Medium-resolution studies of extreme ultraviolet emission from N2 by electron impact: Vibrational perturbations and cross sections of the c4' 1 Sigma u+ and b' 1 Sigma u+ states.

Abstract

We have studied in a crossed-beam experiment the electron-impact-induced fluorescence spectrum of ${\mathrm{N}}_{2}$ in the extreme ultraviolet (euv) at a spectral resolution of up to 0.03 nm. The optically thin experiment allowed us to obtain the highest-resolution electron-impact emission spectrum of the Rydberg and valence states of ${\mathrm{N}}_{2}$. The spectral measurements provide the emission cross sections of each of the vibrational transitions of the ${c}_{4}^{\mathcal{'}}$ $^{1}\ensuremath{\Sigma}_{u}^{+}$\ensuremath{\rightarrow}X $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ Carroll-Yoshino band system and the b' $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$\ensuremath{\rightarrow}X $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ Birge-Hopfield-II band system. The ${c}_{4}^{\mathcal{'}}$ $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ and b' $^{1}\ensuremath{\Sigma}_{u}^{+}$ states strongly perturb one another by homogeneous configuration interactions. This perturbation leads to vibrational-excitation cross sections whose dependence on v' is quite different from the variation of the unperturbed Franck-Condon factors.The laboratory cross sections were measured from 10 to 400 eV. A modified Born approximation analytic model is given for the ${c}_{4}^{\mathcal{'}}$ $^{1}\ensuremath{\Sigma}_{u}^{+}$ and b' $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ vibrational-excitation cross sections. The modified Born approximation analysis leads to accurate band-system oscillator strengths. The emission cross section for the ${c}_{4}^{\mathcal{'}}$(0) level is found to be the same to within 10% by the relative flow technique and the theoretical excitation cross section using the modified Born approximation and the published ${c}_{4}^{\mathcal{'}}$(0,0) oscillator strength. The relative emission cross section for each of the vibrational levels (v'=0--4,6) of the ${c}_{4}^{\mathcal{'}}$ $^{1}\ensuremath{\Sigma}_{u}^{+}$ state closely agrees with the relative excitation cross section from electron-energy-loss experiments. Predissociation for the vibrational levels of the ${c}_{4}^{\mathcal{'}}$ $^{1}\ensuremath{\Sigma}_{u}^{+}$ state is estimated to be less than 10%. On the other hand, a comparison of emission and excitation cross sections shows that the b' $^{1}\ensuremath{\Sigma}_{u}^{+}$ state is 84% predissociated and that the predissociation yield generally increases with vibrational quantum number.In addition, we have measured at low resolution (0.5 nm) the cross section of the atomic dissociation fragments (N ,N II,N III) from 40 to 102 nm, and we have measured at medium resolution (0.05 nm) the emission cross section of the ${c}_{4}$ $^{1}\ensuremath{\Pi}_{u}$, ${c}_{5}^{\mathcal{'}}$ $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$, ${c}_{5}$ $^{1}\ensuremath{\Pi}_{u}$, and ${c}_{6}^{\mathcal{'}}$ $^{1}\ensuremath{\Sigma}_{u}^{+}$\ensuremath{\rightarrow}X $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$(0,0) transitions. The emission and excitation cross sections represent a substantial improvement in the available data base. Previous electron-impact measurements were hampered by an incorrect value of the Lyman-\ensuremath{\alpha} standard cross section, uncertain band-system oscillator strengths, incorrect identification of strongest spectral features, and unresolved structure. This problem has led aeronomers of the Earth's upper atmosphere to overestimate the euv photon flux and predissociation yields from Rydberg and valence states of ${\mathrm{N}}_{2}$ in the photoelectron-excited dayglow and aurora by a factor of 2.