  This paper has been temporarily withdrawn. Highly nonlinear dependences of
4He 2^3S - 2^3P lines on pressure were observed in vapor cell saturation
spectroscopy. Line shifts agree with those expected from light-pressure-induced
perturbation of atomic velocity distribution. In the lin-perp-lin pump and
probe polarization configuration, multilevel atomic structure must be taken
into account. For a two-level atom, the shift magnitude is determined by the
collisional time constant, but when the system has dark states there is an
additional time constant due to optical pumping. The collisional and optical
time constants compete, and become equal at a certain pressure p_0. Saturation
signal vanishes at p_0 whereas signal due to light pressure remains. This leads
to a dependence of line centers on pressure that exhibits apparently singular
behavior at p_0. The 2S - 2P(J=1) line shift reaches the natural half width.
Vapor cells may be more suitable for investigation of this effect than atomic
beams since the shifts vanish at sufficiently high pressures. Two ways of
controlling the shifts for the helium 2P fine structure measurement are
discussed.
