Photoionization of Ca in a static electric field

We present a joint theoretical and experimental investigation for electric-field effects on ground-state photoionization of Ca. For an electric field with its direction along the z axis, the dominant field-free, doubly excited, odd-parity (i.e., 3dnp and/or 3dnf) resonances of the 1,3LJ=1o (i.e., 1,3PJ=1o and 3DJ=1o) symmetries are coupled with the even-parity (i.e., 3dns, 3dnd, and/or 3dng) resonances of the 1,3LJ=0e (i.e., 1SJ=0e and 3PJ=0e) and 1,3LJ=2e (i.e., 3PJ=2e, 1,3DJ=2e, and 3FJ=2e) symmetries. Using a B-spline-based complex-rotation method with spin-dependent interaction, our theoretically calculated spectrum is found to be in good agreement with the observed spectrum from a cross-beam photoionization experiment for field strengths up to 25 kV/cm. We present in detail a number of qualitative features of the field-induced level crossing and avoided crossing in energy between neighboring resonances, their corresponding changes in width, and the resulting variation in resonance structure profiles. A few ''hidden'' resonances due to strong overlap with more prominant resonances are also identified theoretically.