Gas–surface
interactions are some of the most important
yet complex chemical processes to occur, as they intrinsically involve
many-body phenomena across a wide spectrum of energies and length
scales. To understand these complicated interfacial interactions,
we often use model systems such as thiolate self-assembled monolayers
(SAMs) to study phenomena like reactivity and passivation, as these
systems afford fine control over the surface parameters governing
the events in question. In this study, we examine the effect of chain
length on the reactivity of alkanethiolate SAMs with atomic hydrogen
by monitoring morphological surface evolution throughout the reaction.
These spatiotemporal data were obtained using ultrahigh vacuum scanning
tunneling microscopy (UHV-STM) with directed in situ atomic hydrogen
dosing. For a series of alkanethiolate SAMs 8- to 11-carbons long,
we find that small increases in chain length cause disproportionately
large decreases in reactivity. These reaction trends led us to develop
a kinetic model characterized by two rate constants: a slow rate for
hydrogen reactivity with close-packed domains, which is chain-length
dependent, and a fast rate for reactivity with low-density regions,
which is the same for all samples examined. In addition to reaction
rates, we also tracked chain-length-dependent changes in surface morphology,
notably how the size and shape of the SAMs’ etch pits evolved
following hydrogen exposure. Few differences were observed in the
10C and 11C samples, while there was a significant increase in the
mean etch pit area of the 8C and 9C SAMs. Overall, this study provides
important quantitative insights into how surface packing and dynamic
disorder of organic thin films can influence their passivation capabilities.