Different Generation and Ionization Methods Reveal Naphthalene–Water Interaction in Clusters

The interactions between naphthalene (Np) and water molecules play a role in astrochemistry and atmospheric chemistry. Mixed naphthalene–water clusters allow investigation of these interactions at the molecular level. We use different generation and ionization methods to study (Np)m(H2O)n clusters: (I) They are formed in charged (both positively and negatively) He nanodroplets (HND) after uptake of naphthalene and water molecules. (II) Free clusters are formed by supersonic expansions either (A) directly in coexpansion of naphthalene and water in Ar buffer gas, or (B) large naphthalene clusters are formed in supersonic expansion and they pick up individual water molecules. In experiment (II), the clusters are charged positively by electron impact ionization at 70 eV or negatively by electron attachment at ≈2 eV. All mass spectra, independent of cluster generation and ionization technique, show cluster ion series [(Np)m(H2O)n]± with a strong propensity for the magic n = 4 mass peak. The negative ion [(Np)m(H2O)n]− spectra of the clusters generated in HND (I) and in coexpansion (II.A) exhibit striking similarity, with n = 4 and 8 magic clusters. These findings are substantiated by theoretical calculations of the [(Np)m(H2O)n]+/0/– clusters. Complex information revealed by the experiment and calculations offers detailed insight into the naphthalene–water interaction.