We have observed a striking line narrowing in the photoionization spectrum of sodium atoms in an electric field. At crossings of two relatively broad resonances, one of them is narrowed by 2 to 3 orders of magnitude as a result of interference between the discrete and continuum coupling amplitudes that mediate their ionization. The observations agree quantitatively with WKB quantumdefect Stark theory and also with a simpler model involving two discrete states and two continua. 32.70.Jz, 32.80.Fb In the course of investigating resonance widths in the sodium Stark photoionization spectrum, we have observed a striking line-narrowing effect. Near the crossings of otherwise relatively broad Rydberg-Stark levels having the same orbital magnetic quantum number (m L ), we have measured decreases of up to 2 orders of magnitude in the spectral width of one of the resonances over a few percent change in the electric field. This narrowing is a spectral manifestation of a phenomenon first observed temporally by Feneuille et al. 1 in Rb, where the ionization rates of certain levels in the Rb Stark spectrum were observed to decrease by factors up to 50 over a narrow range of fields. We have found excellent agreement of our observations and those in Ref. 1 with calculated spectra based on the recently developed WKB quantum-defect (WKB-QD) theory of the Stark effect in alkali metals. 2 These calculations confirm the interpretation of this narrowing (or stabilizing) effect as an interference between the ionization processes of nearby hydrogenic resonances mixed by the core interaction.In the field-energy domain (FED) under consideration, the dominant contribution to the spectral width of observed Stark resonances with m L = 0, 1, or 2 is from core-induced ionization via degenerate continua. The coupling to these continua arises from the nonhydrogenic part of the potential due to the core electrons which mixes the parabolic basis states of the hydrogen-Stark problem. In the vicinity of an avoided crossing of two such levels, the bound-continuum coupling amplitudes will interfere to some extent as a result of mixing of the anticrossing states. This produces a partial decoupling of one of these levels from the continua resulting in a decrease of its ionization rate and corresponding spectral width. This interference is analogous to that in dipole transition matrix elements which can sharply inhibit radiative transitions. 3 Related effects have been observed in the autoionization spectra of strontium 4 and barium 5 in an electric field, and in cesium 6 at low fields where a large number of closely spaced states approximate a continuum.In our experiments, Na atoms in a thermal beam are stepwise excited by pulsed dye lasers via the 3 2 Py 2 state to high-lying levels in an electric field. 7 ' 8 The photoionization signal is measured as a function of the frequency of the linearly polarized second laser ( ~ 410 nm). We operate in a FED situated about 150 cm -1 below the zero-field ionization limit, E 0 , with a field of from 2 to 4 ...