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

Using the laser photoelectron attachment method, we have investigated the formation of cluster anions in low-energy free electron attachment (E = 1-200 meV) 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 ≤2 meV). 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 (0 1 0), symmetric stretch (1 0 0), and asymmetric stretch (0 0 1). 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 1.7 meV 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.