Revisiting the modeling of quasielastic neutron scattering from bulk water

¹ Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark
² STFC ISIS Facility Rutherford Appleton Laboratory, Harwell Campus, Oxfordshire, OX11 0QX, UK
³ Centre de Biophysique Moléculaire, CNRS and Université d’Orléans, Rue Charles Sadron, 45071 Orléans, France
⁴ Synchrotron Soleil, L’Orme de Merisiers, 91192 Gif-sur-Yvette, France
⁵ European Spallation Source ERIC Box 176, SE-221 00, Lund, Sweden

^* e-mail: martinhp27@gmail.com
^** e-mail: mark.telling@stfc.ac.uk
^*** e-mail: gerald.kneller@cnrs.fr
^**** e-mail: bordallo@nbi.ku.dk

Published online: 17 November 2022

Abstract

Quasi-elastic neutron scattering (QENS) from bulk-water at 300 K, measured on the IRIS backscattering neutron spectrometer (ISIS, UK), is interpreted using the jump diffusion model (JDM), a “minimalistic” multi-timescale relaxation model (MRM) and molecular dynamics simulations (MD). In the case of MRM data analysis is performed in the time domain, where the relaxation of the intermediate scattering function is described by a stretched Mittag-Leffler function, E_α(−(|t|/τ)^α). This function displays an asymptotic power law decay and contains the exponential relaxation function as a special case (α = 1). To further compare the two approaches, MD simulations of bulk water were performed using the SPCE force field and the resulting MD trajectories analysed using the nMoldyn software. We show that both JDM and MRM accurately describe the diffusion of bulk water observed by QENS at all length scales, and confirm that MD simulations do not fully describe the quantum effects of jump diffusion.