P. E. Siska

An improved scattering potential for He*(2 3S) + Ar

Abstract Differential scattering experiments in crossed atomic beams in the energy range 0.5–2.8 kcal mol -1 (0.02–0.12 eV) are used to derive an optical potential for He * (2 3 S) + Ar based on a highly constrained, realistic model potential function. The potential gives a good account of other cross section and rate data.

Collision energy dependence of Penning ionization electron spectra in crossed supersonic beams: He*(2 1S)+N2

Penning ionization electron spectra are reported at five collision energies E in the range 1.6–4.9 kcal/mol. A preliminary analysis of the E dependence of the intensities, shapes, and positions of the vibronic lines reveals details of the ionization dynamics, features of the highly anisotropic potential surface, and connections with N2 orbital structure.

Differential nonreactive scattering of He*(2 1S, 2 3S) by D2 and H2: Anisotropic optical potentials and comparison with ab initio theory

Angular distributions of He*(2 1S, 2 3S) scattered by D2 and H2 measured in crossed supersonic molecular beams at collision energies in the range 1.0–2.4 kcal/mol are analyzed to yield anisotropic optical potentials that simultaneously reproduce these data along with quenching rates and ionization cross sections. Comparison with ab initio calculations of the potentials by Cohen and Lane and Hickman, Isaacson, and Miller shows very good agreement. The results are combined with one‐electron model potential calculations to probe the nuclear and electronic dynamics involved in these collisions. Quenching or Penning ionization is found to occur mainly through broadside attack of He* on H2, despite the fact that the occupied σg orbital on H2 has greater spatial extent along the bond axis than perpendicular to it. An implication is that Penning ionization electron spectroscopy (PIES) experiments on larger molecules cannot be interpreted simply on the basis of van der Waals radii and spatial extent of various mol...

Collision energy dependence of product branching in Penning ionization: He*(2 1S, 2 3S) + H2, D2, and HD

Relative ionization cross sections for the title systems with articulation of all product ion channels have been measured in the collision energy range 1.5–4.0 kcal/mol using crossed supersonic molecular beams; the H2 results have been extended down to 0.5 kcal/mol by the use of a 10% H2/Ar seeded beam. The data are interpreted with a microscopic two‐step model that assumes ionization near the turning point in the excited state, a centrifugal barrier criterion for ionic complex formation, and statistical partitioning of flux among the possible ionic products, i.e., phase‐space theory. A full statistical calculation underestimates the amount of rearrangement ionization He*+H2→HeH++H+e− by a factor of 2, but one which excludes antiparallel coupling of orbital and rotational angular momenta in the H+2 channel is in better accord with the data. A substantial isotope effect favoring HeD+ over HeH+ in the HD reactions by a factor of 1.9±0.2 is well represented by the model.

Potential energy curves for the a 3Σu+ and c 3Σ+g states of He2 consistent with differential scattering, ab initio theory, and low‐temperature exchange rates

Crossed beam scattering experiments on He*(2 3S)+He at energies of 0.94, 1.51, and 2.24 kcal/mol have been combined with ab initio theory and kinetic data on metastability exchange rates to produce new estimates of the title potential energy curves. The long‐range potential barrier in the a state is found to be 1.43±0.05 kcal/mol at a separation of 2.717±0.04 A. Combining these results with those for the corresponding singlet states from an earlier paper [J. Chem. Phys. 80, 5027 (1984)], we discuss the long‐range behavior of the curves, particularly with respect to curve crossing between a given pair of g and u states. These crossings, which conform to theoretical predictions, appear to be essential for a consistent description of all available data.

Repulsive potentials for Ne*(3P)—He and Ar*(3P)—He from differential scattering

Abstract Crossed beam measurements of differential scattering of metastable Ne and Ar by He are used to derive the repulsive parts of the respective interatomic potentials. The potentials are similar to one obtained for Li—He. They show non-exponential behavior at small r suggestive of a core-interaction effect.

Close‐coupling optical model analysis: A refined optical potential surface for He*(2 1S)+H2

A complex‐arithmetic version of Johnson’s log–derivative integration method has enabled a straightforward extension of the close‐coupling approximation to complex‐potential surfaces. The complex‐potential close‐coupling (CPCC) method is applied to the determination from our earlier scattering measurements of an improved anisotropic optical potential for the He*(2 1S)+H2 Penning ionization system, with the aid of a new, nonpiecewise potential surface function based on the improved Tang–Toennies model. The potential is compared to earlier ab initio results, and the CPCC method is used to assess the shortcomings of the complex‐infinite‐order‐sudden (CIOS) approximation. Variation of differential and ionization cross sections with the initial rotational state of H2 predicted by CPCC may be experimentally observable, and may also provide an explanation for the observed H2/D2 isotope effect in quenching rates.

Magnetic deflection analysis of supersonic metastable atom beams

Deflection of 3S1 and 3P2 metastable rare‐gas atoms in an electron‐impact‐excited thermal supersonic expansion by an inhomogeneous magnetic field produces well‐defined daughter beams for each M state. The deflection patterns are spatially resolved with a translatable detector and used to derive fine‐structure populations in the 3P0 and 3P2 states, and to measure absolute efficiency of state selection by optical depletion. The fine‐structure population ratio 3P2/3P0 is measured for electron excitation energies from 100–400 eV, showing a 20%–40% decline with increasing energy for all the rare gases. Radiative decay due to perturbation by the magnetic field appears to be negligible, but the M‐state‐resolved populations are unequal and asymmetric, perhaps due to Majorana transitions at the entrance to the field. The method may also be useful in polarized angular momentum studies of metastable atom–molecule collisions.

Ab initio/spectroscopic interaction potential for He+Ne+

High-level ab initio calculations have been carried out on the lowest Σ and Π states of HeNe+. These have been used to construct a new interaction potential in a Hund’s case (e) representation, by fitting spectroscopic vibrational spacings ΔGv+1/2 and rotational constants Bv using a close-coupling method and a potential function whose form is established by the ab initio data and a long-range analysis. The characteristics of the resulting Born–Oppenheimer potential curves, particularly for the X state, where only the higher vibrational levels were observed, differ considerably from those derived by extrapolation of the experimental spectroscopic constants. A new set of constants is proposed, and functions for the X-state G(v) and Bv are given that are well-behaved from the bottom of the well to the dissociation limit. The asymptotic formula for Bv of Le Roy is extended to improve its accuracy. The X state of 4HeNe+ is predicted to support 15 bound vibrational states, the A2 state 7. Good agreement with ex...

NASCENT VIBRATIONAL POPULATIONS IN HE*(2 1,3S)+H2, HD, D2 PENNING IONIZATION FROM ELECTRON SPECTROSCOPY IN CROSSED SUPERSONIC MOLECULAR BEAMS

Penning ionization electron spectroscopy (PIES) in crossed supersonic molecular beams has been used to measure nascent H+2, HD+, and D+2 vibrational populations, line shifts, and their collision energy dependence in the case of H2 (E=1.6, 2.1, and 2.6 kcal/mol), in the title systems. The use of optical spin–state selection and beam modulation enabled estimates of vibrational populations to be obtained out to the dissociation limit, although the low electron energy resolution (90 meV) employed and low counting rates allowed typically only 12 vibrational states for 1S, 9 for 3S, to be resolved. With a few mild exceptions, the following properties were found. The vibrational populations, which are characteristic of the Penning ionization transition state rather than the asymptotic final state, follow Franck–Condon behavior quite closely, more so than do He i photoionization populations, independent of spin and isotope, and nearly independent of E in the range examined. As expected from the E independence, th...