Alexei A. Buchachenko

Ar–I2 interactions: The models based on the diatomics‐in‐molecule approach

Semiempirical model is developed for studying the electronic structure of the rare gas atom–halogen molecule systems. It is formulated in the frame of diatomics‐in‐molecule (DIM) approach and takes explicitly into account strong spin–orbit coupling pertinent to heavy halogen molecules. The consistent DIM scheme is realized for intermolecular interactions, whereas the description of valence electronic states of halogen molecule is more approximate being based on the asymptotic wave functions. The corresponding perturbation theory is also put forward. The model is applied to analysis of several features of the Ar...I2 van der Waals complex. First, the calculations on the spectroscopic constants of the B←X transition in the complex reveal the quantitative performance of the model. Second, mechanisms of nonadiabatic dynamics are examined. The results are qualitatively consistent with the current view on the Ar...I2 electronic predissociation and one‐atom cage effect. Third, the prediction is made on the valen...

Vibrational predissociation dynamics of the He79Br2 van der Waals molecule: A quantum mechanical study

The vibrational predissociation of the HeBr2 van der Waals complex is studied by means of both accurate and approximate three dimensional quantum mechanical calculations. Simple atom–atom potentials have been tested for matching experimental measurements at low Br2 vibrational excitations. The fragmentation dynamics when the bromine subunit is close to its B state dissociation limit is then explored and compared with experiments. For low to intermediate vibrational states v, good agreement with most of the data (spectral shifts, lifetimes, average structures, average product energies) is achieved. The closing of the Δv=−1 channel at v=44 and the binding energy at that position are successfully reproduced, although calculated and experimental blueshifts and linewidths are not in such good accordance in the v≳38 range. For these high v excitations, fragmentation cross sections exhibit complicated structures indicating strong interactions among different quasibound states. In addition, interesting threshold ...

Ab initio potential energy and dipole moment surfaces, infrared spectra, and vibrational predissociation dynamics of the 35Cl−⋯H2/D2 complexes

Three-dimensional potential energy and dipole moment surfaces of the Cl−–H2 system are calculated ab initio by means of a coupled cluster method with single and double excitations and noniterative correction to triple excitations with augmented correlation consistent quadruple-zeta basis set supplemented with bond functions, and represented in analytical forms. Variational calculations of the energy levels up to the total angular momentum J=25 provide accurate estimations of the measured rotational spectroscopic constants of the ground van der Waals levels n=0 of the Cl−⋯H2/D2 complexes although they underestimate the red shifts of the mid-infrared spectra with v=0→v=1 vibrational excitation of the monomer. They also attest to the accuracy of effective radial interaction potentials extracted previously from experimental data using the rotational RKR procedure. Vibrational predissociation of the Cl−⋯H2/D2(v=1) complexes is shown to follow near-resonant vibrational-to-rotational energy transfer mechanism so...

Ar⋯I2: A model system for complex dynamics

We review spectroscopic and photodissociation dynamical studies in the region of the B L X transition of the Ar ··· I 2 van der Waals complex, both below and above the dissociation limit of the state. This very simple system constitutes a prototype for a wide range of molecular processes: vibrational predissociation involving intramolecular vibrational relaxation, electronic predissociation, cage effect, …. Each of these processes has been or still is the subject of differing interpretations: intramolecular vibrational relaxation involved in the vibrational predissociation of this system can be in the sparse or statistical regime, vibrational and electronic predissociation are in competition and a direct, ballistic interpretation of the cage effect as well as a non-adiabatic one have been proposed. The study of the dependence of these dynamical processes on the relative orientation of the two partners of the complex (stereodynamics) is made possible by the coexistence of two stable Ar ··· I 2 ) isomers. Experimental as well as theoretical results are reviewed. Experiments range from frequency-resolved to time-dependent studies, including the determination of final state distributions. Theoretical studies involve potential energy surface calculations for several electronic states of the complex and their couplings and adiabatic as well as non-adiabatic dynamical simulations.

