  Literature mentions only incidentally a sub-Doppler contribution in the
excitation spectrum of the backward fluorescence of a dense vapor. This
contribution is here investigated on Cs vapor, both on the first resonance line
(894 nm) and on the weaker second resonance line (459 nm). We show that in a
strongly absorbing medium, the quenching of excited atoms moving towards a
window irradiated under near normal incidence reduces the fluorescence on the
red side of the excitation spectrum. Atoms moving slowly towards the window
produce a sub- Doppler velocity-selective contribution, whose visibility is
here improved by applying a frequency-modulation technique. This sub-Doppler
feature, induced by a surface quenching combined with a short absorption length
for the incident irradiation, exhibits close analogies with the narrow spectra
appearing with thin vapor cells. We also show that a normal incidence
irradiation is essential for the sub-Doppler feature to be observed, while it
should be independent of the detection geometry
