Journal of Physical Chemistry B, Vol.104, No.2, 220-227, 2000
Ultrafast optical response of carbon films
The ultrafast photoinduced optical response of heat (infrared) treated polyacrylonitrile films (PAN-IR) is studied by the femtosecond pump-supercontinuum probe (PSCP) technique in the probe energy region 1.6-3.2 eV. The samples were excited by pulses with duration of 50 fs and photon energy of (h) over bar omega(pump) = 2.34 eV. Two types of films were used corresponding to 600 and 700 degrees C heating. The structure of the films was investigated with atomic force microscopy, micro-Raman spectroscopy, and optical absorption spectroscopy. The size of the graphitic clusters was evaluated as similar 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(probe) approximate to 1.95 eV. The region 1.6-1.95 eV shows photoinduced darkening, while the region 1.95-3.2 eV shows photoinduced bleaching. For the second type of film, only photoinduced bleaching is observed, with the fast relaxation I att: increasing monotonically. From the analysis of the transient spectra and relaxation rates, two types: of optical transitions are proposed. The first is the normal intramolecular absorption when photoexcited electrons stay within the same cluster. The second one is a charge-transfer transition, when the photoexcited electron is transferred from one cluster to another closest one. For large size clusters, the contribution from these transitions vanishes and only intramolecular excitations survive.
Keywords:AMORPHOUS-CARBON;RAMAN-SPECTROSCOPY;METASTABLE GROWTH;C-60FILMS;GRAPHITE;POLYACRYLONITRILE;RELAXATION;DIAMOND;DEPENDENCE;LIFETIME