Sequential and nonsequential contributions to double ionization in strong laser fields

We demonstrate experimentally the difference between a sequential interacti on of a femtosecond laser field with two electrons and a nonsequential proc ess of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon io ns created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an in crease in the observed ion momenta. Al the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distribut ions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself.