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Abstract

We use infrared/infrared double-resonance population labeling (IR²MS²) spectroscopy in the spectral region of the free and hydrogen-bonded OH stretching fundamentals (2880–3850 cm^-1) to identify the number and to isolate the vibrational signatures of individual isomers contributing to the gas-phase IR spectra of the cryogenically cooled protonated water clusters H⁺(H₂O)_n·H₂/D₂ with n = 7–10. For n = 7, four isomers are identified and assigned. Surprisingly, the IR²MS² spectra of the protonated water octa-, nona-, and decamer show no evidence for multiple isomers. The present spectra support the prediction that the quasi-2D to 3D structural transition occurs in between n = 8 and 9 in the cold cluster regime. However, the same models have difficulty explaining the remarkable size dependence of the isomer population reported here.