Femtosecond dynamics of correlated many-body states in C$_{60}$ fullerenes

Abstract

Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C$_{60}$ by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of \emph{ab initio} calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of $\tau_\mathrm{el}=10^{+5}_{-3}$ fs. Energy dissipation towards nuclear degrees of freedom is studied in time-resolved techniques. The evaluation of the non-linear autocorrelation trace gives a characteristic time constant of $\tau_\mathrm{vib}=400\pm100$ fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg-Teller coupling), and their interplay.