A new look at N2+ electronic transitions: An experimental and theoretical study

Neutral and ionic N₂ species exhibit a rich spectrum as a result of the large density of couplings between states with different multiplicities. In this sense, spectra of the molecular ion N${}_2^{+}$ are investigated combining Fourier transform spectroscopy and ab initio methods. We have reanalyzed the First Negative band System (B${}^2{\Sigma }_u^{+}$ $\to$ X${}^2{\Sigma }_g^{+}$) including five bands not reported previously by Fourier spectroscopy. The spectra were recorded using a resolution of 0.6 cm⁻¹ and accuracy of 0.005 cm⁻¹. These results are then compared with new MRCI+Q/AV6Z calculations. For the first time, transition probabilities are computed for the previously observed 2${}^2{\Pi }_g$-A${}^2{\Pi }_u$ band system. The 2${}^2{\Pi }_g$ state ($T_e$ = 67,029 cm⁻¹) has a dissociation energy of 24,787 cm⁻¹ at $R_e$ = 2.7332 a₀. The predicted lifetimes for the 2${}^2{\Pi }_g$-A${}^2{\Pi }_u$ emissions are of the order of 170 ns. The calculated transition probabilities A($v^{\prime }$=0, $v^{\prime \prime }$=0) for the B${}^2{\Sigma }_u^{+}$-X${}^2{\Sigma }_g^{+}$ and 2${}^2{\Pi }_g$-A${}^2{\Pi }_u$ bands are 1.156 × 10⁷ and 1.716 × 10³ s⁻¹, respectively. The role of spin–orbit (SO) matrix elements in the spectroscopic data of N${}_2^{+}$ is discussed, including results for SO constants as a function of vibrational level of A${}^2{\Pi }_u$ state. Our theoretical SO constant A₀(A${}^2{\Pi }_u$) = −73.40 cm⁻¹ reproduces well the experimental one (−74.67 cm⁻¹). SO calculations are also used to investigate spin-forbidden transitions on N${}_2^{+}$. The obtained $<A^2{\Pi }_u\left|{\hat{H}}_{\mathrm{SO}}\right|a^4{\Sigma }_u^{+}>\approx$ 30 cm⁻¹. Following a sum-over-states (SOS) methodology, the best estimate for the spin-rotation constant ${\gamma }_0$ of the X${}^2{\Sigma }_g^{+}$ and B${}^2{\Sigma }_u^{+}$ states are 0.0096 and 0.0211 cm⁻¹, respectively, in quantitative agreement with the present experimental data of 0.00917(36) and 0.0206(9).