Centering of ultrafast time-resolved pump-probe electron diffraction patterns

Pump-probe electron diffraction is an exciting new technique to probe the structural dynamics of gas-phase molecules in transient states. A difficult aspect of this experiment is that pump-probe electron diffraction patterns are sensitive toward minute pointing instabilities of the electron beam. Displacements of the beam in the range of microradians, depending on the electron beam energy, can cause artifacts in the pump probe difference patterns that are larger than the desired pump-probe diffraction signature. We describe a solution to the problem, which uses a novel centering algorithm that takes advantage of the inherent symmetry of the patterns and that acts only on the molecular component of the diffraction signal. The implementation of the algorithm is illustrated using the pump-probe diffraction signature obtained in the ultrafast time-resolved study of the ring-opening reaction of 1,3-cyclohexadiene to form 1,3,5-hexatriene. (C) 2004 Elsevier B.V. All rights reserved.