Multiple Excitation and Ionization of Atoms by Strong Lasers: Is There Any New Physics?

Abstract

Although, under certain conditions a transition probability per unit time averaged over the laser pulse may be useful in interpreting general features of experimental results, in general events during the evolution of the pulse must be followed in detail because total ionization of various ionic species is apt to occur.1 As a result, at different stages of the pulse, processes of different order may appear. The end result is ionization. But a number of other processes such as wave mixing and lasing can coexist or even compete with ionization under the appropriate conditions. Employing calculated generalized cross sections, sequential ionization is calculated and shown to be the dominant mechanism under the conditions of recent experiments.2,3 But even in sequential processes, multielectron excitations may play a role in determining the cross section and the state of the ion. Examples of calculations in multielectron atoms illustrate the role of more then one electron excitation. Doubly excited states, for example, can in principle be employed in a sequence of transitions leading to a high lying doubly excited state. The probability of such transitions is estimated and the importance of various competing processes leading to sequential electron ejection is also discussed. In all cases, the structure of a particular atom and the frequency of the laser are found to play an essential role in the details of the process. Specific results pertaining to He, C, Sr and Xe are discussed as examples of the behavior of multielectron atoms in strong laser fields.