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Molecular Physics

Non-destructive quantum reflection of helium dimers and trimers from a plane ruled grating

Bum Suk Zhao, Weiqing Zhang, Wieland Schöllkopf

Published Jun 19, 2013• 14 citations
Publisher

Abstract

We report on the non-destructive scattering and diffraction of He, He2 and He3 from a plane ruled reflection grating. At grazing incidence the normal component of the particle’s wave-vector is sufficiently small to allow for quantum reflection at the attractive Casimir–van der Waals particle–surface interaction potential. Quantum reflection occurs tens of nanometres in front of the surface, before the dimers and trimers reach the region where the surface-induced forces would inevitably cause the breakup of the fragile bonds. The reflected particles are identified via their mass-dependent diffraction angles and by mass spectrometry. The intensity distributions of the observed diffraction patterns are discussed in terms of the grating’s blaze angle.

DOI
10.1080/00268976.2013.787150

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We report on the non-destructive scattering and diffraction of He, He2 and He3 from a plane ruled reflection grating. At grazing incidence the normal component of the particle’s wave-vector is sufficiently small to allow for quantum reflection at the attractive Casimir–van der Waals particle–surface interaction potential. Quantum reflection occurs tens of nanometres in front of the surface, before the dimers and trimers reach the region where the surface-induced forces would inevitably cause the breakup of the fragile bonds. The reflected particles are identified via their mass-dependent diffraction angles and by mass spectrometry. The intensity distributions of the observed diffraction patterns are discussed in terms of the grating’s blaze angle.
Quantum reflection allows an atom or molecule to be reflected from a solid before it reaches the region where it would encounter the repulsive potential of the surface. We observed nondestructive scattering of the helium dimer (He₂), which has a binding energy of 10−7 electron volt, from a solid reflection grating. We scattered a beam containing the dimer as well as atomic helium and larger clusters, but could differentiate the dimer by its diffraction angle. Helium dimers are quantum reflected tens of nanometers above the surface, where the surface-induced forces are too weak to dissociate the fragile bond.
We observe high-resolution diffraction patterns of a thermal-energy helium atom beam reflected from a microstructured surface grating at grazing incidence. The grating consists of $10\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-wide Cr strips patterned on a quartz substrate and has a periodicity of $20\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Fully resolved diffraction peaks up to the seventh order are observed at grazing angles up to 20mrad20\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}20mrad. With changes in de Broglie wavelength or grazing angle the relative diffraction intensities show significant variations which shed light on the nature of the atom-surface interaction potential. The observations are explained in terms of quantum reflection at the long-range attractive Casimir--van der Waals potential.
Quantum reflection allows an atom or molecule to be reflected from a solid before it reaches the region where it would encounter the repulsive potential of the surface. We observed nondestructive scattering of the helium dimer (He(2)), which has a binding energy of 10(-7) electron volt, from a solid reflection grating. We scattered a beam containing the dimer as well as atomic helium and larger clusters, but could differentiate the dimer by its diffraction angle. Helium dimers are quantum reflected tens of nanometers above the surface, where the surface-induced forces are too weak to dissociate the fragile bond.
We report coherent reflection of thermal He atom beams from various microscopically rough surfaces at grazing incidence. For a sufficiently small normal component k(z) of the incident wave vector of the atom the reflection probability is found to be a function of k(z) only. This behavior is explained by quantum reflection at the attractive branch of the Casimir-van der Waals interaction potential. For larger values of k(z) the overall reflection probability decreases rapidly and is found to also depend on the parallel component k(x) of the wave vector. The material specific k(x) dependence for this classic reflection at the repulsive branch of the potential is discussed in terms of an averaging out of the surface roughness under grazing incidence conditions.
Quantum reflection is a universal property of atoms and molecules when scattered from surfaces in ultracold collisions. Recent experimental work has documented the quantum reflection and diffraction of He atoms, dimers, trimers, and neon atoms when reflected from a grating. Conditions for the observation of emerging beam resonances have been discussed and measured. In this paper, we provide a theoretical simulation of the quantum reflection from a grating for those systems. We confirm the universal dependence on the incident de Broglie wavelength with the threshold angles where the emerging beam resonances are observed. However, the angular dependence of the reflection efficiencies, that is, the ratio of scattered intensity into specific diffraction channels relative to the total intensity is found to be dependent on the details of the particle surface interaction.
Abstract Molecular beams of He and D2 are scattered from a ruled diffraction grating in conical‐mount geometry under grazing‐incidence conditions. Fully resolved diffraction patterns as a function of detection angle are recorded for different grating azimuth angles and for two different kinetic energies of the particle beams. Variations in diffraction peak widths are traced back to different velocity spreads of He and D2 determined by time‐of‐flight measurements. A comprehensive analysis of diffraction intensities confirms universal diffraction, that is, for identical de Broglie wavelengths, the relative diffraction intensities for He and D2 are the same. Universal diffraction results from peculiarities of quantum reflection of the atoms and molecules from the diffraction grating. In quantum reflection particles scatter many nanometers in front of the surface from the long‐range attractive branch of the particle–surface interaction potential without probing the potential well and the short‐range repulsive branch of the potential.