The influence of stagnation pressure and electron energy on the unimolecular decomposition of CO 2 microclusters

Abstract

Electron impact mass spectra of carbon dioxide clusters, generated by supersonic beam expansion, were investigated at various electron energies between 30 and 100 eV and stagnation pressures between 0.1 and 3.0 bar. The distribution of cluster ions (CO 2) N + ( N ≦ 8) was observed to depend on electron energy most strongly at high stagnation pressures. In contrast to monomeric CO 2 +, fragmentation of cluster ions to product ions (CO 2) N−2 O 2 + and (CO 2) N−1 X + (X = O, CO) is influenced by electron energy variations in the range studied. The dependence of relative ion intensities on the electron energy is strongest at high stagnation pressures. It is shown that phase transitions or evaporation reactions in the clusters can be employed for the interpretation of the experimental results. At low stagnation pressures the fragmentation behaviour of the clusters is mainly influenced by their intrinsic properties. At high stagnation pressures evaporation processes become more important.