L. Yu. Rusin

Ion source with longitudinal ionization of a molecular beam by an electron beam in a magnetic field

A high-efficiency ion source for a mass-spectrometer’s detector of molecular beams and their scattering products is described. The ion source is designed according to a scheme of impact ionization of a beam particle by a longitudinal electron beam in a magnetic field with a strength of up to 130 mT. The design of the source developed is very flexible and has no limitations for use in any experiments with molecular beams. An ionization efficiency of particles of an atomic helium beam of 10−3 ions/atom has been achieved. The useful signal-to-background ratio in the detector’s chamber is 3 × 104 during detection of ions with mass-to-charge ratio m/q = 4 amu.

The dynamics of direct three-body recombination of ions

The trajectory simulation of the direct recombination of the Cs+ and Br− heavy ions in the presence of third-body atoms R = Kr, Xe, Hg was performed on the potential energy surface that controlled the dissociation of CsBr induced by collisions with the same R. The results showed that the probability of recombination decreased as the energy of the approach of ions to each other and the energy of a third body with respect to the center of mass of the ion pair increased. Direct recombination proceeds according to at least two mechanisms of the stabilization of the molecule formed. The first mechanism involves the collision of the R atom with both ions at low impact parameters with respect to the center of gravity of the ion pair. In the second mechanism, impact parameters are large, and energy is removed through a collision with one of the ions of the pair. The products formed have a strongly nonequilibrium vibrational distribution and almost equilibrium rotational energy distribution.

A hard sphere model for direct three-body recombination of heavy ions

We describe a hard sphere model of direct three-body recombination of the Cs+ and Br− ions in the presence of neutral atoms Hg, Xe, Kr, or Ar as the third bodies. Calculations are carried out for the ion approach energy and the third body energy in the range from 1 to 10 eV under the assumption of non-central approach of the ions. The calculation results include the dependences of the total recombination probability on these energies as well as the opacity functions for two impact parameters and the dependences of the recombination probability on the angles determining the mutual orientation of the velocities of the reagents. The classification of the three-body collisions according to the sequences of pairwise encounters of the particles is considered. The most widespread mechanism of energy removal from the ionic pair is a single impact of the third body with the Br− ion.

Stabilization of diatomic products in recombination of heavy ions

We have studied the kinematic conditions for the formation of the most stable products of direct three-body recombination of ions Cs+ and Br− in the presence of a third-body Xe atom. As the energy of the ions’ central encounters and the energy of the third body range from 1 to 10 eV, the minimal residual energy of the recombination products is found to lie in the interval 0–0.7 eV. The formation of the recombination products with the minimal internal energies is yielded by collisions of the three particles in a triangular configuration in an impact parameter range from 0 to 2.7 a.u. The mutual orientation angles of the vector of the relative ion velocity and the vector of the third body velocity affect the formation of products with the minimal internal energy rather slightly. In most cases, stabilization of the CsBr molecules and high efficiency of the energy transfer to the third body are observed in configurations of the closest approach of the particles with interionic distances close to the equilibrium distance in the CsBr molecule.

A study of the detailed dynamics of the collision-induced dissociation of CsBr by the visualization of elementary process trajectories

A method for studying the detailed dynamics of an elementary process by the visualization of the trajectory of collision of initial particles was suggested. The method has serious advantages compared with graphic processing of the calculated data and provides a high degree of visualization of the calculation results and the possibility of direct comparison between changes in interaction configuration and the energy characteristics of each pair of interacting particles and the total potential energy of a system. The method was used to analyze the detailed dynamics of the collision-induced elementary process of CsBr dissociation. The possibility of elementary process branching because of a change in the sign of the radial component of the generalized momentum of a pair of atoms in a molecule was analyzed.

