Quantum dynamics of proton migration in H₂O dications: H₂⁺ formation on ultrafast timescales

Irradiation of isolated water molecules by few-cycle pulses of intense infrared laser light can give rise to ultrafast rearrangement resulting in formation of the H₂⁺ ion. Such unimolecular reactions occur on the potential energy surface of the H₂O²⁺ dication that is accessed when peak laser intensities in the 10¹⁵ W cm⁻² range and pulse durations as short as 9–10 fs are used; ion yields of ∼1.5% relative to the H₂O⁺ ion are measured. We also study such reactions by means of time-dependent wavepacket dynamics on an ab initio potential energy surface of the dication and show that a proton, generated from O–H bond rupture, migrates towards the H-atom, and forms vibrationally excited H₂⁺ in a well-defined spatial zone.