A photoion–photoelectron coincidence study of (N2)2 and (N2)3

Abstract

The photoion–photoelectron coincidence (PIPECO) spectra for (N2)+2 in the wavelength range 650–866 Å have been measured at different nozzle stagnation pressures. The formation of stable (N2)+2 from fragmentation of excited (N2)+n cluster ions initially produced by photoionization of (N2)n, n≥3, is efficient. For nozzle expansion conditions which minimize the production of (N2)n, n≥3, the intensities for the N+2(Ã,B̃)⋅N2 PIPECO bands are found to be negligibly small compared to that of the N+2(X̃)⋅N2 PIPECO band, indicating that the electronically excited N+2(Ã,B̃)⋅N2 dimer ions are dissociative in temporal ranges <42 μs. Assuming that the radiative lifetimes for N+2(Ã,B̃) and N+2(Ã,B̃)⋅N2 are identical, we estimate that the dissociative lifetimes for N+2(Ã)⋅N2 and N+2(B̃)⋅N2 are ≲10 μs and ≲60 ns, respectively. The ionization energy for (N2)2 is determined to be 14.50±0.08 eV (855±5 Å), suggesting that N+2(X̃)⋅N2 is bound by 1.09±0.08 eV. The PIPECO data for (N2)+2 presented here and those for (CO)+2 reported previously support the perturbed monomer ion model for the photoionization of a van der Waals cluster. Namely, the formation of N+2⋅(N2)n−1 by photoionization of (N2)n, n≥2, can be viewed as a photoionization process of N2 perturbed by the presence of other N2 molecules in the clusters. We suggest that the rapid dissociation of electronically and vibrationally excited dimer ions is a general mechanism for the suppression of autoionization features in the photoionization efficiency spectrum for an ionized van der Waals dimer.