Covariance Velocity Map Imaging Measurements of Strong Field Double Ionization

Double ionization has been of long outstanding interest as a probe of electron correlation in molecules exposed to strong laser fields. However, state resolved measurements of the double ionization yield have been quite elusive. Recently, progress has been made on both single photon and strong field state resolved double ionization on water molecule. Here we have extended our experiment to more complicated molecular structures such as deuterated formaldehyde (CD2O). We doubly ionize the molecules with pulses over a range of pulse energies and durations (5-30 fs, 44 TW/cm2 - 240 TW/cm2 ). We measure the full 3D momentum of the fragments using velocity map imaging and a Timepix camera, and we make use of covariance analysis to separate different dissociation channels. By performing trajectory surface hopping (TSH) calculations on the low lying states of the dication, we can determine the kinetic energy release (KER) of fragments associated with each state of the dication. Fitting the measured KERs for different dissociation channels allows us to determine the relative population of the different dication states produced by the strong field double ionization. We find that the populations do not vary monotonically with energy.