Frequency shift in saturation spectroscopy induced by mechanical effects of light

We report on a substantial light-force-induced line-center shift of the sub -Doppler dip in a saturation spectroscopy configuration where mechanical ef fects are expected to be negligible. We observe the shift on the 2 S-3(1)-- >2 P-3(1) Open transition of He-4 that is relevant for a determination of t he fine-structure constant alpha [Phys. Rev. Lett. 82, 1112 (1999)]. We dis cuss the physical origin of the shift and we provide a numerical analysis t o support our interpretation. The shift can be quite important for various other high-precision spectroscopy measurements. [S1050-2947(99)08811-3].