Cluster formation and distributions in field ionization of coadsorbed methanol and water on platinum

• Published in: Surface science 2016-08, Vol.650, p.130-139
• Main Authors: Rothfuss, C.J., Medvedev, V.K., Stuve, E.M.
• Format: Article
• Language: English
• Subjects: Bonding; Clusters; Field ionization; Ion clusters; Methanol; Methyl alcohol; Platinum; Pulsed field desorption; Rate constants; Scission; Time lag; Tradeoffs; Water
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2015.12.023

Pure and mixed clusters of methanol and water were examined with pulsed field desorption time-of-flight mass spectrometry (TOF-MS) as a function of adlayer composition varying from pure water to nominally pure methanol. The experiments were performed on a Pt tip at 165K and total pressure of approximately 5 × 10–6 Torr. Protonated clusters of up 7 water molecules and up to 4 methanol molecules were detected. For mixed adlayers, mixed clusters involving 1 or 2 water and methanol molecules were observed. The hydronium cluster (H2O)H+ exhibited unusual behavior in that its maximum intensity occurred for an approximately equimolar mixture. This was attributed to direct ionization of a methanol monohydrate species, (CH3OH⋅H2O). Water production was observed in methanol-rich layers and ascribed to scission of the C–O bond to produce CH3 and OH. The TOF-MS data exhibited significant time lags for most higher mass clusters. The time lags for pure H2O were analyzed in terms of a two-step mechanism involving a trade-off of ion cluster emission and growth, from which the rate constant for cluster growth was estimated as 9 × 10–6 s–1. [Display omitted] •High electric fields yield ionic clusters of water and methanol solvated protons.•Cluster sizes of up to (H2O)7H+, (CH3OH)4H+, and (H2O)2(CH3OH)2H+•Hydronium ion H3O+maximized for equimolar water/methanol mixture.•Time lags in TOF mass spectroscopy of H2O occur for trimer and higher ions.•Time lags follow cluster growth kinetics of large water clusters.