Argon predissociation and electron autodetachment spectroscopy of size-selected CH3NO2−⋅Arn clusters

Photodetachment spectra of CH3NO2−⋅Arn clusters in the mid-IR are dominated by three strong resonances. These are assigned to autodetaching (AD) C–H stretching vibrational transitions in the valence (as opposed to dipole-bound) form of the molecular anion on the basis of a H/D isotopic substitution study and their solvation dependence. The AD resonances disappear promptly upon addition of the third argon atom, while the resonant structure appears in the action spectrum for formation of CH3NO2− photoproducts for n⩾2. The strong argon solvation dependence of the photoproducts is traced to the rapidly changing endoergicity of the electron loss channel due to the differential solvation behavior of the valence anion relative to the neutral. We discuss a statistical limit for this competition, and introduce an intramolecular vibrational energy redistribution mediated AD mechanism unique to polyatomic anions.