Electronic spectra of N[sup + ][sub 2]--(He)[sub n] (n=1, 2, 3)

Ionic clusters of nitrogen and helium have been investigated by recording their electronic spectra in the near UV. Structured bands belonging to 14N+2 –He, 14N+2–(He)2, 14N+2–(He)3, and 15N+2 –He have been measured between 390 and 392 nm close to the N+2 B 2Σ+u←X 2Σ+g band origin. Spectra were obtained by exciting mass selected cluster cations with tunable laser radiation and recording the photodissociation cross section as a function of wavelength. The data support the hypothesis that the He– – –N+2 interaction potential has only a small barrier to internal rotation in both the X and B electronic states. A lower estimate for the dissociation energy of the N+2 –He cluster of 101 cm−1 is inferred.