From Above Threshold Ionization to Statistical Electron Emission: The Laser Pulse-Duration Dependence of $C_{60}$ Photoelectron Spectra

The photoelectron spectra of ${\mathrm{C}}_{60}$ ionized using a 790 nm laser with pulse durations varying from 25 fs to 5 ps have been determined. For 25 fs pulses, in the absence of fragmentation, the ionization mechanism is direct multiphoton ionization with clear observation of above threshold ionization. As the pulse duration is increased, this becomes dominated by a statistical ionization due to equilibration among the electronic degrees of freedom. For pulse durations on the order of a ps coupling to the vibrational degrees of freedom occurs and the well-known phenomenon of delayed $\left(\mu \mathrm{s}\right)$ ionization is observed.