Ultrafast Electron Diffraction. 5. Experimental Time Resolution and Applications
This paper, the fifth in a series, is concerned with the experimental description of ultrafast electron diffraction and its application to several isolated chemical systems. We present a detailed description of the Caltech apparatus, which consists of a femtosecond laser system, a picosecond electron gun, and a two-dimensional charge-coupled device ( CCD) detection system. We also discuss the analysis of the scattering patterns. Ultrafast diffraction images from several molecules (CCl_4, I_2, CF_3l, C_2F_4I_2) are reported. For our first study of a chemical reaction in a molecular beam, we show the change in the radial distribution function following the formation of CF_3 radical after dissociation of CF_3l. The total experimental temporal resolution is discussed in terms of the electron pulse width and velocity mismatch. The electron pulse was characterized temporally with a streaking technique that yielded the width as a function of the number of electrons per pulse. Experimental results show that the electron source produces picosecond (or less) pulses at densities of 100 electrons per pulse and 10-ps pulses at 1000 electrons per pulse. We also report our observation of a novel photoionization-induced lensing effect on the undiffracted electron beam, which we have used to establish time zero for UED when reactions are initiated by a laser pulse.
© 1994 American Chemical Society. Received: January 13, 1994. Abstract published in Advance ACS Abstracts, March 1, 1994. This work was also presented at the 1993 Pacific Conference on Chemistry and Spectroscopy, Pasadena, CA, U.S.A., October 1993. This work was supported by a grant from the U.S. Air Force Office of Scientific Research and the National Science Foundation. The authors extend their appreciation to a number of individuals who contributed to the success of this undertaking over the past several years: Jim Janesick and Tom Elliot for their assistance with the CCD chips; Dr. H. Elsayed-Ali for his generous help and time in the initial design; Dr. John Ewbank for his suggestions and advice regarding gas-phase electron diffraction; and Dr. V. Schomaker for his interest in this project. J.C.W. thanks the National Science Foundation for providing a Graduate Student Fellowship. National Science Foundation Pre-Doctoral Fellow. Contribution No. 8917.