Time-resolved spectroscopy in synchrotron radiation

Abstract

Synchrotron radiation (SR) from large-diameter storage rings has intrinsic time structure which facilitates time-resolved measurements from milliseconds to picoseconds and possibly below. The scientific importance of time-resolved measurements is steadily increasing as more and better techniques are discovered and applied to a wider variety of scientific problems. This paper presents a discussion of the importance of various parameters of the SR facility in providing for time-resolved spectroscopy experiments, including the role of beam-line optical design parameters. Special emphasis is placed on the requirements of extremely fast time-resolved experiments with which the effects of atomic vibrational or relaxational motion may be studied. Before discussing the state-of-the-art timing experiments, we review several types of time-resolved measurements which have now become routine: nanosecond-range fluorescence decay times, time-resolved emission and excitation spectroscopies, and various time-of-flight applications. These techniques all depend on a short SR pulse length and a long interpulse period, such as is provided by a large-diameter ring operating in a “single-bunch” mode. In most cases, the pulse shape and even the stability of the pulse shape is relatively unimportant as long as the pulse length is smaller than the risetime of the detection apparatus, typically 1 to 2 ns. For time resolution smaller than 1 ns, the requirements on the pulse shape become more stringent. Experiments requiring time resolution in the 10–100 ps range are conveniently done via observation of the phase shift of high harmonics of the storage-ring orbit frequency. Fast fluorescence decay time measurements and reflectance phase-change measurements have been done in this way. Because of the sensitivity of the harmonic amplitude to the pulse shape, these experiments require the SR pulse shape to stable. With the same phase-shift technique, measurements attaining time resolutions as small as 0.1 ps could be feasible if the SR pulse shape were made sufficiently stable. Observation of the shape of individual pulses of SR from the Stanford Positron Electron Accelerator Ring has been made under a variety of conditions using an Imacon 600 streak camera. These data are not completely analyzed at this time, but preliminary results are presented and discussed. Evidence is given of various modes of electron bunch-shape oscillation. These are apparently sensitive to beam current, kinetic energy, and accelerator cavity voltage.