  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% 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.
