Quasimolecular Systems Research Articles

Continuous absorption and emission of light by the heteronuclear rare gas (ArXe)+ and (KrXe)+ ions

Studies of radiative transitions between ground and charge transfer excited terms of heteronuclear rare-gas (ArXe)+ and (KrXe)+ ions with small and moderate dissociation energies (D0 ArXe+ =179 and D0 KrXe+ =389 meV) are reported. Potential energy curves and dipole transition matrix elements are evaluated using ab initio calculations based on the CASSCF method followed by the NEVPT2 treatment and perturbative description of relativistic effects as implemented in the ORCA suite. We develop an efficient approach for the calculations of Boltzmann-averaged cross sections, rate constants, and absorption and emission coefficients associated with the integral contributions of bound-bound and bound-free transitions from a manifold of rovibrational states of the (RgXe)+ ion. A similar approach is used for the description of the free-free and free-bound transitions between different electronic terms of a quasimolecular (RgXe)+ ion temporarily formed during a collision of Ar or Kr atoms with Xe+ ions. Our method is based on the quantal version of the theory of non-adiabatic transitions in molecular and quasimolecular systems combined with the approximation of a quasicontinuum for rovibrational energy levels. This allowed us to obtain a semianalytic expression for the integral absorption properties particularly relevant at elevated temperatures. We perform a comprehensive analysis of the identified radiative processes in local thermodynamic equilibrium plasmas of rare gas Rg/Xe mixtures containing both atomic, Xe+, and molecular, RgXe+, ions and determine their role in the formation of the absorption and emission spectra in a wide range of wavelengths and gas temperatures. The results obtained are in good agreement with the available experimental data.

Asymptotics of a semiclassical type for wave functions of quasimolecules in problems of ion-molecular collisions

Purpose. To construct an asymptotics of a quasiclassical type for the two-center wave function of the tunneliding electron used for calculating of the two-electron exchange interaction in the problems of ion-molecular collisions with particle rearrangement. Methods. The semiclassical method and the Green’s function formalism have been used for construction of the asymptotically correct two-center wave functions of a homonuclear diatomic molecule and a dipole-bound anion in the vicinity of a perturbing multiply charged ion. Results.We have constructed the semiclassical representation for the two-center one-electronic wave function of the quasimolecular system, which consist of the atomic ion and diatomic homonuclear molecule or dipole-bound anion, in the whole region of configuration space at large separation between colliding particles. Conclusions. An analytical study of the asymptotic properties (at large distances between interacting particles) of the quasimolecular systems that consists of an ion and a homonuclear diatomic molecule or dipole-bound anion has been carried out. The new analytical representations for the two-center one-electronic wave functions of the quasimolecular systems in the whole region of configuration space at large separation between colliding particles has been obtained in the framework of the semiclassical version of the asymptotic approach and the method of Green’s function

Dynamic Instability of Rydberg Atomic Complexes

Atoms and molecules in highly excited (Rydberg) states have a number of unique characteristics due to the strong dependence of their properties on the values of principal quantum numbers. The paper discusses the results of an investigation of collisional Rydberg complexes specific features, resulting in the development of dynamic chaos and the accompanying diffusion autoionization processes. It is shown (experiment and theory) that, in subthermal low energies, the global chaotic regime that evolved in quasimolecular systems leads to significant changes in the Rydberg gases radiation/ionization kinetics. The effect of Förster resonance on the width of the fluorescence spectra and stochastic ionization processes in Rydberg systems is also discussed.

Strong Enhancement of Electron–Ion Recombination Owing to Free–Bound and Bound–Bound Resonance Transitions

The effect of free–bound and bound–bound resonance nonadiabatic transitions of an electron on electron–ion recombination rates in the plasma of a Ne/Xe and Ar/Xe inert gas mixture has been studied. A kinetic model of recombination has been proposed including energy relaxation in collisions with electrons, resonant electron capture to Rydberg states through three-body collisions of Xe+ ions with Ne or Ar atoms and dissociative recombination of NeXe+ or ArXe+ ions, and n → n' resonance transitions. It has been shown that effective resonance processes occurring in quasimolecular systems sharply increase both the recombination coefficient and the effect of collisions with neutral particles even at quite high degrees of ionization of the plasma.

Two-electron exchange interaction in the quasimolecular systems with long-range dipole potential

In the framework of the asymptotic theory the analytic representation for the matrix element of exchange interaction responsible for two electron capture in slow collision of polar molecule with polar molecular ion has been obtained.

High-pressure electrides: the chemical nature of interstitial quasiatoms.

