Nonsequential double ionization of small molecules induced by a femtosecond laser field

Nonsequential double ionization of several molecules such as N₂,CO₂,C₂H₂, and C₃H₄ in strong laser fields has been unambiguously identified from the ion-yield measurements in comparison with a sequential ionization model in the 10¹³--10¹⁵ W/cm² laser intensity range. This effect is observed mainly using linearly polarized laser light. In circular polarization, nonsequential double ionization might be present, for instance for the C₂H₂ molecule, but with a much lower contribution than in linear polarization. The experimental method allows us to detect all the double-ionization decay channels that are the molecular dication and the two-missing electron fragmentation channels. The validity of a single-ionization tunneling model developed for atoms has been extended to molecules, and a good agreement is observed in laser intensity ranges where single-ionization processes take place.