Ejection anisotropy in three-atom Coulomb explosions

Coulomb explosion imaging has been used to explore anisotropies in the fragment angular distribution associated with the symmetric six-electron Coulomb explosion channel of CO₂ and NO₂ induced by 100 fs, 10¹⁵ W∕cm² radiation at 800 nm and leading to doubly charged atomic ions. Specific precursor molecular geometries (bond angles prior to explosion) and orientations of the molecular axis (the line connecting the two outer atoms in the system) relative to the polarization axis were isolated for analysis by exploiting the correlation among the atomic ions ejected simultaneously. The effective orientation of the precursor molecular axis is preferentially along the polarization axis for both CO₂ and NO₂ with respective distributions and widths of cos³⁹ θ (Δθ≃22°) and cos²⁵ θ (Δθ≃27°), which are substantially narrower than that of H₂, cos¹⁹ θ (Δθ≃31°). The widths and distributions are found to be nearly independent of bond angle over a wide range of bond angles (35° and 55° for CO₂ and NO₂, respectively). The narrowing of the width of the distribution from H₂ to CO₂, combined with the independence of bond angle are consistent with (1) an increasing moment of inertia, (2) an increasing final charge state, and (3) an increasing precursor molecular ion ionization stage.