Thermionic electron emission of small tungsten cluster anions on the milliseconds time scale

Small tungsten cluster anions W-n(-) (n=4-9 and 18-20) are stored in a Penn ing trap and electronically excited by photoabsorption (E-hv = 1.815, 2.33, 3.5, and 4.66 eV). Delayed electron emission is observed on the millisecon ds time scale by systematic variation of the storage duration between laser excitation and ion detection. Even if the photon energy exceeds the electr on detachment energy, electrons are emitted several milliseconds after lase r excitation. The electron emission time constant is determined as a functi on of the laser pulse energy. An Arrhenius analysis suggests that the obser ved delayed electron emission is a thermal process in analogy to thermionic emission of bulk materials. As shown by these experiments there is a simpl e rule for the dominating cooling channel of laser excited clusters: thermi onic emission generally occurs as long as the electron binding energy is lo wer than the dissociation energy. (C) 1999 American Institute of Physics. [ S0021-9606(99)70616-1].