Self-assembled monolayers of straight-chain alkanethiolates (RS) on gold were studied by nitrogen laser desorption followed by vacuum ultraviolet (118 nm) photoionization of secondary neutrals in a time-of-flight mass spectrometer. The dominant feature of all photoionization mass spectra were the dimers (RSSR+) for all thiolates studied. No monomer ions (RS+) were observed, in contrast with previous results from time-of-flight secondary ion and direct laser desorption mass spectrometry. Results are presented from experiments on the tetradecyl (C14) and hexadecyl (C16) mercaptan monolayers. Mixed C14/C16 monolayers generated both pure and mixed dimers, indicating a facile association between different monomer chain lengths.
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Trevor et al. (1997) studied this question.