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We have observed the deflection of Rydberg atoms towards a metallic surface by the van der %'aals force. Cs and Na atoms in states of principal quantum number n were sent between two parallel gold-coated mirrors, spaced by a gap m (2.1 pm & au & 8.5 pm). %e measured the value n at which the transmission cuts off' and from the variation of n versus u, we obtained a measure of the atom-surface interaction. For 12' n g30 this interaction is 3-4 orders of magnitude larger than for ground-state atoms"and it obeys the scaling laws of the Lennard-Jones model. The van der Waals interaction between a metallic surface and an atom is an Important process in atomic physics. If the metal behaves as a perfect conductor at characteristic atomic frequencies, the interaction can be viewed, after Lennard-Jones, ' as resulting from the coupling of the atomic dipole with its electrostatic images in the surface. This coupling gives rise to energy-level shifts proportional to z 3 (where z is the atom-surface dis-tance}, and the derivative of these shifts with respect to the distance corresponds to the dipole-image van der Waals force Fvw-z that puBs the atom towards the metal. This assumes that z is small compared with any characteristic atomic-transition wavelength so that retardation is negligible.For ground-state atoms or molecules, Fvw becomes large when z is smaller than a few Bohr radii. It plays an important role in a variety of physicochemical processes in which atoms come close to surfaces, such as adsorption phenomena. 3 In these processes, however, the simple Lennard-Jones interaction is complicated by short-range atom-surface coupling terms that depend upon the detailed microscopic structure of the surface. At distances larger than a few tens of angstroms, Fvw takes its simple asymptotic form, but becomes exceedingly small.To our knowledge only one experiment has demonstrated the existence of the long-range (» 100 A} van der Walls attraction between free atoms and a metal. Kusch et al. observed the very small deflection experienced by a beam of cesium atoms at grazing incidence, passing within a few hundred angstroms of a gold-coatcd cylinder. This was a very diScult experiment, with a very low signal due to exceedingly small atomic cruxes in the relevant atom-metal distance range, and only qualitative information about the van der %aals force could be obtained from it.A simple way of increasing the van der %aals force is to excite the atoms into a Rydberg state with a large effective quantum number n (n is the principal quantum number of the level minus its quantum defect). The electric dipole of the Rydberg atom scales as n and hence the van der %aals force, proportional to the square of this dipole, is expected to increase as n and to scale as (n/z), provided z remains small compared with the characteristic Rydberg transition wavelength (in the mm domain). As a result we expect the range of the van der Waals attraction to be extended by a factor equal to the ratio of excited-state to ground-state e8'ective-n values. In ...

Phys. Rev. Lett. 58, 487 (1987)]. The factor (y -1 )/p in the equation for Da/Dt should multiply only the first two terms. The equation should read Da _ y-\ Dt p bX at-T xn bX + T]J 2 + 7711 oi£+ 02 M bX bX ba bX'

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