Charged cores in ionized $\mathsf{{^{4}He}}$ clusters III: A quantum modeling for the collisional relaxation dynamics

When a pure ⁴He droplet is ionized by electron impact, the most abundant fragment detected in mass spectra after ionization is He₂ ⁺ . All the models that have been proposed thus far to explain the experimental evidence therefore involve the formation of the He₂ ⁺ molecular ion. The understanding of the interactions between this ion and the surrounding He atoms in the cluster and of their dynamical behavior during cluster break-up is an important element for the modeling of the cluster evolution after the ionization event. In previous works [1,2] we have computed and described the Potential Energy Surface (PES) of the electronic ground state for the He₃ ⁺ system that provides the required forces between He₂ ⁺ and He. After ionization He₂ ⁺ is presumably formed by association of an He ⁺ and any of the neutral atoms in the cluster via a 3-body collision process. The ensuing vibrational quenching of the hot molecular ion may release the energy necessary to evaporate the entire droplet, or most of it, and give the fragmentation patterns detected by experiments. We present here a model quantum dynamics that generates vibrational deexcitation cross-sections and the corresponding rate coefficients for the collision of He₂ ⁺ with He. A timescale of the cluster evaporation due to vibrational relaxation is estimated and the present findings are compared with earlier studies on the same system.Received: 15 June 2004, Published online: 24 August 2004PACS: 31.15.Qg Molecular dynamics and other numerical methods - 34.50.Ez Rotational and vibrational energy transfer - 36.40.Wa Charged clusters     

Charged clusters in liquid helium

The appropriateness of the experimental study of charged clusters in liquid helium has been supported. The interaction potential of negative ions (electron bubbles) with inert clusters formed by Ne, Ar, Kr, and Xe atoms or H₂ and N₂ molecules has been found. Small clusters levitate at a distance of 13–16 Å above the negative ion. The scalings laws for the properties of charged inert clusters have been discovered and grounded; the number of quantum levels and states of such clusters has been determined. The mobility measurement of charged clusters may provide a new technique of probing the properties of impurity nanoparticles in helium.     

Modelling singly ionized coronene clusters

We introduce a model combining a density functional based tight-binding method with a configuration interaction like scheme to treat charge delocalization in ionized molecular clusters. As an application, we determine the size-evolution of some properties of stack coronene clusters, namely charge delocalization, binding energies and ionization potentials.     

Stability and fragmentation of multiply ionized clusters

The stability of neutral, singly and multiply ionized silicon clusters, (N = 2-7, M = 0, , , ), has been investigated using an ab initio density functional method. We show that the fragmentation effect significantly affects the structure of mass-spectra of multiply ionized silicon clusters. For clusters, the clusters with a large fragmentation energy are found to correspond to the high peaks at N = 4 and 6 in mass-spectra. For clusters, a peak at N = 5 in mass-spectra has been predicted to be especially high. Received: 9 June 1997 / Revised: 8 January 1998 / Accepted: 25 February 1998     

Bubble formation and decay in 3He and 4He clusters

The energy transfer in 3He and 4He clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of approximately 7 m/s of bubbles in 4He clusters could be deduced. In the nonsuperfluid 3He clusters this velocity was observed to be significantly lower.     

Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

Observation of Andreev-Pushkarov vacancy clusters in phase-separated solid solutions of 4He in 3He

New features are observed for the pressure in a phase-separated dilute solid solution of ⁴He in ³He subjected to multiple temperature cycling within the phase-separation region. The results are explained within the framework of the hypothesis of A.F. Andreev and D.I. Pushkarov that the vacancies in a crystal without ideal periodicity are surrounded by clusters with a periodic structure. The equation for determining the radius of a cluster of pure ⁴He in a solution of ⁴He in ³He is refined. This hypothesis is shown to provide quantitative agreement between the calculated and experimental data under the assumption that the homogenization of the phase-separated solution is accompanied by the formation of metastable vacancies with a concentration of ～(4–5)×10^?5.     

Nucleation of He clusters at111In in metals

Helium has been implanted in copper. Its interaction with¹¹¹In atoms during isochronal annealing is observed by the perturbed angular correlation (PAC) technique. The results reveal that the nucleation of He-clusters at the¹¹¹In site is accompanied by reactions of intrinsic defects. Therefore, in the case of Cu, where the knowledge of intrinsic defects is nearly complete, the successive building-up of He-clusters can be pursued.This work was supported by the Bundesminister für Forschung und Technologie.     

Low field laser ionization of argon clusters: the remarkable fragmentation dynamics of doubly ionized clusters

We have investigated the fission following a Coulomb explosion in argon clusters (up to Ar800) irradiated by a femtosecond infrared laser with moderate intensity IL approximately 10(13) W cm(-2). We report the a priori surprising observation of well-defined velocity distributions of the ionized fragments Ar+n<50. This is interpreted by the formation of a valence shell excited charged ion, followed by relaxation, charge transfer by autoionizing collision at very short distance, and asymmetric fission.