Isolation of site-specific anharmonicities of individual water molecules in the I−·(H2O)2 complex using tag-free, isotopomer selective IR-IR double resonance

Abstract

We reveal the microscopic mechanics of iodide ion microhydration by recording the isotopomer-selective vibrational spectra of the I−·(H2O)·(D2O), I−·(HOD)·(D2O), and I−·(DOH)·(H2O) isotopologues using a new class of ion spectrometer that is optimized to carry out two-color, IR-IR photodissociation in a variety of pump-probe schemes. Using one of these, we record the linear absorption spectrum of a cryogenically cooled cluster without the use of a messenger “tag”. In another protocol, we reveal the spectra of individual H2O and D2O molecules embedded in each of the two possible binding sites in the iodide dihydrate, as well as the bands due to individual OH and OD groups in each of the four local binding environments. Finally, we demonstrate how temperature dependent isotopic scrambling among the spectral features can be used to monitor the onset of large amplitude motion, heretofore inferred from changes in the envelope of the OH stretching vibrational manifold.