Nanometer-scale helium droplets [1, 2, 3] provide a unique solvent in which to perform molecular spectroscopy and are likely to become widely used by the molecular spectroscopic community in the coming years. Currently, this new technique is in its early stage of development and is being practiced in only a few laboratories, but the early results firmly support our view that this type of spectroscopy combines many of the most attractive features of more traditional molecular beam and matrix isolation spectroscopies. In particular, unstable and sometime exotic molecules can be synthesized in the condensed fluid nano-matrices without giving up the possibility of obtaining rotational resolution and therefore of obtaining direct structural information. A review describing the early results has been published .
Large He clusters, , are formed upon the expansion of low-temperature gas ( K) or liquid He through a m nozzle into a vacuum. Such clusters cool by evaporation, reaching temperatures of 0.4 K for He and 0.15 K for He . Uniquely, they remain liquid, as He will not freeze even at zero K, except above 25 atm. In order to emphasize the liquid nature of these clusters, and also their characteristic size, they are referred to as nanodroplets. The He nanodroplets are believed to be superfluid , while the colder and less dense He clusters are believed to be well above their superfluid transition temperature.
As for their heavier noble-gas counterparts, helium nanodroplets can be doped with one or more atoms or molecules using the pick-up technique , in which a beam of droplets passes undeflected through a scattering chamber containing a small pressure ( mtorr) of the dopant species. Spectroscopic study of doped He nanodroplets began with the work of Goyal et al. , where line-tunable CO lasers were used to probe the spectra of SF and its dimer. This work demonstrated that the SF spectrum in He clusters was qualitatively different from that observed in other inert-gas nanoclusters  in that the lines were much narrower and the matrix-induced shifts smaller. However, the line-tunable lasers did not provide enough spectral coverage to resolve the spectrum rotationally. This was achieved at the Max Planck Institute in Göttingen using a tunable diode laser counterpropagating with the droplet beam, which allowed for a continuous scan of the spectrum at high resolution . In that investigation it was discovered that the spectrum had the characteristic rovibrational structure of a gas-phase spherical top molecule, including the first-order Coriolis splitting . This work provided the first measurement of the cluster temperature, which was found to be in excellent agreement with that predicted by both classical  and quantum  evaporation models. Rovibrational spectra of a range of other molecules have now confirmed that He nanodroplets have a temperature of 0.38K, essentially independent of the dopant, but somewhat dependent upon cluster size. The SF spectrum of ref.  also showed that the effective rotational constant measured in He droplets is 1/3 of the gas phase value, while the centrifugal distortion constant is four orders of magnitude larger than that of the isolated dopant. These trends have been confirmed by the recent experiments in Göttingen, Chapel Hill, and our own laboratory, where the rotationally resolved spectra for several other molecules have been obtained [14, 15, 16]
Many of the spectroscopic experiments performed to date on doped He nanodroplets have focussed on electronic spectroscopy. This is largely due to the fact that the highly sensitive method of laser-induced fluorescence could be used to detect absorption and possible subsequent dynamics. We mention in particular only the work of our laboratory on alkali-doped droplets, where spectra of Li, Na, and K atoms, dimers and trimers have been extensively studied, including the Jahn-Teller coupling and time-resolved photochemistry of the quartet state of Na , which had never previously been observed [17, 18, 19]. One problem with electronic excitation, however, is that the transition-induced changes in He solvation can result in most of the intensity being found in the `phonon side band', though `zero phonon lines' have been observed in several systems. This makes it difficult to extract unambiguous rotational information in such experiments, particularly since the low-energy excitation region of the spectra, in the neighborhood of the zero phonon line, is not yet understood [17, 18]. For transitions that substantially change the size of the electronic cloud of the atom or molecule, very broad structures are observed and have been explained by the `bubble' model originally developed to treat electrons solvated in bulk liquid He . In contrast, vibrational transitions induce only small changes in the solvation of He around the dopant and, as a result, infrared spectra have almost all their intensity in the `zero phonon' line, where no direct optical excitation of the internal degrees of the He droplet occurs. In fact, to date no `phonon side band' has been observed in the IR.
IR spectra are normally observed by the method of beam depletion. In the time between IR absorption and detection of the droplet beam (typically s), vibrational relaxation of the dopant leads to He evaporation. Each departing He atom reduces the internal energy of the droplet by cm , so near-IR excitation reduces the size of the droplets by about one thousand atoms. This photoinduced beam depletion has been detected either optothermally with a bolometer  or by reduction in the electron impact ionization yield combined with detection of all ions above a certain mass threshold .