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The Journal of Chemical Physics

The influence of embedded atoms, molecules, and clusters on the lifetimes of electron bubbles in large He4 droplets

Michal Fárnı́k, Jan Peter Toennies

Published Feb 14, 2003• 25 citations
Publisher

Abstract

The lifetimes of electrons injected into large (≈106 atoms) superfluid He4 droplets have been measured in the presence of different prior embedded rare gas atoms, their clusters, and the molecules O2, H2O, and SF6. For the light rare gas atoms Ne and Ar the lifetimes of about 6⋅10−2 s, found previously for pure droplets, are reduced to between 1–2⋅10−2 s. Single molecules with large electron affinities, such as SF6 and O2 and small clusters of H2O lead to a significant increase in the lifetimes. In the case of Kr and Xe, larger clusters with n̄>14 are needed before the lifetimes increase. A simple model, which takes account of the energy levels occupied by the electrons in the corresponding molecules or the clusters, assumed to be solid, can qualitatively explain the observations.

DOI
10.1063/1.1539851

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The lifetimes of electrons injected into large (≈106 atoms) superfluid He4 droplets have been measured in the presence of different prior embedded rare gas atoms, their clusters, and the molecules O2, H2O, and SF6. For the light rare gas atoms Ne and Ar the lifetimes of about 6⋅10−2 s, found previously for pure droplets, are reduced to between 1–2⋅10−2 s. Single molecules with large electron affinities, such as SF6 and O2 and small clusters of H2O lead to a significant increase in the lifetimes. In the case of Kr and Xe, larger clusters with n̄>14 are needed before the lifetimes increase. A simple model, which takes account of the energy levels occupied by the electrons in the corresponding molecules or the clusters, assumed to be solid, can qualitatively explain the observations.
The rate of detachment of electron bubbles inside size-selected 4He{}^{4}\mathrm{He}4He and 3He{}^{3}\mathrm{He}3He droplets ( 105−−107{10}^{5}--{10}^{7}105−−107 atoms) has been measured. At zero electric field the lifetimes of 4He{}^{4}\mathrm{He}4He droplets increase with size from ${10}^{\ensuremath{-}3}$ to 0.2 s, orders of magnitude smaller than previously predicted. With increasing fields the detachment gradually increases for 4He{}^{4}\mathrm{He}4He droplets, whereas a sharp threshold was found at 1.45 kV/cm for 3He{}^{3}\mathrm{He}3He droplets, independent of size. A simple dynamical model which accounts for superfluidity in the 4He{}^{4}\mathrm{He}4He droplets versus a viscous dynamics in 3He{}^{3}\mathrm{He}3He can explain the observations.
Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of Hen+ ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 105 or more atoms), however, the He4+ signal intensity is anomalously large. We have studied the mechanism for the formation of He4+ ions in large helium droplets by varying the duration of the electron impact excitation pulse. Droplets of different average sizes were generated using the expansion of helium at 20 bars and 9-20 K through a pulsed valve nozzle. The resulting ions were analyzed by time-of-flight mass spectroscopy (TOFMS) and quadrupole mass spectroscopy (QMS). The intensity distributions obtained with the TOFMS technique initially showed much smaller He4+ signals than those obtained using QMS. However, we discovered that the intensity anomaly is associated with the duration of the electron bombardment pulse in the TOFMS instrument. Measurements with different electron bombardment pulse durations enabled us to discern a characteristic time of ∼10 μs for enhanced He4+ production in large droplets under our experimental conditions. A qualitative model is presented in which metastables interact on droplet surfaces, yielding two He2+ cores that share a Rydberg electron while minimizing repulsion between the cores. This is the He4+(4A2) state suggested by Knowles and Murrell.
The lifetimes of electron bubbles in large size selected HeN−4 droplets have been measured in a molecular beam experiment to lie between 2⋅10−3 and 0.2 s in droplets with sizes between N≈105 and 107 atoms, respectively. The dependence of the lifetimes on the electric fields used to size select the droplets, on the energies of the incident electrons, and on the infrared radiation emitted from the apparatus walls were investigated and taken into account. The experimental lifetimes are many orders of magnitude smaller than predicted by earlier theories but consistent with a recent theory published by the authors in Phys. Rev. Lett. 81, 3892 (1998).
Many microscopic properties of the only natural superfluid element helium have become available within the last 18 years through a number of novel experiments. The diffraction of matter-waves of small helium clusters has established the unusually large bond distance (5.2 nm) and extremely weak binding energy (10−7 eV) of the dimer and other exotic properties of small helium clusters. The high-resolution spectroscopy of closed-shell chromophore molecules attached to small clusters or embedded inside large superfluid droplets have revealed that as a result of superfluidity, molecules rotate virtually without friction and that only a handful of He atoms are required for microscopic superfluidity. The frictionless translational motion of atoms and molecules inside the droplets has also been observed. These and a number of other experiments demonstrate that helium droplets are the most gentle and coldest of all cryomatrices with enormous potential for a wide range of future applications.
The infrared spectrum of single oxygen carbon sulfide (OCS) molecules was measured inside large superfluid pure helium-4 droplets and nonsuperfluid pure helium-3 droplets, both consisting of about 10(4) atoms. In the helium-4 droplets, sharp rotational lines were observed, whereas in helium-3 only a broad peak was found. This difference is interpreted as evidence that the narrow rotational lines, which imply free rotations, are a microscopic manifestation of superfluidity. Upon addition of 60 helium-4 atoms to the pure helium-3 droplets, the same sharp rotational lines were found; it appears that 60 is the minimum number needed for superfluidity.
In this thesis, we investigate the formation process of snowballs (solid-like ordered helium atoms) when embedding ions within helium droplets. This is achieved through a semi-classical methodology, namely the Gaussian-Imaginary Time-dependent Hartree approach. Subsequently, we propose a modified Density Functional, built upon the well-established Orsay-Trento functional, to better capture the primary effects of the snowball on the surrounding helium droplet. In such systems, a local supersolid layer emerges in the transition region between the snowball (near the ion) and the superfluid (far from the ion). We also investigated Rydberg atoms embedded in helium droplets. These atoms were modeled as an ionic core with a weakly bound electron. In this case, the helium density profile arranges similarly to that observed for ionic impurities, as the perturbation induced by the highly excited Rydberg electron on the droplet is negligible. At the same time, the energy spectrum associated with the Rydberg electron undergoes significant modifications. To better capture the perturbation of the Rydberg electron's states induced by the helium droplet, we introduce the droplet-dressed hydrogenic states. These states incorporate the average effect of the droplet on the highly excited electronic states, significantly reducing the computational effort. We propose a standard methodology that employs these states as a basis for efficiently describing Rydberg electrons in both discrete- and continuous-density environments. Furthermore, we demonstrate the existence of droplet-dressed Rydberg molecules, in which a Rydberg atom located at the center of the droplet binds to a ground-state atom located beyond the droplet's surface via the Rydberg electron. In this context, the helium droplet itself acts as an environment that promotes the stability of such molecular structures and enhances some of its properties like bond length and electric dipole moment.