The Electron-Phonon Interaction at Vicinal Metal Surfaces Measured with Helium Atom Scattering.

Recently, it was demonstrated that inelastic helium atom scattering from conducting surfaces provides a direct measurement of the surface electron-phonon coupling constant (mass enhancement factor ) via the temperature or the incident wave vector dependence of the Debye-Waller exponent. Here, previous published as well as unpublished helium atom scattering diffraction data from the vicinal surfaces of copper (Cu(11α), with α = 3, 5, 7) and aluminum (Al(221) and Al(332)) were analyzed to determine . The results suggested an enhancement with respect to the corresponding data for the low-index surfaces (111) and (001) above the roughening transition temperature.

A semiempirical potential for alkali halide diatoms with damped interactions I. Rittner potential.

A new semiempirical potential is described for the ground state XΣ of the alkali halide diatoms. The model potential is the first to account for the damping of all the electrostatic and induction potential terms as well as of the long-range dispersion potential. Accordingly, the potential does not have a negative singularity at vanishingly small internuclear distances and is the first Rittner-type model with a realistic dependence of the repulsion at short distances.

Magic Numbers in Boson He Clusters: The Auger Evaporation Mechanism.

The absence of magic numbers in bosonic He clusters predicted by all theories since 1984 has been challenged by high-resolution matter-wave diffraction experiments. The observed magic numbers were explained in terms of enhanced growth rates of specific cluster sizes for which an additional excitation level calculated by diffusion Monte Carlo is stabilized. The present theoretical study provides an alternative explanation based on a simple independent particle model of the He clusters.

Accurate semiempirical potential energy curves for the aΣ-state of NaCs, KCs, and RbCs.

A five parameter semiempirical Tang-Toennies type model is used to describe the potential curves of the aΣ-state of the heteronuclear polar molecules NaCs, KCs, and RbCs. These molecules are of current interest in experiments at ultra-cold conditions to explore the effects of the strong dipole-dipole forces on the collective many-body quantum behavior. New quantum phenomena are also anticipated in systems consisting of atomic species with different fermion/boson statistics.

Conformal Analytical Potential for All the Rare Gas Dimers over the Full Range of Internuclear Distances.

An analytical model for the potential between two rare gas atoms at distances between R=0 to R→∞ is assumed to be conformal with the previously published potential for He_{2} [J. Chem. Phys.

An accurate semiempirical potential energy curve for the aΣ-state of KRb.

A semiempirical potential energy curve for the aΣ-state of the KRb molecule with only five parameters is reported. The potential is continuous over the entire range of internuclear distances and has the correct long-range attractive dispersion potential from established theory. The potential provides an equally good fit of the laser induced fluorescence Fourier transform spectroscopic data of Pashov et al.

An accurate semi-empirical potential model for the a Σ state of the alkali dimers Na, K, Rb, and Cs which reproduces the scattering length.

A new four parameter Tang-Toennies type potential model is described for the a  Σ triplet state of the alkali dimers Na, K, Rb, and Cs. Compared to an earlier three parameter semi-empirical model based on the experimental well depth, D, well location, R, and the harmonic vibrational frequency, ω [Lau et al., J.

An accurate potential model for the aΣ state of the alkali dimers Na, K, Rb, and Cs.

A modified semi-empirical Tang-Toennies potential model is used to describe the aΣ potentials of the alkali dimers. These potentials are currently of interest in connection with the laser manipulation of the ultracold alkali gases. The fully analytical model is based on three experimental parameters, the well depth D, well location R, and the harmonic vibrational frequency ω of which the latter is only slightly optimized within the range of the literature values.

Communication: A simple full range analytical potential for H2b(3)∑u (+), H-He (2)∑(+), and He2 (1)∑g (.).

The Tang-Toennies potential for the weakly interacting systems H2b(3)Σu (+), H-He (2)Σ(+), and He2 (1)Σg (+) is extended down to the united atom limit of vanishing internuclear distance. A simple analytic expression connects the united atom limiting potential with the Tang-Toennies potential in the well region. The new potential model is compared with the most recent ab initio calculations for all three systems.

Para-hydrogen and helium cluster size distributions in free jet expansions based on Smoluchowski theory with kernel scaling.

