Role of excited states in multiphoton dynamics

Multiphoton ionization of hydrogen, at laser frequencies, is studied numerically using the stretched hydrogen atom Hamiltonian. In particular, the role of excited bound states in short-pulse, above-threshold ionization (ATI) is emphasized. These states are shown to play a dynamical role both in the production of the higher ATI peaks as well as in the appearance of peak substructure. A simple criterion is used to assess the importance of this process, as a function of field strength and frequency. Arguments are presented for the interpretation of ionization characteristics for short pulses which suggest important differences from that for longer pulses. The extension of these ideas to other frequency regimes is also discussed.