  We measure the modification of the transmission spectra of cold $^{87}$Rb
atoms in the proximity of an optical nanofiber (ONF). Van der Waals
interactions between the atoms an the ONF surface decrease the resonance
frequency of atoms closer to the surface. An asymmetric spectra of the atoms
holds information of their spatial distribution around the ONF. We use a
far-detuned laser beam coupled to the ONF to thermally excite atoms at the ONF
surface. We study the change of transmission spectrum of these atoms as a
function of heating laser power. A semi-classical phenomenological model for
the thermal excitation of atoms in the atom-surface van der Waals bound states
is in good agreement with the measurements. This result suggests that van der
Waals potentials could be used to trap and probe atoms at few nanometers from a
dielectric surfaces, a key tool for hybrid photonic-atomic quantum systems.
