Nonadiabatic Radial Coupling Research Articles

Ab initio study of charge transfer in low energy collisions with atomic hydrogen

Charge transfer cross sections for collisions of ground state and excited state with atomic hydrogen are presented for energies less than . The cross sections are calculated in a diabatic representation using a fully quantum-mechanical, molecular-orbital, close-coupled method. Completely ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained with the spin-coupled valence-bond method are incorporated. Inclusion of the and closed-channels results in oscillations in the charge transfer cross section for collision energies above the separated-atom energy of the lowest closed-channel . Rate coefficients for temperatures between 500 and 100 000 K and cross sections for collisions with isotopic D are presented. Results for the reverse process, charge transfer ionization, are also given.

Potential energy curves and ∂/∂R couplings for electron capture in low-energy collisions of silicon ions with helium and atomic hydrogen

The spin-coupled valence bond approach has been used to investigate the adiabatic potential curves, avoided crossings and nonadiabatic radial couplings that were required for fully quantal scattering treatments of the Si2+/H, Si3+/He and Si4+/He charge transfer systems. The molecular data are provided as electronic supplementary information.

Ab initio study of charge transfer in low-energy collisions ofSi4+with helium

We present state-dependent and total charge transfer cross sections for the process ${\mathrm{Si}}^{4+}$(${2\mathrm{p}}^{6}$)+He(${1\mathrm{s}}^{2}$)\ensuremath{\rightarrow}${\mathrm{Si}}^{3+}$(3l)+${\mathrm{He}}^{+}$(1s), where l=s,p, in the collision energy range 0.004--10 eV/amu. The cross sections are determined using a close-coupled quantal method. Fully ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained with the spin-coupled valence-bond method are incorporated. Rate coefficients for temperatures between 1000 and 50 000 K and results for isotopic $^{3}\mathrm{He}$ are also presented. Astrophysical applications are briefly discussed.

Theoretical treatment of inelastic thermal collisions

The production of excited mercury ions in thermal collisions is studied theoretically. The adiabatic potentials and the nonadiabatic radial couplings are calculated within the multielectron pseudopotential method. The coupled equations for the nuclear motion are solved fully quantum mechanically in a diabatic representation. The partial transition probabilities, the incoming and outgoing currents, the partial cross sections and the partial rate constants are determined in the range of interest and compared with the available experimental data. It is found that the total rate constant is mainly determined by the partial rate constant for excitation of the state.

Potential curves and nonadiabatic matrix elements for collisions involving fragments of the HeN+ molecular ion

Ab initio multireference CI calculations have been carried out for the HeN+ molecular ion in order to describe collision processes between its constituent neutral and ionized atoms. The accuracy of these calculations is evaluated by means of a comparison of results obtained at large internuclear separations with the corresponding asymptotic energies deduced from atomic spectral data. Energy values are computed for the eleven lowest He++N and He+N+ atomic limits and average discrepancies relative to the experimental excitation energies up to 110 000 cm−1 are found to lie in the 1000–3000 cm−1 range, of which only 200 cm−1 appears to be the fault of the configuration interaction (CI) technique itself, with the main portion of the error stemming from the choice of atomic orbital (AO) basis instead. The HeN+ X 3Σ− ground state is calculated to have a De value of only 1414 cm−1, but the excited 2 3Π state has a much larger value of 22 133 cm−1 by virtue of an avoided crossing with the lower state of this symmetry. The corresponding radial nonadiabatic coupling is responsible for a large cross section for an excitation process between the N+(3Pg)+He and N+(3Du)+He channels which indirectly provides an efficient electron-capture mechanism leading to the N(4Su)+He+ exit channel. Additional nonadiabatic matrix elements for rotational and spin–orbit coupling have also been obtained and analyzed, as well as transition moments between the various HeN+ molecular states calculated.

Cooperative fluorescence in nonadiabatic dissociation of alkali diatoms.

We predict a new type of time-resolved cooperative fluorescence (CF) obtainable when a homonuclear alkali-metal diatom dissociating via a $^{1}\mathrm{\ensuremath{\Sigma}}^{\mathrm{*}}$ state evolves (by nonadiabatic radial coupling) into a superposition of two excited adiabatic states correlated to different doublet levels. CF intensity then combines ``ringing'' oscillations, caused by time variation of the interference phase between the receding emitting atoms, with quantum beats associated with fine-structure splitting. The calculated CF patterns for ${\mathrm{Li}}_{2}$ are sensitive to the amplitudes and relative phase of the two superposed adiabatic states.

Theoretical dipole transition moments for transitions between bound electronic states and non-adiabatic coupling matrix elements between2Σ+of HeH

Dipole transition moments and radial non-adiabatic coupling matrix elements between electronic states of HeH are presented. The estimated radiative lifetimes of the excited states of HeH are in good agreement with the available experimental data.

Laser-assisted charge-transfer reactions (Li3++H): Coupled dressed-quasimolecular-state approach.

A semiclassical coupled dressed-quasimolecular-states (DQMS) approach is presented for nonperturbative treatment of multichannel charge-transfer reactions at low collision velocities and high laser intensities, incorporating the implementation of the generalized Van Vleck (GVV) nearly degenerate perturbation theory. The GVV technique allows block partitioning of the infinite-dimensional Floquet Hamiltonian into a finite-dimensional model DQMS space, and thereby reduces greatly the number of effective coupled channels. Further, the GVV-Floquet basis allows minimization of the (usually large in amplitude) field-induced nonadiabatic radial couplings without the need to explicitly construct the transformation between the adiabatic and diabatic DQMS basis. This yields a new set of coupled GVV-DQMS equations (neither adiabatic nor diabatic) which are particularly convenient for multichannel calculations. The method is applied to the study of the laser-assisted charge-transfer process: Li/sup 3 +/+H(1s)+h..omega -->..Li/sup 2 +/(n = 3)+H/sup +/, using 2-, 5-, and 15- GVV-DQMS basis. It is found that while the 5-state results agree well with the 15-state calculations even up to very high intensities for the (LiH)/sup 3 +/ system, the 2-state basis is inadequate at high-intensity and lower-wavelength regimes.

Nonadiabatic semiclassical scattering. II. Solution of two-dimensional models and comparison with quantum results

The generalized surface-hopping procedure for semiclassical nonadiabatic scattering is applied to a two dimensional model for comparison with quantum results. The model consists of two adiabatic electronic potential energy surfaces with a radial nonadiabatic coupling between the electronic states. Both adiabatic potential surfaces are taken to be constant. The coupling is assumed to be weak. This model is sufficient to explore many of the important features of the generalized surface-hopping procedure. Comparison of the semiclasssical and quantum results demonstrates good agreement, and the first order quantum scattering amplitude is analytically found to be the ℏ→0 limit of the first order semiclassical scattering amplitude. The behavior of the wave function near singularities in the transition amplitudes is explored in a general context, independent of these simplified models. These singularities are found to be necessary to produce the correct behavior in the wave function.