Production and vibrational predissociation spectra of solvated alkali ions

Abstract

An ion beam apparatus has been constructed and demonstrated to produce copious alkali ions clustered by wide varieties of solvent molecule, such as H$\sb2$O, CH$\sb3$OH, C$\sb2$H$\sb4$ and SF$\sb6$. The absorption spectra of CO stretching mode of methanol and ethanol molecules in the solvated alkali ions have been collected by the vibrational predissociation method using a modulated C.W. CO$\sb2$ laser.

The CO stretching mode of the alcohol molecules is very sensitive to the chemical environment it experiences. By observing a clear variation of the absorption peak positions of CO stretching mode as a function of metal ions those alcohol molecules attached to and the number of alcohol molecules clustering together, following conclusions can be drawn:

Ten methanol or six ethanol molecules are required to fill the 1-st solvation shell of the Cs$\sp+$ ion in the gas phase and eight methanol molecules are needed to complete the 1-st solvation shell of the Rb$\sp+$ ion in the gas phase. The K$\sp+$ and Na$\sp+$ ions require ten and eight methanol molecules, respectively, to fill their 1-st and 2-nd solvation shell in the gas phase.

Based on the observation that there has only one absorption peak associated with each solvation shell, either it is partially or completely filled, of all the methanol and ethanol solvated alkali ions (except ethanol solvated potassium ions) and each solvation shell peak can be fitted quite well by a single Lorentzian function, the chemical environment in each solvation shell is quite uniform and methanol or ethanol solvent molecules within the same solvation shell are equivalent from one another.