Laser-induced continuum structures and double poles of theS-matrix

Double poles of the S-matrix and avoided level crossings are related to branch points in the complex plane. Although the number of these branch points is very small, they determine the dynamics of the system in a decisive manner. We study, in the adiabatic regime, the effects induced by two strong laser fields in the continuum of the H atom. In the photoabsorption spectra of the probe field from the ground 1s state, the coupling to the 2s, 5s, 5d and 5g states is considered. The quasi-level avoided crossing in the laser-induced continuum structures is traced theoretically as a function of the intensities of the strong laser fields. Under certain conditions, the quasi-levels cross and the S-matrix has a double pole. The relation of the laser-induced continuum structures to branch points in the complex plane is discussed.