Chemiluminescent ion–molecule reactions: Rotational–vibrational population distributions of CH+(A 1Π) and CD+(A 1Π) from C++H2 and C++D2 collisions

Abstract

Optical emission in the 3000–5000 Å range was observed as a result of low‐energy (3–8 eV_CM) ion–molecule reactions of C⁺ ions with H₂ and D₂ molecules. Most of the emission is due to (A ¹Π→X ¹Σ⁺) transitions of CH⁺ and CD⁺, which were each studied with 6 Å and 20 Å FWHM optical resolution. Rotational lines of much higher J were observed than are present in spectra from conventional CH⁺ light sources. The observed spectra were simulated on a computer, taking into account the strong dependence of the electronic transition moment on the internuclear distance, as given by ab initio calculations. Relative band oscillator strengths f_v′v^″(J) were calculated for each rotational line J and were found to depend strongly on J as result of centrifugal stretching. These calculations were done using the R centroid approximation as well as the exact band oscillator strength formula. From the spectrum simulations, accurate rotational and vibrational population distributions were derived. A simple model is proposed which explains the isotope effect on the rotational distributions and gives directly the distribution of impact parameters leading to reaction.