Light-force-induced shift in laser spectroscopy of atomic helium

Abstract

A laser-power-dependent shift has been observed in various atomic beam spectroscopy measurements, which was often treated by extrapolating the results to the zero-field limit. Here we present an experimental and theoretical study of this effect in the measurements of the $2{\phantom{\rule{0.16em}{0ex}}}^{3}S\ensuremath{-}2{\phantom{\rule{0.16em}{0ex}}}^{3}P$ transition frequency of $^{4}\mathrm{He}$. The light-force-induced shift was attributed as a result of modulated atomic trajectories induced by the standing-wave laser field and consequent distortions in the lineshape. Modulation of the beam spatial distributions was detected by imaging atoms at high laser power and was also simulated by the Monte Carlo wave-function approach. The nonlinear behavior of the light-force-shift was observed experimentally and reproduced by the simulations. The systematic shift in the extrapolated result at the zero-field limit was analyzed. As a consequence of this effect, a correction of +0.50(80) kHz was added to our previous result on the $2{\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1}\ensuremath{-}2{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{1}$ transition frequency [Phys. Rev. Lett. 119, 263002 (2017)], and the reevaluated value is 276 734 477 704.3(1.6) kHz.