Anomalous attenuation at low temperatures in high-intensity helium beam sources

Hedgeland, Holly; Jardine, A. P.; Allison, W.; Ellis, John

doi:10.1063/1.2149008

Cited by 16 publications

(27 citation statements)

References 18 publications

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“…3 Attenuation effects are seen to be much weaker than in previous measurements using 4 He. 6 The maximum useful flowrate corresponds to a beam intensity of 2.5 ϫ 10 19 atoms/ sr/ s, which is almost double the value achieved using the previous arrangement, despite the reduction in pumping speed. The inset to Fig.…”

Section: Performancementioning

confidence: 74%

An improved high intensity recycling helium-3 beam source

Hedgeland

Kole

Allison

et al. 2009

Review of Scientific Instruments

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We describe an improved high intensity, recycling, supersonic atomic beam source. Changes address several issues previously limiting performance and reliability of the apparatus, including the use of newly available vacuum pumps and modifications to the recycling system. We achieve a source intensity of 2.5 x 10(19) atoms/s/sr, almost twice that previously achievable during recycling. Current limits on intensity are discussed.

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Section: Performancementioning

confidence: 74%

An improved high intensity recycling helium-3 beam source

Hedgeland

Kole

Allison

et al. 2009

Review of Scientific Instruments

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“…Such interactions can become significant at high nozzle pressures. There have been various approaches for accounting for this, from DSMC simulations, to simpler numerical models based on assumptions on the scattering properties of the skimmer walls [14,38]. Analytical models for the skimmer contributions are so far non-existent due to the difficulty of solving the Boltzmann equation analytically in a typical nozzle-skimmer geometry.…”

Section: Scattering Contributionsmentioning

confidence: 99%

“…Considering the reflection of particles from the skimmer wall makes solving the Boltzmann equation difficult, as DSMC methods are often computationally heavy. Some work has been done regarding the effect of skimmer geometries [11,[14][15][16]. However, much of this work lacks extensive validation due to the lack of experimental data.…”

Section: Introductionmentioning

confidence: 99%

Center-line intensity of a supersonic helium beam

et al. 2018

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Supersonic helium beams are used in a wide range of applications, for example surface scattering experiments and, most recently, microscopy. The high ionization potential of neutral helium atoms makes it difficult to build efficient detectors. Therefore, it is important to develop beam sources with a high centre line intensity. Several approaches for predicting the centre line intensity exist, with the so-called quitting surface model incorporating the largest amount of physical dependencies in a single analytical equation. However, until now only a limited amount of experimental data has been available. Here we present a comprehensive study where we compare the quitting surface model with an extensive set of experimental data. In the quitting surface model the source is described as a spherical surface from where the particles leave in a molecular flow determined by Maxwell-Boltzmann statistics. We use numerical solutions of the Boltzmann equation to determine the properties of the expansion. The centre line intensity is then calculated using an analytical integral. This integral can be reduced to two cases, one which assumes a continuously expanding beam until the skimmer aperture, and another which assumes a quitting surface placed before the aperture. We compare the two cases to experimental data with a nozzle diameter of 10 µm, skimmer diameters ranging from 4 µm to 390 µm, a source pressure range from 2 to 190 bar, and nozzleskimmer distances between 17.3 mm and 5.3 mm. To further support the two analytical approaches, we have also performed equivalent ray tracing simulations. We conclude that the quitting surface model predicts the centre line intensity of helium beams well for skimmers with a diameter larger than 120 µm when using a continuously expanding beam until the skimmer aperture. For the case of smaller skimmers the trend is correct, but the absolute agreement not as good. We propose several explanations for this, and test the ones that can be implemented analytically.

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“…After a prototype was presented by De Kieviet and coworkers [20,21], the Surface Physics group at Cambridge developed the first HeSE apparatus with su cient momentum transfer space resolution to map out adsorbate di↵usion, where accessible timescales range from 0.01 ps to 680 ps [13,23]. helium-3 gas which is then expanded into the source chamber through a cooled nozzle and recirculated back into the system after expansion [23][24][25]. The nominal beam energy for the spectrometer is 8 meV, for which energy the magnets provide optimal focussing.…”

Section: The Cambridge Helium-3 Spin-echo Spectrometermentioning

confidence: 99%

“…The position of the dip can be used to estimate the coverage, 24) from the average nearest-neighbour distance, r, and the substrate lattice constant, a [31].…”

Section: Lateral Interactions: De Gennes Narrowingmentioning

confidence: 99%

Studying Complex Surface Dynamical Systems Using Helium-3 Spin-Echo Spectroscopy

Lechner¹

2014

Springer Theses

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The aim of the present thesis is to extend the range of application of the helium-3 spin-echo (HeSE) technique to complex surface dynamical systems. A twofold approach is presented: the development of improved experimental equipment and the investigation of a series of prototypical aromatic adsorbate systems. Chapter 1 discusses the motivation for this work and compares HeSE with other techniques probing surface motion. Subsequently, an introduction to the HeSE method is given in Chapter 2, explaining the theoretical background and describing the main components of the Cambridge spectrometer as well as the principle data aquisition and interpretation methods. Chapters 3 and 4 describe the development and testing of two new pieces of equipment. MiniScat, a compact helium atom scattering apparatus, has been designed as a sample preparation facility to enable structural studies and increase the experimental throughput. A new supersonic helium beam source improves the resolution of the HeSE spectrometer by a factor of 5.5 and extends accessible timescales into the nanometre range. The dynamics of cyclopentadienyl (Cp) on Cu(111) are presented in Chapter 5. The ionically bound Cp is remarkably mobile, moving in single jumps between adjacent hollow sites over an energy barrier of 41 ± 1 meV. The data exhibit multicomponent lineshapes that allow the determination of the energy di↵erence between fcc and hcp sites of 12.3±0.3 meV in a Bayesian method probing the probability space of all data combined. Molecular dynamics (MD) simulations provide a friction coe cient of 2.5 ± 0.5 ps 1. Chapter 6 shows that a physisorbed pyrrole hops between bridge sites on Cu(111). Strong lateral interactions alter the lineshapes from a predicted double exponential towards an apparent single exponential decay. First principles density functional theory calculations by Sacchi and Jenkins reveal that a large contribution to the experimentally determined apparent activation energy of 53 ± 4 meV arises from a site-dependence in the zero point energies, primarily of the vibrational C-H and N-H out-of-plane bending and ring torsion modes which are not directly involved in the di↵usion process. The surface dynamics of thiophene/Cu(111) are investigated in Chapter 7. Thiophene adsorbs on top sites and forms a covalent S-Cu bond. Two competing activated processes manifest themselves in a kink in the Arrhenius plot: jump di↵usion between adjacent top sites over a barrier of 59 ± 4 meV and rotation around the S-Cu anchor point over a barrier of 22 ± 2 meV. In addition, vertical motion relative to the surface is observed. MD simulations of the di↵usive motion reveal an exceptionally high friction of 5 ± 2 ps 1 .

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Anomalous attenuation at low temperatures in high-intensity helium beam sources

Cited by 16 publications

References 18 publications

An improved high intensity recycling helium-3 beam source

An improved high intensity recycling helium-3 beam source

Center-line intensity of a supersonic helium beam

Studying Complex Surface Dynamical Systems Using Helium-3 Spin-Echo Spectroscopy

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