The size distribution of para-H2 (pH2) clusters produced in free jet expansions at a source temperature of T0 = 29.5 K and pressures of P0 = 0.9-1.

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He and He.

Helium droplets provide the possibility to study phenomena at the very low temperatures at which quantum mechanical effects are more pronounced and fewer quantum states have significant occupation probabilities. Understanding the migration of either positive or negative charges in liquid helium is essential to comprehend charge-induced processes in molecular systems embedded in helium droplets. Here, we report the resonant formation of excited metastable atomic and molecular helium anions in superfluid helium droplets upon electron impact.

The response of a (3)He Fermi liquid droplet to vibronic excitation of an embedded glyoxal molecule.

The zero-phonon line (ZPL) and the sideband in the vibronic spectrum of a single glyoxal molecule inside a (3)He droplet are analyzed within the framework of the Lax formalism. The new theory takes full account of the coupling of the molecule to the single particle-hole (PH) and collective excitations of the doped Fermionic droplet. The effect on the coupling of the wavevector dependence of the effective (3)He mass and the large local density of the first (3)He shell, resulting from the interaction with the chromophore, are also included in the theory.

A density functional study of the structure of small OCS@3He(N) clusters.

Kohn-Sham density functional calculations are reported for the structures of clusters consisting of a carbonyl sulfide (OCS) molecule with N = 1, 8, 18, and 40 attached (3)He atoms. The N = 1 cluster ground state is highly localized at the molecular waist (donut ring position), but for higher levels of excitation becomes increasingly delocalized. The first magic cluster with 8 atoms has a significant density at both ends of the molecule in addition to the donut ring.

Infrared spectroscopy of carbonyl sulfide inside a pure 3He droplet.

The infrared spectrum of the ν(3) band of an OCS (carbonyl sulfide) molecule embedded inside pure (3)He droplets of about 12 × 10(3) atoms reported in 1998 [S. Grebenev, J. P.

Why are para-hydrogen clusters superfluid? A quantum theorem of corresponding states study.

The quantum theorem of corresponding states is applied to N=13 and N=26 cold quantum fluid clusters to establish where para-hydrogen clusters lie in relation to more and less quantum delocalized systems. Path integral Monte Carlo calculations of the energies, densities, radial and pair distributions, and superfluid fractions are reported at T=0.5 K for a Lennard-Jones (LJ) (12,6) potential using six different de Boer parameters including the accepted value for hydrogen.

Spectroscopic investigation of OCS (p-H(2))(n) (n=1-16) complexes inside helium droplets: Evidence for superfluid behavior.

Up to 16 parahydrogen and orthodeuterium molecules have been assembled around an OCS carbonyl sulfide chromophore molecule inside the pure (4)He and mixed (4)He(3)He droplets at temperatures of 0.38 and 0.15 K, respectively.

Superfluid helium droplets: a uniquely cold nanomatrix for molecules and molecular complexes.

Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.

Electron impact ionization of size selected hydrogen clusters (H2)N: ion fragment and neutral size distributions.

Clusters consisting of normal H2 molecules, produced in a free jet expansion, are size selected by diffraction from a transmission nanograting prior to electron impact ionization. For each neutral cluster (H2)(N) (N=2-40), the relative intensities of the ion fragments Hn+ are measured with a mass spectrometer. H3+ is found to be the most abundant fragment up to N=17.

XUV laser-plasma source based on solid Ar filament.

We present a laser driven soft x-ray source based on a novel solid argon filament. The continuously flowing micron-sized filament (diameter approximately 56 microm, flow speed approximately 5 mms) was used as a laser target in order to generate a plasma source of high brightness in the "water window" (2.2-4.

Rotational transitions and diffraction in D2 scattering from the LiF(001) surface: theory and experiment.

High probabilities of energy transfer from translation to molecular rotations are observed in the scattering of n-D(2) from LiF(001) at an incident beam energy of 85.3 meV. For the 100 incidence direction, close-coupling calculations yield ratios of the rotationally inelastic (j=0-->2) and (j=1-->3) peaks to the rotationally elastic specular peaks (G=0) that are in reasonable agreement with experiment, as are the ratios of the rotationally elastic diffraction peak intensities to the specular peak intensities.