ELSEVIER Physica B 228 (1996) 312-318 Coherent effects and relaxation processes in liquid potassium Arkady G. Novikov a, Vadim V. Savostin a'*, Alexander L. Shimkevich a, Renat M. Yulmetyev b, Timur R. Yulmetyev b " State Scientific Centre, Institute of Physics and Power Engineering, Bondarenko Square 1, 249020 Obninsk, Kaluga Region, Russian Federation b Department of Theoretical Physics, Kazan State Pedagogical University, 420021 Kazan, Russian Federation Received 23 June 1995; revised 4 April 1996 Abstract The coherent dynamic structure factor of liquid potassium has been obtained from inelastic neutron scattering data at temperatures of 340, 440 and 550 K. The parts of dispersion curves for collective excitations have been plotted and some of their characteristics have been analysed. Represented in relative units, our experimental points are in an agreement with the ones for liquid rubidium and cesium. The molecular memory effects are described within a framework of theoretical representations of a spatial dispersion for the relaxation parameter of non-Markovian process. It has been found that molecular memory effects are important for relaxation processes which are represented in inelastic both coherent and incoherent neutron scattering. Keywords: Liquid potassium; Dispersion curve; Non-Markovian process 1. Introduction For accessible liquid alkali metals, potassium is only the one which was not investigated practically by inelastic neutron scattering method. With the exception of Ref. [1], we are not familiar with such experiments. One of the possible explanations of this fact consists in that potassium is a mixed scatterer so its scattering cross-section includes both coherent and incoherent components. In this case, the detail and corrected data processing re- quires the separation of these effects and their indi- vidual analysis, being rather difficult and time con- suming procedure. At the same time, the neutron dynamic experiment for mixed scatterers opens a chance to get information about both individual * Corresponding author. and collective atomic motions, i.e. leads to deeper understanding of its microdynamics. Performing the neutron inelastic scattering ex- periment for liquid potassium, we pursued the goal to analyse the coherent as well as incoherent com- ponents of its double-differential scattering cross- section (DDSCS). In [2], the frequency spectrum of the velocity autocorrelation function for liquid po- tassium have been extracted from inelastic incoher- ent component of DDSCS. In so doing, coherent effects were accounted for by the viscoelastic model [3]. Then in [4], we have separated quasi-elastic incoherent component of DDSCS and analysed that, using a number of simple diffusion models and the mode-coupling theory. The aim of present work consists in extraction of a coherent component from experimental DDSCS and its subsequent analysis, keeping in mind to get 0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S092 1-4526(96)00469- 3