Cross Sections for the Formation of Excited States in a Nitrogen Target by the Impact of 0.15- to 1.0-MeV Protons

Abstract

Protons in the energy range 0.15-1.0 MeV were incident on a molecular nitrogen target and the resulting emission was analyzed spectroscopically. Absolute measurements were made of the intensity of certain spectral lines. Cross sections are presented for the emission of two prominent lines of singly ionized atoms and for nine lines from the ${\mathrm{N}}_2^{+}$ first positive band system. Cross sections for the population of the $v^{\prime }=0\mathrm{}$ and $v^{\prime }=1\mathrm{}$ vibrational levels of the $B{}_{}{}^2{}_{}{}^{}\Sigma$ excited state of $N_2^{}{}_{}{}^{+}$ are obtained from the emission functions. In the present work, the proportion of ${\mathrm{N}}_2^{+}$ ions formed in the excited state remains almost independent of impact energy, indicating that the excited ${\mathrm{N}}_2^{+}$ ions are produced by simultaneous ionization and excitation of the target. The measured relative populations of two vibrational levels in the ${\mathrm{N}}_2^{+}$ ion are shown to be in good agreement with the ratios of the relevant overlap integrals. An investigation of the emission of the ${\mathrm{N}}_2$ second positive system shows that it is mainly excited by secondary-electron impact, the cross section for proton impact excitation of the $C{}_{}{}^3{}_{}{}^{}\Pi$ state of the ${\mathrm{N}}_2$ molecule being some three orders of magnitude less than that for electrons at the same impact velocity. This indicates the importance of spin conservation in governing the transitions which occur in collisional excitations by heavy-particle impact.