Multiphoton ionization as time-dependent tunneling

A semiclassical approach to ionization by an oscillating field is presented. An asymptotic analysis is performed with respect to a quantity h, defined (up to a factor) as the ratio of photon energy to ponderomotive energy. This h appears formally equivalent to Planck's constant in a suitably transformed Schr\"odinger equation and allows us to formally use semiclassical methods. Systematically, a picture of tunneling wave packets in complex time is developed, which by interference account for the typical ponderomotive features of ionization curves. For a one-dimensional {δ}-function atom, these analytical results are compared to numerical simulations [Scharf, Sonnenmoser, and Wreszinski, Phys. Rev. A 44, 3250 (1991)] and are shown to be in good agreement.