Isolating the spectra of cluster ion isomers using Ar-“tag” -mediated IR-IR double resonance within the vibrational manifolds: Application to $NO2−⋅H2O$

We demonstrate a method for isolating the vibrational predissociation spectra of different structural isomers of mass-selected cluster ions based on a population-labeling double resonance scheme. This involves a variation on the “ion dip” approach and is carried out with three stages of mass selection in order to separate the fragment ion signals arising from a fixed-frequency population-monitoring laser and those generated by a scanned laser that removes population of species resonant in the course of the scan. We demonstrate the method on the Ar-tagged $NO2−⋅H2O$ cluster, where we identify the spectral patterns arising from two isomers. One of these structures features accommodation of the water molecule in a double H-bond arrangement, while in the other, $H2O$ attaches in a single ionic H-bond motif where the nominally free OH group is oriented toward the N atom of $NO2−$⁠. Transitions derived from both the $NO2−$ and $H2O$ constituents are observed for both isomers, allowing us to gauge the distortions suffered by both the ion and solvent molecules in the different hydration arrangements.