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Quasi-Elastic Neutron Scattering
Niina Jalarvo
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OverviewINTRO to QENS
QENS Instruments
QENS Theory
Science Examples
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What is QENS
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Quasi Elastic Neutron ScatteringQENS
β’ Quasi elastic neutron scattering is a limiting case of inelastic neutron scattering
β’ Doppler type of broadening of the elastic line due to a small energy transfer between the neutrons and the atoms in the sample
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Neutron-Material Interaction β’ Cross section (Ο) β Area related to the
probability that a neutron will interact with a nucleus in a particular way (e.g. scattering or absorption)
β’ Light element sensitivity in presence of heavy elements
β’ Systems containing a reasonable proportion of H atoms, scattering from H tends to dominate
β’ Isotopic sensitivity β’ H-D contrast, H large incoherent cross-section
β’ Use of deuteration/selective deuteration to suppress incoherent scattering
β’ Thermal neutron wavelengths (few Γ βs) are comparable to interatomic and intermolecular distances
β’ Thermal neutron energies (few meVβs) comparable to energies of excitation in materials
=> vibrations, librations, reorientations, diffusion, and relaxational processes can be observed
total
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What is QENS used for
Probes slow dynamics β’ Translational diffusionβ’ Molecular reorientationsβ’ Relaxation processes
Applicable to wide range of scientific topicsβ’ Materials science: fuel cells,
batteries, hydrogen storage, β’ Soft Matter: polymer
nanocomposites and blends, organic photovoltaics, polymer electrolytes
β’ Biology: hydration water, dynamics of proteins
β’ Chemistry: water interfaces, ionic liquids, clays, porous media, complex fluids, surface interactions
Results are comparable to Molecular Dynamics simulations
Nature Communications 6, 7124 (2015)
Journal of Physical Chemistry C, 123, 2019 (2019).
Soft Matter, 7, 12, 5745-5755 (2011)
QENS spectra
Elastic intensity
Quasielastic intensity
Quasielastic broadening
Debye-Waller factor: Vibrational amplitudes
A0 = EISF (ratio elastic/total): Geometry of motion
Width: Characteristic time scale / diffusion
Dynamic scattering function provides information on the sample states
QENS Instruments
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QENS Instruments
Backscattering Spectrometers
β’ High energy resolutionβ Resolution determined by the
instrument (final energy of neutrons fixed)
β Access to slower dynamics on nanosecond to picosecond time scale
β Dynamic range limited
Time-of-FligthSpectrometers
β’ Lower energy resolutionβ Resolution can be varied by
changing the energy of incoming neutrons
β Access to dynamics on picosecond time scale
β Larger dynamic range accessible
Currently about 20 QENS spectrometers in the world (approximately 4 in the U. S., other locations include Germany, France, Switzerland, Japan and Australia)
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QENS and Neutron Scattering Instruments
β’ Dynamics in sample measurable β Length scales set by Q range (depends on neutron Ξ»)
β’ Q = 2Ο/dβ’ 0.1 Γ -1 < Q < 4 Γ -1 β 60 Γ > d > 1.6 Γ
β Time scales set by the elastic energy resolutionβ’ higher resolution β longer times/slower motion (ns time scales accessible)β’ lower resolution β shorter times/faster motion (ps time scales accessible)
β’ interchangeβ dynamic range / resolution / count rateβ Neutron Ξ» vs Q
β’ large Ξ» β> high resolution -> long times/slow motionsβ’ large Ξ» β> limited Q-range, limited length scales
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The SNS Inelastic Instrument Suite
Momentum Distributions
