Size-Specific Fragmentation and Intracluster Reactions of Methanol Clusters in Superfluid Helium Nanodroplets

In this work, methanol clusters containing up to ten molecules were formed inside superfluid helium nanodroplets. The fragmentation dynamics following electron ionization of droplets doped with these clusters were systematically investigated. The mass spectrum is dominated by the protonated (CH₃OH)_n-1H⁺ fragments, consistent with the previous molecular beam studies. However, in helium droplets, intact cluster cations (CH₃OH)_n^•+, and deprotonated cations (CH₃OH)_n-1CH₃O⁺ are also observed. An investigation of the pressure dependence of relevant signals in the mass spectra led to the determination of the parent cluster(s) for each of the observed mass fragments. This approach provided comprehensive insight into the size-dependent fragment pathways of methanol clusters. Notably, the deprotonated dimethyl ether CH₃OCH₂⁺ fragment, dimethyl ether molecular radical CH₃OCH₃^•+, and protonated dimethyl ether (CH₃OCH₃)H⁺ fragment are found to originate exclusively from methanol dimers and trimers. Furthermore, we demonstrate that beyond a certain size, (CH₃OH)_n clusters forming (CH₃OH)_n-1H⁺, (CH₃OH)_n-1CH₃O⁺, and (CH₃OH)_n^•+ species are hosted by droplets of different mean sizes. We also observe weak helium-ion adduct fragments, He_kCHO⁺, He_kCH₂O^•+, and He_kCH₃O⁺. The pressure dependence of these fragments shows that they originate from droplets containing methanol monomers, dimers, and trimers.