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

摘要

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 165 K and total pressure of approximately 5 x 10(-6) Torn 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 center dot 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 x 10(-6) s(-1). Published by Elsevier B.V.