Quantum interference and light polarization effects in unresolvable atomic lines: Application to a precise measurement of the6,7LiD2lines

Abstract

We characterize the effect of quantum interference on the line shapes and measured line positions in atomic spectra. These effects, which occur when the excited-state splittings are of order of the natural line widths, represent an overlooked but significant systematic effect. We show that excited-state interference gives rise to non-Lorentzian line shapes that depend on excitation polarization, and we present expressions for the corrected line shapes. We present spectra of ${}^{6,7}$Li $D$ lines taken at multiple excitation laser polarizations and show that failure to account for interference changes the inferred line strengths and shifts the line centers by as much as 1 MHz. Using the correct line shape, we determine absolute optical transition frequencies with an uncertainty of $\ensuremath{\le}$25 kHz and provide an improved determination of the difference in mean-square nuclear charge radii between ${}^{6}$Li and ${}^{7}$Li. This analysis should be important for a number of high-resolution spectral measurements that include partially resolvable atomic lines.