Time resolved spectroscopy using synchrotron radiation

Abstract

Measurements have been carried out in the time domain by groups in Orsay, Stanford and Hamburg and the scientific significance of such experiments using synchrotron radiation has been considered in recent review articles [1–7]. At present, time resolved excitation and emission studies can be carried out routinely with a resolution of at least 1 ns, and measurements have been made on the fluorescence decay of individual vibronic levels of small molecules, on emission from low quantum yield vapours (∼10 −4) at low pressure (∼1 Torr) and on quenching mechanisms in rigid matrices. Pure rare gas solids and 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. Using polarised light for excitation, orientated atomic states can be produced by photoselection and periodic fluorescence modulation can then be seen in the presence of an external applied field. The data give information about lifetimes, spins, multipolarities and “ g” factors — even when the substates are unresolved in energy. Photoselection of an individual residue within a protein with pulsed, polarised light can, and 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. The best time resolution obtained so far using standard photon counting techniques is about ±50 ps and has been limited by the time response of the photodetector used. The narrow pulse width and the associated broad frequency spectrum (extending beyond 1 GHz) from storage ring sources has led to the study of phase shift and modulation techniques at a number of different frequencies. Preliminary results have revealed that in the frequency domain times of the order of a few ps can be measured. It is highly probable that, in the future, time domain measurements will be improved to give a resolution of about 1 ps and will be extended to include excitation over an extensive range of wavelengths in the VUV and X-ray regions. In this paper, a description of the actual time modulation of synchrotron radiation is followed by a comparison between several subnanosecond timing techniques which have been used at synchrotron radiation sources. The broad range of possible applications of these methods is illustrated by some slected examples. Finally a discussion is included on the prospects for reducing the bunch length into the picosecond region in future storage rings.