This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.32.2769.Accurate ab initio nonperturbative L2 non-Hermitian Floquet calculations for intensity-dependent threshold shifts and ground-state total ionization widths (rates) for one-, two-, and three-photon-dominant intense-field ionization of atomic hydrogen are presented. The results show the importance of both the ac Stark shift and the pondermotive potential in the determination of the net threshold shift. In addition, branching ratios to individual continua have been estimated, yielding physical insights regarding the general features and mechanisms of the frequency- and intensity-dependent continuum-continuum transitions and ‘‘peak switching’’ phenomena in the above-threshold ionization processes
