letters to nature 312 NATURE | VOL 398 | 25 MARCH 1999 | www.nature.comComparing Figs 2a, b and 3, we see that the initially thin but highly strained layer of lattice expansion produces the broad, lowintensity lines at early time delays. As the strain wave propagates away from the surface, the layer of surface expansion thickens and these lines correspondingly become narrower and more intense, but diffract at smaller angular deviations due to the weaker average strain. In parallel, the compression wave leading the propagation into the bulk contributes to the slight shift to higher angles seen in the main Ka lines. At the longest delays, the strain wave has largely moved beyond the depths probed by the X-ray pulse, and we see diffraction lines broadened and shifted slightly to lower angles due to the exponential surface strain of the relaxed lattice. The strain for these late times is simply proportional to the temperature distribution. Finally, the theoretical angle-integrated diffraction signal similarly reproduces the monotonic increase and plateau behaviour seen in the experimental data.Additionally, we performed an iterative genetic-algorithm inversion, obtaining the strain from the measured data. The angle-and time-resolved diffraction curves corresponding to the retrieved strain are shown in Fig. 2c. Spectral and geometrical broadening of the diffraction lines signi®cantly limits the uniqueness of the result. Nevertheless, we obtain qualitatively similar strain behaviour to that shown in Fig. 3. We retrieve an exponential surface strain, with 8-mA Ê peak strain, and a unipolar, 3-mA Ê strain pulse which propagates into the bulk at several thousand metres per second. Although the physical model that we used is fully consistent with the measured data (to within our experimental resolution), the spectral and geometrical broadening mentioned above mean that it is not a unique interpretation of that data.This work, using a table-top laboratory apparatus, demonstrates with a simple crystalline material the direct observation of millia Êngstro Èm atomic motion on the picosecond timescale. Further developments of the technique should permit direct observation by ultrafast X-ray diffraction of the ultrafast atomic motions accompanying a wide variety of physical, chemical, and perhaps biological processes.M

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The electrochemical doping of polypyrrole has been studied using simultaneous a.c. impedance and in-situ electrochemical e.s.r. measurements. A quantitative estimate of the polypyrrole conductivity as a function of doping level has been obtained and the corresponding change in the polaron concentration measured by in-situ electrochemical e.s.r. It was found that at low doping levels there was a good correlation between these two properties. It is suggested that the polaron may be responsible for the conduction process under such conditions. At higher doping levels conduction would appear to involve the bipolaron.

Theory is presented for the shape of current-voltage curves measured at a rotating-disc electrode for the cases where the substrate reacts via either an ECE or a DISPl process in which the kinetic step is sufficiently fast to allow the use of the reaction-layer concept in the theoretical treatment. When the electrode potential for the electrode reaction can be found under conditions where there are no complications of coupled kinetics, analysis of the shift in half-wave potential with disc rotation speed allows the deduction of the rate constant for the kinetics of the homogeneous chemical process involved in each mechanism. If an independent measurement of this rate constant is available (e.g. from spectroelectrochemistry) then discrimination between ECE and DISPl pathways is possible. Experiments involving the reduction of fluorescein in aqueous carbonate/bicarbonate buffers were found to be in good agreement with theory. Furthermore, comparison of the kinetic data with transient electrochemical e.s.r. data revealed the reaction mechanism to be DISPl, in agreement with previous observations.

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