Calculation of the probability of photoprocesses induced by ultrashort electromagnetic pulses

An expression for the total probability of photoprocesses induced by ultrashort electromagnetic pulses in terms of the excited transition cross-section has been derived by using perturbation theory. The expression obtained is valid for any radiation spectral width and for various types of transition: bound-bound and bound-free. By way of illustration the excitation of a two-level system by an ultrashort pulse with a controlled carrier–envelope phase and the photoionization of a hydrogen atom under the action of a sequence of two-cycle electromagnetic pulses are considered.