Helium clusters

Rare-gas clusters can be produced easily using supersonic expansion. They are attractive to study theoretically because the interaction potentials are relatively simple and dominated by the van der Waals interactions. The Lennard-Jones pair potential describes the stmctures of the rare-gas clusters well and predicts magic clusters with icosahedral stmctures [139, 140]. The first five icosahedral clusters occur at 13, 55, 147, 309 and 561 atoms and are observed in experiments of Ar, Kr and Xe clusters [1411. Small helium clusters are difficult to produce because of the extremely weak interactions between helium atoms. Due to the large zero-point energy, bulk helium is a quantum fluid and does not solidify under standard pressure. Large helium clusters, which are liquid-like, have been produced and studied by Toennies and coworkers [142]. Recent experiments have provided evidence of... 

Rick SW, Lynch DL, Doll JD (1991) A variational Monte Carlo study of argon, neon, and helium clusters. J Chem Phys 95 3506-3520. 

Figure 4-19. Resonance-enhanced two-photon ionization spectra of ions issued from fluorobenzene/methanol d4/helium clusters, measured by scanning the laser near the 00 transition of fluorobenzene [0]. Bands [4-6] are due to the FB+(CD3OD)2 precursor which totally fragments, either by evaporation of one methanol-d4 molecule or reaction leading to anisole + DF + CD3OD. Bands [7-9] are due to the 1-3 precursor also losing one CD3OD molecule or reacting. Bands [1-3] are more likely attributed to the 1-1 complex (isomer( ) [1], hot band [2], 0q [3]) (from Brutschy et al. 1991).

The ability to dope impurity atoms or molecules into large helium clusters by a pick-up method, pioneered by the groups of Toennies and Scoles, has helped make studies in superfluid helium clusters more accessible. In this method, an expansion through a nozzle produces a beam of helium clusters. Under appropriate conditions, helium droplets comprising up to 10 helium atoms can be formed. These droplets then traverse a collision cell containing a foreign gas at a pressure of 10 -10 Pa. Atoms or... 

This review is devoted to Br2 He,. clusters and is organized as follows. In section 2 we briefly overview, for N = 1, the photo-dissociation process Bn (10 He + hco -+Bi2(B,vB,jB) + He and discuss the different PES s used, namely empirical and ah initio surfaces. Then, in section 3, we show, through ab initio calculations, how the full Br2(X) -He2 surface can be accurately described by the addition of two triatomic Br2(A) He potentials plus the He-He interaction, enabling the study of larger clusters. Then in section 4, we present our Hartree approach for describing doped bosonic helium clusters and their energetics and density distributions are analyzed. 

Fig. 1.6 isothermal chromatography of short-lived cyclotron produced nuclides transported by helium (cluster) jet [23]. 

A. Excess Electron Localization on and in Bulk Liquid Helium and Helium Clusters... 

E. Ultraslow Electron Tunneling Rates from Normal Fluid Helium Clusters... 

We now introduce an excess electron into the bubble, which is located in the center of the helium cluster at a fixed nuclear configuration of the helium balloon. The electronic energy of the excess electron will be calculated within the Born-Oppenheimer separability approximation. We modified the nonlocal effective potential developed by us for surface excess electron states on helium clusters [178-180] for the case of an excess electron in a bubble of radius Rb... 

Electron tunneling dynamics from electron bubbles in helium clusters strongly depends on the transport dynamics of the electron bubble within the cluster. In normal fluid ( He) and ( He)jy clusters the electron bubble motion is damped, while in (" He)jy superfluid clusters this motion is nondissipative [99]. Accordingly, bubble transport dynamics in ( He) clusters dominates the time scale for electron tunneling from the bubble, providing a benchmark for superfluidity in finite boson systems [245, 251]. In this chapter we address (a) the dynamics of electron tunneling from bubbles in ( He) and ( He) clusters [99, 209, 242-245, 251] and (b) the role of intracluster bubble transport on the lifetime of the bubble states. Our analysis provides semiquantitative information on electron bubbles in (" He) clusters as microscopic nanoprobes for superfluidity in finite quantum systems, in accord with the ideas underlying the work of Toennies and co-workers [99, 242-245]. 

Following the injection of an excess electron into a helium cluster, a sequence of dynamic processes is realized which involve ... 

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