Photoionization spectroscopy of Yb I from the $6s7s{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ state in the presence of a static electric field

Laser investigation of the photoionization spectra in the presence of an electric field has been performed in $\pi$ and $\sigma$ polarizations on an ytterbium atomic beam from the particular level $4f^{14}6s7s{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ which was populated by a two-step excitation. Broad modulations (in $\pi$ polarization) and Stark resonances in the vicinity of the zero-field ionization limit are interpreted satisfactorily as in the case of alkali atoms. However, the photoionization spectra are deeply affected by the presence of two perturbing levels belonging to the configuration $4f^{13}5d^{2}6s$. This phenomenon is explained by strong interference effects between the structured Stark continuum and the field-induced autoionizing states corresponding to the doubly excited levels.