Z. Ben Lakhdar

Rydberg states of small NaArn* clusters

The 4s and 5s Rydberg excited states of NaAr(n)* clusters are investigated using a pseudopotential quantum-classical method. While NaAr(n) clusters in their ground state are known to be weakly bound van der Waals complexes with Na lying at the surface of the argon cluster, isomers in 4s or 5s electronically excited states of small NaAr(n)* clusters (n< or =10) are found to be stable versus dissociation. The relationship between electronic excitation and cluster geometry is analyzed as a function of cluster size. For both 4s and 5s states, the stable exciplex isomers essentially appear as sodium-centered structures with similar topologies, converging towards those of the related NaAr(n)+ positive ions when the excitation level is increased. This is consistent with a Rydberg-type picture for the electronically excited cluster, described by a central sodium ion solvated by an argon shell, and an outer diffuse electron orbiting around this NaAr(n)+ cluster core.

Rotational excitation and de-excitation of C2(X Σ1g+) by para-H2(j=0)

For the van der Waals C(2)(X (1)Sigma(g)(+))-H(2) molecular system, we generated a new ab initio potential energy surface (PES). We mapped this PES at the multireference internally contracted configuration-interaction method including the Davidson correction together with a large diffuse basis set. Then, we incorporated our PES into quantum scattering calculations at the close coupling and infinite order sudden approximation methods to cover collision energies ranging from 0.1 up to 4000 cm(-1). After Boltzmann thermal averaging, rate coefficients for temperatures of up to 1000 K are deduced. Discrepancies between our new rates and those computed previously are noticed. This should induce deviations in astrophysical modeling.

Rotational excitation of HOCO+ by helium at low temperature

Context. It has been shown that the HOCO + ion is present in interstellar space. As a large number of HOCO + lines can be observed in the millimeter and submillimeter wavelengths, this molecule is a useful tracer for both the temperature and the density structure of the clouds. Modeling of the spectra will require accurate radiative and collisional rates of species of astrophysical interest. Aims. The paper focuses on the calculation of rotational excitation rate coefficients of HOCO + by He, useful for studies of low-temperature environments. Methods. Cross sections are calculated using the quantum Coupled States approach for a total energy range 0-200 cm -1 . These calculations are based on a new ab initio CEPA (coupled electron pair approach) potential energy surface. It was assumed that the HOCO + ion was fixed at its theoretical equilibrium geometry. Results. Thermally averaged rate coefficients were calculated from the cross sections, at kinetic temperatures up to 30 K.

Low-Temperature Rate Constants for Rotational Excitation and De-excitation of C3 (X1Σg+) by Collisions with He (1S)

The low-temperature rotational (de-) excitation of C3 (X1Σg+) by collisions with He (1S) is studied using an ab initio potential energy surface (PES). This PES has been calculated using the single- and double-excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] and the augmented correlation-consistent triple-ζ basis set (aug-cc-pVTZ) with bond functions. This PES is then incorporated in full close-coupling quantum scattering calculations for collision energies between 0.1 and 50 cm−1 in order to deduce the rate constants for rotational levels of C3 up to j = 10, covering the temperature range 5-15 K.

Net emission of H2O–air–MgCl2/CaCl2/NaCl thermal plasmas

This paper is devoted to the calculation of the net radiation emitted by water–air–MgCl2/CaCl2/NaCl mixtures. Assuming a spherical, homogeneous and isothermal plasma, the net emission coefficient (NEC) is calculated for several pressures between 1 and 10 bar in the temperature range 300–30 000 K under the assumption of local thermodynamic equilibrium. The various contributions to the NEC (atomic continuum, molecular continuum, molecular band and spectral lines) are presented and described. The influence of the pressure and of the proportion of air, MgCl2, CaCl2 or NaCl is analysed. It is also demonstrated that the alkaline salt resonance lines (Ca+, Ca2+, Na+, Mg+, Mg2+, Cl+ and Cl2+) have a significant effect on the NEC value in spite of their strong self-absorption.

Spectroscopy and metastability of BeO

The potential energy curve of the ground electronic state of BeO and those of the lowest electronic states of the BeO+ cation are computed using the CASSCF/MRCI methods and a large basis set. For the cation, the spin–orbit coupling and the transition momentum integrals are also evaluated. These data are used later to deduce an accurate set of spectroscopic constants and to investigate the spin–orbit-induced predissociation of the lowest electronic excited states of BeO+. Our calculations show that the high-rovibrational levels of the BeO+ (12Σ+) electronic state exhibit rapid predissociation processes forming Be+(2S) + O(3P). Our curves are also used for predicting the single ionization spectrum of BeO.

