MULTIPHOTON IONIZATION OF C60

The multiphoton ionization photoelectron spectrum (MPI-PES) of C60 using pulsed lasers of nanosecond duration shows prompt electrons resulting from direct two-photon ionization, as well as a dominant low-energy delayed electron signal which follows an approximately thermal energy distribution with average energy epsilonBAR = 0.30 +/- 0.05 eV. The MPI-PES together with the mass spectrum at lambda = 193 nm suggests an important role of intersystem crossing from the singlet to triplet manifold of C60. MPI at lambda = 193 nm is believed to result from such intersystem crossing followed by one-photon ionization of triplet C60. Under this assumption, the photoionization cross section for C60 in the lowest triplet state is estimated to be (5.1 +/- 2.0) x 10(-17) cm-2.