Itinerant Magnets
Crystal Fields
Molecular VibrationsLattice and Spin Excitations
Small Molecule Diffusion
Large Scale MotionsPolymers and Biological Systems
Tunneling Spectroscopy
Electron-Phonon Interactions
Hydrogen Modes
Molecular Reorientation
Ultracold Neutrons Fundamental Physics
Slower Motions
Larger Objects
Glassy Dynamics
ARCS Fermi Chopper
SEQUOIA Fermi Chopper
HYSPEC
Cold Neutron Chopper Spectrometer
Backscattering
Neutron Spin Echo
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BASIS backscattering
spectrometer at SNS
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BASIS
SNS
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BAckscattering SIlicon Spectrometer is a high-energy resolution, wide-dynamic range inverted geometry neutron spectrometer built on BL2 and facing a decoupled supercritical hydrogen, centerline-poisoned moderator
β’ Incident Flight Path - 84 m moderator-sample positionβ Curved Guide: 10 cm wide x 12 cm
tall, 1000 m radius of curvature, line-of-sight at 31 m
β Straight Guide: 10 cm wide x 12 cm tall
β Converging Funnel: last 7.7 m; exit 3.25 cm x 3.25 cm, stops 27.5 cm from sample
β’ Chopper System - 3 bandwidth/frame overlap choppers at 7, 9.25 and 50 m- Operation at 60 (standard), 30, 20, 15, 12, or 10 Hz- Bandwidth (full choppers transmission) of about 0.5 Γ at 60 Hz
steel, heavy concrete, concrete shielding
steel shieldingcurved guide
3rd chopper and secondary shutter
final evacuated flight path
BASIS overview
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β’ Radial Collimator βrestricts analyzer view of the sample
β’ Final Evacuated Flight Path - 2.5 m sample -analyzer, βΌ 2.23 m analyzer β detector
β’ Detector Choice βLPSD 3He tubes
Instrument Specifications
Si111 Si311Elastic energy 2.08 meV 7.63 meV
Bandwidth60 Hz30 Hz
Β±100 Β΅eV Β±200 Β΅eV
Β±660 ΞΌeVΒ±1700 ΞΌeV
Elastic resolution (HWHM)
3.6 Β΅eV 15 ΞΌeV
Q range (elastic) 0.2 Γ -1 < Q < 2.0 Γ -1 0.4 Γ -1 < Q < 3.8 Γ -1
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β’ Typical sample holders: annular cylindrical or flat plate
β Keep sample thin to avoid multiple scattering effects
β Transmission probability ~ 95 %
Sample
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β’ Energy resolution (Q-averaged): 3.6 Β΅eV (FWHM)
β’ Signal-to-background ratio (at the elastic line): better than 1000:1
β’ Dynamic range: variable (affects counting statistics, but not the energy resolution)
Vanadium Standard
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1 Hz signal (no frame overlap), 1/v efficiency corrections applied
β’ Incident band (full chopper opening): (60/Ξ½)*[0.5 Γ ], where Ξ½ = 60, 30, 20, 15, 12, 10 Hz
β’ Inelastic resolution: Ξ΄E β 0.001*Ei at high energy transfers
Incident spectrum and (Q,Ο) coverage
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CNCS Cold Neutron Chopper
Spectrometer
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β’ direct-geometry, multi-chopper inelastic / QENS spectrometer designed to provide flexibility in the choice of energy resolution
β’ best at low incident energies (2 to 50 meV).
β’ typical experiments use energy resolutions between 10 and 500 Β΅eV.
Instrument Description
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Specifications
Source-sample distance 36.2 mSample-detector distance 3.5 mAngular coverage Horizontally:β50Β° β +140Β°
Vertically:Β±16Β°Energy resolution 10 β 500 Β΅eVIncident energy range 0.5 β 80 meVMomentum transfer range 0.05 β 10 Γβ1
QENS theory
Scattering Kinematics
Neutron scattering events are described by means of energy andmomentum transfer.
01
01
EEkkQ
β=β=
Ο
000
ββ =
ΟΟΟ
ELASTIC scattering
INELASTIC scattering
QUASIELASTICscattering
The collision of two objects (e.g. neutron and sample atom) can be described in terms of momentum and energy conservation.
