Coherent vibrational dynamics of intermolecular hydrogen bonds in acetic acid dimers studied by ultrafast mid-infrared spectroscopy

Ultrafast vibrational motions and the underlying microscopic couplings in hydrogen-bonded cyclic dimers. of acetic acid are studied by pump-probe and photon echo experiments in the mid-infrared. Upon feratosecond excitation of the O-H stretching mode, we demonstrate coherent nuclear motions along the in-plane dimer stretching mode which persist for picoseconds. The anharmonic coupling of the O-H stretching and the low-frequency mode is isolated in the nonlinear vibrational response, whereas other couplings are suppressed. Three-pulse photon echo experiments demonstrate a dephasing of O-H stretching excitations on a femtosecond timescale and quantum beats due to the anharmonically coupled dimer stretching mode.