Ultrafast coherent excitation and probing of molecular vibrations with isotopic substructure

Adjacent vibrational modes of different isotope species are coherently excited by an intense ultrashort light pulse. The free decay of the vibrational system is observed by coherent probe scattering of a second delayed interrogating pulse. Coherent superposition of the excited vibrational states gives rise to a beating effect of the collectively vibrating isotope components. The phenomenon is observed for the first time in several tetrahalides on a time scale of several 10^-12 sec. The experimental information obtained by the probing technique depends on the wave-vector geometry of the material excitation and the interrogating pulse: One individual vibrational component is studied under a selective →k condition while coherent superposition of the vibrational components is observed for nonselective →k matching. Our technique provides new information on the vibrational system which is not obtained by other experimental methods.