A 118 nm vacuum ultraviolet laser/time-of-flight mass spectroscopic study of methanol and ethanol clusters in the vapor phase

Clusters of methanol and ethanol formed above neat liquid samples were entrained in a supersonic jet of helium and probed in the expansion using 118 nm vacuum ultraviolet laser single-photon ionization/time-of-flight (TOF) mass spectrometry. Almost every cluster ion observed in the TOF mass spectra could be represented by the formula H(ROH)(n)(+), where R=CH3 or C2H5, and n=1-5. Formation of these species is attributed to a well-established ionization pathway where each protonated (n-1)-mer originates from its n-mer neutral parent. Signals in the TOF mass spectra due to the protonated trimers H(CH3OH)(3)(+) and H(CH3CH2OH)(3)(+) were found to be the most intense and provides direct evidence that these particular cluster ions are "magic-number" structures. The possible relationships between the observed ion data and the neutral cluster vapor phase distributions are discussed. In this context, methanol and ethanol vapor cluster distributions at 298.15 K and at several pressuresgreater than or equal tothe equilibrium vapor pressure were computed using the grand canonical Monte Carlo and molecular dynamics techniques. Lastly, differences between these experiments and the results of bimolecular reaction studies are discussed. (C) 2002 American Institute of Physics.