  Elevated temperature STM measurements have shown that one key phase during
gas-source homoepitaxy of Si(001) is the formation of clean Si ad-dimers from
hydrogenated ad-dimers, though the mechanism for this formation is unknown. We
present ab initio density functional calculations designed to explore this
mechanism. The calculations show that there is a pathway consistent with the
experimentally observed reaction rates, which proceeds via a meta-stable
intermediate, and is effectively irreversible. This result fills a vital gap in
our understanding of the atomic-scale details of gas-source growth of Si(001).
