Ultrafast optical response of carbon films

The ultrafast photoinduced optical response of heat (infrared) treated poly acrylonitrile films (PAN-IR) is studied by the femtosecond pump-supercontin uum probe (PSCP) technique in the probe energy region 1.6-3.2 eV. The sampl es were excited by pulses with duration of 50 fs and photon energy of (h) o ver bar omega(pump) = 2.34 eV. Two types of films were used corresponding t o 600 and 700 degrees C heating. The structure of the films was investigate d with atomic force microscopy, micro-Raman spectroscopy, and optical absor ption spectroscopy. The size of the graphitic clusters was evaluated as sim ilar to 13 Angstrom for the first and > 17 Angstrom fur the second sample. The study of the photoinduced response shows that its temporal evolution is characterized by two-stage relaxation with fast and slow relaxation rates. For the first type of film, the spectral dependence of the fast relaxation rate shows nonmonotonic behavior with a maximum at (h) over bar omega(prob e) approximate to 1.95 eV. The region 1.6-1.95 eV shows photoinduced darken ing, while the region 1.95-3.2 eV shows photoinduced bleaching. For the sec ond type of film, only photoinduced bleaching is observed, with the fast re laxation I att: increasing monotonically. From the analysis of the transien t spectra and relaxation rates, two types: of optical transitions are propo sed. The first is the normal intramolecular absorption when photoexcited el ectrons stay within the same cluster. The second one is a charge-transfer t ransition, when the photoexcited electron is transferred from one cluster t o another closest one. For large size clusters, the contribution from these transitions vanishes and only intramolecular excitations survive.