Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

Abstract

In this article, recent developments in HElium NanoDroplet Isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its isotropy) that acts on a solute inside a droplet, and of the energy levels that arise because of this confinement. Second, we treat the question of the homogeneous versus inhomogeneous broadening of the spectral profiles, moving after this to a discussion of the rotational dynamics of the molecules and of the surrounding superfluid medium. The change in rotational constants from their gas phase values, and their dependence on the angular velocity and vibrational quantum number are discussed. Finally, the spectral shifts generated by this very gentle matrix are analyzed and shown to be small because of a cancellation between the opposing action of the attractive and repulsive parts of the potential of interaction between molecules and their solvent. The review concludes with a discussion of three recent applications to (a) the synthesis of far-fromequilibrium molecular aggregates that could hardly be prepared in any other way, (b) the study of the influence of a simple and rather homogeneous solvent on large amplitude molecular motions, and (c) the study of mixed 3 He/ 4 He and other highly quantum clusters (e.g., H2 clusters) prepared inside helium droplets and interrogated by measuring the IR spectra of molecules embedded in them. In spite of the many open questions, we hope to convince the reader that HENDI has a great potential for the solution of several problems in modern chemistry and condensed matter physics, and that, even more interestingly, this unusual environment has the potential to generate new sets of issues which were not in our minds before its introduction.

Photoelectron Imaging of Helium Droplets Doped with Xe and Kr Atoms

“…Owing to the ability of He droplets to pick up foreign species, first demonstrated in the early 1990s, 4,5 there has been much interest in their application as an ultracold (0.37 K) and weakly interacting medium for performing rotational, vibrational, and electronic spectroscopy on droplets doped with atoms, molecules, and clusters. [6][7][8][9][10] Through their capability to dissipate energy efficiently, He droplets can also serve as a cryogenic platform to synthesize unusual complexes by trapping target species in local minima. [11][12][13] Considerably stronger interactions with the droplet environment are expected for charged particles, both ions and electrons, and many experiments have been performed to unravel these interactions.…”

Interaction-induced localization of an impurity in a trapped Bose-Einstein condensate

“…Some atom impurities reside on the cluster surface while others migrate to the center of the helium cluster. Spectroscopic measurements of molecules located at the center of the cluster have, e.g., shown unambiguously that 4 He clusters with about 60 atoms are superfluid [5].…”

“…Electron impurities have also played a key role in the study of liquids, in particular liquid 4 He [2]. More recently, the study of doped mesoscopic helium clusters has attracted much attention [3,4]. Some atom impurities reside on the cluster surface while others migrate to the center of the helium cluster.…”

“…In 1933, Landau predicted, using quantum mechanical arguments, that the localization of electron impurities in a crystal could be used to probe the activation energy of solids [1]. Electron impurities have also played a key role in the study of liquids, in particular liquid 4 He [2]. More recently, the study of doped mesoscopic helium clusters has attracted much attention [3,4].…”

Evaporative Cooling of Helium Nanodroplets with Angular Momentum Conservation

“…Brink and Stringari 4 used a statistical evaporation model and predicted a terminal temperature of 4 He nanodroplets of 0.4 K, in excellent agreement with the value of 0.38 K later deduced from the rotational structure in vibrational spectra of SF 6 and other molecules. 5,6 Despite the obvious success of this theoretical work, the model used is clearly incomplete in that the constraint of angular momentum conservation was not imposed. The need for a more complete evaporative cooling study was made evident by the recent observation of a polarization anisotropy in the absorption spectrum of pentacene in helium droplets.…”