We present experiments and calculations of the deposition and aggregation of silver clusters embedded in helium droplets onto an amorphous carbon surface at room temperature. Calculations were also performed for deposition onto a graphene surface. They involve potentials for the interaction of carbon atoms with silver and helium atoms, provided by ab initio calculations. The numerical simulations were performed for few-nanometer-sized silver clusters including up to 5000 Ag atoms and He droplets with up to 105 4He atoms. The fluid nature of the 4He droplet is accounted for by the renormalization of the He–He interaction potential. The numerical results are consistent with deposition experiments with an average number of 3000 Ag atoms per 4He droplet and indicate that the aggregation of the silver clusters on the carbon surface is mediated by secondary droplet impacts. They also reveal nontrivial dynamics of the Ag clusters within the carrier droplet, showing a tendency to drift toward the droplet surface. These findings are of relevance in understanding the heterogeneous deposition patterns (large ramified islands) developed for very large droplets with an average number of Ag atoms per droplet within the million range. Finally, the simulations of large (5000 atoms) Ag cluster deposition on graphene reveals strong superdiffusive behavior. In stark contrast, the diffusion is negligible on the amorphous carbon surface.

Ethane core–silver shell clusters consisting of several thousand particles have been assembled in helium droplets upon capture of ethane molecules followed by Ag atoms. The composite clusters were studied via infrared laser spectroscopy in the range of the C–H stretching vibrations of ethane. The spectra reveal a splitting of the vibrational bands, which is ascribed to interaction with Ag. A rigorous analysis of band intensities for a varying number of trapped ethane molecules and Ag atoms indicates that the composite clusters consist of a core of ethane that is covered by relatively small Ag clusters. This metastable structure is stabilized due to fast dissipation in superfluid helium droplets of the cohesion energy of the clusters.

Helium droplets were used to assemble composite metal–molecular clusters. Produced clusters have several hundreds of silver atoms in the core, immersed in a shell consisting of methane, ethylene, or acetylene molecules. The structure of the clusters was studied via infrared spectra of the C–H stretches of the hydrocarbon molecules. The spectra of the clusters containing methane and acetylene show two distinct features due to molecules on the interface with silver core and those in the volume of the neat molecular part of the clusters. The relative intensities of the peaks are in good agreement with the estimates based on the number of the captured particles. Experiments also suggest that selection rules for infrared transitions for molecules adsorbed on metal surfaces are also valid for silver clusters as small as 300 atoms.

Nitrogen oxide clusters (NO)n have been studied in He droplets via infrared depletion spectroscopy and by quantum chemical calculations. The ν1 and ν5 bands of cis-ON-NO dimer have been observed at 1868.2 and 1786.5 cm–1, respectively. Furthermore, spectral bands of the trimer and tetramer have been located in the vicinity of the corresponding dimer bands in accord with computed frequencies that place NO-stretch bands of dimer, trimer, and tetramer within a few wavenumbers of each other. In addition, a new line at 1878.1 cm–1 close to the band origin of single molecules was assigned to van der Waals bound dimers of (NO)2, which are stabilized due to the rapid cooling in He droplets. Spectra of larger clusters (n > 5), have broad unresolved features in the vicinity of the dimer bands. Experiments and calculations indicate that trimers consist of a dimer and a loosely bound third molecule, whereas the tetramer consists of two weakly bound dimers.

Methane is one of the very few substances that show rotation of individual molecules in the crystalline phase. Here we explore the evolution of the rotation spectrum of methane from single molecules to clusters containing up to about 4 × 103 molecules. The clusters were assembled in He droplets at T = 0.38 K and studied via infrared laser spectroscopy in the ν3 region of the methane molecules. Well-resolved rotational structure in the spectra was observed in clusters containing up to about 50 molecules. We have concluded that in distinction to the crystals molecular rotation in methane clusters is confined to the surface and is enabled by low coordination of the molecules. On the contrary the molecules in the cluster’s interior are in amorphous state wherein the rotation is quenched. These results demonstrate that even at very low temperature the surface of the methane clusters remains fluxional due to quantum effects.

