Formation of a linear LiOH compound on Cu(001): reaction of H₂O with Li adatoms at low coverages

The reaction of H₂O with Li adatoms at low coverages on Cu(001) has been studied at room temperature by using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and work-function measurement. The coverage (θ) of Li was 0.125 except for the experiment of the work function (θ = 0.25). The O 1s XPS spectra from the H₂O exposed Li/Cu(001) surface showed a single peak at 531.5 eV. It was found by comparing the area of the O 1s peaks of H₂O/Li/Cu(001) with that of (√2 × 2√2) R45°O Cu(001) that the stoichiometry of oxygen to Li is 1:1. HREELS spectra showed a strong peak at 600 cm^-1 and small peaks at 3600, 1300 and 1100 cm^-1. The work function increased with increasing H₂O exposure. These observations and the results of previous studies lead to the conclusion that the reaction product as a result of interaction of H₂O with Li adatoms on Cu(001) is a linear triatomic molecule of LiOH which sits on the surface upright with Li down. The reaction scheme is expressed as Li(a) + H₂O(a) → LiOH(a) + H(a), where (a) denotes adspecies. The strong 600 cm^-1 peak in HREELS spectra is assigned to the Li-OH stretching mode. The intense loss-peak indicates the LiOH molecule formed on Cu(001) has an ionic-bond character. In fact the effective dynamic charge of the Li-OH stretching vibration is estimated to be ∼ 0.5e, and this is larger than that of the Li stretching vibration in the Li/Cu(001) system, ∼ 0.3e. Force constants of the LiOH admolecule are estimated. A transition state for the reaction is proposed.