Coherent dynamics of the localized vibrational modes of hydrogen in CaF2

We report the observation of giant quantum coherence effects in the localiz ed modes of ionized hydrogen in synthetic fluorite. Infrared free induction decay experiments on the substitutional H+ center thew dramatic modulation s at negative delay times due to interference between multiple vibrational levels. Spectrally resolving the degenerate four wave mixing signal allows unambiguous assignments of the participating vibrational states. The depend ence of the signal intensity upon the delay path between the exciting free electron laser pulses can be accounted for in terms of the resonant third o rder polarization with a common dephasing rime for the excited states.