The B←X electronic spectrum of N2+–He

Abstract

The electronic spectrum of N₂⁺–He has been measured in the region corresponding to the N₂⁺ B ²Σ⁺_u←X ²Σ⁺_g origin transition. The spectrum was recorded by photoexciting a mass selected beam of N₂⁺–He ions and detecting N₂⁺ fragments. A likely process for the fragmentation involves fluorescence to a vibrationally excited level of the ground electronic state followed by vibrational predissociation. The observed spectrum exhibits well resolved discrete structure and bears a remarkable resemblance to a cold N₂⁺ spectrum suggesting that the potential between the N₂⁺ ion and helium atom in both the X and B electronic states, has at most only a small barrier to internal rotation. Measurement of the shift of N₂⁺–He transitions with respect to the corresponding N₂⁺ lines indicates that the binding energy of the helium atom to the N₂⁺ ion is almost the same in both the B and X electronic states.