Nonlinear Variation of Light-Pressure-Induced Multilevel Atomic Line   Shifts with Vapor Pressure in Helium Spectroscopy
  This paper has been temporarily withdrawn.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. 
T. Zelevinsky