Thermionic Emission from Free, Photoexcited Tungsten Clusters

We report on delayed electron emission from free 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 μs after the laser pulse without interference from prompt ions. All clusters of size 5≤n≤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. REFERENCE

Hot Tungsten Clusters: Competition between Atom Ejection and Thermionic Emission

This is a first report concerning the thermionic emission of electrons from hot metal clusters. Tungsten clusters were exposed to a 10 ns laser pulse, and the delayed emission of electrons from the clusters was recorded as a function of time after the excitation. A large yield of W +n cluster ions, which were born as late as microseconds after the laser pulse, has been detected forn\u3e4. Tungsten cluster ions created via thermionic emission show no measurable metastable decay during the flight time in the mass spectrometer. This indicates that they are colder than expected, if evaporative cooling after prompt ionization would prevail

Electronic Shell Structure in Multiply Charged Silver Clusters

Silver clusters are generated by standard laser vaporization technique and ionized via multiphoton ionization. Time-of-flight mass spectrometry reveals singly, doubly and triply charged clusters, Ag z+n (z=1,2,3). The spectra show, for all charge states, intensity variations, indicating enhanced stabilities for cluster sizes with closed electronic configurations in accord with the spherical jellium model