  Atomic-level structure of solids is typically determined by techniques such
as X-ray and electron diffraction,1, 2, 3, 4 which are sensitive to atomic
positions. It is hardly necessary to mention the impact that these techniques
have had on almost every field of science. However, the bonds between atoms are
critical for determining the overall structure. The dynamics of these bonds
have been difficult to quantify. Here, we combine second-harmonic generation
and the bond-charge model of nonlinear optics5, 6 to probe, in real time, the
dynamics of bond-by-bond chemical changes during the oxidation of H-terminated
(111)Si, a surface that has been well characterized by static methods. We thus
demonstrate that our approach provides new information about this exhaustively
studied system. For example, oxidation is activated by a surprisingly small
applied macroscopic strain, and exhibits anisotropic kinetics with one of the
three equivalent back-bonds of on-axis samples reacting differently from the
other two. Anisotropic oxidation kinetics also leads to observed transient
changes in bond directions. By comparing results for surfaces strained in
different directions, we find that in-plane control of surface chemistry is
possible. The use of nonlinear optics as a bond-specific characterization tool
is readily adaptable for studying structural and chemical dynamics in many
other condensed-matter systems.
