Charge Resonance Enhanced Multiple Ionization of H₂O Molecules in Intense Laser Fields

We perform a kinetically complete measurement on the fragmentation of Coulomb explosion of H₂O molecules in intense few-cycle linearly and circularly polarized laser fields. Both the fragmentations of H₂O³⁺ and H₂O⁴⁺ reveal the concerted pathway of dissociation. The length of the O–H bond prior to the Coulomb explosion of both molecular ions is sensitive to the laser pulse duration and laser intensity. However, the bending angle of H–O–H is less sensitive to the pulse duration and laser intensity. We introduce the mechanism of charge resonance enhanced double ionization to elucidate the triple (or quadruple) dissociative ionization dynamics of H₂O, in which two electrons are non-adiabatically localized at the protons of the precursor ion H₂O⁺ (or H₂O²⁺) and are released simultaneously due to the over barrier ionization in the combined laser field and molecular ionic potential. Such charge resonance enhanced multiple ionization is not suppressed in few-cycle laser fields and elliptically polarized laser fields.