Sequential and nonsequential contributions to double ionization in strong laser fields

Th Weber, M Weckenbrock, A Staudte, L Spielberger, O Jagutzki, V Mergel, F Afaneh, G Urbasch, M Vollmer, H Giessen, R Dörner
2000 Journal of Physics B: Atomic, Molecular and Optical Physics  
We demonstrate experimentally the difference between a sequential interaction of a femtosecond laser field with two electrons and a nonsequential process of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon ions created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an increase in the observed ion momenta. At the onset
more » ... enta. At the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distributions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself. Coupling of a time-dependent field to a quantum mechanical many-body Coulomb system can lead to two-body transitions in at least two ways. One way is an independent coupling of the external field to each of the two particles that are to be excited. The other way is the interaction of a single particle, which then excites the second particle by correlation within the composed system. Prominent examples for such behaviour can be found in atomic physics if two electrons in an atom are ejected by time-dependent fields, for example, from an intense laser or a traversing charged particle. Generally, the contribution due to two interactions with the field decreases much faster with decreasing perturbation strength than the process involving internal correlation. For double ionization by a short (femtosecond) intense laser (10 14 -10 16 W cm −2 ), the process involving two interactions with the field is referred to as sequential double ionization (see, e.g., [1, 2] and [3] for a recent review), while for charged particle impact it is commonly called TS2 (two step two) (see, e.g., [4, 5] and [6] for a recent review). For laser impact, the distinction of the sequential and nonsequential double-ionization mechanisms is so far based solely on the experimental observation of a stepwise increase of the ratio (R) of the rates of doubly-to-singly charged ions with increasing perturbation strength and its theoretical interpretation (see [1, 2] for experiments on helium and [7, 8] for experiments on argon and other noble gas targets (cf also [9] ). Theoretically, a clear cut distinction between the sequential and nonsequential process can be made in terms of Feynman diagrams [9, 10]. Recently, Dundas and co-workers have analysed sequential and nonsequential contributions in an elaborate three-dimensional time-dependent calculation [11] . The main finding of an 0953-4075/00/040127+07$30.00
doi:10.1088/0953-4075/33/4/104 fatcat:k5ou3fix7vgwfbd53uw73dux4i