Building on our previous chemical and physical model of high-pressure electrides (HPEs), we explore the effects of interaction of electrons confined in crystals but off the atoms, under conditions of extreme pressure. Electrons in the quantized energy levels of voids or vacancies, interstitial quasiatoms (ISQs), effectively interact with each or with other atoms, in ways that are quite chemical. With the well-characterized Na HPE as an example, we explore the ionic limit, ISQs behaving as anions. A detailed comparison with known ionic compounds points to high ISQ charge density. ISQs may also form what appear to be covalent bonds with neighboring ISQs or real atoms, similarly confined. Our study looks specifically at quasimolecular model systems (two ISQs, a Li atom and a one-electron ISQ, a Mg atom and two ISQs), in a compression chamber made of He atoms. The electronic density due to the formation of bonding and antibonding molecular orbitals of the compressed entities is recognizable, and a bonding stabilization, which increases with pressure, is estimated. Finally, we use the computed Mg electride to understand metallic bonding in one class of electrides. In general, the space confined between atoms in a high pressure environment offers up quantized states to electrons. These ISQs, even as they lack centering nuclei, in their interactions with each other and neighboring atoms may show anionic, covalent, or metallic bonding, all the chemical features of an atom.

Dynamic resonances in the autoionization Rydberg states of atomic systems

Contemporary views on the formation of dynamic nonlinear resonances in isolated intermediate Rydberg quasi-molecular complexes formed by a single elementary act upon the slow collision highly excited atoms are discussed. A relation between the emergence of dynamic resonance and the quasi-intersection of atomic terms is revealed in the quasi-classical approximation. Relationships between the widths of dynamic resonances and the dipole matrix element leading to the corresponding quasi-intersections are determined. The physical reasons for the dynamic chaos regime are formulated. A quasi-classical statistical description is given of the diffusion ionization of an atomic complex with a dense network of quasi-intersections of terms. Development of the stochastic instability of the Rydberg electron trajectories due charge exchange inside the collision complex is considered. The effect the narrowing region of stochastic motion has when passing through the Coulomb convergence of the levels with a simultaneous drop in the ionization rate of quasi-molecular systems is predicted for external statistical magnetic and electric fields under conditions of Forster resonance.

Specifics of the stochastic ionization of a Rydberg collision complex with Förster resonance

The physics of the formation of dynamic nonlinear resonances in an isolated Rydberg collisional complex is described. The development of the stochastic instability of Rydberg electron trajectories due to charge exchange in the complex is considered. The realization of the resonance in external statistic magnetic and electric fields is predicted to be accompanied by a significant narrowing of areas of stochastic motion with a concurrent decrease in the rates of the ionization of real quasimolecular systems proceeding through the migration over the Rydberg crowding of quantum states.

Deep level promotion mechanism in sputtering

We have applied a double decoupled localized level Anderson–Newns Hamiltonian to the analysis of surface effects upon the ionized fraction R+ of sputtered atoms from a metal surface. Electronic excitations, induced in the conduction band by the transient formation of quasi-molecular systems, between substrate and emitted atoms, in the collision cascade generated by the primary incident beam, have been explicitly included into an instantaneous transition matrix peaked at the Fermi level of the material. The interaction dynamics seem to take place over two different time scales, one related to sputtered atom trajectories and the other to recoiled substrate particles. Finite temperature calculations have suggested, at very low ejection energies, a power law dependence of the final charge state of the sputtered beam on its detected velocity. This result is in agreement, in the zero temperature limit, with some previously published papers and its validity has been compared with other theoretical outcomes and tested on SIMS data.

On the stability of rotating nuclei against fission through creviced shapes

The stability of rotating nuclei against fission through creviced shapes is investigated within the generalized liquid-drop model including the nuclear proximity energy. The potential barrier corresponds to large deformations hindering the fission process even at very high spins. At intermediate angular momenta, the deformation energy is relatively constant up to beta approximately 0.7 and shell effects can generate secondary minima where superdeformed states may survive. With increasing spins, the potential pocket moves towards the barrier foot leading to hyperdeformed equilibrium shapes. The characteristics of these quasi-molecular nuclear systems are given in the whole mass range. For an intermediate nuclear mass, asymmetric fusion allows the potential surface to be penetrated more deeply and higher spins to be reached.

