SUBPICOSECOND DYNAMICS OF EXCITONS AND PHOTOEXCITED INTRINSIC POLARONS IN THE QUASI-ONE-DIMENSIONAL SOLID PTCL

Subpicosecond transient absorption (TA) spectroscopy was used to ident ify excited states, and to measure their relaxation kinetics as a func tion of excitation wavelength in the halogen-bridged transition-metal (MX) compound PtCl. For pump photon energies E-pump between 1.3 and 1. 8 eV, below the threshold for exciton formation, intrinsic polarons ar e photoexcited and decay within 1-3 ps. Excitation into the exciton ab sorption tail (2.0-2.3 eV) yields singlet excitons that rapidly decay into an even-parity state, which has a lifetime (similar to 100 ps) va rying with the intial photon energy. Excitation (3.1 eV) well above th e exciton threshold appears to generate more slowly decaying excitons, as well as polarons that are formed upon exciton dissociation. The TA spectra also indicate that relaxation of excitons for E-pump>2 eV is mediated by large-amplitude vibrations perpendicular to the chains. Us ing third-harmonic generation spectroscopy in addition to TA, we devel op an energy-level diagram for the excited states, including polaron l evels, and the lowest odd-and even-parity excitons.