Photoionization and fragmentation of (N2O)n clusters

Abstract

Photoionization efficiency curves for (N₂O)_n, n=1,...,6, in the wavelength region λ=55–110 nm have been measured using a supersonic molecular beam and monochromatized synchrotron radiation from the electron storage ring BESSY at Berlin. The dependence of the photoionization cross section on λ in the threshold region varies significantly with cluster size. By measuring the energy loss of cluster ions which fragment in a field free drift region of the ion source, we were able to detect delayed unimolecular and collision induced fragmentations of the cluster ions (N₂O)⁺_n→(N₂O)⁺_n−m+(N₂O)_m with n up to 6 and m up to 3. From a study of the dependence of the ‘‘metastable’’ signals on background pressure it appears that only the removal of a single N₂O at a time can occur by unimolecular decay. For n≥3 PIE curves for the metastable ions show no significant difference when compared with those of the ‘‘prompt’’ (zero energy loss) ions. For n=2 the metastable spectrum exhibits clear autoionization peaks, whereas in the prompt spectrum the peaks are washed out by (pre)dissociation and—at stagnation pressures of 1 bar and above—by fragments of heavier n clusters.