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 effects are expected to be negligible. We observe the shift on the $2{}^3{S}_1\to 2{}^3{P}_1$ open transition of ${}^4\mathrm{He}$ that is relevant for a determination of the fine-structure constant $\alpha$ [Phys. Rev. Lett. 82, 1112 (1999)]. We discuss the physical origin of the shift and we provide a numerical analysis to support our interpretation. The shift can be quite important for various other high-precision spectroscopy measurements.