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 (CCl4, I2, CF3I, C2F4I2) 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 CF3 radical after dissociation of CF3I. 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.