Complete basis set extrapolation limit for electronic structure calculations: Energetic and nonenergetic properties of HeBr and HeBr2 van der Waals dimers

The lowest 2Σ+ and 2Π electronic states of the HeBr molecule have been calculated by the ab initio coupled cluster approach in conjunction with a series of increasing size augmented correlation-consistent basis sets of double through quintuple zeta quality. Different extrapolation formulas to the complete basis set limit have been tested by comparing estimated and actual quintuple zeta quality counterpoise corrected interaction energies. Frozen-core approach is checked by performing calculations in which all electrons are correlated. The potential energy surfaces of the HeBr2 van der Waals complex have been obtained from the HeBr potentials by means of the diatomic-in-molecule approach. Finally, transport, scattering, and spectroscopic properties of HeBr and HeBr2(B) systems derived from ab initio data for different basis sets are examined. It is shown that their convergence closely follows the convergence of corresponding potential energy surfaces.

He79Br2 B,v=8←X,v″=0 excitation spectrum: Ab initio prediction and spectroscopic manifestation of a linear isomer

The B←X rovibronic excitation spectrum of the HeBr2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology (linear and T-shaped wells) for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of t...

A combined experimental-theoretical study of the vibrational predissociation and product rotational distributions for high vibrational levels of He79Br2

Pump-probe spectra of HeBr2 in vibrational states v′=10 and 39 through 48 of the B electronic state are reported and the fragment rotational distributions from vibrational predissociation of the cluster are extracted from the measured E(0g+)←B(3Π0u+) spectra of Br2. The experimental results are compared to theoretical calculations on the B←X spectra using atom-atom model potentials and performing a thermal average over transitions that contribute to the net excitation. Very good agreement between experiment and theory is obtained, except in the region of v′=44, where the Δv=−1 channel closes, and in the region of v′=48 where the Δv=−2 channel closes. For v′=43, and v′=44, the agreement is less satisfactory because the dynamics are extremely sensitive to details of the potential energy surface due to threshold effects associated with the Δv=−1 channel closing. Similar sensitivity to the potential due to the Δv=−2 channel closing impairs the agreement between experiment and theory for v′=48. Below v′=43, th...

Collision and transport properties of Rg+Cl(2P) and Rg+Cl−(1S) (Rg=Ar, Kr) from ab initio potentials

Highly accurate ab initio coupled cluster theory calculations, with single, double and noniterative triple excitations [CCSD(T)], and with the extended basis set augmented by the bond functions, were performed for the interactions of chlorine atom and chloride anion with Ar and Kr. Analytical fits to the ab initio points were shown to provide the consistent and reliable set of multiproperty potentials capable of reproducing all the available experimental data, namely: Total cross sections for Rg+Cl, reduced mobilities and diffusion coefficients for Rg+Cl−, as well as the data of spectroscopic zero electron kinetic energy experiments [see Buchachenko et al., J. Chem. Phys. 114, 9929 (2001), following paper]. The relative accuracy of available interaction potentials is discussed.

RG+CL(2P) (RG=HE, NE, AR) INTERACTIONS : AB INITIO POTENTIALS AND COLLISION PROPERTIES

The lowest states of Σ and Π symmetry of Rg⋯Cl (Rg=He, Ne, Ar) complexes were investigated using the coupled cluster approach with single, double, and noniterative triple excitations (CCSD(T)) in an extended basis set including bond functions. The Σ states possess deeper minima at shorter interatomic distances than the corresponding Π states. The Σ-Π splittings, which for He⋯Cl and Ar⋯Cl are significantly larger than previously deduced, are mainly due to differences in the exchange repulsion terms. The total energies were dissected into electrostatic, exchange, induction, and dispersion components. The calculated potentials have been used in the calculations of collision properties of Rg⋯Cl systems. Absolute total cross sections, spin-orbit quenching rate constants, and diffusion coefficients were evaluated from both the ab initio and previously available empirical potentials. The performance of ab initio potentials in these calculations proved to be very reasonable.

Interaction potentials of the RG–I anions, neutrals, and cations (RG=He, Ne, Ar)

Interaction potentials of the iodine atom, atomic cation, and anion with light rare-gas atoms from He to Ar are calculated within the unified ab initio approach using the unrestricted coupled-cluster with singles and doubles and perturbative treatment of triples correlation treatment, relativistic small-core pseudopotential, and an extended basis set. Ab initio points are fit to a flexible analytical function. The calculated potentials are compared with available literature data, assessed in the I(-)-and I+-ion mobility calculations and the Ar-I(-)-anion zero electron kinetic-energy spectra simulations, and analyzed using the correlation rules. The results indicate a high precision of the reported potentials.