High resolution low-energy electron attachment to molecular clusters of sulfur dioxide

Abstract

Using the laser photoelectron attachment method, we have investigated the formation of cluster anions in low-energy free electron attachment (E=1–200meV) as well as in Rydberg electron transfer (at principal quantum numbers 20–260) to molecular clusters of sulfurdioxide (SO2) in a collimated supersonic beam at high electron energy resolution (energy width ≤2meV). At these low energies (almost) only homogeneous cluster anions (SO2)q− (q≥1) are found with energy dependences which show – superimposed on a monotonically decreasing continuum cross section – characteristic peaks which are interpreted as vibrational Feshbach resonances (VFR), associated with excitation of the three fundamental vibrational modes in the SO2 molecule, namely bending (010), symmetric stretch (100), and asymmetric stretch (001). The VFRs are clear for q=2–12, but only weak for q=1. The energy positions of the VFRs are progressively red-shifted with increasing cluster anion size q by about 1.7meV per added molecular unit, and the peaks broaden towards larger q. The cross section enhancement in the VFRs – compared to the size of the continuum – is substantial, indicating that the VFRs act as important doorway states for electron capture. Model R-matrix calculations are presented which recover the main features of the experimental attachment spectra for cluster anion formation and suggest that VFRs occur for neutral (SO2)N clusters with sizes N≥4. With growing N, the electron binding energy increases and the capture cross section decreases. In contrast, the Vogt–Wannier model for electron capture into a polarization well predicts a growth of the cross section with the cluster size.