Analysis of mixed nitric oxide–water clusters by complementary ionization methods

Abstract

Mixed clusters of nitric oxide and water are investigated by mass spectrometry using different ionization methods, namely (1) the electron ionization at 70eV (EI); (2) the doping with sodium atoms and subsequent photoionization (NaPI); and (3) the low energy electron attachment (EA). All these methods applied to the same molecular beam generated by mixed NO–water expansion yield distinctly different mass spectra. The EI spectra are dominated by the protonated water (H2O)nH+ ions, nevertheless, they show some evidence for the mixed NO–water clusters. The NaPI spectra exhibit only (H2O)n·Na+ ions which originate from the pure water clusters (H2O)N. The abundance of the pure water clusters generated in the mixed expansions is small compared to the abundance of the mixed species. The negative EA spectra clearly demonstrate the dominance of the mixed clusters, namely (H2O)n·NO2− and (H2O)n·(NO)m−, m=2, 3. None of the above mentioned methods alone can reveal the neutral cluster composition and size, however, based on their unique combination in our experiment, we can discuss the neutral clusters and ionization processes. In addition, we have investigated the negative ion intracluster chemistry by the theoretical calculations. Energetically favorable pathways from the (NO)3− ionic core to the NO2− in the presence of water are identified, justifying the generation of the (H2O)n·NO2− series. Further series in the negative ion spectra are discussed.