Mainz 1997 – wissenschaftliches Programm
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A: Atomphysik
A 7: Cluster I
A 7.4: Vortrag
Dienstag, 4. März 1997, 15:00–15:15, P5
Electron Bubble Lifetime Measurements in Large HeN− Droplets — •M. Farnik, U. Henne, B. Samelin, and J. P. Toennies — Max-Planck-Institut für Strömungsforschung Göttingen, Bunsenstr. 10, D-37073 Göttingen
Excess electrons in liquid helium droplets form bubbles with a ground state energy of approximately 0.1 eV above the vacuum level. Therefore the electrons are only metastably bound and can decay by autodetachment. Calculations of Northby et al. [1] suggest lifetimes of 1012 s for droplets consisting of N=106 He atoms with an internal temperature of 0.5 K.
We have measured the lifetimes for size selected negatively charged He droplets as a function of their size N. The He droplets are created with standard molecular beam methods and the electron attachment is achieved by electron impact at energies of 1-40 eV. We have found that the measured lifetimes τ are 13 orders of magnitude smaller than the predicted lifetimes, e.g. τ=40 ms for N=106. The observed lifetime dependence τ (N) is a simple exponential function of the droplet radius R∝ N1/3 and increases strongly with size N.
Additionally the stability of the negatively charged He droplets in electric fields of up to 104 V/cm has been measured as a function of the droplet size N. A strong decrease of τ is found for electric fields exceeding 6000 V/cm for negatively charged droplets with N<107.
[1] J. A. Northby, C. Kim, T. Jiang, Physica B 194, 426 (1994)