LETTER TO THE EDITOR Sequential and nonsequential contributions to double

Abstract. 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 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 interaction