Fernand Spiegelman

Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium

Aims. The competition between the formation and destruction of coronene clusters under interstellar conditions is investigated theoretically. Methods. The unimolecular nucleation of neutra! clusters is simulated with an atomic model combining an explicit classical force field and a quantum tight-binding approach. Evaporation rates are calculated in the framework of the phase space theory and are inserted in an infrared emission model and compared with the growth rate constants. Results. It is found that, in interstellar conditions, most collisions lead to cluster growth. The time evolution of small clusters (containing up to 312 carbon atoms) was specifically investigated under the physical conditions of the northern photodissociation region of NGC 7023. These clusters are found to be thermally photoevaporated much faster than they are reformed, thus providing an interpretation for the lowest limit of the interstellar cluster size distribution inferred from observations. The effects of ionizing the clusters and density heterogeneities are also considered. Based on our results, the possibility that PAH clusters could be formed in PDRs is critically discussed.

K-H2 quasi-molecular absorption detected in the T-dwarf ε Indi Ba

Context. T-type dwarfs present a broad and shallow absorption feature centred around 6950 A in the blue wing of the K doublet at 0.77 μm which resembles in depth and shape the satellite absorption predicted by detailed collisional broadening profiles. In our previous work, the position of the predicted line satellite was however somewhat too blue compared to the observed feature. Aims. In this paper, we investigate whether new calculations of the energy surfaces of the potentials in the K-H 2 system, including spin-orbit coupling, result in a closer coincidence of the satellite with the observed position. We also investigate the extent to which CaH absorption bands contribute to the feature and at what T eff these respective opacity sources predominate. Methods. We present model atmospheres and synthetic spectra, including gravitational settling for an improved description of depth-dependent abundances of refractory elements, and based on new K-H 2 line profiles using improved interaction potentials. Results. By comparison with a high signal-to-noise optical spectrum of the Tl dwarf sIndiBa, we find that these new models do reproduce the observed feature, while CaH does not contribute for the atmospheric parameters considered. We also find that CaH is settled out so deep into the atmosphere that even turbulent vertical mixing would appear insufficient to bring significant amounts of CaH to the observable photosphere in dwarfs of later type than ∼L5. Conclusions. We conclude that previous identification of the feature at this location in the spectra of T dwarfs as well as the latest L dwarfs with CaH was erroneous, as expected on physical grounds: calcium has already condensed onto grains in early L dwarfs and thus should have settled out of the photosphere in cooler brown dwarfs. This finding revokes one of the observational verifications for the cloud-clearing theory assumption: a gradual clearing of the cloud cover in early T dwarfs.

Correction for dispersion and Coulombic interactions in molecular clusters with density functional derived methods: Application to polycyclic aromatic hydrocarbon clusters

The density functional based tight binding (DFTB) is a semiempirical method derived from the density functional theory (DFT). It inherits therefore its problems in treating van der Waals clusters. A major error comes from dispersion forces, which are poorly described by commonly used DFT functionals, but which can be accounted for by an a posteriori treatment DFT-D. This correction is used for DFTB. The self-consistent charge (SCC) DFTB is built on Mulliken charges which are known to give a poor representation of Coulombic intermolecular potential. We propose to calculate this potential using the class IV/charge model 3 definition of atomic charges. The self-consistent calculation of these charges is introduced in the SCC procedure and corresponding nuclear forces are derived. Benzene dimer is then studied as a benchmark system with this corrected DFTB (c-DFTB-D) method, but also, for comparison, with the DFT-D. Both methods give similar results and are in agreement with references calculations (CCSD(T) and symmetry adapted perturbation theory) calculations. As a first application, pyrene dimer is studied with the c-DFTB-D and DFT-D methods. For coronene clusters, only the c-DFTB-D approach is used, which finds the sandwich configurations to be more stable than the T-shaped ones.

One-electron pseudopotential calculations of excited states of LiAr, NaAr, and KAr

The potential curves and spectroscopic constants of the excited states of alkali–argon diatomics MRg (M=Li, Na and K, Rg=Ar) are calculated using usual semilocal single valence electron pseudopotentials on alkali atoms [M+]-core pseudopotentials), semilocal pseudopotentials replac(ing all the electrons of argon ([Ar]-core pseudopotentials), and core polarization pseudopotentials on both centers. All states dissociating into Li(2s, 2p, 3s, 3p, 3d, 4s, and 4p), Na(3s, 3p, 3d, 4s, 4p, 4d, 5p) and K(4s, 4p, 5s, 3d, 5p, 4d, 6s, 4f, 6p, 5d, 7s, 5f) are considered. The core–core interactions for Li+Ar and Na+Ar are included using the accurate ab initio potentials of Ahmadi et al. [G. R. Ahmadi, J. Almlof, and I. Roeggen, Chem. Phys. 199, 33 (1995); G. R. Ahmadi and I. Roeggen, J. Phys. B 27, 5603 (1994)] while the K+Ar ion data are determined by MP2 all-electron calculations.

Two-electron pseudopotential investigation of the electronic structure of the CaAr molecule

The electronic structure of the Ca-Ar molecule is investigated using [Ca2+] and [Ar] core pseudopotentials complemented by core polarization operators on both atoms, considering the molecule to be a two-electron system. The electronic two-body problem is solved by achieving a full configuration interaction with extensive Gaussian basis sets. The potential energy curves and the molecular constants of all CaAr states dissociating into atomic configurations ranging between the ground state 4s2u200a1S and the doubly excited state 4p2u200a3P are determined. Spin–orbit coupling is also included in an atom-in-molecule scheme for states dissociating into the 4s4p and 4s3d configurations. The present theoretical results show good overall agreement with experimental data. They also help to clarify the very complicated spectroscopy of the CaAr system in the 38u200a000u2009cm−1 energy range where many states correlated with the 4s4d, 3d4p, and 4p2 atomic configurations interact with or cross one another. As a by-product of the prese...

