Luminescence in near-thermal charge exchange. III. CH(+) and CD(+) A → X emission from He+ C2H2 and He+ + C2D2

Abstract A pulsed ICR cell fitted with synchronous photon counting equipment is used to investigate the emission produced between 185 and 500 nm by near-thermal charge exchange between He + and C 2 H 2 (C 2 D 2 ). The emission bands observed are A 2 Δ → X 2 π and (weakly) B 2 Σ − → X 2 π in CH(CD) and A 1 π → X 1 Σ in CH + (CD + ). Wavelength measurements on the bandheads of the (0,0) and (0,1) bands of CD + A → X are used to evaluate vibrational constants of CH + (CD + ) X 1 Σ + . The results are (in cm −1 ): ω e = 2869 ± 27 (2106 ± 20); ω e χ e = 65 ± 13 (35 ± 7). These constants are used to calculate Morse-potential Franck—Condon factors and vibrational branching ratios for CH + and CD + A → X emission. The spectral distributions and the (relatively low) absolute emission rates produced by He + /C 2 H 2 (C 2 D 2 ) charge exchange are briefly discussed in the light of presently available information on the charge transfer reaction and on the excited states of C 2 H 2− +