Study of the A → X Transitions in N2+ and CO+

Abstract

Optical emissions from long-lived, excited states of CO+ and N2+ in ion beams have been studied. CO or N2 is ionized by electron impact, and the ions are removed from the ion source to the observation region. Spectra of emission from CO+ and N2+ are obtained for wavelengths in the ranges 3200–5200 Å and 6000–8700 Å, respectively. All of the spectral features can be ascribed to the CO+ comet-tail and N2+ Meinel emission systems. The electronic transition moments, R̄e, for these two emission systems are determined and are found to vary rather slowly with wavelength over the wavelength ranges for which we have data. Lifetimes of the v′=1, 2, 4, and 6 vibrational levels of CO+ (A 2π) are found to be 3.49, 2.78, 2.63, and 2.41 μsec, respectively. Lifetimes of the v′=2, 3, 4, and 5 vibrational levels of N2+(Aπu2) are found to be 12.28, 10.70, 10.08, and 9.14 μsec, respectively. The dependences of the measured lifetimes on v′ are consistent with the experimental values of Re and with available Franck-Condon factors. Relative cross sections for excitation of various emission bands have been derived from the spectra and lifetimes presented in this report; these cross sections agree fairly well with theory and with previous measurements. The A states of CO+ and N2+ appear to constitute about 46% and 30%, respectively, of the ions produced by 46.5-eV electrons impinging on CO and N2. The electron energy dependences of the light output in the (2, 0) Meinel band and in the (3, 0) and (4, 0) comet-tail bands have been measured from threshold to 200 eV. From our data and total ionization cross sections we obtain maximum cross sections for excitation of the (2, 0) Meinel and the (3, 0) comet-tail bands of 0.073 and 0.133 Å2, respectively.

Ab initiocalculations of radiative transition probabilities in theX2Σ+andA2Π electronic states of CO+

“…For v' ~> 5 the agreement between the theoretical and experimental lifetimes is excellent, the deviations are less than 5 per cent in all cases. For lower vibrational states the calculated lifetimes are in satisfactory agreement with the experimental values of Bondybey and Miller [5], but they are 10-20 per cent larger than those of other authors [2,3,7]. We found that the experimental lifetimes of M6hlmann and de Heer [3] for all vibrational levels can be welt reproduced by using a constant transition moment of 0.274 ea0, that is the value calculated at the equilibrium distance of the ground state.…”

“…In view of the experimental difficulties arising from the spacial dissipation of the ions, caused by their mutual electrostatic repulsion, the measured"lifetimes of all the above mentioned groups (With the exception of [6]) are in surprisingly good agreement. It has been found that the observed v' dependence of the lifetimes can be well reproduced within the Franck-Condon approximation [2,3,8], which implies that the electronic transition moment does not vary much in the investigated range of vibrational levels (v'=0-9). However, the true r-dependence of the transition moment function, which approaches zero at large internuclear distances, is so far unknown.…”

“…However, the electronic transition probabilities into the X ~Z + state are 2-3 orders of magnitude larger and therefore the fluorescence within the A 311 state will not play any significant r61e. F r o m the fact that the v' dependence of the measured radiative lifetimes of the A ~II state of CO + can be well reproduced within the Franck-Condon approximation it has been concluded that the transition moment function should be almost constant in the vicinity of the equilibrium distances [2,3,8]. However, since both the X eE+ and the ~/2II states dissociate into the ground states of the C ÷ and O atoms, the transition moment function approaches zero at large internuclear distances.…”

“…In particular, much effort has been devoted to the investigation of the fluorescence from the first excited A ~Yl state into the X 2y,+ ground state [2][3][4][5][6][7][8], which contributes to the visible emission of comet tails [9]. The first accurate lifetime measurements for the A 2II state were performed by Holland and Maier [2], who observed the fluorescence from a CO + ion beam at very low pressures. M6hlmann and de Heer [3] measured the lifetimes for ten vibrational states using the delayed coincidence technique [4].…”

Influence of multiple scattering on the radiation of relativistic particles in amorphous and crystalline media

“…Rosen et al argued that this indicates that some of the ions used in the Mitchell and Hus study were in the A 2 state that has a lifetime of between 3.82 µs for the v = 0 level and 2.1 µs for the v = 9 level [7,8]. These times are longer than the flight time from the ion source to the interaction region in the single-pass experiment but much shorter than the beam lifetime in the storage ring experiment.…”