Electron spectroscopies analyze the electrons that are ejected from a material for qualitative or semi-quantitative analysis. In general an excitation source such as x-rays or electrons will eject an electron from an inner-shell orbital of an atom. Detecting photoelectrons that are ejected by x-rays is call x-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA). Detecting electrons that are ejected from higher orbitals to conserve energy during electron transitions is called Auger electron spectroscopy (AES). These electron processes are described below. Ejected electrons can escape only from a depth of approximately 3 nm or less, making electron spectroscopy most useful to study surfaces of solid materials. Depth profiling is accomplished by combining an electron spectroscopy with a sputtering source that removes surface layers.