High resolution study of cluster anion formation in low-energy electron collisions with molecular clusters of CO 2, CS 2, and O 2

Abstract

Using a high resolution (Δ E=1–2 meV) laser photoelectron attachment (LPA) method, we have studied the formation of (CO 2) q − ( q=4–32), (CS 2) q − ( q=1, 2, 3, 5), and (O 2) q − ( q=5–14) cluster anions in collisions of low-energy electrons (0–200 meV) with molecular clusters of CO 2, CS 2, and O 2 molecules in a skimmed supersonic beam. For CO 2 clusters, the attachment spectrum is dominated by rather narrow vibrational Feshbach resonances (VFRs) of the type [(CO 2) N−1 CO 2( ν i )] − which involve a vibrationally excited molecular constituent ( ν i ≥1 denotes excited vibrational mode). The size-dependent values of the observed redshifts of the VFRs (about 12 meV per added CO 2 unit for small q) in conjunction with their narrow widths and with simple model calculations of the electron binding energies suggest that the size q of the observed (CO 2) q − anions is essentially the same ( q= N or N−1) as the size N of the neutral precursor (CO 2) N which captures the electron. For cluster sizes q above 24, a doubling of the VFR peak structure is observed which we tentatively attribute to the transition from icosahedral (low N) to bulk cubic (high N) cluster structure, previously predicted to occur for N around 30. The size distribution in the (CO 2) q − anion mass spectrum, due to attachment of electrons with well-defined energies, shows undulatory structure which reflects the presence or absence of VFRs at that particular energy. In contrast, the energy-integrated cluster anion mass spectrum shows a rather smooth dependence on cluster size q. The yield spectra for (CS 2) q − cluster anions exhibit an s-wave attachment peak near 0 eV and a smoothly falling cross section towards higher energies without significant structure. In the yield for (O 2) q − anions we observe a rise towards 0 eV, indicative of s-wave electron capture, and a slowly varying and essentially structureless attachment continuum between 10 and 200 meV. The attachment characteristics of the different clusters systems are discussed in the light of the basic properties of the monomers.