DELAYED IONIZATION OF PHOTOEXCITED NIOBIUM AND NIOBIUM OXIDE CLUSTERS

The delayed emission of electrons from hot refractory metal clusters can be regarded as the molecular equivalent of ,,thermionic emission", which is well-known for metal surfaces. Niobium clusters - generated via pulsed laser vaporization technique - were exposed to a 10 ns laser pulse, and the delayed emission of electrons from the clusters was recorded as a function of cluster size as well as time after the multiphoton excitation. A large yield of NbnOx+ (x = 0, 1, 2, and 3) cluster ions, which were born as late as microseconds after the laser pulse, has been detected for n > 5. The data are discussed in terms of a model based on the Richardson-Dushman-equation.