Infrared spectroscopy of mass-selected Cs(CH3OH)+N N=4–16

Abstract

The infrared spectra of mass‐selected cesium cations solvated by 4–16 methanol molecules have been obtained from vibrational predissociation spectroscopy. The solvated ions were prepared in a molecular beam apparatus equipped with a quadrupole mass spectrometer. A cw CO₂ laser operating in the 9.6 μ region was used to excite the methanol C–O stretch in the solvated ions. Depletion of the mass‐selected solvated ion signal due to vibrational predissociation was used to generate the infrared absorption spectra. This work represents the first infrared study of metal ion–solvent clusters. The spectra display a clear variation with the number of solvents throughout the range of 4–16. A single peak at ∼1031 cm⁻¹ steadily grew in intensity from four to ten methanol molecules. The presence of additional methanol molecules beyond ten did not affect the intensity of the 1030 cm⁻¹ peak, but a new spectral feature became observable to higher frequency. The spectroscopic data are consistent with a first solvation shell size of ten methanol molecules. This is the first direct experimental observation of a filled solvent shell associated with the cesium cation. The C–O stretching frequencies of the methanol molecules are compared with existing experimental results involving neutral clusters.