Determination of Atom-Surface van der Waals Potentials from Transmission-Grating Diffraction Intensities

Abstract

Molecular beams of rare gas atoms and D2 have been diffracted from 100 nm period SiNx transmission gratings. The relative intensities of the diffraction peaks out to the 8th order depend on the diffracting particle and are interpreted in terms of effective slit widths. These differences have been analyzed by a new theory which accounts for the long-range van der Waals −C3/l 3 interaction of the particles with the walls of the grating bars. The values of the C3 constant for two different gratings are in good agreement and the results exhibit the expected linear dependence on the dipole polarizability. where l is the distance from the surface. This potential plays an important role in understanding virtually all static (thermodynamical) and dynamical aspects of gas adsorption phenomena. Despite its importance, very few experimental determinations of C 3 have so far been reported and most of our present knowledge is based on theoretical estimates [2]. The pioneering experiments by Raskin and Kusch on the deflection of Cs atoms from a conducting metal surface [3] have recently been extended to alkali atoms in high Rydberg states by measuring the transmission through 8 mm long narrow (2 − 9 µm) channels as a function of their principal quantum number n [4]. Similar techniques have also been applied to the interaction of alkali atoms in their ground state [5,6] or in low excited states [7]. Although the scattering of many different atoms and molecules from solid single crystal surfaces has been extensively studied, the reflection coefficients are relatively insensitive to the weak long range attractive forces since the collisions are largely determined by the reflection from the hard repulsive wall close to the surface [8].Here, a new atom optical technique using transmission grating diffraction [9,10] of molecular beams is employed. The van der Waals force causes a change in the diffraction intensities just as a smaller slit width would. A newly developed theory makes it possible to interpret measurements over a range of different beam energies in terms of the potential constant C 3 . For an incident plane wave the diffraction peak heights depend on the number of illuminated slits N , as N 2 . With N = 100 slits the gain in sensitivity is about four orders of magnitude over previous experiments.The measurements were made with a previously described [10] molecular beam diffraction apparatus. The beams are produced by a free jet expansion of the purified gas through a 5 µm diameter, 2 µm long orifice from a source chamber at a temperature T 0 , into vacuum of about 2 × 10 −4 mbar. At T 0 = 300 K the source pressure P 0 was 140 bar for He, Ne, Ar and D 2 and 50 bar for Kr. At lower source temperature P 0 was reduced to avoid cluster formation. The atomic beams are characterized by narrow velocity distributions with ∆v/v ≈ 2.1 % (He), 5 % (Ne), 7.6 % (D 2 ), 7.7 % (Ar), and 10 % (Kr) at T 0 = 300 K, where ∆v and v denote the full half width and the mean value, respectively. After passing through the 0.39 mm diameter skim...

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