With a view to improved parity violation measurements in Cs, we have considered the 6S-7S-6P3/2 « forbidden three-level system », in which one laser excites the forbidden 6S-7S transition while a second, colinear laser probes the excited 7S atoms. This paper presents an analytical calculation of the 7S → 6P ½ fluorescence intensity and of the probe amplification as functions of the laser frequencies. The collisional processes in the 6P 3/2 resonance level, as well as the level multiplicity and the laser polarizations are taken into account. The quantitative agreement with experiment is good ; in particular, the observed sub-Doppler structures are correctly described. The damping rates of the 7S-6P3/2 cohérence and of the trapped 6P3/2 atoms are extracted. New détection schemes for detecting a 7S orientation, with direct possible extension to an alignment, are demonstrated.