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

A new semiempirical potential is described for the ground state X1Σ+ 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. The new potential is tested by comparing with ab initio potentials, which presently are only available in the well region for the molecules LiF, LiCl and CsI. The three parameters of the new potential are determined by fitting the latest experimental parameters for the well depth De, bond distance Re and vibrational frequency ωe. The new potential is in good agreement with the ab initio potentials.

Accurate semiempirical potential energy curves for the a3Σ+-state of NaCs, KCs, and RbCs.

A five parameter semiempirical Tang-Toennies type model is used to describe the potential curves of the a3Σ+-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. The model parameters are obtained by simultaneously fitting all five of the parameters to the extensive LIF-Fourier transform spectroscopy published by Tiemann and collaborators [e.g., Docenko et al. J. Phys. B: At., Mol. Opt. Phys. 39, S929-S943 (2006)], who also report best fit potential curves. Although the new potentials are in good agreement with the earlier potentials, they have the advantage that they are continuous over the entire range of internuclear distances and have the correct long-range behavior. The scattering lengths for all isotope combinations show good agreement with dedicated experiments where available. The new potentials are also in excellent agreement with combining rules based on the potentials of the homonuclear systems.

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. 142, 131102 (2015)JCPSA60021-960610.1063/1.4916740]. The potential curves of the rare gas dimers all have the same shape and only depend on the well parameters D_{e} and R_{e}. The potentials and the vibrational levels for the 11 homonuclear and heteronuclear dimers for which recent ab initio calculations are available agree, within several percent, with the ab initio results. For the other rare gas dimers, the new potential provides the first realistic estimates for the potentials.

An accurate semiempirical potential energy curve for the a3Σ+-state of KRb.

A semiempirical potential energy curve for the a3Σ+-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. [Phys. Rev. A 76, 022511 (2007)] as their multi-parameter potential. The new potential is supported by the good agreement of the well parameters De, Re and the harmonic vibrational constant ωe with combining-rule estimates. The scattering lengths for all six isotopologues are also in excellent agreement with experiment with a 0.2% adjustment within theoretical uncertainty of the leading dispersion coefficient C6. An analysis of the harmonic vibrational constant ωe and the constant ωexe of the potential of Pashov et al. reveals a significant difference to the present potential which turned out to be due to an oscillatory deviation in their potential in the vicinity of the potential minimum. The new potential is, thus, the best available because its simplicity is ideal for further applications.

An accurate semi-empirical potential model for the a 3Σu + state of the alkali dimers Na2, K2, Rb2, and Cs2 which reproduces the scattering length.

A new four parameter Tang-Toennies type potential model is described for the a 3Σu + triplet state of the alkali dimers Na2, K2, Rb2, and Cs2. Compared to an earlier three parameter semi-empirical model based on the experimental well depth, De, well location, Re, and the harmonic vibrational frequency, ωe [Lau et al., J. Chem. Phys. 145, 194308 (2016)], the new model is also adjusted to be consistent with the scattering length. The results are shown to have a similar good agreement with the spectroscopic term values as the earlier model with the advantage that the scattering length is properly described. The deviations from recent potentials for Cs2 are discussed.

Surface lattice dynamics and electron-phonon interaction in cesium ultra-thin films.

The phonon dispersion curves of ultrathin films of Cs(110) on Pt(111) measured with inelastic helium atom scattering (HAS) are reported and compared with density-functional perturbation theory (DFPT) calculations. The combined HAS and DFPT analysis also sheds light on the bulk phonon dynamics of bcc-Cs, on which very little is known from neutron scattering due to its large neutron capture cross-section. Moreover the temperature dependence of the elastic HAS Debye-Waller exponent of Cs(110)/Cu(111) ultrathin films allows for an estimation of the electron-phonon coupling strength λ as a function of the film thickness.

An accurate potential model for the a3Σu+ state of the alkali dimers Na2, K2, Rb2, and Cs2.

A modified semi-empirical Tang-Toennies potential model is used to describe the a3Σu+ 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 De, well location Re, and the harmonic vibrational frequency ωe of which the latter is only slightly optimized within the range of the literature values. Comparison with the latest spectroscopic data shows good agreement for Na2, K2, Rb2, and Cs2, comparable to that found with published potential models with up to 55 parameters. The differences between the reduced potential of Li2 and the conformal reduced potentials of the heavier dimers are analyzed together with why the model describes Li2 less accurately. The new model potential provides a test of the principle of corresponding states and an excellent first order approximation for further optimization to improve the fits to the spectroscopic data and describe the scattering lengths and Feshbach resonances at ultra-low temperatures.

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 = 0.9-1.96 bars is reported and analyzed according to a cluster growth model based on the Smoluchowski theory with kernel scaling. Good overall agreement is found between the measured and predicted, Nk = A k(a) e(-bk), shape of the distribution. The fit yields values for A and b for values of a derived from simple collision models. The small remaining deviations between measured abundances and theory imply a (pH2)k magic number cluster of k = 13 as has been observed previously by Raman spectroscopy. The predicted linear dependence of b(-(a+1)) on source gas pressure was verified and used to determine the value of the basic effective agglomeration reaction rate constant. A comparison of the corresponding effective growth cross sections σ11 with results from a similar analysis of He cluster size distributions indicates that the latter are much larger by a factor 6-10. An analysis of the three body recombination rates, the geometric sizes and the fact that the He clusters are liquid independent of their size can explain the larger cross sections found for He.

