State selected ion–molecule reactions by a TESICO technique. X. O+2(v)+CH4

Abstract

Vibrational state selected (relative) reaction cross sections have been determined for v=0–3 of the O+2 ion, for each of the three product channels of the reaction O+2(v)+CH4, viz. O+2(v)+CH4→CH3O+2+H (1) →CH+3+HO2 (2) →CH+4+O2 , (3) using the TESICO (threshold electron–secondary ion coincidence) technique. At a fixed collision energy of 0.27 eV, it has been found that the cross section of exoergic channel (1) increases most prominently with increasing vibrational quantum number v in the range v=0–2, but decreasees sharply in going from v=2 to v=3. The cross sections of endoergic channels (2) and (3) also increase with increasing v but their rates of increase are much smaller than that of channel (1) in the range v=0–2. When v is increased to 3, however, charge transfer channel (3) is enhanced dramatically and the CH+4 ion becomes the most abundant product ion. The cross section of channel (2) also increases more sharply in going from v=2 to v=3 than in the range v=0–2, but the CH+3 ion still remains the least abundant of the three product ions. As a result of these variations in the individual cross sections, the overall cross section for the O+2+CH4 reaction increases monotonically with increasing v throughout the range studied (v=0–3). The results are compared with that of the collision energy dependence as obtained in drift and flow-drift experiments and the implications are discussed in conjunction with the structure of the CH3O+2 ion and the relevant potential energy surfaces.