  We calculated the dynamics of nuclear wave packets in coupled
electron-vibration systems and their nonlinear optical responses. We found that
the quantized nature of the vibrational modes is observed in pump-probe spectra
particularly in weakly interacting electron-vibration systems such as cyanine
dye molecules. Calculated results based on a harmonic potential model and
molecular orbital calculations are compared with experimental results, and we
also found that the materials parameters regarding with the geometrical
structure of potential energy surfaces are directly determined by accurate
measurement of time-resolved spectra.
