Simple method for constructing the ionization spectra of driven time-periodic Hamiltonians

Using a general Green's-function formalism for time-periodic Hamiltonians, it is shown that the above-threshold ionization spectra can be generated from time-independent calculations. Taking only the leading-order term of the corresponding Born-series expansion leads to a simple closed analytical expression that accounts for the essential features of the ionization spectrum and their connection to the time-independent quantities in question.