  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 the excited state line widths, represent
an overlooked but significant systematic effect. We show that excited state
interference gives rise to non-Lorenztian 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 lineshape, we determine absolute optical transition frequencies
with an uncertainty of <= 25kHz 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.
