H. Berriche

AB INITIO STUDY OF THE LIH+ MOLECULE, ELECTRONIC INTERACTION ANALYSIS AND LIH UV PHOTOELECTRON SPECTRUM

Abstract All adiabatic curves of LiH + dissociating into Li(2s, 2p, 3s, 3p, 3d) + H + and Li + + H (1s, 2s, 2p) are determined by an ab initio approach involving a non-empirical pseudopotential for the Li(ls 2 ) core and core valence correlation corrections. The resulting spectroscopic constants and vibrational level spacings of all these states are presented. From the usual semiclassical approximations an analysis of the high energy vibrational level spacing is performed allowing for accurate long range extrapolations. For the lowest curves dissociating into Li + + H (1s) and Li (2s) + H + an analysis of the main electronic interactions is carried out from a diabatic model and reveals the importance of the binding charge delocalisation effects versus the polarisation (charge localised) ones. In addition the LiH photoelectron spectrum is calculated. An interesting feature of that spectrum is that both bound-bound and bound-free transitions coexist due to the particular shape of the LiH and LiH + potential energy curves.

Nonradiative lifetimes for LiH in the A state using adiabatic and diabatic schemes

Accurate positions and nonradiative lifetimes of states belonging to the adiabatic A state of LiH are estimated. The results coming from a Golden Rule treatment in the adiabatic scheme present excellent agreement with those obtained through a diabatic close coupling calculation. That confirms the accuracy reached in both approaches and also in the treatment of the diabatic–adiabatic transformation. It involves, in particular, an effective phase control that is needed to properly estimate nonadiabatic couplings. Also, a powerful numerical procedure to obtain energy profiles in the diabatic close coupling frame is described and applied in this work.

Ab initio adiabatic and diabatic permanent dipoles for the low-lying states of the LiH molecule. A direct illustration of the ionic character

Abstract The permanent dipole moments of the eight low-lying 1 Σ states of the LiH molecule are calculated by an ab initio approach for both the adiabatic and the diabatic representations. The results shed light on the interplay between the ionic and the neutral states producing a direct illustration of the ionic character of the electronic wavefunction. Our results suggest that the location and the width of the avoided crossings for the potential energy curves could be experimentally derived.

Theoretical study of the NaLi molecule: potential energy curves, spectroscopic constants, dipole moments and radiative lifetimes

The adiabatic potential energy curves and the permanent and transition dipole moments of the low-lying electronic states of the NaLi molecule dissociating into Na(3s, 3p, 4s, 3d, 4p) + Li(2s, 2p, 3s, 3p, 3d) have been investigated. The molecular calculations are performed using an ab initio approach based on non-empirical pseudopotentials for Na+ and Li+ cores, parameterized l-dependent polarization potentials and full configuration interaction calculations through the CIPCI quantum chemistry package. The derived spectroscopic constants of the ground state and lower excited states are in good agreement with available experimental and theoretical works. However, the 7–101,3Σ+, 5–71Π, 2–63Π and 1–31,3Δ excited states are studied for the first time. The permanent dipole moment of NaLi has revealed both ionic characters relating to electron transfer and yielding Na+Li− and Na−Li+ arrangements. The transition dipole moment is used to evaluate the radiative lifetimes of the vibrational levels trapped in the 21Σ+ excited state for the first time. In addition to the bound–bound contribution, the bound-free term has been evaluated exactly using the Franck–Condon (FC) approximation and added to the total radiative lifetime.

Theoretical study of the LiCs molecule: adiabatic and diabatic potential energy and dipole moment.

For nearly all the states dissociating into Cs (6s, 6p, 5d, 7s, 7p, 6d, 8s) and Li (2s, 2p, 3s), we present an extensive adiabatic study for (1,3)Sigma(+), (1,3)Pi, and (1,3)Delta symmetries of the LiCs molecule. We have used an ab initio approach based on nonempirical pseudopotentials, parametrized l-dependent polarization potentials, and full configuration interaction calculations. A diabatisation procedure based on the effective Hamiltonian theory and an effective metric is used to produce the quasi-diabatic potential energy for all studied states. The spectroscopic constants (R(e), D(e), T(e), omega(e), omega(e)x(e), and B(e)) of these states are derived and compared with the available theoretical and experimental works. In addition to the potential energies, accurate permanent and transition dipole moment have been determined for a wide range of internuclear distances. The adiabatic permanent dipole moment for the first 10 (1)Sigma(+) electronic states has revealed ionic characters relating to electron transfer and yielding both Li(-)Cs(+) and Li(+)Cs(-) arrangements. The quasi-diabatic permanent moments show linear behaviors, especially at intermediate and large distance. The transition dipole moment between neighbor states has revealed many peaks located around the avoided crossing positions.

