# Light-Induced Increase of the Local Molecular Coverage on a Surface

## TL;DR

This study shows how light can increase the concentration of molecules on a surface, with the reaction progress limited by molecular movement and incomplete chemical changes.

## Contribution

The paper reveals how light-induced reactions and molecular diffusion interact to influence surface coverage during on-surface polymerization.

## Key findings

- Local molecular coverage increases significantly at the center of the laser spot with illumination time.
- Thermal diffusion and reduced mobility of reaction products control material accumulation.
- Debromination of adsorbed species does not reach completion even after extended irradiation.

## Abstract

Light is a versatile
tool to remotely activate molecules
adsorbed
on a surface, for example, to trigger their polymerization. Here,
we explore the spatial distribution of light-induced chemical reactions
on a Au(111) surface. Specifically, the covalent on-surface polymerization
of an anthracene derivative in the submonolayer coverage range is
studied. Using scanning tunneling microscopy and X-ray photoemission
spectroscopy, we observe a substantial increase of the local molecular
coverage with the sample illumination time at the center of the laser
spot. We find that the interplay between thermally induced diffusion
and the reduced mobility of reaction products steers the accumulation
of material. Moreover, the debromination of the adsorbed species never
progresses to completion within the experiment time, despite a long
irradiation of many hours.