OPTICAL-EMISSIONS FROM THE DISSOCIATIVE ELECTRON RECOMBINATION OF N2H+ AND HCO+

Emission spectroscopy in the wavelength range 190-800 nm has been used to detect electronically excited products of the dissociative recombination of N2H+ and HCO+ with electrons. Flowing afterglow techniques were used to generate recombining plasmas consisting only of the ions of interest and electrons. Population of the excited states N-2(A(3) Sigma(u)(+), (BIIg)-I-3, W-3 Delta(u), B'(3) Sigma(u)(-) and a'(1) Sigma(u)(-)) and NH(a(1) Delta) is energetically possible for N2H+ and CO(a(3)II, and a'(3) Sigma(+)) for HCO+. Of these, very obvious emissions were observed from the N-2 B State (the B --> A(3) Sigma(u)(+) transition) and the CO a state (the spin-forbidden a --> X(1) Sigma(+) transition). Detection of the a-state emissions shows that the technique can detect emissions from long-lived excited states. A degree of vibrational excitation in the product ions was detected which could only originate from vibrationally excited ions. This excitation in the N2H+ and HCO+ was quenched by addition of large concentrations of N-2 and CO, respectively. Such excitation could be responsible for the factor of 2 discrepancy in the literature values of the recombination rate coefficient of HCO+. Information on the vibrational product distribution for vibrationally relaxed ions was obtained for the N-2 B State with relative populations of 100, 28, 24, and 14 for v' from 2 to 5. For the CO a state, emissions from vibrational levels up to v' = 7 were detected; however, not all of these were fully resolved. Data for the lowest vibrational levels showed that the populations of the v' = 0 and 2 levels were approximately equal with that of v' = 1 being about 30% smaller. For N2H+, weak emissions were also detected from overlapping N-2(B-->X, W-->X, a'-->X) bands.