Eric A. Gislason

Vibronic coupling at intersections of covalent and ionic states

Abstract The relationship between vibrational motion or vibrational excitation and electronic transitions is the subject of this review. Collisions betweens atoms and molecules in the energy range of an eV to 5keV, where vibronic processes occur, are discussed. An overview of the available theories is presented. The collision models are introduced using atom-atom collisions. The discussion of atom-molecule interactions starts with an introduction of the potential surfaces. Most of the features of the dynamics of these collisions are then discussed in classical terms. Finally quantal methods to describe vibronic excitation are discussed. In most cases the discussion refers to experimental and theoretical investigations of alkali-atom halogen, O2 or N2 collisions.

Multiple‐crossing electron‐jump model for reactions of metal atoms with diatomic halogen molecules

The reaction of metal atoms with diatomic halogen molecules via an electron jump is examined using the multiple‐crossing formalism of Bauer and Fisher. This permits the calculation of reactive cross sections from known properties of the atom and the molecule. Application of the model to reactions of I2 yields cross sections in good agreement with available experimental data. The model predicts that vibrational excitation via curve crossings should become an important process at energies above thermal, and cross section estimates are obtained. In addition, the relationship between the location of the electron jump and the dynamics of the reaction is examined. Finally, the model presented here is compared with other electron‐jump models.

A time-dependent quantal analysis of vibronic excitation via charge transfer in ion-molecule collisions

Abstract A semi-classical multiple state model describing charge transfer in ion-molecule system is presented. Analytical expressions for the state-to-state transition probabilities are given for both the weak-coupling and the high-velocity limits. The expressions are compared to a two-state model describing charge transfer in ion-atom systems. Some numerical calculations are presented to illustrate the various phenomena which can occur in multiple state charge transfer processes. These numerical calculations will be based upon a simple system derived from the system (Ar + N 2 ) + .

An exact trajectory surface hopping procedure: Comparison with exact quantal calculations

A new exact trajectory surface‐hopping procedure is presented. The method is used to run test calculations on two classic (Landau–Zener – and Demkov‐type) atom–atom systems. Transition probabilities as a function of impact parameter show an excellent agreement with quantal results.

Scattering of fast molecular hydrogen from Ag(111)

Abstract We present experiments on the scattering of fast (70 eV ⩽ E ⩽ 1 keV) neutral and ionic molecular hydrogen from Ag(111). In the experiments ions are, rather surprisingly, easily formed. The recorded distributions are greatly influenced by the initial charge state of the incoming molecule. In the H + 2 experiments the observed dissociation of the product ions is mainly caused by the dissociative neutralization into a repulsive state. A minority of the ionic molecules that neutralize into the singlet ground state proceed to scatter as neutral molecules. The dissociated fraction in the neutral beam experiments shows a scaling with E θ 2 where θ is the total scattering angle. This scaling indicates the impulsive and atomic nature of the collision between the incoming molecule and the individual target atoms. The dissociation of the product ions is initiated by rotational excitation in the collision between the incident species and an independent Ag atom. The ionization step and the dissociation step in these experiments seem to be decoupled.

A trajectory surface‐hopping study of H+2+He collisions with identification of the product electronic state in dissociation processes

A trajectory surface‐hopping study of collisions of H+2 (v)+He for v=0, 3, 6, and 10 has been carried out on the two lowest potential‐energy surfaces at relative collision energies of 3.1, 5, and 10 eV. The diatomics‐in‐molecules (DIM) surfaces of Whitton and Kuntz, suitably modified at large internuclear distances, were used in the calculations. The probability for hopping between the two surfaces was calculated using the Demkov formalism. Both total cross sections and velocity vector distributions are reported. The reactive cross sections to give HeH++H were not affected by the accessibility of the excited potential surface. By comparison, the results for collision‐induced dissociation (CID) to give He+H++H were quite revealing. In an earlier paper we have shown that it is possible to distinguish the two (nearly) degenerate product states in CID. The present work shows that between 33% and 45% of the CID products appear in the excited electronic state. The H+ velocity distributions are quite different i...

Determination of molecular quadrupole moments from ion-molecule scattering experiments

Abstract Incomplete total cross sections have been measured for K + ions scattered by several molecules. By comparing the cross sections at low energy with those measured for the heavier rare gases, it is possible to determine accurate quadrupole moments for the molecules. A novel method for analyzing the experimental cross sections is described.

The role of electron transfer stabilization in several gas phase ion–molecule reaction processes

A simple theoretical method is presented for estimating the bond energies of ion–molecule complexes such as O+2–M and NO+–M, where M is a neutral molecule. The theory has one adjustable parameter, H12, which is the electronic coupling between the state O+2–M (or NO+–M) and the charge– transfer state O2–M+ (or NO–M+). H12 has a fixed value for each ion. Good agreement is obtained with experimental bond energies where available. The theoretical bond energies are then compared with vibrational quenching rate constants and with three‐body association rate constants measured for O+2–M and NO+–M systems. In each case there is a strong correlation, in agreement with earlier predictions. A similar comparison is made using the incremental bond energy which can be attributed to the H12 term. The correlation is even better, suggesting that the anisotropy in the ion–molecule interaction plays an important role in stabilizing the collision complexes.

Theoretical state‐to‐state charge transfer cross sections for collisions of Ar+ (2P3/2, 2P1/2) with N2

State‐to‐state cross sections have been calculated for collisions of Ar+(2P3/2, 2P1/2) with N2 over the relative collision energy range 1–4000 eV. The computations have been done by means of the vibronic semiclassical method recently used by Parlant and Gislason for N+2+Ar collisions. The translational motion is treated classically, and the time‐dependent Schrodinger equation is solved exactly for the vibronic states of the system. The potential energy surfaces utilized are those of Archirel and Levy. The results for the total charge transfer cross sections are in fairly good agreement with experimental data over the whole energy range. An unexpected participation of the A state of N+2 at low collision energy is observed. The charge transfer cross section ratio for the two spin–orbit states is discussed in a comparison with the available experimental data. In addition, the vibrational state distributions of N+2(X;v’) show good agreement with the recent measurements of Liao et al.

Determination of cesium ion--rare gas potentials from total cross section measurements

Total cross sections have been measured for Li+ ions scattered by He, Ne, Ar, Kr, and Xe in the range EϑR = 5–1000 eV deg. Here E is the laboratory energy of the Li+ beam, and ϑR is the resolution angle of the apparatus. The cross sections have been inverted to obtain accurate estimates of the potential V(R) over a wide range of R including the attractive well region. The results are compared with other theoretical and experimental work on these systems.