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.