Asymmetry in the molecular-frame photoelectron angular distribution for oxygen 1s photoemission from CO ₂

Molecular-frame photoelectron angular distributions (MFPADs) for photoemission from the O (1s) ¹ hole in CO ₂ measured in coincidence with the O ⁺ and CO ⁺ fragments, produced in the subsequent Auger decay of the core-hole state, are not symmetric with respect to the reflection plane of symmetry, which is perpendicular to the molecular axis and contains the C atom, found in the CO ₂ molecule in its equilibrium structure. To understand the observed asymmetries, we have considered a one-dimensional vibrational model of the CO ₂ system where we have only included the anti-symmetric stretch. For asymmetric geometries, i.e. when the two CO bonds have different lengths, O 1s ionization of CO ₂ leads to hole states with the hole localized on the left O atom in one state and on the right O atom in the other state. Computed fixed-nuclei MFPADs from these two hole states show significant left-right asymmetries. Using a modified version of the lifetime-vibrational interference equation and the diabatic vibronic states, we then computed the dependence of the MFPAD on the lifetimes of the core holes. We could model the experimental MFPADs using computed Auger decay rates for the decay to a number of CO ^{2 +} ₂ states and by assuming that the decay to a directly fragmenting state leads to the dissociation of the CO bond that is longer at the time of decay and that the decay to a double-ion state which has a minimum at the equilibrium geometry leads to pre-dissociation with an equal probability of breaking each CO bond.