Physical Properties of Nitrogen Molecules and Ions in External Electric Fields

Based on density functional theory (DFT) with the B3LYP functional and the 6-311G+(d) basis set, this work systematically examines the evolution of physical properties—including bond length, total energy, energy level distribution, infrared spectra, and tunneling effects—in N₂ molecules and N₂⁺ ions under an external electric field (ranging from 0 to 0.03 a.u.) applied perpendicular to the N–N bond axis. The results indicate that as the electric field strengthens, the bond length of the N₂ molecule exhibits stepwise variations, whereas that of N₂⁺ increases monotonically without such discrete transitions. Concurrently, the total energy of N₂⁺ gradually decreases, the dipole moment rises steadily, the energy gap of α-spin orbitals narrows, and the gap for β-spin orbitals widens. The infrared absorption intensity of N₂⁺ is significantly enhanced and exhibits a red shift. Changes in frontier orbital energies further elucidate the redistribution of the electron density and variations in the phase difference of wave functions. Additionally, this study analyzes the electric-field-induced tunneling effect in N₂ molecules and their transition to N₂⁺, along with the evolution of potential energy curves for both species under strong field conditions (0–0.08 a.u.). A comparison is also made between the effects of electric fields applied perpendicular and parallel to the molecular axis on the physical properties of N₂.