Formation ofN2+BΣu+2andN2CΠu3in collisions ofH+and H withN2

Abstract

The 200-500-nm radiation excited by collisions of beams of 1-25-keV ${\mathrm{H}}^{+}$, ${\mathrm{D}}^{+}$, H, and D with ${\mathrm{N}}_{2}$ has been studied under thin-target conditions with a viewing geometry chosen to minimize polarization effects. For both ion and neutral impact, the $\mathrm{N}_{2}^{}{}_{}{}^{+}$ ($B^{2}\ensuremath{\Sigma}_{u}^{+}\ensuremath{-}X^{2}\ensuremath{\Sigma}_{g}^{+}$) first negative ($1n$) bands are the most intense spectral features in this wavelength range. As expected from consideration of electron-spin conservation, the probability of excitation of the ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}\ensuremath{-}B^{3}\ensuremath{\Pi}_{g}$ second positive ($2p$) bands by H or D impact greatly exceeds that for ${\mathrm{H}}^{+}$ or ${\mathrm{D}}^{+}$ bombardment. Relative emission cross sections for the (0,0) bands of the $1n$ system at 391.4 nm and the $2p$ system at 337.1 nm were determined and made absolute via normalization to measurements reported at higher energies by previous workers. The relative vibrational population $P({v}^{\ensuremath{'}})$ for ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}$ formed in 3-25-keV H and D impact is in good agreement with the prediction of a simple, Franck-Condon excitation mechanism; however, for $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ formed in collisions with 4-25-keV H and D, a dramatic enhancement in $P({v}^{\ensuremath{'}})$ for ${v}^{\ensuremath{'}}>0$ over the Franck-Condon values is observed at low projectile velocities. The higher-velocity onset and greater magnitude of this effect for H and D in comparison with that observed for ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ are in disagreement with the predictions of a modified Franck-Condon mechanism that allows for polarization of ${\mathrm{N}}_{2}$ by the incident projectile. The measured values of $P({v}^{\ensuremath{'}})$ were used in conjunction with the (0,0) band cross sections to derive total cross sections for formation of $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ and ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}$. A maximum in the cross section for formation of $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ of 1.12\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 10 keV was found for ${\mathrm{H}}^{+}$ impact, while for H, the cross section for formation of this state rises steadily with increasing collision energy until reaching a nearly constant value of 3.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ in the 15-25-keV range. The fraction of the total $\mathrm{N}_{2}^{}{}_{}{}^{+}$ yield that is formed in the $B$ state increases from about 0.03 to 0.08 in the energy range studied. For formation of ${\mathrm{N}}_{2}C^{3}\ensuremath{\Pi}_{u}$ the cross section has a maximum value of 2.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 5 keV. At H-atom energies below 7 keV, exchange excitation of ${\mathrm{N}}_{2}$ to the $C^{3}\ensuremath{\Pi}_{u}$ state is more probable than ionization to yield $\mathrm{N}_{2}^{}{}_{}{}^{+}$ in the $B$ state while, at higher energies, ionization to yield the $B$ state is the more probable process. Comparison of our results with values calculated via the application of recently developed, semiempirical scaling relationships to electron impact cross-section data indicates that the latter approach yields a good prediction of the energy dependence and an acceptable prediction of the magnitude of the $C^{3}\ensuremath{\Pi}_{u}$ cross section for 1-100-keV H impact. The semiempirical approach also makes a good prediction of the cross section for formation of $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ in ${\mathrm{H}}^{+}$ or H impact above 10 keV; however, primarily because of the lack of experimental data at lower energies, previous semiempirical predictions have overestimated the ionization contribution to the over-all $\mathrm{N}_{2}^{}{}_{}{}^{+}B^{2}\ensuremath{\Sigma}_{u}^{+}$ cross section.