Efficiency of Thermionic Emission from C<INF>60</INF>

We have determined the number of delayed electrons emitted from an ensemble of gas-phase C<INF>60</INF>, 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 μs ≤ t ≤ 80 μs. 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/cm², equivalent to a power density of 14 MW/cm², where delayed electron emission reaches a probability of 2.6 ± 1.1% per multiphoton-excited C<INF>60</INF>. This experimental value is interpreted as a lower limit to the quantum efficiency for electron emission from C<INF>60</INF>; it provides an upper limit to the efficiency of competing reactions such as dissociation of C<INF>60</INF> into C<INF>58</INF> + C<INF>2</INF>. The dependence of the electron number on the temperature of the fullerene source is in good agreement with the number density of C<INF>60</INF> in the molecular beam, as computed from published equilibrium vapor pressures.