Vibronic shifts for LiH in X and A states

The energy shift of the vibrational levels of the X and A states due to the vibronic interactions within the manifold of states is accurately determined using a diabatic representation. These vibronic shifts which measure the breakdown of the Born - Oppenheimer approach are compared with the usual estimation where only the adiabatic correction is taken into account. The adiabatic corrections lead to a systematic overestimation of the full vibronic shift for the ground state and to an underestimation for the A one. These results have been related to the interstate dynamical couplings and rationalized using perturbative arguments. Their validity is expected to be very general.

Structural and spectroscopic study of the LiRb molecule beyond the Born-Oppenheimer approximation.

Adiabatic and diabatic potential energy curves and the permanent and transition dipole moments of the low-lying electronic states of the LiRb molecule dissociating into Rb(5s, 5p, 4d, 6s, 6p, 5d, 7s, 6d) + Li(2s, 2p) have been investigated. The molecular calculations are performed with an ab initio approach based on nonempirical pseudopotentials for Rb(+) and Li(+) cores, parametrized l-dependent core polarization potentials and full configuration interaction calculations. The derived spectroscopic constants (R(e), D(e), T(e), ω(e), ω(e)x(e), and B(e)) of the ground state and lower excited states are in good agreement with the available theoretical works. However, the 8-10(1)Σ(+), 8-10(3)Σ(+), 6(1,3)Π, and 3(1,3)Δ excited states are studied for the first time. In addition, to the potential energy, accurate permanent and transition dipole moments have been determined for a wide interval of internuclear distances. The permanent dipole moment of LiRb has revealed ionic characters both relating to electron transfer and yielding Li(-)Rb(+) and Li(+)Rb(-) arrangements. The diabatic potential energy for the (1,3)Σ(+), (1,3)Π, and (1,3)Δ symmetries has been performed for this molecule for the first time. The diabatization method is based on variational effective Hamiltonian theory and effective metric, where the adiabatic and diabatic states are connected by an appropriate unitary transformation.

Theoretical Study of the CsLi Molecule Beyond the Born‐Oppenheimer Approximation

The adiabatic and diabatic, potential energy curves, the spectroscopic constants and the transition dipole moments of the lowest electronic states of the CsLi molecule dissociating into Cs (6s, 6p, 5d, 7s, 7p, 6d, 8s) and Li (2s, 2p, 3s) have been performed. We have used an ab initio approach based on non‐empirical pseudopotential, parameterized l‐dependent polarization potentials and full configuration interaction calculations. Our spectroscopic constants of the ground and the first excited states are in good agreement with the available theoretical works.

Interactions and low-energy collisions between an alkali ion and an alkali atom of a different nucleus

We study theoretically interaction potentials and low-energy collisions between different alkali atoms and alkali ions. Specifically, we consider systems such as X + , where X( is either Li(Cs+) or Cs(Li+), Na(Cs+) or Cs(Na+) and Li(Rb+) or Rb(Li+). We calculate the molecular potentials of the ground and first two excited states of these three systems using a pseudopotential method and compare our results with those obtained by others. We derive ground-state scattering wave functions and analyze the cold collisional properties of these systems for a wide range of energies. We find that, in order to get convergent results for the total scattering cross sections for energies of the order 1 K, one needs to take into account at least 60 partial waves. The low-energy scattering properties calculated in this paper may serve as a precursor for experimental exploration of quantum collisions between an alkali atom and an alkali ion of a different nucleus.

Electronic structure and spectra of the RbAr van der Waals system including spin-orbit interaction.

The potential energy curves and spectroscopic constants of the ground and excited states of the RbAr van der Waals system have been determined using a one-electron pseudopotential approach. This technique is used to replace the effect of the Rb(+) core and the electron-Ar interactions by effective potentials. The core-core interaction for Rb(+)Ar was incorporated using the accurate CCSD(T) potential of Hickling et al. [Hickling, H. L.; Viehland, L. A.; Shepherd, D. T.; Soldán, P.; Lee, E. P. F.; Wright, T. G. Phys. Chem. Chem. Phys. 2004, 6, 4233-4239]. This model reduces the number of active electrons of the RbAr van der Waals systems to just the single valence electron, permitting the use of very large basis sets for the Rb and Ar atoms. Using this approach, the potential energy curves of the ground and excited states dissociating into Rb(5s, 5p, 4d, 6s, 6p, 6d, and 7s) + Ar are calculated at the SCF level. Spin-orbit interaction was also considered within a semiempirical scheme for the states dissociating into Rb(5p) and Rb(6p). Spectroscopic constants are derived and compared with the available theoretical and experimental data. Such comparisons for RbAr show very good agreement for the ground and the first excited states. Furthermore, we have predicted the B(2)Σ(+)(1/2) ← X(2)Σ(+), A(2)Π(1/2) ← X(2)Σ(+), A(2)Π(3/2) ← X(2)Σ(+), A(2)Π(3/2) ← X(2)Σ(+), 5(2)Σ(+) ← X(2)Σ(+), 3(2)Π(1/2) ← X(2)Σ(+), and 3(2)Π(3/2) ← X(2)Σ(+) absorption spectra.