Coincidence Momentum Imaging of Four- and Three-body Coulomb Explosion of Formaldehyde in Ultrashort Intense Laser Fields

Abstract Ion-coincidence momentum imaging of Coulomb explosion of formaldehyde (H 2 CO) in femtosecond intense laser fields (800 nm, 1.3 × 10 15  W/cm 2 ) is performed with two different laser pulse durations (7 fs and 35 fs). In the 7-fs laser fields, the full fragmentation pathway from H 2 CO 4+  → 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, H 2 CO 3+  → 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.