Double poles of theSmatrix in laser-assisted electron-atom scattering

We discuss two double-resonance scenarios within the context of laser-assisted electron-atom scattering in which double poles of the S matrix occur. When the energy of the incident electron is chosen to coincide with an autoionizing resonance and the frequency of the laser field resonantly couples this autoionizing state to a quasibound state (i. e. , a state that is bound in the absence of the laser) or another autoionizing resonance, double poles occur in the S matrix for particular laser frequencies and intensities. We demonstrate the effect of the double poles on the resonant contribution to the cross section as well as on the time delay matrix.