SPring-8, the large synchrotron radiation facility

Date&Time: 4th August 2003, 13:30-15:00

Speaker: Prof. Dietrich Menzel
Affiliation: Technische Universitat Munchen

Abstract
　　Charge transfer (CT) between an adsorbed atom or molecule and its substrate is of direct importance for the understanding of photochemical surface processes and more generally of the adsorbate-substrate coupling. A direct measurement of its extremely short timescale (of order of a few fs or even less) is very difficult, even for present-day laser techniques. It can be accomplished by measuring core excitation and decay spectra under narrow-band conditions - widths below the core hole lifetime width; so-called Auger resonant Raman conditions - using the core hole lifetime as an internal time standard. Tuning through the resonance allows one to distinguish those parts in the decay spectra which correspond to decay before, vs. after, transfer of the locally excited electron from the adsorbate into the substrate. The ratio of intensities of these two types of spectra gives directly the ratio of the CT time and the core hole decay time. For adsorbate resonances between the Fermi and the vacuum level of the substrate, values from below 1 fs (strong chemisorption), via the range of some fs to some 10 fs (physisorption with graded coupling), to above 50 fs (decoupled condensate) have been found; their dependence on the actual excitation energy can also be determined. Screening of suddenly created charge in an adsorbate is even faster.
I shall give a survey of these processes and results and discuss their interpretation.