Strong-Field Double Ionization of hydrogen: Wavelength Dependent Study

We studied double-ionization of H2 and D2 by intense femtosecond laser pulses of different wavelengths (500, 550, 600, 650, 800, 1300, 2000 nm) and peak intensities, by measuring kinetic energy release of resulting fragments. In addition to fragments from the well known enhanced ionization channel (5-6 eV), higher energy protons/deuterons (8-10 eV) were for the first time observed when using shorter wavelengths (500-650 nm) at high-peak intensities. This channel exhibited wavelength dependence, with KER decreasing for longer wavelengths. The channel was observed with both linear and circular polarization. We attribute this phenomenon to 3-photon excitation of molecular ion sequentially followed by the second ionization.