High resolution study of cluster anion formation in low-energy electron collisions with OCS clusters

Using the laser photoelectron attachment (LPA) method involving a supersonic target beam with carbonyl sulfide molecules (OCS) seeded in helium, we have studied the formation of cluster anions in low-energy electron attachment (E=1-200 meV) to molecular clusters of OCS at high resolution (energy width 1-2 meV). Homogeneous cluster anions (OCS)q- (q=1-12) are predominantly formed. In addition, due to CS2 impurities in the OCS sample, mixed cluster anions ((OCS)q-1CS2-, q>=1) are observed at levels of a few percent. The energy dependences of cluster anion formation are characterized by a strong rise towards zero energy attributed to s-wave attachment as well as--below the onsets of the (0 1 0), (0 0 1), and (0 2 0) vibrational modes of the OCS molecule--by vibrational Feshbach resonances (VFR) whose importance decreases towards larger cluster sizes q. Formation of the (OCS)2- anion is especially enhanced at near-zero electron energies; moreover, the attachment spectrum exhibits a sharp VFR resonance just below the onset for excitation of the (0 2 0) mode in the OCS molecule. The electron attachment behaviour of OCS clusters is intermediate between that of CO2 clusters (which is dominated by VFRs) and that of CS2 (which exhibits a strong zero energy peak, but no VFRs). This finding is correlated with the properties of the respective molecular neutrals and anions which have been studied using high level ab initio methods. We also report the energetics of the neutral and the anion dimer species. The role of VFRs as doorway states into valence-type anion states is discussed.