Synchrotron Radiation as a Source to Study Time-Dependent Phenomena

Abstract

A number of important preliminary measurements have been carried out by groups at Orsay, Stanford and Hamburg and the scientific benefits of time-domain measurements using synchrotron radiation have been discussed in several recent review articles.1–7 These works have revealed that time-resolved excitation and emission measurements can be carried out routinely with a resolution of at least 1 ns or better. For example, measurements have been made on the fluorescence decay of individual vibronic levels of small molecules, on emission from low quantum yield gases (∿10−4) at low pressure (∿ 1 torr) and on quenching mechanisms in rigid solutions. Pure rare gas solids and also mixtures have been studied extensively in an attempt to understand the kinetic processes associated with exciton formation. Also for the first time, quantum coherence effects have been seen in rare gases. Oriented atomic states can be produced by photoselection using polarised light for excitation and periodic fluorescence modulation can be seen in the presence of an external applied field. Analysis of these data yields lifetimes, spins, multipolarities and “g” factors — even when the substates are unresolved in energy. More recently, photoselection of an individual residue within a protein with pulsed, polarised light has led to an understanding of the microenvironment within the large molecule and has enabled initial estimates to be made of the flexibility of such large structures as well as of their overall size and shape.