Subpicosecond dynamics of excitons and photoexcited intrinsic polarons in the quasi-one-dimensional solid PtCl

Subpicosecond transient absorption (TA) spectroscopy was used to identify excited states, and to measure their relaxation kinetics as a function of excitation wavelength in the halogen-bridged transition-metal $\mathrm{}\left(\mathrm{MX}\right)\mathrm{}$ compound PtCl. For pump photon energies $E_{\mathrm{pump}}$ between 1.3 and 1.8 eV, below the threshold for exciton formation, intrinsic polarons are photoexcited and decay within 1–3 ps. Excitation into the exciton absorption tail (2.0–2.3 eV) yields singlet excitons that rapidly decay into an even-parity state, which has a lifetime (∼100 ps) varying with the intial photon energy. Excitation (3.1 eV) well above the 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 $E_{\mathrm{pump}}>2\mathrm{}\mathrm{}\mathrm{eV}$ is mediated by large-amplitude vibrations perpendicular to the chains. Using third-harmonic generation spectroscopy in addition to TA, we develop an energy-level diagram for the excited states, including polaron levels, and the lowest odd- and even-parity excitons.