  The van der Waals atom-surface attraction, scaling as C3 z-3 for z the
atom-surface distance, is expected to be valid in the distance range 1-1000 nm,
covering 8-10 orders of magnitudes in the interaction energy. A Cs vapour
nanocell allows us to analyze the spectroscopic modifications induced by the
atom-surface attraction on the 6P3/2->6D5/2 transition. The measured C3 value
is found to be independent of the thickness in the explored range 40-130 nm,
and is in agreement with an elementary theoretical prediction. We also discuss
the specific interest of exploring short distances and large interaction
energy.
