Reactions of O+ with CnH2n+2, n=2-4: a guided-ion beam study

We have measured absolute reaction cross sections for the interaction of O(+) with ethane, propane, and n-butane at collision energies in the range from near thermal to approximately 20 eV, using the guided-ion beam (GIB) technique. We have also measured product recoil velocity distributions using the GIB time-of-flight (TOF) technique for several product ions at a series of collision energies. The total cross sections for each alkane are in excess of 100 A(2) at energies below approximately 2 eV, and in each case several ionic products arise. The large cross sections suggest reactions that are dominated by large impact parameter collisions, as is consistent with a scenario in which the many products derive from a near-resonant, dissociative charge-transfer process that leads to several fragmentation pathways. The recoil velocities, which indicate product ions with largely thermal velocity distributions, support this picture. Several product ions, most notably the C(2)H(3) (+) fragment for each of the alkanes, exhibit enhanced reaction efficiency as collision energy increases, which can be largely attributed to endothermic channels within the dissociative charge-transfer mechanism.