Wavepacket Dynamics at Low Temperature in Localized Excitons of the Quasi-One-Dimensional Br-Bridged Pt Complex Studied by Femtosecond Luminescence Spectroscopy

Abstract

We report on the femtosecond time-resolved luminescence spectroscopy in a quasi-one-dimensional halogen-bridged mixed valence platinum complex, [Pt(en)₂][Pt(en)₂Br₂](ClO₄)₄ at low temperatures. From the temperature dependence of the lifetime of the self-trapped excitons (STEs), we estimated the potential barrier height in the direction to the non-radiative path to be 99±10 meV. The observed time-evolutions of the luminescence excited by 1.57 eV photons at 4 K show complicated behavior, which is considerably different from that at room temperature. From Fourier transform of the time domain data, a low frequency mode was found at 0.7 THz in addition to the main mode at 3.4 THz, which has been assigned to the motion of the halogen ions in the self-trapped excitons. Incorporating two modes into the model based on the theory of resonant secondary radiation, the experimental data are well reproduced. This result shows that the wavepacket moves on the two-dimensional adiabatic potential surface of the self-trapped exciton, following a Lissajous-like trajectory. Under excitation with 3.14 eV photons, the degradation of the coherence and existence of long-lived excess vibrational energy were found. These results give information about the STE formation process from the free exciton state.