Comparison of the cis-bending and C–H stretching vibration on the reaction of C2H2+ with H2 using laser induced reactions

Abstract

Laser induced reaction (LIR) of C₂H₂⁺ + H₂ in a 22-pole ion trap at 90 K has been employed to detect the ν₃ C–H stretching vibration and the ν₅cis bending vibration of the acetylene parent ion using the wide tunability of the free electron laser FELIX. The vibrational frequency of the bending vibration, ω₅, and the corresponding Renner–Teller parameter, ε₅, are determined to be 710 cm⁻¹ and 0.03, respectively. These results differ quite substantially from previous experimental work but are in line with the most recent and advanced theoretical work. The dependence of the LIR-signal of the two vibrational modes is studied systematically with respect to the laser power, storage time, and number density of the hydrogen collision partner. A reaction scheme describing all steps involved in the LIR process is set up. The corresponding rate equation system is solved numerically. From this solution the lifetimes for the vibrational excited states, τ₃ = (3 ± 1) ms and τ₅ = (200 ± 50) ms and the vibrational dipole moments μ₃ = 0.19(2) D and μ₅ = 0.21(2) D are determined under the assumption that the excited parent ion relaxes or reacts with a net rate coefficient equal to the Langevin limit. The lifetime for the C–H stretching vibration is in agreement with a previous LIR experiment and with ab initio calculations. C–H stretching turns out to be about an order of magnitude more efficient than bending in promoting hydrogen abstraction. This strong mode dependence is discussed on the basis of the energetics for hydrogen abstraction and a possible inhibition of complex formation in the entrance channel of the C₂H₂⁺··H₂ collision system.