  We study theoretically and experimentally the infrared (IR) spectrum of an
adamantane monolayer on a Au(111) surface. Using a new STM-based IR
spectroscopy technique (IRSTM) we are able to measure both the nanoscale
structure of an adamantane monolayer on Au(111) as well as its infrared
spectrum, while DFT-based ab initio calculations allow us to interpret the
microscopic vibrational dynamics revealed by our measurements. We find that the
IR spectrum of an adamantane monolayer on Au(111) is substantially modified
with respect to the gas-phase IR spectrum. The first modification is caused by
the adamantane--adamantane interaction due to monolayer packing and it reduces
the IR intensity of the 2912 cm$^{-1}$ peak (gas phase) by a factor of 3.5. The
second modification originates from the adamantane--gold interaction and it
increases the IR intensity of the 2938 cm$^{-1}$ peak (gas phase) by a factor
of 2.6, and reduces its frequency by 276 cm$^{-1}$. We expect that the
techniques described here can be used for an independent estimate of substrate
effects and intermolecular interactions in other diamondoid molecules, and for
other metallic substrates.
