State selected ion–molecule reactions by a TESICO technique. IX. Vibrational state dependence of the cross sections in the reaction C2H+2(ν2)+D2(H2)

Abstract

Vibrational-state selected reaction cross sections have been measured for four product channels of the reaction C2H+2+D2, by use of the threshold electron–secondary ion coincidence (TESICO) technique. The ν2 vibrational states of the C2H+2 ion were selected up to v=2 and the collision energies were changed from 0.2 to 3.7 eV. At low collision energies close to 0.2 eV, considerable enhancement of the cross sections for the channels producing C2H2D+ and C2HD+2 was observed when the vibrational quantum number was increased successively. As the collision energy was increased, the extent of this enhancement diminished gradually and the cross sections finally became almost independent of v at about 2 eV. The cross section for the H/D exchange channel, on the other hand, was found to decrease with increasing vibrational quantum number at low collision energies. However, the v=0 and v=1 cross sections of this channel, especially the former, decayed very quickly as the collision energy was increased, reaching essentially zero at ∼2 eV, while the cross section for v=2 remained substantial even at this and higher collision energies. These results have been discussed in terms of two concurrent reaction mechanisms, i.e., a complex mechanism predominating at low collision energies and a direct one predominating at high collision energies. The ν2 state selected cross sections for the reaction C2H+2+H2 →C2H+3+H have also been studied in a similar collision energy range for comparison.