In the first step, we performed an ALD experiment on CoO(100) at 400 K under UHV conditions. Previously, Heinz et al. investigated the structure and properties of cobalt oxides grown on Ir(100) using LEED, scanning tunneling microscopy (STM), and density functional theory (DFT). They found that CoO(100) is non-polar and exhibits a bulk-like termination with oxygen and cobalt ions in the same plane. Water adsorbs molecularly on CoO(100) below 210 K, exposing only very few OH groups at defect sites that completely disappear from the surface above 360 K. We chose the reaction temperature at 400 K based on previous studies that have shown that the reactions of the ALD process are most efficient at this temperature. Additionally, at 400 K, the CoO(100) surface does not exhibit acidic protons. In total, we performed 10 full ALD cycles, each consisting of 4 steps, i.e., (i) dosing TDMAH for 5 min, (ii) pumping for 2 min, (iii) dosing D2S for 10 min, and finally (iv) pumping for 2 min. While performing ALD, we continuously recorded IR spectra (1 min per spectrum, 19 spectra every full ALD cycle). The recorded set of IR spectra is shown as difference spectra (spectrum(n)spectrum(n-1)) in Figure . By referencing against the previous spectrum, negative IR bands indicate the adsorption of a species, while positive IR bands indicate the loss or consumption of a species.