We present a theoretical and experimental investigation of the postselection effect in precision spectroscopy of the $2{}^{3}S-2{}^{3}P$ transition in helium using an atomic beam. By employing the Monte Carlo wave function (MCWF) method, we simulate the time evolution of the ground and excited amplitudes in the presence of laser fields, revealing a systematic dependence of the measured transition frequency on the spatial parameters of the detection slit, including its position and width. Our results demonstrate that postselection—an inherent aspect of quantum projection in such measurements—can induce frequency shifts on the order of tens of kilohertz. By elucidating the underlying physical mechanisms, our study offers a practical framework for quantitatively correcting the postselection-induced shifts, thereby improving the accuracy of atomic beam spectroscopy.

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• Atomic spectra