Hidden crossings and excitation of hydrogen by a slow electric pulse

The complex eigenenergy surfaces of quasistationary states for the hydrogen ic Stark problem as a function of complex field strength have been calculat ed. It is shown that the location of 'hidden crossings' of energy surfaces connected with different states can be obtained from a semiclassical quanti zation of 'top of the barrier' motion. A numerical calculation of the excit ation of hydrogenic states by an electric field pulse shows an unexpected t hreshold-like behaviour in the adiabatic regime. In addition, the lower the peak-field strength, the higher is the state populated maximally. It is fo und that these effects are correlated to the position of the 'top of the ba rrier' hidden crossings as a function of field strength.