An ion-molecule reaction has been studied by measuring the momentum of both the reactant and the product ions. This is carried out in an ordered molecular film of CD3I where electron stimulated desorption causes the reaction CD+3+ CD3I--> C2D+5+DI. The close similarity of the normal momentum of CD+3 and C2D+5 indicates that a sticky collision occurs in which, to within 10%, the momentum of the reactant ion is transferred to the momentum of the product ion. The measurement represents the first use of molecularly aligned species to study momentum effects in an ion-molecule reaction.
Three definitive experiments have been performed to investigate the possibility of dissociative adsorption of methanethiol (CH3SH) on clean Ag(110). On the clean Ag(110) surface, the adsorption in the first layer occurs to 0.5 ML, producing a (2 x 1) low-energy electron diffraction (LEED) structure. The undissociated molecule desorbs starting at approximately 140 K, and only tiny quantities of other gaseous products are desorbed, and only tiny quantities of S-containing species remain. Using a 50:50% mixture of CH3SD and CD3SH, we find no evidence of S-H or S-D bond scission between these molecules upon desorption. And finally, when the CH3SH molecule is incident on the clean Ag(110) surface in the temperature range of 230-400 K, less than 1% of the incident molecules dissociate to produce adsorbed sulfur-containing species. The results influence our thinking about the surface bonding of alkanethiol-based self-assembled monolayers (SAMs) on noble metals.
The adsorption and thermal decomposition of methanethiol (CH3SH) on Ag(110) and on Cu(110) surfaces
have been investigated by low energy electron diffraction, Auger electron spectroscopy, and temperature
programmed desorption methods. On the clean Ag(110) surface, methanethiol nondissociatively adsorbs at
25 K, forming a (2 × 1) overlayer structure and then molecularly desorbs at ∼140 K. On the partially sulfur-covered Ag(110) surface, methanethiol readily decomposes below 240 K, producing CH4 (g), H2S (g), and
sulfur atoms on the surface as well as small amounts of carbon. The sulfur overlayer formed by the
decomposition of methanethiol exists as a p(3 × 2) ordered structure at the saturation coverage of sulfur. On
this fully sulfur-covered Ag(110) surface, the thermal decomposition of methanethiol is completely suppressed.
An autocatalytic mechanism involving silver sites activated by neighboring adsorbed sulfur is proposed to
explain the sulfur-catalyzed thermal decomposition reaction of methanethiol on Ag(110). In contrast, on the
clean Cu(110) surface, methanethiol decomposes below 320 K, producing CH4 (g) and C2H6 (g), and leaving
sulfur overlayers, either a c(2 × 2) structure at low coverage or a c(8 × 2) structure at high coverage. A
complete layer of chemisorbed sulfur suppresses further methanethiol decomposition on both the silver and
the copper surfaces.
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