Volume 115, Issue 22, 08 December 2001
Index of content:
- SPECIAL TOPIC: HELIUM NANODROPLETS: A NOVEL MEDIUM FOR CHEMISTRY AND PHYSICS
115(2001); http://dx.doi.org/10.1063/1.1418249View Description Hide Description
In this report I will review experimental studies of free helium droplets, with the exception of spectroscopic studies of helium droplets that contain impurities. This particular topic, as well as theoretical studies of helium droplets, will be reviewed separately elsewhere in this issue.
115(2001); http://dx.doi.org/10.1063/1.1424926View Description Hide Description
Heliumnanodroplets and trapped Bose–Einstein condensates in dilute atomic gases offer complementary views of fundamental aspects of quantum many-body systems. We discuss analogies and differences, stressing their common theoretical background and peculiar features. We briefly review some relevant concepts, such as the meaning of superfluidity in finite systems, the behavior of elementary excitations and collective modes, as well as rotational properties and quantized vorticity.
115(2001); http://dx.doi.org/10.1063/1.1418746View Description Hide Description
In this article, recent developments in heliumnanodroplet 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 superfluidhelium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its anisotropy) 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-from-equilibrium 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 and other highly quantum clusters (e.g., clusters) prepared inside heliumdroplets 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.
115(2001); http://dx.doi.org/10.1063/1.1418443View Description Hide Description
Path integral Monte Carlo is an exact simulation method for calculating thermodynamic properties of bosonic systems. Properties such as superfluidity and bose condensation are directly related to multiparticle exchange cycles of individual particle paths. Such calculations of bosonic systems in confined geometries, such as helium and hydrogen on surfaces and in droplets are reviewed.
115(2001); http://dx.doi.org/10.1063/1.1415433View Description Hide Description
Recent experiments on the electronic spectroscopy of atoms, clusters, and organic molecules embedded in heliumnanodroplets are reviewed. Electronic transitions imply a larger degree of distortion of the helium environment as compared to vibrational and rotational excitations. Thus new phenomena arise such as the appearance of side bands in the spectra, which are due to the excitation of helium collective vibrations, large changes of the effective molecular rotational constants and even the expulsion of an atom (or molecule) from the cluster upon excitation. These features make it possible to probe the helium environment and its interactions with molecular chromophores on the atomic scale. Real-time studies of the manifestations of superfluidity and of chemical processes in the droplets via femtosecond excitation techniques, provide a new perspective to this field. The considerable amount of data available so far shows the large potential of heliumdroplets for isolation and spectroscopy of large molecules and clusters. The low temperature and the high spectral resolution achievable because of the relative homogeneity of this medium, are instrumental for separating solvation effects that are obscured by the presence of much larger fluctuations in more classical environments. Hence, electronic and geometrical structures of even large entities become accessible.