THERMIONIC EMISSION FROM FREE, PHOTOEXCITED TUNGSTEN CLUSTERS

We report on delayed electron emission from tree tungsten clusters, excited by light from a Q-switched YAG laser. Using a novel ion extraction 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 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate for different wavelengths and fluences. 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 electronically excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Good agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed deexcitation channels, namely, emission of atoms or photons, are not significant under our experimental conditions.