Excitation of the CO Fourth Positive Band System by Electron Impact on Carbon Monoxide and Carbon Dioxide

Abstract

Absolute excitation cross sections were measured for five vibrational bands of the $\mathrm{CO}{\mathrm{(A}}^1{\mathrm{\Pi –X}}^1{\Sigma }^{+})$ Fourth Positive band system. The bands were produced by electron impact on CO and CO₂ and the cross sections were measured from threshold to 350 eV. Relative intensity measurements on 28 bands were used to determine the dependence of $R_e$ on the $r$ centroid, which is $R_e{\text{\hspace{0.17em}=\hspace{0.17em}1.9(1.0\hspace{0.17em}−\hspace{0.17em}0.6r̄}}_{\mathrm{\upsilon \prime \upsilon ″}})$ . Absolute transition probabilities were computed using this functional form for $R_e$ and normalizing the total transition probability of the $\text{υ′\hspace{0.17em}=\hspace{0.17em}2}$ level to published experimental results. The total f‐value for this transition is then 0.15. Absolute excitation cross sections for the first five vibrational levels $\text{(υ′\hspace{0.17em}=\hspace{0.17em}0\hspace{0.17em}→\hspace{0.17em}4)}$ of the $A^1\Pi$ state were determined for each parent gas using the measured cross sections of specific bands and the absolute transition probabilities. When CO was the parent gas, the relative populations of these vibrational levels were proportional to the Franck–Condon factors, $q_{\text{υ′0}}$ . The sum of the cross sections for these levels is 1.87 × 10⁻¹⁷ cm² ± 15% at 300 eV. This represents 87.4% of the total excitation cross section for the $A^1\Pi$ state. The relative populations of the first five vibrational levels were also determined for dissociative excitation of CO₂ and they differed from the relative populations which resulted from direct excitation of CO. The sum of the cross sections for these five levels is 1.4 × 10⁻¹⁸ cm² ± 17% at 300 eV.