Sodium-doping as a reference to study the influence of intracluster chemistry on the fragmentation of weakly-bound clusters upon vacuum ultraviolet photoionization

Abstract

The fragmentation of methanol, water, dimethyl ether, and acetic acid clusters upon photoionization with a single vacuum ultraviolet (VUV) photon of 10.1 eV, 13.3 eV, or 17.5 eV energy is studied with mass spectrometry. The sodium-doping method is used as an independent approximate measure of the original cluster size distribution providing information on the degree of fragmentation upon VUV ionization. The experimental results show strong fragmentation for (CH₃)₂O and CH₃CO₂H clusters but minor fragmentation for H₂O and CH₃OH clusters. The pronounced cluster decay for (CH₃)₂O and CH₃CO₂H is explained by additional intracluster chain reactions that occur after the initial fast proton transfer in the ionic clusters, i.e. the decay of (CH₃)₂O molecules into H₂, CO, and CH₄ catalyzed by the methoxymethyl radical, and the decay of CH₃CO₂H molecules into CO₂ and CH₄ catalyzed by the acetyloxy radical. The absence of equivalent reaction cycles in ionic H₂O and CH₃OH clusters after the fast proton transfer is consistent with the much less pronounced cluster fragmentation observed upon VUV ionization. The study shows that VUV photoionization even at threshold cannot in general be considered a soft ionization method for weakly-bound clusters, largely because of potential intracluster reaction.