High resolution kinetic energy release spectra and angular distributions from double ionization of nitrogen and oxygen by short laser pulses

We have used momentum imaging techniques to measure in high resolution the kinetic energy release spectra and angular distributions of coincident O+ and N+ ion pairs produced by short laser pulses (8-35 fs) on targets of N-2 and O-2 at peak intensities between 1 and 12 x 10(14) W cm(-2). We record the full momentum vectors of both members of each pair and achieve a kinetic energy release resolution of less than 0.3 eV We find that the process proceeds through well-defined electronic states of the excited molecular dications. Using linear and circularly polarized light, we identify two mechanisms for the production of these states, rescattering and sequential ionization. By using 8 fs pulses, we observe that the internuclear distance can be frozen during the pulse. For low intensities and 8 fs pulses, emission from N-2 is strongly directed along the polarization vector, while that for O-2 is not, a result we interpret as being due to the different symmetries of the outer orbitals of these molecules. For high intensities and longer pulses, the distributions increasingly fold towards the polarization vector, ultimately peaking at zero degrees for both molecules. For oxygen, a local peaking for molecules aligned at right angles to the polarization vector is seen. A discussion and interpretation of the results are presented.