Delayed electron emission of negatively charged tungsten clusters

The delayed electron emission of negatively charged tungsten clusters has been investigated on a time scale from 1 to 500 ms. After being stored in a Penning trap clusters ions W-n(-) (4 less than or equal to n less than or equal to 14, 18 less than or equal to n less than or equal to 20) were heated via multiphoton absorption (hv = 1.81 eV). In contrast to alkali and coinage metals no photofragmentation could be detected. Instead, for all cluster sixes studied so far only a decrease in the initial ion intensity as a function of time after excitation was observed. This decrease is not caused by ion loss from the trap, but has to be attributed to neutralization via delayed electron emission. The presented results strongly suggest that this process can be viewed as ''thermionic emission'' known from bulk-metal surfaces.