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 fluctua tions. Measurements of these fluctuations by diffraction techniques ha ve recently been performed for studying the dynamics of phase transiti ons at surfaces using various diffraction techniques. For this case, t he characteristic time constant of the probed system is conveniently e stimated by calculating the intensity-fluctuation autocorrelation func tion for a wave vector that is sensitive to the physical process under investigation. The analysis is carried out in terms of a separation d istribution of events so that it can be widely adapted for many physic al processes. Both the correlation in time of the primary train of eve nts (which yields the true correlation time constant), and the correla tion modified by using a finite gate time are studied. It is shown tha t it is in practice nontrivial to choose a gate time to measure the tr ue time constant and to avoid integrating out the high frequency spect ral content of the process. (C) 1998 Elsevier Science B.V. All rights reserved.