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If the scattered neutron waves from the different nuclei have definite relative phases, they can interfere => COHERENT SCATTERING
If the scattered neutron waves from the different nuclei have RANDOM relative phases (no interference)=> INCOHERENT SCATTERING
STRUCTUREDYNAMICS
Incoherent vs Coherent Neutron ScatteringDifferent atoms and isotopes have different coherent and incoherent scattering cross sections
Element Οcoh (barns) Οinc (barns)
Hydrogen (H) 1.8 79.9
Deuterium (D) 5.6 2.0
Carbon (C) 5.6 0.001
Oxygen (O) 4.232 0
Deuterated sample to obtain structure and collective excitations.
Protonated sample to observe single particle dynamics (quasielastic) and for the inelastic spectrum to weighthydrogen vibrations.
Deuteration can help to suppress dynamics of particular groups
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QENS - Incoherent and Coherent Scattering
β’ Large proportion of QENS experiments focus on dynamics of hydrogenous samples
ππ ππ,ππ = ππππππππ ππ,ππ + πππππππ ππ,ππ
No information about structureDominated by H dynamics
Contaminates QENS signal (Bragg peaks)
β’ Coherent QENS signal observable e.g. oxide ion diffusionβ’ Mixed coherent and incoherent QENS signal e.g. lithium or sodium diffusion
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QENS diffusion models
β’ (TD) Translational Diffusion following Fickβs lawβ’ (CE) Chudley-Elliot -model, jump diffusion on a
latticeβ’ (SS) Singwi-SjΓΆlander -model, alternation
between oscillatory motion and directed motionβ’ (HR) Hall-Ross βmodel, jump diffusion within a
restricted volume
Spatially restricted diffusionβ’ Jumps between 2, 3, β¦ n sitesβ’ Rotational diffusion on a circleβ’ Diffusion on a sphereβ’ Diffusion inside a sphere, cylinder
Angular dependency gives access to fundamental processes
TDCE
HR
SS
QENS - Science examples
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Science Example 1: Molecular Reorientation
β’ Polyoligosilsesquioxane Ligand Dynamics
β’ How do ligand dynamics contribute to the functionalities of POSS?
Jalarvo N., et al, Journal of Physical Chemistry C, 118, 10, 5579-5592 (2014)
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Science Example 1: Molecular Reorientation
0.3 Γ -1 1.9 Γ -1
300 KOctamethyl POSS @ BASIS
Jalarvo N., et al, Journal of Physical Chemistry C, 118, 10, 5579-5592 (2014)
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πΈπΈπΈπΈπππΈπΈ =ππππππππππππππ ππππππππππππππππππ
ππππππππππππππ ππππππππππππππππππ + ππππππππππππππππππππππππ ππππππππππππππππππ
Jalarvo N., et al, Journal of Physical Chemistry C, 118, 10, 5579-5592 (2014)
Science Example 1: Molecular Reorientation
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Science Example 1: Molecular Reorientation BASIS
= 53 meV
-> V3 = 74.4 meV (7.18 kJ/mol)
Barrier for methyl rotation
Jalarvo N., et al, Journal of Physical Chemistry C, 118, 10, 5579-5592 (2014)
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Science example 2:Elucidate Li+ vs H+ ion diffusion
H+ ions were found to be immobile while Li+ ion maintained a desired mobility in the solid electrolyte (Li6.25-xHxAl0.25)La3Zr2O12
Pristine LLZO
Hx-LLZO
Xiaoming Liu, et al, Energy & Environmental Science, 12, 3, 945-951 (2019).
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10
20
30
40
300 K350 K
400 K450 K
HWHM
(ueV
)
(a) 500 K
0.00 0.25 0.50 0.75 1.00
10
20
30
40
300 K350 K400 K450 K
HWHM
(Β΅eV
)
Q2(A-2)
(b) 500 K
LLZO
Hx-LLZO
Science example 2:Elucidate Li+ vs H+ ion diffusion
H+ ions are found to be immobile while Li+ ions maintain mobility in H-LLZO at theoperating temperature range of ALBs.
