Accelerators for ultrafast phenomena

Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973,U. S. A. 

The purpose of this chapter is to describe the technology that enables the investigation of radiation chemical phenomena at picosecond and femtosecond timescales. Several research groups have used femtosecond laser photoionization techniques to examine electron solvation dynamics and other processes relevant to early radiation chemistry events [1,2]. This chapter will focus on ultrafast studies using ionizing radiation, primarily electron beams, as the excitation source. 

Empirical measurements with near-Gaussian optical and electron pulses at 800 nm with 8.5 MeV electrons indicate that the FWHM response function broadening increases by 700 fs for every millimeter of travel through water [3]. Therefore, time resolution is ultimately limited by sample depth, the choice of which is affected by considerations such as detection sensitivity, signal strength and the ability of the sample to tolerate signal averaging. Because of this limitation, ultrafast in the context of accelerators refers to timescales from a few hundred femtoseconds to tens of picoseconds. 

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