J. P. Toennies

Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.15 K (3He droplets), lower than possible in most solid matrices. Thus the helium-droplet technique combines the benefits of both the gas phase and the classical matrix-isolation techniques. Most important, the superfluid helium facilitates binary encounters, and absorbs the released binding energy upon recombination. Thus the droplet can be viewed as an isothermal nanoscopic reactor, which isolates single molecules, clusters, or even a single reactive encounter at ultralow temperatures.

The interatomic van der Waals potentials for all the possible 21 homogeneous and heterogeneous pairs of rare gas atoms including radon are determined using the Tang–Toennies potential model and a set of previously derived combining rules. The three dispersion coefficients and the two Born–Mayer parameters needed for calculating the potential curves are listed.

The multiple pickup of up to 30 foreign particles by large (N≳103 atoms) liquid helium clusters and the formation of clusters of the foreign particles within the helium cluster has been observed in a molecular beam experiment for Ar, Kr, Xe, H2O, and SF6. Evaporation of helium atoms due to the pickup process has been measured quantitatively by two methods. The measured apparent cross sections for capture and coagulation of these particles are significantly smaller than the total scattering cross sections of the clusters. After electron impact ionization the size distribution of the foreign particle cluster ion fragments can be well described by a Poisson distribution. This is interpreted as indicating negligible fragmentation of the coagulated foreign particle clusters. The measured coagulation cross sections are smaller than the capture cross sections. This is explained through a model which involves only partial coagulation of the foreign particles. The observed processes allow the preparation of foreign particle clusters with well-defined size distributions inside large helium clusters.

A simple analytical expression for the entire van der Waals potential curve of the helium dimer is derived from perturbation theory. The repulsive part is essentially from Duman and Smirnov [Opt. Spectrosc. 29, 229 (1970)] who assume the exchange of only one pair of electrons at any one time. The potential depends only on the known dispersion coefficients and the amplitude of the asymptotic wave function and the ionization energy of the atoms. Without any adjustable parameters the potential is in excellent agreement with the latest experimental and theoretical results and predicts the existence of the He dimer. The same method can be applied to heavier rare gases.

The recently developed technique of accessing volatile liquids in a high vacuum environment by using a very thin liquid jet is implemented to carry out the first measurements of photoelectron spectra of pure liquid water, methanol, ethanol, 1-propanol, 1-butanol, and benzyl alcohol as well as of liquid n-nonane. The apparatus, which consists of a commercial hemispherical (10 cm mean radius) electron analyzer and a hollow cathode discharge He I light source is described in detail and the problems of the sampling of the photoelectrons in such an environment are discussed. For water and most of the alcohols up to six different electronic bands could be resolved. The spectra of 1-butanol and n-nonane show two weakly discernable peaks from which the threshold ionization potential could be determined. A deconvolution of the photoelectron spectra is used to extract ionization potentials of individual molecular bands of molecules near the surface of the liquid and shifts of the order of 1 eV compared to the gas phase are observed. A molecular orientation for water molecules at the surface of liquid water is inferred from a comparison of the relative band strengths with the gas phase. Similar effects are also observed for some of the alcohols. The results are discussed in terms of a simple “Born-solvation” model.