A collaborative theoretical and experimental study of the structure and electronic excitation spectrum of the Bar and Bar2 complexes

Accurate ab initio structural parameters of the diatomic and triatomic van der Waals molecules (11)BNg (X(2)Π, A(2)Σ(+)) and (11)BNg2 (X̃(2)B1), Ng = (4)He, (20)Ne, (40)Ar, (84)Kr, and (132)Xe

The weakly interacting BNg and BNg2 molecular systems, Ng = He, Ne, Ar, Kr, and Xe, have been thoroughly studied through coupled-cluster RCCSD(T) calculations and large correlation consistent basis sets. For the BNg diatomics, the states examined are the X(2)Π and A(2)Σ(+), and the X̃(2)B1 state for the C2v BNg2 triatomics. A series of corrections render our final results reliable, judging as well from the (limited) experimental numbers available. Both BHe and BHe2 are marginally unbound, whereas the attractive interactions of the BNg X(2)Π states, where Ng = Ne, Ar, Kr, and Xe, are D0 = 19.8, 98.2, 141.9, and 209.1 cm(-1), respectively. For the BRn (Rn = radon) species, an estimated value of interaction energy D0 ≈ 280 cm(-1) is obtained by a D0 versus static polarizability (α) extrapolation. Corresponding atomization energies of the BNg2 (X̃(2)B1) molecules are AE0 = 52.0 (BNe2), 263.4 (BAr2), 384.6 (BKr2), and 576.9 (BXe2) cm(-1).

The OH radical-H2O molecular interaction potential

The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between the rigid OH radical and H(2)O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there is a low-lying excited state for the H(2)O.OH complex, the impact of the excited state on the chemical behavior of the OH radical can be very important. We perform a potential energy surface scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole-type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H(2)O for both ground and excited states. The stationary points of (H(2)O)(n)OH clusters were identified as a benchmark of the potential.

Path integral Monte Carlo simulation of the absorption spectra of an Al atom embedded in helium

We use a multilevel path integral Monte-Carlo (PIMC) method to simulate the arrangement of He atoms around a single Al atom doped in a He cluster. High-level ab initio Al-He pair potentials and a Balling and Wright pairwise Hamiltonian model are used to describe the full potential and the electronic asymmetry arising from the open-shell character of the Al atom in its ground and excited electronic states. Our calculations show that the doping of the Al 3p electron strongly influences the He packing. The results of the PIMC simulation are used to predict the electronic excitation spectrum of an Al atom embedded in He clusters. With inclusion of tail corrections for the ground and excited states potentials, the calculated 3d<--3p spectrum agrees reasonably well with the experimental spectrum. The blueshift of the calculated spectrum associated with the 4s<--3p transition of solvated Al is about 25 nm (2000 cm-1) larger than seen in experiments on Al embedded in bulk liquid He. We predict that the spectrum associated with the 4p<--3p transition will be blueshifted by approximately 7000 cm-1 (nearly 1 eV).

Spectroscopic investigation of nonbonding interactions of group-14 atoms with rare gases: the SnAr van der Waals complex

The laser fluorescence excitation spectra of the SnAr van der Waals complex, in the vicinity of the individual fine-structure lines of the Sn 5s25p6s3P0<-- 5s25p2(3)P atomic resonance transition in the spectral region 317-270 nm are reported. Excited-state (v',0) progressions of bands built upon the individual J'<-- J" fine-structure atomic lines were observed. Because the collisional spin-orbit relaxation was slow, transitions were observed out of the lower SnAr states built upon all the J'' atomic asymptotes. The spectra were interpreted through model potential energy curves based on the isoelectronic SiAr system. Lower bounds to the dissociation energies of all lower SnAr states were determined. The binding energies of the group-13, and -14-atom-argon complexes and the effect of the spin-orbit interaction on moderating nonbonding interactions are discussed.

Modeling of the Three-Body Effects in the Neutral Trimers in the Quartet State by ab initio Calculations. H3, Na3, and Na2B

The Na2B, Na3, and H3 trimers in the lowest quartet states were studied by ab initio methods, using both the supermolecular approach and the intermolecular Møller-Plesset perturbation theory. Partitioning of the nonadditive contribution into the orientational two-body part and the genuine three-body part was proposed. The lowest quartet state of the Na3 trimer and all the three lowest quartet states of the Na2B trimer are bound, and the forms of these clusters are essentially determined by two-body forces. In the case of the Na2B trimer the orientational two-body nonadditivity proved to be crucial. In addition, in the title metal trimers, in the region of the van der Waals minima, the genuine nonadditivity is very important, and amounts to 30% in Na2B and up to 70% in Na3. The leading nonadditive term is the triple-exchange Heitler-London exchange term. For triangular arrangements it considerably enhances the total stabilization. The single-exchange term and the SCF deformation play only a secondary role. The dispersion nonadditivity is negligible. The isotropic part of the basis set superposition error (BSSE) is large and must be corrected by the counterpoise method. The anisotropic contribution to BSSE is practically negligible.