  Despite inherently complex multiphoton dynamics, our observations show that
Ba double ionization with an infrared laser (8800-8920 cm^{-1}) resembles a
single-photon process; namely, its probability is proportional to the laser
intensity. In this regime, single-electron ionization is due to a four-photon
resonant transition through the highly perturbed state 6p^2 1D_2, whereas
double ionization is realized by the two-electron mechanism. Furthermore, we
argue that these conclusions are valid for other alkaline-earth-metal atoms and
other parameters of laser radiation.
