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 dynam ics of the self-trapping process in a quasi-one-dimensional system, the hal ide-bridged mixed-valence transition metal linear chain (MX) complex [Pt(en )(2)][Pt(en)(2)Br-2].(PF6)(4), (en: ethylenediamine, C2H8N2) using femtosec ond spectroscopic techniques in the vibrationally impulsive limit. In these experiments, we impulsively excite the optical intervalence charge-transfe r transition with light pulses 35 fs in duration, short compared to the per iod of the characteristic chain-axis vibrational motion. The red-shifted ab sorbance of the self-trapped exciton state, which forms on a time scale of similar to 200 fs, is modulated by vibrational wavepacket oscillations that correspond to lattice motions induced by the optical excitation. In additi on to detecting an oscillatory response consistent with impulsive stimulate d Raman excitation of the ground-stare symmetric chain-axis stretching mode at similar to 175 cm(-1), and its harmonics, we find that the self-trapped exciton absorbance is strongly modulated by a heavily damped, low frequenc y wavepacket component at similar to 110 cm(-1). The coherence time of this new frequency component closely parallels the induction of the self-trappe d exciton absorbance, consistent with a wavepacket corresponding to the lat tice motion that carries the excited system to the self-trapped state. The spectral evolution of the low-frequency wavepacket oscillation provides a d etailed picture of the coupled electron-lattice dynamics of the photo-excit ed state. (C) 2001 Elsevier Science B.V. All rights reserved.