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integral by measuring the intensity of the outgoing beam resolved into different spin states. The spin echo case is referred to as a type of 'tilted projection measurement'. Spin echo measurements are an appropriate tilted projection for quasi-elastic measurements of surface dynamics because the raw data are closely related to the
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Conventional 'spin echo' measurements are a common special case of the more general scattering-with-precession measurements, in which the incoming and outgoing magnetic field integrals are constrained to be equal. The polarization of the outgoing beam is measured as a function of the precession field
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HeSE has been used to study the diffusion rates and mechanisms of atoms and molecules ('adsorbates') at surfaces. A non-exhaustive list of the research themes associated with HeSE diffusion measurements include: nuclear quantum effects in the surface diffusion of atomic hydrogen; benchmarking the
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experiment. The raw data of the experiment are the spin-resolved scattered helium intensities as a function of the incoming magnetic field integral, outgoing field integral and any other variable parameters relevant to specific experiments, such as surface orientation and temperature. In the most
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Alexandrowicz, Gil; Kole, Pepijn R.; Lee, Everett Y.M.; Hedgeland, Holly; Ferrando, Riccardo; Jardine, Andrew P.; Allison, William; Ellis, John (May 6, 2008). "Prev. Article Next
Article Table of Contents Observation of Uncorrelated Microscopic Motion in a Strongly Interacting Adsorbate System".
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general kind of scattering-with-precession experiment, the data can be used to construct the 2D 'wavelength intensity matrix' for the surface scattering process, i.e. the probability that a helium atom of a certain incoming wavelength scatters into a state with a certain outgoing wavelength.
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beam is created by a supersonic expansion followed by a spin-filtering stage (polariser). The helium scatters from the experimental sample and is detected at the end of the beamline after another spin-filtering stage (analyser). Before and after the scattering process, the beam passes through
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Riley, D.; Jardine, A.P.; Dworski, S.; Alexandrowicz, G.; Fouquet, P.; Ellis, J.; Allison, W. (13 March 2007). "A refined He–LiF(001) potential from selective adsorption resonances measured with high-resolution helium spin-echo spectroscopy".
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Jardine, A.P.; Lee, E.Y.M.; Ward, D.J.; Alexandrowicz, G.; Hedgeland, H.; Allison, W.; Ellis, J.; Pollak, E. (24 September 2010). "Determination of the
Quantum Contribution to the Activated Motion of Hydrogen on a Metal Surface: H/Pt(111)".
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HeSE has been used to construct empirical helium-surface scattering potentials through the measurement of selective adsorption resonances (bound state resonances) on the clean LiF(001) surface and the hydrogenated Si(111) surface.
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The experimental principles of the HeSE experiment are analogous to those of neutron spin echo, differing in details such as the nature of the probe/sample interactions that give rise to scattering. In outline, a polarized
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in which the
Doppler-like energy gain and loss of the helium atoms is small compared to the beam energy. More strongly inelastic measurements can provide information about energy loss channels on the surface such as
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The surface processes that HeSE can measure can be broadly divided into elastic, quasielastic and inelastic processes. Measurements in which the predominant signal is elastically scattered include standard helium
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Alexandrowicz, G.; Jardine, A.P.; Hedgeland, H.; Allison, W.; Ellis, J. (10 October 2006). "Onset of 3D collective surface diffusion in the presence of lateral interactions: Na/Cu(001)".
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adsorbate/surface free energy landscape; energy exchange ('friction') between adsorbates and the surface; pairwise and many-body inter-adsorbate interactions.
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Kole, P.R.; Jardine, A.P.; Hedgeland, H.; Alexandrowicz, G. (2010). "Measuring surface phonons with a 3He spin echo spectrometer: a two-dimensional approach".
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Jardine, A.P.; Hedgeland, H.; Alexandrowicz, G.; Allison, W.; Ellis, J. (2009). "Helium-3 spin-echo: Principles and application to dynamics at surfaces".
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Tuddenham, F.E.; Hedgeland, H.; Knowling, J.; Jardine, A.P.; Maclaren, D.A.; Alexandrowicz, G.; Ellis, J.; Allison, W. (11 June 2009).
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Alexandrowicz, G.; Jardine, A.P. (2007). "Helium spin echo spectroscopy: studying surface dynamics with ultra-high-energy resolution".
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Hedgeland, H.; Kole, P.R.; Davies, H.R.; Jardine, A.P.; Alexandrowicz, G.; Allison, W.; Ellis, J.; Fratesi, G.; Brivio, G.P. (2009).
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31:. The information available from HeSE complements and extends that available from other inelastic scattering techniques such as
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337:"Surface dynamics and friction of K/Cu(001) characterized by helium-3 spin-echo and density functional theory"
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McIntosh, Eliza; Wikfeldt, K. Thor; Ellis, John; Michaelides, Angelos; Allison, William (April 19, 2013).
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Lechner, B.A.J.; Kole, P.R.; Hedgeland, H.; Jardine, A.P.; Allison, W.; Hinch, B.J.; Ellis, J. (2014).
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