Prediction of a new sequence of peaks in above-threshold detachment spectrum in an intense laser field

We predict the presence of a new, equally spaced sequence of peaks in the above-threshold detachment (ATD) spectra in a very intense laser of a fixed frequency. Using a general theoretical argument, originally made by Wigner in the context of nuclear reaction theory, we identify the new comblike peak sequence as due to Wigner cusps at the thresholds of stimulated multiphoton bremsstrahlung channels in the intermediate continuum states. Our prediction is supported by solving the Schrödinger equation of a 3D model of ${\mathrm{H}}^{\mathrm{-}}$ in an intense infrared laser (ħω=0.35 eV). A way of experimentally distinguishing the new sequence from the usual ATD peaks is pointed out.