Accurate ionization thresholds of atoms subject to half-cycle pulses

Abstract

The evolution of Rydberg states of hydrogen and alkali-metal atoms subject to short half-cycle pulses is studied. The convergence of the numerical solutions of the time-dependent Schrodinger equation based on an expansion of the electronic wave function in a finite basis set of Sturmian functions is analyzed in detail. It is shown that the accuracy of such calculations can be established by investigating the stabilization of the transition probabilities with respect to the parameters that define the basis set. The dependence of the quantum and classical ionization thresholds on the pulse shape is investigated. The calculations are compared with experimental data for various pulse profiles, which feature slow or fast rise times. The results show that the ionization thresholds for long pulses are very sensitive to the rise time of the electric field. @S1050-2947~98!00309-6# PACS number~s!: 32.80.Rm, 02.60.2x During the last few years, the ionization and excitation of Rydberg atoms by pulsed unidirectional electric fields, termed half-cycle pulses ~HCPs!, have been investigated ex- tensively. Experiments have reached the regime in which the effective duration of the pulses T p and the peak fieldsFp are of the order of the classical electron orbital period Tni 52pn i and the Coulomb electric field Fn i 5n i4 in the