The atom-surface interaction is one of the simplest prototype of the universal dipole-dipole interaction between neutral bodies and a key phenomenon in the ultimate cohesion of the matter. It scales in C{sub}3 z{sup}(-3) (z: the atom-surface distance) in the electrostatic van der Waals (vW) regime, valid typically for a distance range spanning from ～1nm (to smooth down the structural details of the surface) up to ～1000 nm (i.e. when retardation effects can no longer be neglected). Although this attraction law should cover about 10 orders of magnitude in energy, little has been done to test the predicted dependence; for which various subtle corrections are now currently predicted. Following the blossom of Casimir interaction measurements with study on the distance dependence, a very recent investigation has appeared based upon the reflection coefficients of slow atoms on a magnetic mirror; its spatial resolution is related with the size of the magnetic domain, and allows to explore a 20-100 nm distance to a wall for cold atoms [1]. Here, we report on a spectroscopic investigation of the vW interaction exerted onto the excited atoms of a vapour nanocell, whose nanometric thickness varies locally. Our method is based upon a detailed spectral analysis and a fitting of the simultaneously recorded transmission and reflection spectra, through a comparison with predicted lineshapes of a detailed model. These two signals, originating in linearly independent spatial combinations of the atomic response, are processed independently. They yield however consistent results. A full series of measurements was completed for the high-lying state Cs (6D{sub}(5/2)), for a thickness range 40-130 nm (NB: for a 40 nm thickness, 20 nm is the maximal atom-surface distance). As shown in fig.1, a single set of parameters is sufficient to predict the transmission and the refection lineshapes for various thicknesses, showing [2] that the C{sub}3 coefficient appears to be constant for all thickness within our experimental accuracy, yielding a check of the z{sup}(-3) scaling law. Reproducibility of the spectra for various regions of the nanocell, and an estimate of the possible residual Stark shift, demonstrate that the observed spectral shift and distortions can be traced back to the Cs interaction with the YAG surface itself. The estimated value C{sub}3 value (～ 14 ± 3 kHz.μm{sup}3) is found to be in very good agreement with the common theoretical prediction (～15 kHz.μm{sup}3). This is a remarkable agreement, because various subtleties could affect the accuracy of the prediction, such as the anisotropy in the pumping step (to the resonant level 6P{sub}(3/2)), or corrections related to the nonzero-temperature of the vacuum, or short distances (non retarded) contributions of the electronic core to relevant virtual transitions.～
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