The various energy regions of the electromagnetic spectrum are typically defined by wavelength, or energy.  The analytical chemist is interested in interactions of matter with the various energies of radiation since these form the basis of many useful types of spectrographic analysis.   These interactions range from changes in spin of subatomic particles at the low energy end (radio waves) to nuclear events at the high energy end.   The energy utilized in X-ray fluorescence (XRF) spectroscopy) is sufficient to penetrate outer electron shells, but not energetic enough to cause atomic transitions.  The unique atomic event induced in the XRF process is the loss of inner shell electrons.  An element ionized in this way, leaving a vacancy in an inner electron shell, is highly unstable and quickly reverts to a more stable form by transfer of an outer shell electron to fill the inner shell vacancy.  This transition across an energy gap from outer to inner shells within an atom is accompanied by fluorescence emission with an energy unique to the atomic structure of the element.  The sketch of the atom shown above illustrates this excitation and emission process.