Infrared spectroscopy of helium nanodroplets: novel methods for physics and chemistry

Abstract

Helium nanodroplets have emerged as a new and exciting medium for studying the structure and dynamics of both this quantum solvent and impurities that can be doped into (onto) and grown inside (on the surface) of the droplets. Spectroscopic studies of these molecular impurities can provide detailed information on helium as a solvent and its interaction with the solute. This is particularly important given that helium is completely transparent to photons below 20 eV, making the direct spectroscopic study of liquid helium problematic. Since liquid helium is an extremely weak solvent, the corresponding perturbations to the spectrum of the solute molecules are often minor; really only evident because of the high resolution that is often achieved in such studies. As a result, helium nanodroplet spectra often resemble the corresponding gas-phase results. Indeed, for the case of rotational spectroscopy, the gas-phase Hamiltonian is often sufficient to describe the system, with the effects of the solvent being to simply modify the molecular constants, while the molecular symmetry is maintained. In the case of vibrational spectroscopy, the perturbations due to the solvent are often so weak that the results can be compared directly with the theory for the corresponding isolated system. The growth of small clusters and nanoparticles in helium droplets is strongly influenced by the low temperature of the latter (0.37 K), often accentuating the effects of the long-range interactions between the constituent molecules. In many cases, these effects lead to the formation of exotic species that are difficult or impossible to make using more conventional techniques. Overall, helium nanodroplets act as a nearly ideal matrix for the synthesis and spectroscopic characterisation of these new and exotic species. Although there have been a number of previous reviews on helium nanodroplet spectroscopy, there are many important aspects of this emerging field that have yet to be suitably highlighted, making the present review timely. The goal here is to discuss some of the exciting new directions that are being explored using infrared laser spectroscopy as the probe. As noted above, the spectroscopy of impurities can provide interesting and new insights into the properties of liquid helium (including superfluidity, rotons, ripplons, etc.). Perhaps of even greater interest is the use of helium nanodroplets as nanocryostats for the growth of novel species, including those formed from metals, semiconductors, salts, biomolecules, free radicals, ions and hydrogen-bonding molecules. As we will demonstrate herein, helium nanodroplets provide considerable control over how these ‘nanomaterials’ are grown, opening up new possibilities for the formation and study of such species. Contents PAGE 1. Introduction 16 2. Experimental methods 19 2.1. The droplets 19 2.2. The pick-up technique 22 2.3. Detection 24 2.4. Infrared lasers 27 2.5. Pendular state spectroscopy 28 2.6. Vibrational Transition Moment Angles (VTMAs) 29 2.7. Optically Selected Mass Spectrometry (OSMS) 32 3. Molecular dynamics in helium 34 3.1. Rotational dynamics 34 3.1.1. Rotational relaxation rates 34 3.1.2. The effective moment of inertia of solvated rotors 36 3.1.3. Centrifugal distortion in helium solvated rotors 39 3.2. Vibrational dynamics 41 3.3. Photo-induced isomerisation 43 4. Molecular clusters in helium nanodroplets 46 4.1. The dynamics of cluster growth in helium nanodroplets 46 4.2. Hydrogen clusters in helium nanodroplets 51 4.3. Structural determination of metal atom cluster–adsorbate complexes 53 4.4. Biomolecule and hydrated-biomolecule complexes 55 4.5. Entrance and exit channel complexes – free radicals 57 4.5.1. X–HY complexes 59 4.5.2. Hydrogen abstraction reactions – organic radical chemistry 61 4.5.3. Metal atom insertion reactions 63 4.6. Salt clusters 65 4.7. The structure and chemistry of semiconductor clusters 66 5. Summary 67 Acknowledgements 68 References 69