Mixed (HCl)N(H2O)M clusters have been assembled in He droplets from the constituting molecules. Spectra of the clusters were obtained in the range of hydrogen-bonded OH vibrations (3100–3700 cm–1) by infrared laser depletion spectroscopy. The observed bands were assigned to cyclic hydrogen-bonded aggregates containing up to two HCl and three H2O molecules. The obtained frequencies are in good agreement with the results of harmonic quantum chemical calculations upon appropriate uniform shifts mimicking anharmonic corrections. Although larger clusters containing up to six water molecules were also produced in the droplets, their spectra were found to contribute to the unresolved signal in the range 3250–3550 cm–1. The fact that no narrow bands could be unambiguously assigned to the mixed clusters containing more than three water molecules may indicate that such clusters exist in many isomeric forms that lead to overlapped and unresolved bands giving rise to broad structureless features. Another possible explanation includes the formation of elusive zwitterionic clusters, whose bands may have considerable breadth due to electrostatic coupling of different vibrational modes and concomitant intramolecular vibrational relaxation.

Ethane and ethane clusters (N ≈ 102–104) were studied inside helium droplets with infrared laser spectroscopy. The spectra were measured in the 2880–3000 cm–1 range, which covers the ν5, ν8+11, and ν7 vibrational bands of ethane. Partially resolved rotational fine structure in the spectrum of the monomer reveals solvent-induced band origin blue shifts that are each approximately 1 cm–1. The effective BHe and AHe rotational constants were found to be reduced by 52% and 16% in comparison to their gas phase values, respectively. Spectra of the clusters show the same three bands shifted toward low frequency by approximately 10 cm–1 because of intermolecular interactions in the clusters. The spectra of the ethane clusters are dominated by the ν7 band, whereas the relative intensities of the ν5 and ν8+11 bands are about a factor of 5 weaker than for single molecules or for solid ethane, the spectrum of which is also reported here.

Here, we have studied the utility of large He droplets of 105–107 atoms for the growth of composite clusters consisting of an Ag core and a shell of ethane molecules. The clusters have been assembled by doping He droplets with up to 103 Ag atoms and ethane molecules in two sequential pickup cells and studied via infrared spectroscopy in the C–H stretch region of the ethane molecules. We found that the ν7 band of ethane molecules at the interface with the Ag atoms has a low frequency shift of approximately 15 cm–1 with respect to that of more distant ethane molecules away from the interface. The intensity ratio of the two bands was used for evaluation of the Ag core and ethane shell cluster structure. We found that the number of surface ethane molecules is in good agreement with a model that assumes a dense, core–shell structure for clusters containing less than about 100 atoms. However, large Ag clusters consisting of about 3000 atoms have a factor of about 5 larger surface area than that predicted by the model, indicating a ramified structure for such larger Ag clusters obtained in liquid He. Moreover, we demonstrate that He droplets behave as calorimeters for measurements of the number of captured atoms and molecules as well as the amount of absorbed laser energy.

Small (HCl)m(H2O)n clusters have been assembled in He droplets, and their spectra in the HCl stretch range (2500−3000 cm−1) have been obtained. In a recent He droplet study, a band at 2670 cm−1 was assigned to the dissociated H3O+(H2O)3Cl− ion pair. In this work, we have revised the assignment of this band to a cyclic hydrogen-bonded form of the (HCl)2(H2O)2 cluster based on careful measurements of the pickup pressure dependence as well as the transition moment angles associated with the HCl stretch vibrations. A number of vibrational bands due to small mixed clusters have also been observed. As the number of the captured water molecules increases, a broad feature appears that spans the 2550−2800 cm−1 range. The possible origin of this spectral broadening in large (HCl)m(H2O)n clusters is discussed.Keywords (keywords): acid dissociation; acid dissolution; HCl−H2O clusters; helium nanodroplets; water clusters;

In this work, infrared laser spectroscopy in helium droplets is used to study the solvation of HCl with small water clusters. Clusters of HCl(H2O)n with n = 1−3 and (HCl)2H2O have been identified in the free OH stretching spectral range of 3680−3820 cm−1. The assignment of the infrared vibrational bands of the clusters is aided by ab initio calculations.

Spectra of the binary complexes of water and ammonia molecules in the OH stretching region have been studied in helium droplets. The infrared intensity in the hydrogen bond bridge stretching mode was found to be enhanced by a factor of 2.5 with respect to the ν3 band of single water molecules. This enhancement factor is significantly smaller than that obtained in recent theoretical calculations. The observed large band frequency shifts are similar to those measured in other rare gas matrices indicating the strong hydrogen bonding in the complex.Depletion spectra of the H2O and NH3 complexes in He droplets.