Continuum-state selectivity in hydrogen in Stark fields by charge-shape tuning

We present numerical calculations of the photoionization of atomic hydrogen in the presence of a strong dc electric field, using three-photon excitation with two-photon resonance with intermediate Stark states. The systematics of the dependence of the cross section on the intermediate Stark states is calculated for the n=2 to n=9 manifolds. Our results indicate that one can use charge-shape tuning to selectively excite and enhance Stark-induced giant dipoles near E=0 in hydrogen without the excitation of the overlapping continuum. Frequency selectivity can be used to excite from 1s (spherical charge) an intermediate whose charge is focused along the field followed by another excitation to the giant dipoles. Charge tunability can be achieved by controlling the degree of focusing by choice of the field and intermediates.