Vibrational spectroscopy of structurally relaxed self-trapped excitons via excited-state resonant impulsive stimulated Raman spectroscopy.

We probe the vibrational modes of the equilibrated self-trapped exciton (STE) in the mixed-valence linear chain material [Pt(en)(2)][Pt(en)(2)Br(2)]·(ClO(4))(4) using resonantly enhanced impulsive stimulated Raman excitation of the excited electronic state. In these measurements, excitons are created by photoexcitation of the optical intervalence charge transfer band, and after a delay to allow self-trapping and equilibration, the metastable STE is impulsively excited and probed within its red-shifted absorption band. The pump-pump-probe response reveals wavepacket oscillations at a frequency of 125 cm(-1) that are assigned to a Raman-active mode of the STE having Br-Pt-Br symmetric stretching character. This frequency is shifted from the 171 cm(-1) symmetric stretch Raman frequency of the ground electronic state, and from the previously observed 110 cm(-1) wavepacket modulation that accompanies the formation of the STE from the initially excited electronic state, reflecting a new component of the structural relaxation of the exciton.