The van der Waals potential between metastable atoms and solid surfaces: Novel diffraction experimentsvs.theory

Abstract

Highly polarizable metastable He* (2 3 S) and Ne* (2 3 P) atoms have been diffracted from a 100 nm period silicon nitride transmission grating and the van der Waals coefficients C3 for the interaction of the excited atoms with the silicon nitride surface have been determined from the diffraction intensities out to the 10th order. The results agree with calculations based on the non-retarded Lifshitz formula.PACS numbers: 34.50.Dy, 03.75.BeThe van der Waals (vdW) force between atoms, molecules and solid surfaces is of far reaching importance in many branches of physics, chemistry, and biology [1]. For larger distances, retardation due to the exchange of virtual photons has to be included, while for distances much smaller than the smallest wavelength a non-retarded approach can be used. The theoretical foundations for atom-surface interactions were laid in the pioneering work of Lifshitz [2]. In this case the non-retarded vdW potential has the form −C 3 /l 3 in leading order, where l is the atom-surface separation and C 3 depends on the atom, its electronic state, and on the electronic states of the solid.For groundstate rare gas atoms the C 3 coefficients have recently been measured with good accuracy [3]. Less is known about the van der Waals interactions of electronically excited metastable and Rydberg atoms, in particular the C 3 coefficient is not accurately known. Some time ago, transmission through narrow channels [4] and level shifts in closed or semi-infinite cavities [5,6] have been studied. Recently, inelastic electronic transitions on passage over a metal edge [7] and reflection from surfaces and (reflection) gratings [8] have been measured. Currently there is great interest in these potentials, in particular for metastable helium which is widely used in atom optics [9] as well as in surface physics [10] and for which Bose-Einstein condensation has recently been achieved [11]. The atom-surface van der Waals potentials could soon become relevant in guiding slow metastable rare gas atoms along microstructures [12] or in studying collective effects of Bose-Einstein condensed metastable He* atoms in contact with a surface.From a theoretical point of view atoms in excited states are of particular interest. Their polarizability α is expected to increase as n 7 , with correspondingly much stronger interactions [13]. Therefore it is not obvious whether approximate formulae for the groundstate atom-surface vdW potential are still applicable for excited atoms. Moreover, with the much stronger vdW interaction new effects such as higher multipole coefficients [14] can be expected.In this article, an effective but simple experimental method is used to determine the atom-surface vdW coefficient C 3 for metastable rare gas atoms. It is based on diffraction of an atomic beam from a nanostructured transmission grating with a period of only 100 nm. Modifications in the hierarchy of the intensities of the higher order maxima in the diffraction pattern have been shown to be directly related to the strength C 3 of the atom...

A Green's function approach to modeling molecular diffraction in the limit of ultra‐thin gratings

“…In earlier experiments on atom diffraction at 'thick' gratings [17,18] it has been useful to approximate the wall inside of each grating slit as a semi-infinite plane, such that the particle's interaction with one side of the slit can be described by V CP with the Casimir-Polder coefficient…”

“…In case of the grating material SiN x , its magnetic response can be neglected and only the electric part of the Casimir-Polder potential (5) contributes. For the dielectric susceptibility we used the model [18,22] Im…”

Matter-wave diffraction of quantum magical helium clusters

“…Experiments with different rare gases He, Ne, Ar, Kr and D 2 reveal intensity patterns which are modified by the van der Waals interaction with the grating bars, which causes a reduction in the effective slit width. This led to the first determination of the van der Waals coefficients for ground state rare gas and Na atoms [5] and electronically excited metastable atoms [6]. Large differences are also found between the slit function of He atoms and that of the dimers which are best seen at T 0 = 6 K (Fig.3(d)).…”

Optics and Interferometry with Atoms and Molecules

“…Several experiments have measured the intensity ͉A n ͉ 2 in diffraction orders to determine C 3 for various atomsurface combinations, with some results shown in Fig. 65 and 66 ͑Grisenti et al, 1999;Bruehl et al, 2002;Cronin and Perreault, 2004;. The diffraction intensities ͉A n ͉ 2 depend on phase gradients induced by U͑r͒.…”

“…Several experiments can now test these predictions. Atoms transmitted through a cavity ͑Anderson et Sukenik et al, 1993͒, atoms diffracted from a material grating Grisenti, Schöllkopf, Toennies, Manson, et al, 2000;Shimizu, 2001;Bruehl et al, 2002;Cronin and Perreault, 2004;Perreault et al, 2005͒, atoms undergoing quantam reflection ͑Anderson et al, 1986Berkhout et al, 1989;Shimizu, 2001;Shimizu and Fujita, 2002a;Druzhinina and DeKieviet, 2003;Pasquini et al, 2006͒, atoms reflecting from evanescent waves near surfaces ͑Hajnal et al, 1989;Kaiser et al, 1996;Westbrook et al, 1998;Esteve et al, 2004͒, atoms trapped near surfaces ͑Lin et al, 2004McGuirk et al, 2004;Harber et al, 2005͒, andatoms in interferometers ͑Brezger et al, 2002;Kohno et al, 2003;Nairz et al, 2003;Cronin, 2005, 2006͒ have been used to measure atom-surface interaction potentials. For a review of experiments see the CAMS ͑2005͒ proceedings.…”