Conversion of He(23 S) to He2( a3Σ u+) in Liquid Helium.

The report of an anomalously intense He4+ peak in electron impact mass spectra of large helium droplets created a stir 3 decades ago that continues to this day. When the electron kinetic energy exceeds 41 eV, an additional pathway opens that yields He4+ predominantly in an electronically excited metastable state. A pair of He*(23 S) atoms has been implicated based on the isolated He* energy of 19.82 eV and the 41 eV threshold, and the creation of He4+ has been conjectured to proceed via a pair of He2*( a3Σ u+) precursors. The mechanism whereby He* converts to He2* in liquid helium has remained a mystery, however. High level ab initio theory combined with classical molecular dynamics has been applied to systems comprising small numbers of He atoms. The conversion of He* to He2* in such systems is shown to be due to a simple many-body effect that yields He2* rapidly and efficiently.

Effects of the mean free path and relaxation in a model for the aggregation of particles in superfluid media.

In this paper, we study a two-dimensional model for the growth of molecular clusters in superfluid helium at low temperature. In the model, particles of diameter a follow random ballistic moves of length delta = a - 256a. Upon attachment on the cluster surface, particles allow one-step relaxation to the nearest twofold coordinated site. Average coordination numbers of particles show that in the presence of relaxation the screening for incoming particles modifies the microscopic structure of the cluster. These results are in qualitative agreement with experimental aggregation of methane in He droplets, in which large abundance of fully coordinated sites is observed. The average coordination number increases with delta, showing that screening is relevant when relaxation is present. As the cluster size increases, the corresponding structure clusters changes from a compact to a fractal, typical of ballistic and diffusion limited models, respectively. A scaling ansatz describing the crossover between the two regimes is proposed.

The effects of isotope substitution and nuclear spin modifications on the spectra of complexes of tetracene with hydrogen molecules in ultracold 0.37 K He droplets.

The van der Waals complexes consisting of single tetracene chromophore molecules with an attached H(2), HD, or a D(2) molecule have been assembled inside cold (0.37 K), large ( approximately 1.5 x 10(4) atoms) helium droplets. Their laser-induced fluorescence spectra exhibit typically three well isolated fairly sharp [deltanu(full width at half maximum) approximately 0.5 cm(-1)] bands in the spectral region 22220-22300 cm(-1). Their positions differ for each isotopomer and also are different for each of the ortho- and para-spin modifications. The common feature (except for D(2)) with the largest redshift at about 30 cm(-1), found also in other related free complexes, is attributed to a strongly bound site above one of the two central benzene rings. The other major features come in pairs spaced 3 cm(-1) apart and are not found in similar gas phase studies. This doublet is assigned to a less tightly bound peripheral site with either slightly different configurations or states of the aduct or possibly the He atoms which are stabilized by the surrounding helium bath. The common feature and one branch of the doublet exhibit a pronounced narrow fine structure with spacings of only 0.1 cm(-1), which is nearly the same for all complexes as well as for the bare chromophore, and maybe be due to partially resolved rotational structure of the bands.

Spectroscopy of OCS-hydrogen clusters in He droplets.

Clusters of para-hydrogen (pH2) and ortho-deuterium (oD2) have been assembled around an OCS chromophore molecule inside He droplets in a molecular beam and studied via IR diode laser depletion spectroscopy (nu approximately 2060 cm-1). The superfluid 4He droplets provide a gentle host ensuring a constant low temperature of either T = 0.38 K for 4He droplets or T = 0.15 K for both the pure 3He and mixed 4He-3He droplets. The spectra show well resolved rotational structure of the vibrational bands for each attached hydrogen molecule in the range n = 1-8. With only one (n = 1) attached pH2, HD or an oD2 molecule the best fit rotational constants were used to determine the structure of the complex, which was found to be in surprisingly good agreement with quantum chemical calculations for the free complex. With n = 5 and 6 the Q-branch disappears for the pH2 clusters but not for the oD2 clusters which is consistent with a donut model. The moments of inertia of the pH2 and the oD2 complexes are explained by a new model in which each of the 18 attached helium atoms in a shell surrounding the OCS molecule are assigned a mass of 0.55, while each attached H2 and D2 molecule has an effective mass of about 10 and 12 u, respectively.

Spectroscopy of atoms and molecules in liquid helium.

Laser ablation of in situ metals has recently made it possible to immerse a large number of different metal atoms and ions and small clusters of metal atoms in liquid helium (He) and thus study their absorption and emission spectra in the visible region. Atoms and molecules are readily picked up by large (N > or = 103 atoms) He droplets, and their spectra are sensitively detected through the use of either beam depletion following absorption or laser-induced fluorescence. Within the past three years, a wide variety of molecules, ranging from OCS to large organic molecules such as amino acids and a number of van der Waals complexes and even large metal clusters, have been embedded in He droplets and studied either in infrared or in the visible region. These results are discussed here in detail, and the evidence for the effect of superfluidity on the spectral features is reviewed.