O1s

O1s is shorthand for the oxygen 1s core level in atomic and molecular systems. It refers to the innermost electron shell (n=1, l=0) of oxygen, which in neutral oxygen has the electron configuration 1s2 2s2 2p4. The 1s electrons are tightly bound and require several hundred electronvolts of energy to remove, far above the energies of valence electrons.

In spectroscopy, the O1s level is commonly probed by X-ray photoelectron spectroscopy (XPS) and by X-ray absorption spectroscopy (XAS). In XPS, electrons are ejected from the O1s orbital, producing a peak whose binding energy is characteristic of oxygen in the sample. The O1s binding energy typically lies around 530 eV, with shifts of a few eV depending on oxidation state and chemical environment. Oxidized forms such as oxides, hydroxides, and carbonates generally exhibit higher O1s binding energies than non-oxidized form, and different oxygen-containing functional groups can produce distinguishable shifts.

In XAS, the O K-edge corresponds to transitions from O1s to unoccupied states, providing information on the local electronic structure and bonding of oxygen. Edge energy and spectral features depend on chemical state and coordination, helping to identify bonding environments in a material.

O1s data are widely used to characterize materials including metal oxides, ceramics, carbon-based materials, catalysts, and biological samples. While notation is generally O1s, the exact binding energies are instrument- and sample-dependent, requiring proper calibration for accurate interpretation.