Cross Section for Excitation of the Fourth Positive Band System in Carbon Monoxide by 20-120 keV Protons

Excitation cross sections for the fourth positive band system in carbon monoxide have been determined from studies of the energy-loss spectra of 20-120-keV protons incident on gaseous CO targets. The energy-loss spectra had an energy resolution of about 2 eV. Prominent peaks in the spectra were observed at 8.5 and 13.8 eV. The first peak is believed to be due to excitation of the fourth positive band system of $\mathrm{CO} (X{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}\to A{}_{}{}^1{}_{}{}^{}\Pi$, while the 13.8-eV peak covers a number of possible states, including the $B{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ and the $C{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ states. Changes in the slope of the ionization continuum were noted at 16.5 eV, corresponding to the $A{}_{}{}^2{}_{}{}^{}\Pi _i^{}{}_{}{}^{}$ state of C${\mathrm{O}}^{+}$, and at 20.5 eV, corresponding to the $B{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ state of C${\mathrm{O}}^{+}$. Absolute excitation cross sections for the fourth positive band system of CO are presented, as well as the total inelastic cross sections and the total ionization cross sections for 20-120-keV protons incident on CO.