Dynamics of the formation of the self-trapped exciton in the MX complex PtBr(en)

We have directly time-resolved the coupled electronic and vibrational dynamics of the self-trapping process in a quasi-one-dimensional system, the halide-bridged mixed-valence transition metal linear chain (MX) complex [Pt(en)2][Pt(en)2Br2]·(PF6)4, (en: ethylenediamine, C2H8N2) using femtosecond spectroscopic techniques in the vibrationally impulsive limit. In these experiments, we impulsively excite the optical intervalence charge-transfer transition with light pulses 35 fs in duration, short compared to the period of the characteristic chain-axis vibrational motion. The red-shifted absorbance of the self-trapped exciton state, which forms on a time scale of ∼200 fs, is modulated by vibrational wavepacket oscillations that correspond to lattice motions induced by the optical excitation. In addition to detecting an oscillatory response consistent with impulsive stimulated Raman excitation of the ground-state symmetric chain-axis stretching mode at ∼175 cm−1, and its harmonics, we find that the self-trapped exciton absorbance is strongly modulated by a heavily damped, low frequency wavepacket component at ∼110 cm−1. The coherence time of this new frequency component closely parallels the induction of the self-trapped exciton absorbance, consistent with a wavepacket corresponding to the lattice motion that carries the excited system to the self-trapped state. The spectral evolution of the low-frequency wavepacket oscillation provides a detailed picture of the coupled electron-lattice dynamics of the photo-excited state.