What is referred to as the edge region in X-ray spectroscopy?

In X-ray spectroscopy, the term "edge region" specifically refers to the range of photoelectron energy that corresponds to the transition of core electrons to unoccupied states in the material being analyzed. When X-rays are absorbed by an atom, they can impart enough energy to eject core electrons, which creates a photoelectron. The energy of these ejected electrons varies depending on the binding energy of the electrons in their original states and the energy of the incoming X-ray photons.

The edge region is characterized by a sudden increase or "edge" in the absorption spectrum, which indicates the threshold energy required to remove an electron from a core level (such as 1s, 2s levels). This threshold marks the transition from the bound states of electrons to free states. Therefore, this region is critical in understanding electronic structures and bonding in materials, serving as an important diagnostic tool in X-ray spectroscopy.

This definition does not apply to the other choices, as they describe different aspects of electronic transitions or energy levels not specific to the "edge" concept within the framework of spectroscopy.