CLOCKING TRANSIENT-CHEMICAL CHANGES BY ULTRAFAST ELECTRON-DIFFRACTION

With the advent of femtosecond (fs) time resolution in spectroscopic e xperiments, it is now possible to study the evolution of nuclear motio ns in chemical and photobiochemical reactions. In general, the reactio n is clocked by an initial fs laser pulse (which establishes a zero of time) and the dynamics are probed by a second fs pulse; the detection methods include conventional and photoelectron spectroscopy and mass spectrometry(1-4). Replacing the probe laser with electron pulses offe rs a means for imaging ultrafast structural changes with diffraction t echnique(5-8), which should permit the study of molecular systems of g reater complexity (such as biomolecules). On such timescales, observat ion of chemical changes using electron scattering is non-trivial, beca use space-charge effects broaden the electron pulse width and because temporal overlap of the (clocking) photon pulse and the (probe) electr on pulse must be established. Here we report the detection of transien t chemical change during molecular dissociation using ultrafast electr on diffraction. We are able to detect a change in the scattered electr on beam with the zero of time established unambiguously and the timing of the changes docked in situ. This ability to clock changes in scatt ering is essential to studies of the dynamics of molecular structures.