Stochastic dynamics in molten potassium explored by polarized quasielastic neutron scattering

Abstract

The coherent S c ͑Q , ͒ and single-particle S s ͑Q , ͒ dynamic structure factors which contribute to the lowenergy spectrum of molten potassium are separated by means of neutron polarization analysis. The linewidth and amplitude of the single-particle spectra follow an apparent sub-Fickian behavior, with a diffusion coefficient below the value found by macroscopic means. Once this is accounted for, the results are found to conform with predictions made from kinetic theory. Also, the available theoretical recipes to account for the coherent quasielastic intensity are seen to reproduce the experimental observations within a region of momentum transfers located about the location where the static structure factor shows its main peak. From the measured data as well as from computer simulation results which are validated by comparison to measurement, a clear separation of time scales involving rapid rattling motions within the liquid cages from diffusional processes involving collective or single-particle rearrangements is established.

Cation Dynamics in the Pyridinium Based Ionic Liquid 1-N-Butylpyridinium Bis((trifluoromethyl)sulfonyl) As Seen by Quasielastic Neutron Scattering

“…The observed effect was first predicted theoretically by de Gennes . This effect has been observed experimentally measuring the transport properties of H 2 and D 2 in NaX zeolites and in investigations on the stochastic dynamics in molten potassium using polarized QENS …”

“…48 This effect has been observed experimentally measuring the transport properties of H 2 and D 2 in NaX zeolites 49 and in investigations on the stochastic dynamics in molten potassium using polarized QENS. 50 The most interesting finding, in this context, is the fact that the fast dynamic process remains unchanged by the static structure factor, as can be seen by inspection of Figure 7, showing the line widths Γ 1 (Q) that characterize the fast dynamic process. For both the completely protonated and the completely deuterated ILs, we observe the same Q dependence (within the experimental errors) of these line widths.…”

Boiling Temperature As a Scaling Parameter for the Microscopic Relaxation Dynamics in Molecular Liquids

“…On the timescale experimentally accessible by QENS (about a picosecond and longer), the structural correlations decay first to a finite value, through the secondary relaxation process, and then to zero through the main relaxation. With few exceptions, the experimental neutron scattering studies of supercooled liquids tend to analyze the relaxations near the structural maximum of the static structure factor, S ( Q ). − Somewhat surprisingly, the same applies to most molecular dynamics (MD) simulations, − which, unlike experimental studies, are not constrained by the factors such as the predominantly coherent neutron scattering cross sections of the elements involved or the advantage of coherent scattering signal for spin–echo measurements. Only recently have there been MD , and QENS ,− studies that analyze the two-component relaxations, through the secondary and then main processes at relatively low Q , far below the structural maximum of the S(Q) yet above the values dominated by the small-angle coherent scattering.…”

Identifying the Spin-Incoherent Contribution to Quasielastic Neutron Scattering with a Cold Triple-Axis Spectrometer

“…With cold neutrons, triple-axis spectrometers can investigate physical phenomena with high energy and momentum resolution. Previous studies, such as that undertaken at the Institut Laue-Langevin, have taken advantage of the ability of a polarised cold triple-axis spectrometer to separate coherent and single-particle scattering for molten potassium in the QENS regime [11]. The cold-neutron triple-axis spectrometer Sika at the Australian Centre for Neutron Scattering (ACNS) excels in measuring well-defined regions of S(Q, ω) with a very low background, allowing for parametric studies (e.g., varying temperature or magnetic field) to be conducted efficiently in both inelastic and elastic scattering experiments [12,13].…”