Electronic transitions in highly charged ion-atom collisions

Three different aspects of electronic transitions in fast, highly charged ion-atom collisions are discussed. First, experimental data and n-CTMC calculations for differential multiple ionization cross sections of 1.4 MeV u U 32+on rare gas atoms are presented. It is shown that the electronic motion has a dramatic influence on the kinematics of the emitted particles (in particular the nuclei). The possibility is discussed to measure in fast ionizing processes by a recoil ion-projectile coincidence technique the internal sum momentum of “electron clusters” in atoms. This new “technique” opens a new field of atomic structure research at high-energy heavy-ion accelerators. Second, the use of the H-like heavy ions as projectiles is discussed to measure, through observable interference structures, static and dynamic properties of transiently formed superheavy quasimolecular systems. Third, the “ancient” gas target-solid target difference in the impact-parameter dependence of K-shell ionization in nearly symmetric ion-atom collisions is presented. This severe discrepancy between gas and solid still remains an unsolved fundamental problem in the field of inner-shell ionization in the MO regime.

Collective effects in atomic collisions

The dynamics of collective charge flow in slow quasimolecular collision systems is investigated. In a first step, a classical Hamiltonian function is derived from a generalised Born-Oppenheimer approach. In a second step, the resulting coupled equations of motion for the relative coordinate and the charge asymmetry between the two atomic species are solved. As an illustration of the method, trajectories are presented for the model system HF. They exemplify elastic and charge-changing collisions as well as trapping into a molecular state. It is suggested that the mechanism studied here might play a role in slow collisions of multicharged ions with neutral target atoms.

On the adequacy of first-order perturbation theory for calculations of inner-shell ionisation

Transition probabilities for inner-shell electrons during the formation of quasi-molecular systems in heavy-ion collisions are considered. The effect of transitions to the continuum on discrete transition probabilities is given via a simple prescription satisfying unitarity. In this way, it is also shown that the direct ionisation probabilities will be modified by virtual transitions to the continuum. Adopting a scaling model and the authors' prescription, the effect of such virtual processes on the ionisation of 1s sigma electrons can be calculated analytically. As an example, the authors calculated it for the Pb-Pb quasi-molecular system. It is found that such virtual processes suppress the direct 1s sigma ionisation probability for large impact parameters and/or small relative ion velocities.

Doppler-shift analysis of continuum x radiation from quasimolecular systems

Doppler-shift analysis of continuum x radiation from quasimolecular systems

Variational calculation of the 1s? orbital in quasi-molecular systems

We describe a simple analytical model for the ground state energy of an electron in a two-center potential. An effective chargeZ(R) is derived which corresponds to the monopole approximation of this potential. As an example, we study the (H, H) and (Br, Zr) systems.

Theory of Collision Broadening in Molecular-Orbital X-Ray Spectra

The amplitude for emission of molecular-orbital X-rays during slow ion-atom collisions is discussed in the framework of so-called uniform asymptotic approximations. These methods systematically generalize the simple stationary-phase approximation which, when applied in evaluating the molecular-orbital emission amplitude, leads to the “static” approximation. The uniform approximations allow for coherence effects due to nearly coalescing stationary points and enable one to continue the emission amplitude smoothly into the classically forbidden region (“collision broadening”). In simple cases, the resulting differential emission probabilities exhibit rainbowlike structures in their impact-parameter and frequency dependence. As a specific example, photon emission due to 2pσu-1sσg transitions in H2+-like quasimolecular systems is considered.

Relativistic effects in the variable-screening model

The variable-screening model devised for a quantitative treatment of screening effects in slow (quasimolecular) ion-atom collisions is extended to include relativistic effects. Relativistic correction terms corresponding to a relative energy correction of second order in the electron velocity are incorporated into the effective molecular single-electron Hamiltonian. The model is applied to the calculation of relativistic molecular-orbital correlation diagrams for the quasimolecular systems Br+Br, Xe+Xe, and I+Au. A detailed comparison of our results with the results of relativistic calculations for (unscreened) one-electron systems and calculations based on self-consistent field methods is performed. The agreement with Dirac-Fock-Slater results is fairly good, and could be improved further by read-justing the values of the parameters in the atomic potentials used to construct the effective molecular potential.

Variable-screening model for the quasimolecular treatment of ion-atom collisions

An effective single-electron potential is proposed which is appropriate for a realistic description of the electronic motion in quasimolecular systems formed in near-adiabatic ion-atom collisions. The potential is represented as a sum of two spherical potentials, each centered about one of the colliding nuclei. For finite internuclear distances, these potentials are obtained by smoothly interpolating the screening parameters of suitable atomic potentials (Thomas-Fermi or phenomenological potentials) between the united-atom and separated-atom limits (variable-screening model). The use of phenomenological potentials entails a great accuracy in describing the atomic states and opens up the possibility of treating ionized quasimolecular systems which were hitherto inaccessible to molecular-orbital Hartree-Fock calculations. Molecular correlation diagrams calculated from the variable-screening model for various neutral and ionized systems are presented. For the neutral cases the agreement with the results of Hartree-Fock calculations is surprisingly good. The scaling properties of the variable-screening model are discussed.