Frequency shift in saturation spectroscopy induced by mechanical effects of light

none6noWe 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 3S1 →2 3P1 open transition of 4He that is relevant for a determination of the fine-structure constant alpha [Phys. 11 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.openMinardi F.; Artoni M.; Cancio P.; Inguscio M.; Giusfredi G.; Carusotto I.Minardi F.; Artoni M.; Cancio P.; Inguscio M.; Giusfredi G.; Carusotto I