Breakdown of the Coulomb-explosion imaging technique induced by the ultrafast rotation of fragments

Coulomb-explosion imaging is a broadly employed technique to reconstruct the geometry of molecules from the direct multibody breakups of its ions. However, we reveal that this technique fails for a large class of systems, such as (CO)(2)(3+) and ArCO3+, since the events of the "direct breakup channel" are not the real direct breakups but the rapid sequential breakups with a short-lived and ultrafast-rotational fragment. Using Ar-CO as a prototype, we have investigated theoretically its Coulomb-explosion process. We find that due to the interfield between the metastable CO2+ and Ar+, ArCO3+ changes from its initial T shape into the more stable linear shape within 100 fs; such a rotation of CO2+ is hundreds of times faster than the field free case. Further, the advanced three-body surface hopping simulations indicate that the angle angle((Ar,CO)) has rotated from an initial 95 degrees to 110.6 degrees on average before the Coulomb explosion, which agrees well with the one (115 degrees) detected by Gong et al. [Phys. Rev. A 88, 013422 (2013)]. Furthermore, we point out that correct geometry of ArCO can be obtained from the three-body breakups with high kinetic-energy release (>20 eV).