Individual Helium Atoms Research Articles

Nuclear Charge Radius ofHe8

The root-mean-square (rms) nuclear charge radius of ^8He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of ^6He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from ^6He to ^8He is an indication of the change in the correlations of the excess neutrons and is consistent with the ^8He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.

Photodissociation of alkyl iodides in helium nanodroplets. I. Kinetic energy transfer

The photodissociation of (fluorinated) alkyl iodides in helium nanodroplets at a wavelength of 266 nm has been investigated by means of ion imaging techniques. It is found that a significant fraction of the created fragments escapes from the helium droplets. The speed and kinetic energy distributions of these fragments are found to be notably modified with respect to the corresponding gas phase distributions. The fragments, furthermore, show a speed dependent angular distribution. The loss of kinetic energy as well as the reduction of the anisotropy parameter show a strong mass dependence. These observations point to a nonthermal escape process in which the kinetic energy and momentum transfer from the fragments to the solvent is governed by binary collisions with the individual helium atoms making up the droplet. Monte Carlo simulations based on hard-sphere binary collisions substantiate this interpretation of the data.

Metastable helium cluster He 4 *

The existence of a metastable cluster He 4 * with total spin S = 2 is predicted. The cluster consists of two covalently bound excited spin-polarized triplet He 2 * molecules and is rectangular in shape. The electron wavefunctions, the dependence of the energy He 4 * system on the distance between the He 2 * triplet molecules, the atomic spacing, the frequency spectrum of natural oscillations of the cluster, and other characteristics are calculated from first principles. It is shown that the metastable state is formed if one of the excited He 2 * molecules is in the 3Σ u + state, while the other is in the 3Πg state. The radiation lifetime τ of the metastable cluster He 4 * is calculated; it is found to range from 100 to 200 s, which is much longer than the lifetime τ ≈ 20 s of the triplet molecule He 2 * (3Σ u + ). The height U ≈ 0.5 eV of the potential barrier preventing the departure from the local energy minimum is determined. The energy E acc ≈ 9 eV/atom accumulated in the He 4 * cluster is calculated; this energy considerably exceeds the energy of known chemical energy carriers. It is shown that the accumulated energy is released virtually completely during decomposition of the He 4 * cluster into individual helium atoms. This means that helium clusters are a promising material with a high accumulated energy density (HEDM).

Imaging the Translational Dynamics ofCF3in Liquid Helium Droplets

The translational dynamics of CF3 in liquid helium have been investigated by photodissociating CF3I dissolved in helium droplets consisting of several thousands of atoms. The velocity distribution of CF3 fragments that have escaped from the droplets has been determined using ion imaging techniques and is found to be considerably shifted to lower speeds with respect to the photodissociation of gas phase CF3I. The fragments furthermore show a speed dependent angular distribution that is isotropic for the slowest and approaches the gas phase distribution for the faster fragments. These distributions point to a nonthermal escape process in which, at least for the speeds relevant for the present experiment, the kinetic energy and momentum transfer from the fragments to the solvent appears to be governed by binary collisions with the individual helium atoms.

STUDIES OF 2D CRYOCRYSTALS BY STM TECHNIQUES

Scanning tunneling microscopy (STM) studies of two-dimensional (2D) cryocrystals 4 He, Kr, Xe) physisorbed on graphite surfaces are presented. Individual helium atoms, usually thought to be invisible with STM, were recently observed on graphite surfaces at a density corresponding to the \(\sqrt {\text{3}} \times \sqrt {\text{3}}\) commensurate solid. Here we show that a local elastic deformation seems to be the principal mechanism responsible to render the atoms visible. Recent tight-binding calculations of the local density of states (LDOS) of graphite which predict the appearance of an energy gap support this picture. I-z curve measurements for the case of 4 He show a sharp drop (increase) of the tunneling current I at a certain tip-surface distance z during retraction (approach) of the tip. This drop (increase) may be associated with the tunneling of a single He adatom, opening new possibilities to study the quantum tunneling of atoms via STM.

Thermodynamics of boson quantum films

Theoretical studies of films of liquid $^{4}\mathrm{He}$ adsorbed to strongly attractive plane substrates indicate that the growth of such films occurs through a sequence of first-order phase transitions --- ``layering transitions'' --- which are a direct consequence of the short-range, hard-core-like interaction between individual helium atoms. The present work examines the effects of temperature on these transitions. At given temperatures, the spinodal points and phase coexistence boundaries are determined for the transitions. Increasing the temperature tends to decrease the coverage span of the transition regions, signaling the possible existence of a critical point terminating the two-phase equilibrium. The layering transitions depend strongly on the helium-substrate potential; the longer-range helium-magnesium potential yields fewer transitions and noticeably lower transition temperatures than the helium-graphite potential. The temperature dependence of the chemical potential, third sound, static structure function, heat capacities, and superfluid densities are reported. The heat capacities are compared to those measured by Greywall and Busch [Phys Rev. Lett. 67, 3535 (1991)]. The thermal broadening of the film's density profile is also discussed. We find that below 1.2 K, thermal broadening is quite weak for coverages away from the layering transitions. The monolayer film experiences the least broadening whereas double-layer and triple-layer films broaden by increasing the local density in the outer tail of their density profiles, while depleting the local density in the inner portion of the outermost layer.

Thermodynamics of boson liquid films

Theoretical studies of films of liquid4He adsorbed to strong, plane substrates indicate that the growth of such films occurs through a sequence of first-order phase transitions—“layering transitions”—which are a direct consequence of the short-range, hard-core like interaction between individual helium atoms. The present work examines the effects of temperature on these transitions. At given temperatures, the spinodal points and phase coexistence boundaries are determined for the transitions. Increasing the temperature tends to decrease the coverage span of the transition regions, signalling the possible existence of a critical point terminating the two-phase equilibrium.

The effect of helium on regrystallization and grain boundary movement in aluminum

A layer of helium-containing material was formed in samples of cold-worked aluminum by cyclotron α-particle irradiation, and this layer was used to observe the effect of helium on subsequent recrystallization and grain growth in the samples. Helium concentrations > 10 −4 at. % severely inhibited recrystallization of the layer during postirradiation heating of the samples, and helium-filled bubbles inhibited grain boundary movement in recrystallized aluminum. Helium concentrations < 10 −5 at. % had little effect on either recrystallization or grain boundary movement. Addition to the aluminum of impurities other than helium tended to reduce the inhibiting effect of helium on recrystallization. Calculations indicate that the inhibition of recrystallization was not due to the presence of heliumfilled bubbles of radii > 10 −5 cm; it must have arisen either from individual helium atoms in solution or from small clusters of helium atoms.

The transition temperature in liquid helium

The partition function for an imperfect Bose-Einstein assembly of atoms is evaluated following Feynman’s method, as modified by Kikuchi. The ‘effective mass’ approximation is avoided, and instead a factor depending explicitly on interatomic forces is introduced. This factor is evaluated approximately for pairs of atoms interacting with a Lennard-Jones potential. The calculation involves only the properties of the individual helium atoms and disregards collective excitations. No adjustable parameters are used. A peak in the specific heat at 2-0°K is found (experimental, 2·2°K), and this transition temperature decreases with density. There is another transition at about 3·0°K which is probably a liquid-gas transition. If the interatomic forces are very weak one would only get this second transition; the λ transition does not occur at all unless the forces are of the order of magnitude of those actually found in helium.