A beam of ${\mathrm{He}}_N$ clusters ( ${10}^3\lesssim N\lesssim {10}^4$) is seeded with one or more chromophores by sticky collisions with a low pressure gas and is probed, thereafter, by laser-induced fluorescence spectroscopy. Using sodium, we observe the spectra of atoms, dimers, and trimers and find that they are almost unshifted from their gas phase counterparts, but show cluster-induced broadenings in the range of tens of ${\mathrm{cm}}^{-1\mathrm{}}$. Upon formation on the cluster surface, a fraction of the dimers desorbs slowly and becomes part of the beam, allowing narrow, rotationally resolved, spectra to be obtained.

Nanometer-sized helium droplets, each containing about 10 ⁴ helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

Structure, shape and composition are the basic parameters responsible for properties of nanoscale materials, distinguishing them from their bulk counterparts. To reveal these in three dimensions at the nanoscale, electron tomography is a powerful tool. Advancing electron tomography to atomic resolution in an aberration-corrected transmission electron microscope remains challenging and has been demonstrated only a few times using strong constraints or extensive filtering. Here we demonstrate atomic resolution electron tomography on silver/gold core/shell nanoclusters grown in superfluid helium nanodroplets. We reveal morphology and composition of a cluster identifying gold- and silver-rich regions in three dimensions and we estimate atomic positions without using any prior information and with minimal filtering. The ability to get full three-dimensional information down to the atomic scale allows understanding the growth and deposition process of the nanoclusters and demonstrates an approach that may be generally applicable to all types of nanoscale materials.

Sodium dimers in their lowest triplet state are prepared by pick‐up of sodium atoms by a beam of large (n≂10⁴) liquid helium clusters when these cross a scattering box containing sodium vapor. Using laser induced fluorescence detection the vibrationally resolved laser excitation spectrum of the 1 ³Σ⁺_g←1 ³Σ⁺_u transitions of Na₂ is obtained. The spectral shifts with respect to the corresponding gas phase values are surprisingly small. These data show the feasibility of using helium clusters as an almost ideal matrix for the synthesis and spectroscopic analysis of weakly bound complexes.