An Integrating Method of Time-Resolved EXAFS Measurement

Abstract

One of the advantages of using EXAFS as a structure probe is its high event rate. With a beam of 10¹² photons-s⁻¹ −ev⁻¹, it is feasible to measure microsecond- or submicrosecond-resolved transmission EXAFS spectra. However₇ one would need integrating detectors for such measurements. Even in the fluorescence mode, dilute biological samples often yield strong enough fluorescence that an integrating detector is needed in order to take the full advantage of intense synchrotron radiation (SR). For example a SR pulse of 10⁶ photons (of 7.2 Kev) incident on a haemoglobin (Hb) solution of 0.5 mM concentration in Fe yields (1/3)x10³ Fe K-shell fluorescence photons. A detector subtending 5% of 4π at the sample (which is easily achievable with a 1” detector) would intercept about 20 photons. Since these photons are practically inseparable in time, the conventional counting method would have to sacrifice a factor of 20 in the detecting solid angle in order to measure the variations in absorption. As the SR laboratories continue to develop more powerful sources, the need for integrating detectors will undoubtedly intensify. In this paper I will describe an integrating method for measuring 100 μs-resolved EXAFS of photolyzed haemoproteins [1].

This work was supported in part by the Office of Naval Research Contract No. N00014-76-C-0273 and the Robert A. Welch Foundation. The work performed at SSRL was also supported by the Department of Energy, Office of Basic Energy Sciences; the National Science Foundation, Division of Materials Research; and the National Institutes of Health, Biotechnology Resource Program, Division of Research Resources.