Intensity fluctuations from surfaces and the assessment of time constants
This paper describes the effect of the gate time on a correlated train of “events”, which is typical, for example, for equilibrium fluctuations. Measurements of these fluctuations by diffraction techniques have recently been performed for studying the dynamics of phase transitions at surfaces using various diffraction techniques. For this case, the characteristic time constant of the probed system is conveniently estimated by calculating the intensity-fluctuation autocorrelation function for a wave vector that is sensitive to the physical process under investigation. The analysis is carried out in terms of a separation distribution of events so that it can be widely adapted for many physical processes. Both the correlation in time of the primary train of events (which yields the true correlation time constant), and the correlation modified by using a finite gate time are studied. It is shown that it is in practice non-trivial to choose a gate time to measure the true time constant and to avoid integrating out the high frequency spectral content of the process.
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