Resonant structure in electron impact excitation of CO near threshold

Vibrationally resolved photoemissions of N2 (C3Πu → B3Πg) and CO (b3Σ+ → a3Π) by low-energy electron impacts

Vibrationally resolved photoemission spectra of the electronic-state transitions C³Π_u → B³Π_g of N₂ and b³Σ⁺ → a³Π of CO following low-energy electron impacts are measured with a crossed-beam experimental arrangement. The absolute cross sections of C³Π_u (ν') → B³Π_g (ν″) of N₂ are presented for the vibrational state-to-state transitions (ν',ν″) = (0,0), (0,1), (1,0), (1,2), and (2,1). The excitation cross sections of the metastable state C³Π_u of N₂ show the maxima at the electron-impact energies 14.10 (ν' = 0) eV and 14.50 (ν' = 1) eV, which are potentially related to the core-excited vibrational Feshbach resonant state ²Σ_u ⁺ of N₂ ^- formed by electron attachment. The absolute cross sections of b³Σ⁺ (ν' = 0) → a³Π (ν″ = 0, 1, 2, 3, 4) of CO are given by the calibrations with those of N₂ measured in this work. Besides the maximum excitation cross section 5.85 × 10^-18 cm² at 10.74 eV of the CO b³Σ⁺ (ν' = 0) state, some fine structures on the excitation function profile are attributed to different shapes and Feshbach resonant states of CO^- formed by electron attachment, while the others arise from the direct electron-impact excitation. Some discrepancies, particularly for N₂, between the present data and the results available in the literature studies arise from different experimental techniques and data-processing procedures. Furthermore, contributions of physical processes such as wave-packet evolution and non-Franck-Condon dynamics are highlighted here.