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. Although the molecular anion is heliophobic and migrates toward the surface of the helium droplet, the excited metastable atomic helium anion is bound within the helium droplet and exhibits high mobility. The atomic anion is shown to be responsible for the formation of molecular dopant anions upon charge transfer and thus, we clarify the nature of the previously unidentified fast exotic negative charge carrier found in bulk liquid helium.

An analytical model for the potential between two rare gas atoms at distances between $R=0$ to $R\to \infty$ is assumed to be conformal with the previously published potential for ${\mathrm{He}}_2$ [ J. Chem. Phys. 142, 131102 (2015)]. 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.

Thin films of lead have been grown on the Cu(111) surface and their structure and phonon dispersion curves have been investigated with high-resolution helium atom scattering. The diffraction pattern of the first monolayer (ML) indicates a $p\left(4\times 4\right)$ structure. Films consisting of $N=3$, 4, 5, and bulklike 50 ML all have a regular $\left(1\times 1\right)$ structure with the same lattice constant as the bulk and small superimposed $p\left(4\times 4\right)$ peaks for the thinnest films. The time-of-flight spectra reveal an unusually large number of inelastic peaks for all the films. The results have been analyzed in terms of dispersion curves which exhibit more than $\frac{1}{2}$ the total number of expected two $N$ modes active in the planar scattering geometry. Dispersion curves for an unannealed 5 ML film are also reported. A force-constant model fitted to the bulk dispersion curves can only qualitatively reproduce some of the data but help to explain some features. Along the $\overline{\Gamma }-\overline{K}$ direction, a Kohn anomaly is identified for the 3, 4, and 5 ML films at nearly the same wave vector as in the bulk along the equivalent direction.

A new four parameter Tang-Toennies type potential model is described for the $a{}^3{\mathrm{\Sigma }}_u^{+}$ 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, D_e, well location, R_e, and the harmonic vibrational frequency, ω_e [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.