Abstract. This article provides a review of recent work in the field of helium nanodroplet spectroscopy with emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

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.

A nonstandard, high sensitivity, absorption detection technique has been applied to the investigation of the very weak fifth, sixth, and seventh overtones of HCN at 100 Torr and 296 K. The frequency range covered is from 17 500 to 23 000 cm-'. We report high resolution, absolute absorption spectra with a noise equivalent sensitivity as low as -2 X 10m9/cm (recently improved to 7 X IO-"/cm).Band origins, rotational constants, and band intensities are reported and compared with calculated values. The HCN overtone spectra in the present study are not affected by any kind of perturbation, despite the high excitation energy involved.

We describe a three-wave mixing experiment using time-separated microwave pulses to detect the enantiomer-specific emission signal of the chiral molecule using Fourier transform microwave (FTMW) spectroscopy. A chirped-pulse FTMW spectrometer operating in the 2-8 GHz frequency range is used to determine the heavy-atom substitution structure of solketal (2,2-dimethyl-1,3-dioxolan-4-yl-methanol) through analysis of the singly substituted (13)C and (18)O isotopologue rotational spectra in natural abundance. A second set of microwave horn antennas is added to the instrument design to permit three-wave mixing experiments where an enantiomer-specific phase of the signal is observed. Using samples of R-, S-, and racemic solketal, the properties of the three-wave mixing experiment are presented, including the measurement of the corresponding nutation curves to demonstrate the optimal pulse sequence.