Effectiveness of the third body in the direct recombination of ions

The dynamics of the three-body recombination of the Cs+ and Br− ions with the formation of products with the lowest internal energy in the presence of the neutral atoms R = Hg, Xe, and Kr as third bodies is studied. The efficiency of the process is characterized by the effectivity function, which represents the dependence of the internal energy of the nascent molecule on the ion encounter energy and the third body energy. The Hg and Xe atoms are demonstrated to exhibit similar efficiencies in stabilizing the CsBr molecules, significantly superior to that of the Kr atom. The effectivity of each third body as an acceptor of excess energy of the molecules formed in recombination is determined by the structure of the potential energy surface of the individual R-Cs+-Br− system, the masses of the third bodies, and the dynamics of three-body collisions leading to recombination.

Mechanism of the direct three-body recombination of atomic ions in a central collision

The detailed dynamics of direct three-body recombination of the Cs+ and Br− heavy ions in the presence of the Xe atom as a third body is studied by the quasiclassical trajectory method. A potential energy surface that quantitatively correctly describes the dynamics of the reverse process of ion formation induced by collisions of CsBr with Xe is used. Depending on the impact parameter of the third body, the stabilization of the product molecule proceeds by several different mechanisms. At impact parameters of bR ≤ 7 au, the stabilization of the nascent molecule is largely controlled the repulsion potential between one of the ions or both the ions and the third body. The energy transferred to the third body does not depend directly on the repulsive potential energy between the ion and the third body. The phase of collision of the ions at the instant of closest approach plays a key role in the process of energy transfer. For collinear collision configurations of the three particles, the ion-Xe shallow potential wells are demonstrated to produce a noticeable effect.

Evolution of a pair of classical ions in a cavity with elastic walls, crosspieces, and implanted charges

Using the method of classical trajectories, we have simulated the motion of a pair of oppositely charged ions in spherical and ellipsoidal cavities, including cavities that contain cylindrical “crosspieces” and positively charged “nuclei.” We supposed that each of the ions reflected off the cavity wall, crosspieces, and nuclei according to the elastic impact law. Most of attention is focused on the statistics of the events of ion recombination and those of dissociation of the corresponding molecule with an ionic bond. When nuclei are absent, recombination and dissociation events are possible only on collisions of the ions with the cavity wall or crosspieces. In the presence of nuclei, on the other hand, the major part of the events occur in time intervals between the collisions of the ions with the cavity wall, crosspieces, or nuclei.

The optimization of potential energy surface parameters for the CsCl + RbI system with the use of linear regression analysis

The influence of interaction potential parameters of likely charged ions on cross sections of various channels of a reaction of a pair of diatomic molecules with ionic bonds was studied in terms of quasi-classical trajectory simulation with the use of linear mean-square regressions. In the regression approach, the dependence of the cross section of a given reaction channel on potential parameters at each fixed collision energy is approximated by a linear function. We determined the region of softness parameters of the Cs+-Rb+ and Cl−-I− interaction potentials. This region was optimum for the reproduction of experimental excitation functions of atomic and complex positive ions for the CsCl + RbI → products reaction.

The special features of rotationally resolved differential cross sections of the F + H2 reaction at small scattering angles

We studied the nature and collision energy dependence of the maximum that appears in the angular distributions of the HF (v′ = 3) product of the F + H2 (v = 0; j = 0, 1, 2) → H + HF (v′, j′) reaction at small scattering angles θ in the center-of-mass frame. This maximum and its increase as the collision energy increased were discovered in the well-known experiment described by D.M. Neumark, A.M. Wodtke, G.N. Robinson, C.C. Hayden, and Y.T. Lee, J. Chem. Phys. 82 (7), 3045 (1985). In order to determine the nature of the maximum, we performed quantum-mechanical simulation of the reaction on the Stark-Werner ground state potential energy surface at collision energies of 1.84, 2.74, and 3.42 kcal/mol corresponding to the above-mentioned experiment and calculated the vibrationally and rotationally resolved differential cross sections dσv′j′/dΩ of the reaction. The maximum under consideration was found to be due to a superposition of two effects, namely, the absence of HF (v′ = 3; j′) products with large j′ because of energy restrictions and an increase in the relative amplitude of quantum-mechanical oscillations on dσv′j′/dΩ cross sections at small j′ and θ as v′ increased. Oscillations on dσ3j′/dΩ cross sections with small j′ are responsible for the maximum observed.