Wave-packet oscillation in the excited state of KBr F centers

We investigated wave-packet oscillation of KBr F centers at 5 K using up-conversion spectroscopy method. The Fourier power spectrum of the oscillatory signal shows that the predominant peaks are located at 2.6 and 4.8 THz rather than at 3.3 THz, which corresponds to the A1g breathing mode of the lattice around the vacancy. These modes are assigned to the LA phonons near the singularities in (1 0 0) and/or (1 1 0) crystal directions, and the LO phonons near the Γ point, respectively. The result shows that in the excited state the local vibration around the center is described as the interaction mode consisting of a linear combination mainly of these bulk phonon modes, because the electron wavefunction in the excited state spatially spreads out. It is strongly suggested that the luminescence reflects solely the wave-packet motion in the excited state, while the degenerate pump and probe measurements probes mainly the ground state.