various interference effects and possible rainbow scattering. With all phase‐information supressed, the procedure, when compared with the full quantum‐mechanical results, reproduces the background elastic and inelastic scattering in Li+–H2 and in H+–H2collisions. Restoration of the phases, particularly of the eikonal or action phases associated with the different classical paths that contribute to a specified scattering angle, produces the interference oscillations present in the differential cross section for scattering angles less than the rainbow angle. The method, when compared with the full quantal procedure, is remarkably efficient and accurate.

A multistate semiclassical orbital treatment of heavy-particle collisions with application to HH2 rotational transitions

Abstract A new semiclassical description of elastic and inelastic scattering in heavy-particle collisions is developed. The treatment includes appropriate trajectories determined from a certain optical potential designed so as to couple the response of the internal structure to the orbit for the relative motion. The procedure is applied to various rotational transitions in HH 2 collisions at 0.5–1.5 eV and yields results remarkably close to recent quantal calculations.

Differential charge exchange cross sections in AB+–AB collisions

Differential cross sections for vibrational excitation in H+2(X 2Σ+g, ν0′) –H2(X 1Σ+g, ν0″), N+2(X 1Σ+g, ν0′) –N2(X 1Σ+g, ν0″), O+2(X 2Πg, ν0′) –O2(X 3Σ−g, ν0″), CO+(X 2Σ+, ν′0) –CO(X 1Σ+, ν0″), and NO+(X 1Σ+, ν0′) –NO(X 2Πr, ν0″) charge transfer reactions have been determined for incident ions with 0.004 to 2.2 keV kinetic energies. Differential cross sections for excitation of individual charge transfer channels have been computed as a function of scattering angle using a multistate impact parameter treatment of the collision processes. At low kinetic energies, coverged cross sections are obtained with the inclusion of a relatively small number of product vibrational states in the wavefunction expansion of the system while a large number of product states must be included at higher energies. Small angle scattering within several degrees dominates the charge transfer, with the scattering becoming more concentrated in the forward direction at higher kinetic energies.

Direct vibrational excitation of ions and molecules via charge exchange in N+2–N2, O+2–O2, CO+–CO, and NO+–NO collisions

Differential and integral cross sections for direct vibrational excitation in symmetric N+2(X 2Σ+g, υ′0) –N2(X 1Σ+g, υ″0), O+2(X 2Πg, υ′0) –O2(X 3Σ−g, υ″0), CO(X 2Σ+, υ′0) –CO(X 1Σ+, υ″0) and NO+(X 1Σ+, υ′0) –NO(X 2Πr, υ″0) collisions have been determined for ions with 0.004 to 2.2 keV kinetic energies. Inelastic channels in the individual symmetric ion–molecule systems are strongly coupled to each other and to the energy resonant charge exchange channels. Differential cross sections for excitation of individual inelastic product channels have been computed as a function of scattering angle using a multistate impact parameter description of the collision. Small angle scattering dominates and becomes more concentrated in the forward direction with increasing ion kinetic energy. Multistate integral cross sections for individual channels and integral cross sections summed over all product channels are presented as a function of both ion kinetic energy and reactant ion vibrational state.

Charge transfer cross sections in argon ion–diatomic molecule collisions

Charge‐transfer cross sections have been examined for Ar+–H2, N2, O2, CO, and NO collisions, in the energy range 0.5–3.0 keV, using time‐of‐flight techniques in which fast neutral Ar products from the electron transfer processes are measured. Cross sections in Ar+–H2 and N2 systems are comparatively large and decrease with ion kinetic energy while the Ar+–O2, CO, and NO cross sections increase with incident ion kinetic energy. The Ar+ ion beam was composed of 2P3/2 and 2P1/2 states with no evidence of long‐lived, electronically excited states participating in these reactions. A multistate impact parameter treatment of atomic ion–diatomic molecule collisions is presented and applied to the Ar+(2P3/2, 2P1/2)−H2(X 1Σ+g, v0′=0) system. The resulting coupled differential equations have been solved numerically, with convergence of the cross sections achieved by inclusion of a comparatively large number of possible product channels in the wavefunction expansion of the system. At low ion kinetic energies both the...

Theoretical cross sections for ionization of metastable excimers Ne*2 and Ar*2 by electron impact

Cross sections for the ionization of the metastable excimers Ne*2(1,3Σ+u) and Ar*2(1,3Σ+u) by electron impact are calculated in the binary‐encounter approximation for collision energies E (eV) in the range 5⩽E⩽50 eV. They have maximum values ∼10−15 cm2 and are within 50% higher than the cross sections for ionization of the corresponding metastable atomic states.

Direct vibrational excitation via charge exchange in symmetric ion–molecule collisions

Integral and differential cross sections for vibrational excitation in symmetric H+2(X 2Σ+g, v′0) + H2(X 1Σ+g, v″0) collisions have been determined for incident kinetic energies E (keV) in the range 0.01<E<2.0. A multistate impact parameter description of the collision was used. At low kinetic energies, the inclusion of a relatively small number of states in the wavefunction expansion of the system achieves converged cross sections while at high energies a large number of product states are needed. The inelastic channels in this symmetric system are strongly coupled to each other and to the energy resonant charge exchange channels. Differential cross sections for excitation of individual product channels have been computed as a function of scattering angle within the multistate impact parameter approach. Small angle scattering within several degrees is dominant with forward inelastic scattering becoming more pronounced as the ion kinetic energy is increased. Integral cross sections for individual channels...