Resonant dynamic Stark shift as a tool in strong-field quantum control: calculation and application for selective multiphoton ionization of sodium.

A method for determining the resonant dynamic Stark shift (RDSS), based on wave-packet calculations of the populations of quantum states, is presented. It is almost insensitive to variations of the laser pulse profile, and this feature ensures generality in applications. This method is used to determine an RDSS data set for 3s → nl (n ≤ 6) transitions in sodium induced by laser pulses with peak intensities up to 7.9 × 1012 W cm-2 and wavelengths in the range from 455.6 to 1139 nm. The data are applied to analyze the photoelectron spectra (electron yield versus excess energy) of the sodium atom interacting with 800 nm laser radiation. Substructures observed in the experimentally measured spectra are successfully reproduced and related to the resonantly enhanced multiphoton ionization via specific (P and F) intermediate states.