Manifestation of electron-electron interactions in time-resolved ultrafast pump-probe spectroscopy in C$_{60}$

The electron-electron interaction EEI is at the core of modern physics from high-temperature supercon- ductivity to giant magnetoresistance. Nanostructures, in general, and $\rm C_{60}$, in particular, open a new frontier for the study of the electron correlation effect in quasi-zero-dimensional materials. Here, a direct investigation of the time-resolved pump-probe signal in $\rm C_{60}$ shows [1] that the on-site electron-electron interaction manifests itself in two aspects in the early stage of ultrashort laser excitation. First, it pushes the signal peak to an earlier time delay for below-resonance excitation and narrows the peak-time change with probe detuning [2]. Second, it shortens the quasiparticle lifetime and, if the interaction is strong enough, it diminishes the spike in the lifetime at resonance. These features are detectable experimentally, and the findings here suggest a different route to detect dynamical EEI in nanostructure [3].\\[0pt] [1] Zhang and George, Phys. Rev. B {\bf 76}, 085410 (2007); [2] Zhang and George, J. Opt. Soc. Am. B {\bf 24}, 1150 (2007); [3] Zhang {\it et al.}, J. Phys. Chem. A {\bf 113}, 1175 (2009)