THERMIONIC EMISSION FROM FREE, PHOTOEXCITED TUNGSTEN CLUSTERS

We report on delayed electron emission from tree tungsten clusters, ex cited by light from a Q-switched YAG laser. Using a novel ion extracti on lens, electron emission can be analyzed over a time range of 50 ns- 5 mus after the laser pulse without interference from prompt ions. All clusters of size 5 less-than-or-equal-to n less-than-or-equal 40 exhi bit delayed emission on this time scale, while delayed emission from s maller clusters does not occur. We analyze the time dependence and siz e dependence of the emission rate for different wavelengths and fluenc es. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronicall y excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Go od agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other d elayed deexcitation channels, namely, emission of atoms or photons, ar e not significant under our experimental conditions.