Rotationally resolved pulsed field ionization photoelectron study of $CO+(X 2Σ+,v+=0–42)$ in the energy range of 13.98–21.92 eV Available to Purchase

We have obtained rotationally resolved pulsed field ionization–photoelectron (PFI-PE) spectra of CO in the energy range of 13.98–21.92 eV, covering the ionization transitions $CO+(X 2Σ+,v+=0–42,N+)←CO(X 1Σ+,v″=0,N″).$ The PFI-PE bands for $CO+ (X 2Σ+,$ $v+=8–22,$ 24, and 28–39) obtained here represent the first rotationally resolved spectroscopic data for these states. The high-resolution features observed in the PFI-PE spectra allow the identification of vibrational bands for the $CO+ (X 2Σ+,$ $v+=10,$ 14, 15, 17, 18, 21, 24, 25, 29–31, 33, 35–37, and 39) states, which strongly overlap with prominent vibrational bands of the $CO+(A 2Π3/2,1/2,B 2Σ+)$ states. The simulation using the Buckingham–Orr–Sichel model has provided accurate molecular constants for $CO+(X 2Σ+,v+=0–42),$ including ionization energies, vibrational constants (⁠$ωe+=2218.8±3.5 cm−1,$ $ωe+xe+=16.20±0.32 cm−1,$ $ωe+ye+=0.074±0.011 cm−1,$ and $ωe+ze+=−0.001 83±0.000 13 cm−1$⁠), and rotational constants [$Be+=1.9797±0.0051 cm−1,$ $αe+=0.0201±0.0011 cm−1,$ $γe+=0.000 122±0.000 067 cm−1,$ $ze+=−(5.2±1.1)×10−6 cm−1].$ Enhancement of $ΔN<0$ rotational branches, attributable to field-induced rotational autoionization, was clearly discernible in PFI-PE bands for $CO+ (X 2Σ+,$ $v+=0–5,$ 11, and 12). Significant local enhancements due to near-resonance autoionization were observed for low $v+$ (<10) PFI-PE bands of $CO+(X 2Σ+),$ where the density of interloper Rydberg states converging to higher ionic levels is high as manifested in the photoion spectrum. The observation of a long vibrational progression in the Franck–Condon gap region, where strong autoionization states are absent, is consistent with the suggestion that high-n Rydberg states converging to highly excited vibrational levels of $CO+(X 2Σ+)$ are partially populated via direct excitation to a repulsive neutral state. The relatively minor band intensity variation observed for high $v+$ PFI-PE bands is also in accord with the direct excitation model. Since $ΔN=0,$ ±1, ±2, and ±3 rotational branches are observed in the PFI-PE spectra, we conclude that the ejected photoelectrons are restricted to angular momentum continuum states $l=0–4.$