LIGHT-DRESSED STATES OF H₂O IN INTENSE LASER FIELDS

The Coulomb explosion processes of H₂O, i.e., (i) H₂O³⁺ → H⁺ + O⁺ + H⁺ and (ii) H₂O⁴⁺ → H⁺ + O⁺ + H⁺, in ultrashort intense laser fields (∼ 1 PW/cm²) are investigated by the mass-resolved momentum imaging (MRMI) technique. From the analysis of these MRMI maps of the atomic fragment ions, it is found that the ∠H-O-H bond angle of the parent ions exhibits a significantly broad distribution (FWHM ∼ 60°) centered at the linear configuration and the O-H bond lengths for H₂O³⁺ and H₂O⁴⁺ become 1.7 and 2.0 times as long as R_e = 0.958 Å, respectively. The characteristic ultrafast bond angle deformation occurring prior to the Coulomb explosion is interpreted by the formation of the light-dressed potential energy surfaces of H₂O⁺.