Finite-field many-body-perturbation-theory calculation of the static hyperpolarizabilities and polarizabilities of Mg, Al+, and Ca

Electronic structure, cold ion-atom elastic collision properties and possibility of laser cooling of BeCs+ molecular ion

The BeCs⁺ system represents a possible future candidate for the realization of samples of cold or ultra-cold molecular ion species that have not yet been investigated experimentally or theoretically. With the aim of highlighting the spectroscopic and electronic structure of the cesium and beryllium cation BeCs⁺, we theoretically investigate ground and low lying excited states of ^1,3Σ⁺, ^1,3Π and ^1,3Δ symmetries below the first nine asymptotic limits dissociating into Be⁺(2s) + Cs(6s, 6p, 5d) and Be(2s², 2s2p, 2s3s, 2p²) + Cs⁺. We used a quantum chemistry approach based on a semi-empirical pseudo potential for Be²⁺ and Cs⁺ cores, core polarization potentials (CPP), large Gaussian basis sets and full configuration interaction (FCI) method for the valence electrons. Additional calculations have been performed for the ground state using CCSD(T)/CI methods with different basis sets. Adiabatic potential energy curves, spectroscopic constants, vibrational levels, and permanent and transition dipole moments are reported in this work. Furthermore, the elastic scattering properties at low energy for both ground 1¹Σ⁺ and second excited states 3¹Σ⁺, of BeCs⁺ are theoretically investigated, and isotopic effects on cold and ultra-cold energy collisions are also detected. Vibrational lifetimes of the ground state 1¹Σ⁺ are calculated taking into account both spontaneous and stimulated emissions and also the absorption induced by black body radiation at room temperature (T = 300 K). Vibrational radiative lifetimes for the first 2¹Σ⁺ and second 3¹Σ⁺ excited states are also calculated and extensively analyzed. We found that the radiative lifetimes of the lower vibrational levels of the 1¹Σ⁺ state have an order of magnitude of seconds (s), while those of 2¹Σ⁺ and 3¹Σ⁺ states have an order of nanoseconds (ns). The Franck-Condon factors are also calculated for transitions from the low lying excited 2¹Σ⁺, 3¹Σ⁺, 1¹Π states to the ground state 1¹Σ⁺. We found that the favourite vibrational transition to the 1¹Σ⁺(v = 0) ground state is obtained for 1¹Π (v''' = 0)-1¹Σ⁺(v = 0) with a diagonal structure and a large Franck-Condon factor value of 0.94. This Franck-Condon factor value is sufficiently large to make the BeCs⁺ system a favorable candidate for direct laser cooling.

Polarizabilities of the alkali anions: Li− to Fr−

Static dipole polarizabilities are calculated for the ground states of the alkali anions from Li- to Fr-. The polarizabilities include scalar relativistic effects at the second-order Douglas-Kroll level and were computed using the finite-field, coupled-cluster CCSD(T) method with large, carefully optimized basis sets. The relativistic polarizabilities increase with Z, reach a maximum at Cs-, and then decrease again unlike their nonrelativistic counterparts which increase monotonically with Z.

Predicting atomic dopant solvation in helium clusters: The MgHen case

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg-doped helium clusters MgHe(n) with n=2-50. Three high-level [MP4, CCSD(T), and CCSDT] MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He(n). Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited 1P Mg atom and the He moiety has been approximated using the diatomics-in-molecules method and the two excited 1pi and 1sigma MgHe potentials. The shape of the simulated MgHe50 spectra shows a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact density-functional theory has been adapted to the case of doped He(n) and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviors.

Polarizabilities and hyperpolarizabilities for the atoms Al, Si, P, S, Cl, and Ar: Coupled cluster calculations

Accurate static dipole polarizabilities and hyperpolarizabilities are calculated for the ground states of the Al, Si, P, S, Cl, and Ar atoms. The finite-field computations use energies obtained with various ab initio methods including Moller-Plesset perturbation theory and the coupled cluster approach. Excellent agreement with experiment is found for argon. The experimental alpha for Al is likely to be in error. Only limited comparisons are possible for the other atoms because hyperpolarizabilities have not been reported previously for most of these atoms. Our recommended values of the mean dipole polarizability (in the order Al-Ar) are alpha/e(2)a(0) (2)E(h) (-1)=57.74, 37.17, 24.93, 19.37, 14.57, and 11.085 with an error estimate of +/-0.5%. The recommended values of the mean second dipole hyperpolarizability (in the order Al-Ar) are gamma/e(4)a(0) (4)E(h) (-3)=2.02 x 10(5), 4.31 x 10(4), 1.14 x 10(4), 6.51 x 10(3), 2.73 x 10(3), and 1.18 x 10(3) with an error estimate of +/-2%. Our recommended polarizability anisotropy values are Deltaalpha/e(2)a(0) (2)E(h) (-1)=-25.60, 8.41, -3.63, and 1.71 for Al, Si, S, and Cl respectively, with an error estimate of +/-1%. The recommended hyperpolarizability anisotropies are Deltagamma/e(4)a(0) (4)E(h) (-3)=-3.88 x 10(5), 4.16 x 10(4), -7.00 x 10(3), and 1.65 x 10(3) for Al, Si, S, and Cl, respectively, with an error estimate of +/-4%.