Efficiency of thermionic emission from C60

We have determined the number of delayed electrons emitted from an ensemble of gas-phase C-60, excited at 355 nm with light from an unfocused pulsed Nd:YAG laser. Delayed electrons are detected by single particle counting and integrated over a time interval of 0.1 mu s less than or equal to t less than or equal to 80 mu s. At low laser fluence I, the electron number scales as I-p with p = 6.0 +/- 0.4. It approaches saturation at a maximum laser fluence of 100 mJ/cm(2), equivalent to a power density of 14 MW/cm(2), where delayed electron emission reaches a probability of 2.6 +/- 1.1% per multiphoton-excited C-60 This experimental value is interpreted as a lower limit to the quantum efficiency for electron emission from C-60; it provides an upper limit to the efficiency of competing reactions such as dissociation of C-60 into C-58 + C-2. The dependence of the electron number on the temperature of the fullerene source is in good agreement with the number density of C-60 in the molecular beam, as computed from published equilibrium vapor pressures.