Ground state and excitation dynamics in Ag doped helium clusters

Abstract

We present a quantum Monte Carlo study of the structure and energetics of silver doped helium clusters AgHen for n up to 100. Our simulations show the first solvation shell of the Ag atom to be composed by roughly 20 He atoms, and to possess a structured angular distribution. Moreover, the electronic 2 P 1/2 ← 2 S 1/2 and 2 P 3/2 ← 2 S 1/2 electronic transitions of the embedded silver impurity have been studied as a function of the number of helium atoms. The computed spectra show a redshift for n ≤ 15 and an increasing blueshift for larger clusters, a feature attributed to the effect of the second solvation shell of He atoms. For the largest cluster, the computed excitation spectrum is found in excellent agreement with the ones recorded in superfluid He clusters and bulk. No signature of the direct formation of proposed AgHe2 exciplex is present in the computed spectra of AgHe100. PACS numbers: Superfluid4 He clusters represent a gentle environment where high resolution spectroscopic studies of atoms, atomic clusters, and molecules at low temperature can be carried out [1]. In such cold and fluid quantum systems many perturbing effects due to the temperature and solid matrices are absent, allowing therefore for an easier interpretation of the experimentally recorded spectra. Moreover, their superfluid behavior allows interesting quantum effects to take place and to be experimentally probed (for instance see Refs. [2,3]).Whereas the coupling of the rotational and vibrational motion of the molecules with the quantum motion of the solvent is permitted by the similarity between energy levels, the electronic structure of an atom is characterized by energy differences orders of magnitude larger than the ones needed to induce excitation in the atomic motion. Although this difference might seem to work in the direction of simplifying the physical description of the electronic transition processes, many important details still wait to be clarified. As an example, the fluorescent D 2 emission line (i.e. the 2 S 1/2 ← 2 P 3/2 radiative transition) of heavy single valence electron atoms dispersed in superfluid helium is absent, while the D 1 line is sharp and only slightly shifted (1-2 nm) to the blue [4]. This is in contrast with the large broadening and strong blueshift of the absorption lines. Moreover, some features of the LIF spectra of the dispersed Ag were interpreted as signature of the AgHe and AgHe 2 exciplex formation [5].The blueshift and broadening of the absorption lines * Electronic address: Massimo.Mella@unimi.it † Present address: Laboratory of Inorganic Chemistry, ETH Hönggenberg, CH-8093 Zürich, Switzerland; Electronic address: Colombo@inorg.chem.ethz.ch ‡ Electronic address: Gabriele.Morosi@uninsubria.it have been interpreted by means of a "bubble model". Here, the dispersed atom is enclosed in a spherical cavity due to the exchange repulsion of its valence electrons and the He ones. The liquid He around an atom is modeled by an isotropic sharp-edge density profile with no atomic internal structure. How...

Predicting atomic dopant solvation in helium clusters: The MgHen case

“…In both cases a description of the ground state is sought, the low temperature of the clusters (0.37 K) and the large energy gap between vibrational excited states in He n suggesting that thermal excitations should not play a relevant role (for a discussion on this topic see Ref. [7]). …”

“…[7] to compute the Ag spectrum, adapting a technique previously proposed by Cheng and Whaley [38] for the Franck-Condon line shapes of an electronic transition in a condensed phase system. The method was originally presented by Lax [39] and modified to take into account the system temperature of 0 K. In its crudest approximation, the spectral lines of a chromophore are computed collecting the distribution of the differences V exc (R) − V gs (R) over the sampled f (R).…”

“…Despite the merits of simplicity, and of reducing the number of independent variables to a single one, this model was not devised to describe in detail the solvation process, but only to predict whether for a given impurity the solvation occurs or not. Furthermore, the DFT approach does not take into account properly the discrete nature and the anisotropic deformation [6,7] of the He aggregates, and this might lead to overestimate the overall interaction energy with the impurity. The drawbacks of this approximation, as well as the ones cited above, are expected to be particularly relevant in systems with λ close to the critical value 1.9.…”

“…In systems where accurate interaction potentials between helium and the impurity are available, the quantum Monte Carlo (QMC) approach is probably the best suited, and has been already applied successfully to the study of doped helium clusters [6,7,8,9,10,11,12]. Though the DMC simulations cannot recover the temporal evolution of the system, they provide many important quantities that are hardly accessible experimentally, such as radial and angular distribution functions, solvation energies, excitation spectra, as well as their dependence on the size of the helium aggregate.…”

Formation and dynamics of van der Waals molecules in buffer-gas traps

“…Research has included spectroscopic investigations of the formation of RbHe exciplexes on the surface of a superfluid He droplet 37,38 , and of the effects of spin-orbit interactions on the formation of NaHe and KHe excimers upon optical excitation of alkali-doped nanodroplets 39,40 . He nanodroplet spectroscopy can also give information on the character of M-Rg forces, showing, for example, that Ag atoms reside at the center of He nanodroplets [41][42][43] .…”