Modeling Charge Resonance in Cationic Molecular Clusters: Combining DFT-Tight Binding with Configuration Interaction

In order to investigate charge resonance situations in molecular complexes, Wu et al. (J. Chem. Phys. 2007, 127, 164119) recently proposed a configuration interaction method with a valence bond-like multiconfigurational basis obtained from constrained DFT calculations. We adapt this method to the Self-Consistent Charge Density-Functional-based Tight Binding (SCC-DFTB) approach and provide expressions for the gradients of the energy with respect to the nuclear coordinates. It is shown that the method corrects the wrong SCC-DFTB behavior of the potential energy surface in the dissociation regions. This scheme is applied to determine the structural and stability properties of positively charged molecular dimers with full structural optimization, namely, the benzene dimer cation and the water dimer cation. The method yields binding energies in good agreement with experimental data and high-level reference calculations.

Ab initio study of silver bromide AgnBrp(+) clusters (n⩽6,p=n,n−1)

Ab initio calculations in the framework of the density functional theory (DFT) with 19-electron pseudopotential on silver atoms are performed to study the lowest-energy isomers of silver bromide clusters AgnBrp(+)u200a(n⩽6, p=n, n−1). The stability, the structural and electronic properties of neutral, and positively charged systems are examined. The B3LYP functional has been used. For the smallest species, the DFT/B3LYP results are very close to those obtained with multireference configuration interaction or coupled cluster methods. In AgnBrn−1 clusters, the excess electron density shows a picture intermediate between that observed in alkali halide clusters (F-center or localization on a pending alkali atom) and that suggesting partial delocalization on a dimer or trimer silver subunit. Isomer stabilities, fragmentation energies, electric dipole moments, adiabatic and vertical ionization potentials are calculated. The use of 11-electron pseudopotential on silver is discussed.

Structure, stability, and infrared spectroscopy of (H2O)nNH4+ clusters: A theoretical study at zero and finite temperature

The combined effects of size and temperature on the stable structures of water clusters doped with one ammonium molecule have been investigated theoretically using an empirical potential and density-functional theory (DFT) calculations. Global optimization with Monte Carlo methods has been performed using an explicit intermolecular potential based on the Kozack-Jordan polarizable model. Putative lowest-energy structures based on this empirical potential are reported. Our results indicate a high propensity for the NH(4)(+) impurity to be fully solvated by water molecules. Clathratelike patterns are formed for clusters containing more than 11 molecules. Local reoptimizations of candidate structures carried out at the DFT level with the B3LYP hybrid functional and the 6-311++G(d,p) basis set confirm the general trends obtained with the intermolecular potential. However, some reorderings between isomers often due to zero-point energy corrections are found in small clusters, leading to stable geometries in agreement with other first-principles studies. Temperature effects have been assessed using a simple harmonic superposition approximation for selected cluster sizes and using dedicated Monte Carlo simulations for (H(2)O)(20)NH(4)(+). The clusters are found to melt near 200 K, and possibly isomerize already below 50 K. The free energy barrier for core/surface isomerization of the impurity in the 21-molecule cluster is estimated to be only a few kcal/mol at 150 K. The vibrational spectroscopic signatures of the clusters obtained from the electronic structure calculations show the usual four O-H stretching bands. As the cluster size increases, the double acceptor-single donor band near 3700 cm(-1) increasingly dominates over the three other bands. While we do not find conclusive evidence for a O-H stretching spectroscopic signature of the ammonium impurity to be in the core or at the surface in the 20-molecule cluster, a possible signature via the N-H stretching bands is suggested near 2800-2900 cm(-1). In the larger (H(2)O)(49)NH(4)(+) cluster, the impurity is slightly more stable at the surface.

On the premelting features in sodium clusters

Melting in Na(n) clusters described with an empirical embedded-atom potential has been reexamined in the size range 55</=n</=147 with a special attention at sizes close to 130. Contrary to previous findings, premelting effects are also present at such medium sizes, and they turn out to be even stronger than the melting process itself for Na(133) or Na(135). These results indicate that the empirical potential is qualitatively inadequate to model sodium clusters.

Dynamics of highly excited barium atoms deposited on large argon clusters. I. General trends

Ba(Ar)(approximately 750) clusters were generated by associating the supersonic expansion and the pick-up techniques. A femtosecond pump (266.3 nm)-probe (792 or 399.2 nm) experiment was performed to document the dynamics of electronically excited barium within the very multidimensional environment of the argon cluster. Barium was excited in the vicinity of the 6s9p (1)P state and probed by ionization. The velocity imaging technique was used to monitor the energy distribution of photoelectrons and photoions as a function of the delay time between the pump and the probe pulses. A complex dynamics was revealed, which can be interpreted as a sequence/superposition of elementary processes, one of which is the ejection of barium out of the cluster. The latter has an efficiency, which starts increasing 5 ps after the pump pulse, the largest ejection probability being at 10 ps. The ejection process lasts at a very long time, up to 60 ps. A competing process is the partial solvation of barium in low lying electronic states. Both processes are preceded by a complex electronic relaxation, which is not fully unraveled here, the present paper being the first one in a series.