Isomer effects in orbital-resolved photoionization cross sections of C2H4O2: Role of continuum dynamics and shape resonances

Abstract

We present ab initio calculations of total and orbital-resolved photoionization cross sections for the C2H4O2 isomers glycolaldehyde, acetic acid, and methyl formate from threshold up to 40 eV. The calculations were performed using the ePolyScat framework at the static-exchange (SE) and SE-plus-polarization levels. The total photoionization cross sections, obtained as the sum over all twelve valence orbitals, exhibit a similar overall energy dependence for all isomers, with systematic differences near the ionization threshold and in the region of the cross section maximum. In contrast, the orbital-resolved analysis reveals a pronounced isomer dependence. To justify the single-particle picture underlying the scattering calculations, electron propagator theory was employed; the calculated pole strengths for the outer-valence orbitals remain high (∼0.9), validating the use of Hartree–Fock initial states as adequate representations of the Dyson orbitals. The highest occupied molecular orbitals, dominated by nonbonding oxygen lone pairs, give rise to the largest cross sections. Pronounced shape resonances are observed in the HOMO channels in the 15.5–19.3 eV range, with positions and intensities strongly dependent on molecular structure. Partial-wave analysis shows that these features are primarily governed by the l = 4 component, with additional contributions from neighboring channels. These results demonstrate that, despite identical stoichiometry, differences in molecular structure lead to distinct continuum electron dynamics. The computed cross sections provide quantitative input for astrochemical models and provide insights into isomer-dependent behavior in UV-irradiated environments.