Electron bubbles in helium clusters. II. Probing superfluidity
- School of Chemistry, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv (Israel)
In this paper we present calculations of electron tunneling times from the ground electronic state of excess electron bubbles in ({sup 4}He){sub N} clusters (N=6500-10{sup 7}, cluster radius R=41.5-478 A ), where the equilibrium bubble radius varies in the range R{sub b}=13.5-17.0 A. For the bubble center located at a radial distance d from the cluster surface, the tunneling transition probability was expressed as A{sub 0}{phi}(d,R)exp(-{beta}d), where {beta}{approx_equal}1 A {sup -1} is the exponential parameter, A{sub 0} is the preexponential factor for the bubble located at the cluster center, and {phi}(d,R) is a correction factor which accounts for cluster curvature effects. Electron tunneling dynamics is grossly affected by the distinct mode of motion of the electron bubble in the image potential within the cluster, which is dissipative (i.e., {tau}{sub D}<{tau}{sub 0}) in normal fluid ({sup 4}He){sub N} and ({sup 3}He){sub N} clusters, while it is undamped (i.e., {tau}{sub D}>>{tau}{sub 0}) in superfluid ({sup 4}He){sub N} clusters, where {tau}{sub D} is the bubble motional damping time ({tau}{sub D}{approx_equal}4x10{sup -12} s for normal fluid clusters and {tau}{sub D}{approx_equal}10 s for superfluid clusters), while {tau}{sub 0}{approx_equal}10{sup -9}-10{sup -10} s is the bubble oscillatory time. Exceedingly long tunneling lifetimes, which cannot be experimentally observed, are manifested from bubbles damped to the center of the normal fluid cluster, while for superfluid clusters electron tunneling occurs from bubbles located in the vicinity of the initial distance d near the cluster boundary. Model calculations of the cluster size dependence of the electron tunneling time (for a fixed value of d=38-39 A ), with lifetimes increasing in the range of 10{sup -3}-0.3 s for N=10{sup 4}-10{sup 7}, account well for the experimental data [M. Farnik and J. P. Toennies, J. Chem. Phys. 118, 4176 (2003)], manifesting cluster curvature effects on electron tunneling dynamics. The minimal cluster size for the dynamic stability of the bubble was estimated to be N=3800, which represents the threshold cluster size for which the excess electron bubble in ({sup 4}He){sub N}{sup -} clusters is amenable to experimental observation.
- OSTI ID:
- 20783283
- Journal Information:
- Journal of Chemical Physics, Vol. 124, Issue 19; Other Information: DOI: 10.1063/1.2192782; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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