2.0 2.2 2.4 2.6 2.8 3.0
-8.0
-7.6
-7.2
-6.8
Ea = 2.65 kJ/mol
Ea = 20.08 kJ/mol
Log 10
D (m
2 /s)
1000/T (K-1)
(d)
-8.0
-7.6
-7.2
-6.8
Ea = 13.06 kJ/mol
log 10
D (m
2 /s)
LLZO
Hx-LLZO
Xiaoming Liu, et al, Energy & Environmental Science, 12, 3, 945-951 (2019).
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Science Example 3:Water Dynamics in Anion Exchange Membranes Studied by QENS
Ionic conductivity and water transport are key properties for applications
Depends on water contentWater transport in AEM multiscale problem
Investigation of transport in multiple time- and length scales for structure βfunction insight
1 nm
J. Melchior, et al, Journal of Membrane Science 586, Pages 240-247 (2019)
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Science Example 3:Water Dynamics in Anion Exchange Membranes Studied by QENS
0 1 2
10
100
Rotational Dynamics
HW
HM
/ Β΅e
V
Q / Γ -1
jump diffusion
D
ππππππππ
ππππ
resolution
confinement
Rotational Diffusion
-800 -600 -400 -200 0 200 400 600 8000.001
0.01
0.1
1
10
I / a
.u.
energy transfer / Β΅eV
Ξ»water =15TofTof Ξ» = 5 Γ Q = 0.75 Γ -1
ππ ππ,ππ = ππ ππ0(ππ)πΏπΏ + οΏ½ππ
)ππππ(ππππ
Ξππ2 + Ξ2
β π π ππππ + π΅π΅πππππ΅π΅π΅π΅
Ξ~1
ππππ1 β ππβππ
2π·π·ππππ
TOFTOF: time-of-flight spectrometer
J. Melchior, et al, Journal of Membrane Science 586, Pages 240-247 (2019)
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Science Example 3:Water Dynamics in Anion Exchange Membranes Studied by QENS
J. Melchior, et al, Journal of Membrane Science 586, Pages 240-247 (2019)
-20 -15 -10 -5 0 5 10 15 201E-4
0.001
0.01
0.1
elastic Lorentzian lin. background tot. fit
I / a
.u.
energy transfer / Β΅eV
SPHERES: high-resolution backscattering spectrometer
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80
1
2
3
4
5
6
7
Ξ»water = 4
Ξ»water = 4.8
HW
HM
/ Β΅e
V
Q / Γ -1
Ξ»water = 11
Ξ~1
ππππ1 β ππβππ
2π·π·ππππ
2 4 6 8 10 12 14 16
10-6
10-5
D /
cm2 s-1
Ξ»water = [H2O] / [funct. group]
SPHERES
PFG-NMR
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Science Example 3:Water Dynamics in Anion Exchange Membranes Studied by QENS
10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10010-11
10-10
10-9
T = 293 K
FAA-3 OH-
FAA-3 F-TOFTOF
BASIS
BASIS
SPHERES
D /
m2 s-1
Ο / s
PFG-NMR
Ξ» = 5
J. Melchior, et al, Journal of Membrane Science 586, Pages 240-247 (2019)
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Summary:
β’ QENS is an unique technique to measure diffusive dynamics providing exclusive information about the geometry of the diffusionβ accessible through Q-dependenceβ Large range of time scales available depending of the selected QENS
instrument ( sub-picosecond < t < nanosecond (Β΅sec for NSE)β Hydrogen sensitivity
β’ Instrument selection is a critical decision β resolution to match the time scale of the diffusion processβ Q range to match the diffusion length scale
β’ Suitable technique to study dynamics in large variety of materials and science problems.
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β’ Questions?β’ Literature:
β Quasielastic Neutron Scattering, M. Bee (Bristol, Adam Hilger, 1988).
β Quasielastic Neutron Scattering and Solid State Diffusion, R. Hempelmann (2000).