Temporal Characterization of a Plasma Produced by Interaction of Laser Pulses with Water Solutions

The temporal evolution of a plasma formed by the interaction of a Nd-YAG 10 ns laser pulse with the surface of aqueous solutions of CaCl 2 , has been observed by analyzing the variation of the emission spectra of ions and neutral atoms in the time interval after the laser pulse, 500 ns T e using a Saha-Boltzmann analysis. The broadening of the resonance line of CaI at 4227 A was used to deduce the electronic density N e , with the aid of the semiclassical theory of Stark broadening. A value of T e ≈ 28000 K decreasing slightly to 21000 K, and an exponential decay for N e were found, with N e ≈ 1:25 × 10 18 cm -3 at 500 ns and τ e = ( 1200 ± 50 ) ns.

Theoretical investigation of excited and Rydberg states of imidogen radical NH: potential energy curves, spectroscopic constants, and dipole moment functions.

A search is conducted for the calculation of potential energy curves (PECs), spectroscopic constants, and dipole moment functions for excited and Rydberg states of imidogen radical NH, with a particular emphasis on the Rydberg states arising from 3s configuration of nitrogen and 2s and 2p configurations of hydrogen. A range of about 11 eV above the electronic ground state X (3)Sigma- atomic separation limit which corresponds to the first eight asymptotes of dissociation is spanned. Computations are carried out at the internally contracted multireference singles plus doubles configuration interaction level of theory, including the Davidson correction to account for quadruple excitations. The Gaussian basis set used has been modified from a standard basis to give a balanced description of valence-Rydberg interactions. States of (1)Sigma-, (1)Pi, (1)Delta, (3)Sigma-, (3)Pi, (3)Delta, and (5)Sigma- symmetries are computed accurately in the range of energy investigated. PECs of the three lowest (5)Pi states are obtained for the first time. Our spectroscopic constants show good agreement with experimental data in comparison with other theoretical studies reported in the literature. A discussion on the variations of dipole moment functions helps to understand the strong interactions between excited and Rydberg states as well as the avoided crossings. The present study may be of great practical interest for investigations in astrophysical research as well as in laboratory experiments.

Rotationally inelastic collisions of methinoposphide (HCP) with para-H2 at low temperature

The interaction potential energy surface of the methinoposphide (HCP)-H(2) complex is calculated at the ab initio coupled-cluster level of theory with an aug-cc-pVTZ Gaussian basis set. The [H-C] and [C-P] bond lengths of HCP are set to their values at the linear equilibrium ground vibrational level of the molecule. The calculated interaction energy presents two minima located 106.3 and 67.6 cm(-1) below the HCP+H(2) dissociation limit. Using the interaction potential obtained, we have computed collision excitation cross sections in the close-coupling approach and downward rate coefficients at low temperature, i.e., T<or=70 K. These quantities are significantly magnified in comparison with their counterparts for the HCP-He collisions. It is shown that there is a propensity toward DeltaJ=1 transitions.

Ab initio transition state theory calculation of the rate constant for the hydrogen abstraction reaction H2O2+H→H2+HO2

Large basis set and two levels of ab initio calculation (ROHF and MCSCF) are used to determine the electronic structure of reactants, products, and saddle point involved in the hydrogen abstraction reaction H2O2+H→H2+HO2. The calculated ROHF and MCSCF imaginary frequency ω≠ corresponds to the motion of an hydrogen atom between H2O2 and H and has respectively, a magnitude of 6826.5 and 2909.9 cm−1. Calculated (MP2//ROHF and MP2//MCSCF) values of 8.92 and 7.92 Kcal/mol are, respectively, found for the barrier height of the title reaction. The ab initio results are used with the transition state theory (TST) to evaluate the rate constant kTST(T) over the range of temperature 200⩽T⩽2000 K. Tunneling corrections to kTST(T) are considered through the evaluation of the transmission coefficient by Wigner (W) and zero curvature tunneling (ZCT) methods. Our results show that the calculated rate constants based on the ROHF electronic structure results do not agree with the experimental values. The best agreement wit...