Formation of the oxyl’s potential energy surface by the spectral kinetics of a vibrational mode

One of the most reactive intermediates for oxidative reactions is the oxyl radical, an electron-deficient oxygen atom. The discovery of a new vibration upon photoexcitation of the oxygen evolution catalysis detected the oxyl radical at the SrTiO3 surface. The vibration was assigned to a motion of the sub-surface oxygen underneath the titanium oxyl (Ti-O·-), created upon hole transfer to (or electron extraction from) a hydroxylated surface site. Evidence for such an interfacial mode derived from its spectral shape which exhibited a Fano resonance—a coupling of a sharp normal mode to continu-um excitations. Here, this Fano resonance is utilized to derive precise formation kinetics of the oxyl radical and its associ-ated potential energy surface (PES). From the Fano lineshape, the formation kinetics are obtained from the anti-resonance (the kinetics of the coupling factor), the resonance (the kinetics of the coupled continuum excitations), and the frequency integrated spectrum (the kinetics of the normal mode’s cross-section). All three perspectives yield a logistic function growth with a half-rise of 2.3 ± 0.3 ps and rate of 0.48 ± 0.09 ps. A non-equilibrium transient associated with photoexcitation is separated from the rise of the equilibrated PES. The logistic function characterizes the oxyl coverage at the very initial stages (t~0) to have an exponential growth rate that quickly decreases towards zero as a limiting coverage is reached. Such time-dependent reaction kinetics identify a dynamic activation barrier associated with the formation of a PES and quantify it for an oxyl radical coverage.