Coincidence momentum imaging of four- and three-body Coulomb explosion of formaldehyde in ultrashort intense laser fields

Ion-coincidence momentum imaging of Coulomb explosion of formaldehyde (H₂CO) in femtosecond intense laser fields (800 nm, 1.3 × 10¹⁵ W/cm²) is performed with two different laser pulse durations (7 fs and 35 fs). In the 7-fs laser fields, the full fragmentation pathway from H₂CO⁴⁺ → H⁺ + H⁺ + C⁺ + O⁺ is identified. The angles between the fragment momenta are well reproduced by a simple Coulomb explosion model from the geometry of neutral formaldehyde, showing that the molecular structure is virtually frozen along the bending coordinates during the multiple ionization. Three-body Coulomb explosion pathways from triply charged states, H₂CO³⁺ → H⁺ + H⁺ + CO⁺ and H⁺ + CH⁺ + O⁺, are observed in both the 7-fs and 35-fs laser fields. Significant changes in the momentum angle distribution and asymmetric energy partitioning between two H⁺ ions are observed in the 35-fs case, which are attributed to structural deformation prior to the Coulomb explosion in the longer pulse.