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. The agreement is better than 20% (H2 and He2) or comparable with the differences in the available ab initio calculations (H-He) over six orders of magnitude corresponding to the entire range of internuclear distances.

Unveiling mode-selected electron-phonon interactions in metal films by helium atom scattering.

The quasi two-dimensional electron gas on a metal film can transmit to the surface even minute mechanical disturbances occurring in the depth, thus allowing the gentlest of all surface probes, helium atoms, to perceive the vibrations of the deepest atoms via the induced surface-charge density oscillations. A density functional perturbation theory (DFPT) and a helium atom scattering study of the phonon dispersion curves in lead films of up to 7 mono-layers on a copper substrate show that: (a) the electron-phonon interaction is responsible for the coupling of He atoms to in-depth phonon modes; and (b) the inelastic HAS intensity from a given phonon mode is proportional to its electron-phonon coupling. The direct determination of mode-selected electron-phonon coupling strengths has great relevance for understanding superconductivity in thin films and two-dimensional systems.

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. By fitting of a coupling parameter and the phase factor between the PH and collective response functions, the shape and relative intensity of the observed ZPL and its slowly decreasing multiexcitation sideband are well-reproduced. The new theory is consistent with the previous explanation of the surprisingly sharp phonon line superimposed on the sideband in terms of the dense first (3)He shell, which acts as a Helmholtz resonator for the zero sound of the droplet.

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. With N = 18 (3)He atoms the molecule is enclosed by a magic number closed shell. Another magic stable structure consisting of two nearly isotropically spherical closed shells is found at N = 40. A comparison with calculations for the same sized (4)He clusters show some important similarities, e.g., pile up at the donut ring position but altogether a more diffuse, less anisotropic structure. These results are discussed in the light of the recently analyzed infrared spectra measured in large pure (3)He droplets (N ≈ 1.2 × 10(4)) [B. Sartakov, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys. 136, 134316 (2012)]. The moments of inertia of the 11 atom spherical shell structure, which is consistent with the experimental spectrum, lies between the predicted moments of inertia for N = 8 and N = 18 clusters. Overall the calculations reveal that the structures and energies of small doped (3)He are only slightly more diffuse and less energetic than the same (4)He clusters.

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. Toennies, and A. F. Vilesov, Science 279, 2083 (1998)] is carefully evaluated. The spectrum, which consists of a broad central peak and a distinct shoulder at lower energy, was analyzed by assuming unresolved rotational line structure of either a linear or a symmetric top. In each case the spectrum was fitted using either Lorentzian or Gaussian peak shapes with a preassigned fixed temperature of 0.15 K or a best fit temperature. Many of the fits describe the spectra nearly equally well and indicate broad R(0), R(1), and P(1) peaks but no Q-branch, a moment of inertia which is about a factor six greater than for the free molecule, and a temperature of 0.07 ± 0.06 K which is significantly less than 0.15 K determined for mixed (3)He∕(4)He droplets. The increased moment of inertia is consistent with about 11 attached (3)He atoms which take part in the end-over-end rotations of the chromophore. The large line widths are attributed to creation of particle-hole pair excitations in the fermionic droplets.

Mode-selected electron-phonon coupling in superconducting Pb nanofilms determined from He atom scattering.

The electron-phonon coupling (EPC) strength for each phonon mode in superconducting Pb films is measured by inelastic helium atom scattering (IHAS). This surprising ability of IHAS relies on two facts: (a) In ultrathin metal films, the EPC range exceeds the film thickness, thus enabling IHAS to detect most film phonons, even 1 nm below the surface; (b) IHAS scattering amplitudes from single phonons are shown, by first-principle arguments, to be proportional to the respective EPC strengths. Thus IHAS is the first experiment providing mode-selected EPC strengths (mode-lambda spectroscopy).

Evidence for a spin acoustic surface plasmon from inelastic atom scattering.

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).

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. The results indicate that the hydrogen clusters are on the borderline to being a nonsuperfluid solid but that the molecules are sufficiently delocalized to be superfluid. A general phase diagram for the total and kinetic energies of LJ (12,6) clusters encompassing all sizes from N=2 to N=infinity and for the entire range of de Boer parameters is presented. Finally the limiting de Boer parameters for quantum delocalization induced unbinding ("quantum unbinding") are estimated and the new results are found to agree with previous calculations for the bulk and smaller clusters.

Spectroscopy of the copper dimer in normal fluid, superfluid, and solid (4)He.

Copper atoms and molecules are laser ablated into bulk liquid and solid helium, and the emission spectra of the laser excited D→X, B→X, and a→X transitions of Cu(2) are observed to exhibit clearly resolved vibrational bands. Surprisingly, for the D→X and the B→X transitions, no differences were observed for superfluid He at 1.5 K, for the normal liquid at 2.65 K, or for the 1.5 K solid at higher pressures of about 30 bars. An interpretation based on the bubble model indicates that the interaction with the He matrix is much weaker than in the case of the alkali atoms. Compared to other solid rare gas matrices, the line shifts and line widths in condensed helium are much smaller by nearly an order of magnitude.