State-to-state photoionization dynamics of vanadium nitride by two-color laser photoionization and photoelectron methods

We have conducted a two-color visible-ultraviolet (VIS-UV) resonance-enhanced laser photoionization and pulsed field ionization- photoelectron (PFI-PE) study of gaseous vanadium mononitride (VN) in the total energy range of 56900-59020 cm^-1. The VN molecules were selectively excited to single rotational levels of the intermediate VN(D³Π ₀, v′=0) state by using a VIS dye laser prior to photoionization by employing a UV laser. This two-color scheme allows the measurements of rovibronically selected and resolved PFI-PE spectra for the VN⁺(X²Δ; v⁺=0, 1, and 2) ion vibrational bands. By simulating the rotationally resolved PFI-PE spectra, J⁺=3/2 is determined to be the lowest rotational level of the ground electronic state, indicating that the symmetry of the ground VN⁺ electronic state is ²Δ_3/2. The analysis of the PFI-PE spectra for VN⁺ also yields accurate values for the adiabatic ionization energy for the formation of VN⁺(X²Δ_3/2), IE(VN)=56909.5±0.8 cm^-1 (7.05588±0.00010 eV), the vibrational frequency ω_e⁺=1068.0±0.8 cm^-1, the anharmonicity constant ω_e⁺χ_e⁺=5.8±0.8 cm^-1, the rotational constants B_e⁺=0.6563±0.0005 cm^-1 and α_e⁺=0.0069±0.0004 cm^-1, and the equilibrium bond length, r_e⁺=1.529 Å, for VN⁺(X²Δ_3/2); along with the rotational constants B_e⁺=0.6578±0.0028 cm^-1 and α_e⁺=0.0085±0.0028 cm^-1, and the equilibrium bond length r_e⁺=1.527 Å for VN⁺(X²Δ_5/2), and the spin-orbit coupling constant A=153.3±0.8 cm^-1 for VN⁺(X²Δ_5/2,_3/2). The highly precise energetic and spectroscopic data obtained in the present study are valuable for benchmarking the predictions based on state-of-the-art ab initio quantum calculations. © 2013 Chinese Physical Society.