Vibrational dynamics in condensed matter probed with linac based FELs

The super-conducting linac pumped free electron laser (FEL) at Stanford has been used to study the vibrational dynamics of molecules in liquids and glasses. A variety of experiments have been performed, including the worldʹs first vibrational photon echo experiments in liquids and glasses and polarization selective pump-probe experiments. In this paper vibrational photon echo experiments are described and characteristics of the FEL important to these experiments are discuss. Vibrational photon echo experiments were performed on the asymmetric CO stretching mode of tungsten hexacarbonyl (1976 cm−1) in the organic glass dibutylphthalate as a function of temperature using sub-picosecond infrared pulses. The echo decays display pronounced beats and are bi-exponential. The beats and bi-exponential decays arise because the bandwidth of the transform limited FEL pulses exceeds the vibrational anharmonicity, leading to the excitation and dephasing of a multilevel coherence. From the beat frequency, the anharmonicity is determined to be 14.7 cm−1. From the bi-exponential decay components, the temperature-dependent homogeneous vibrational dephasing rates of both the v = 0 → 1 and v = 1 → 2 transitions are determined.