  In the present paper, we study nonresonant corrections for experimental
measurements of the transition frequencies in the helium atom. Having attracted
more attention, such effects can make a significant contribution to experiments
based on one- and two-photon atomic spectroscopy. The quantum interference
effects in the measurements of $2^3S_1-n^3D_1$ ($ n=3,\,4,\,5 $) transition
frequencies based on Doppler-free two-photon spectroscopy, are considered as a
possible source of current discrepancy between the experimental and theoretical
data. We demonstrate that line profile asymmetry caused by the quantum
interference of fine sub-levels of the $ ^3D_{J} $ $ (J=1,2,3) $ state can
reach tenths of a megahertz for different experimental conditions. Thus,
previously unaccounted nonresonant corrections should be taken in
next-generation experimental measurements of transitions frequencies in helium.
However, they could not completely eliminate the current imbalance in the study
of helium spectra and the question is still open.
