Direct observation of the dynamics of excited electronic states in solids: F-sec time resolved photoemission of C60

Time-resolved photoelectron spectroscopy of low-energy excitations of 4×4 C60/Cu(111)

“…Optical absorption in fullerite is best described by the creation of an exciton. An exciton does have an impulse, but the creating photon delivers only a small impulse and the exciton slows down, so after 1 ps it is certainly in the band minimum [39]. Thus, for the present investigation this band minimum is important as depicted in figure 1.3.…”

“…Eberhardt and coworkers [39] reported that an electron excited from the HOMO to the LUMO+1 decays into the LUMO with decay time of 1.1 ps. Sawatzky and coworkers [45] noted that the electron in the LUMO+1 belonging to the exciton lies energetically within the band of free electrons in the LUMO and thus in the course of the photo absorption 1% of the excitons dissociate [11,46].…”

“…The precise percentage depends on the temperature and the applied electric field. Eberhardt and coworkers [39] reported that a lifetime of 134 ps can be assigned to the free electrons in the LUMO band. Henninger and coworkers [46] reported that in the ps and ns time regime the number of free electrons decays proportional to 1/t, with t being the time, which they explained by the spatial distribution of traps and electron hopping.…”

“…Kabler and coworkers [11] determined a lifetime of 25 ns in photopolymerised fullerite at 81 K. 35% of the singlet excitons decay into the triplet exciton with an intersystem crossing time of 2.5 ns. Eberhardt and coworkers [39] found a singlet exciton lifetime of 998 ps without resolving the decay channels. Kabler and coworkers [11] determined that at fluences higher than 1 mJ/cm 2 triplet exciton creation due to collision between a free carrier and a singlet exciton becomes important.…”

“…Eberhardt and coworkers [39] used a list of delays suited for a logarithmic scale. The mechanical delay steps are chosen slightly shorter than the laser pulse duration to reduce the width of the Poisson distribution.…”