Figure On the right: Exemplary Mössbauer signal with the typical doublet structure; the transmission is given on the y-axis and the probe velocity on the x-axis. The isomer shift (violet) refers to the center of the doublet with respect to a reference (typically, α-iron). The quadrupole splitting (green) measures the distance between the doublet minima. On the left: The corresponding energy diagram with the nuclear energy levels; the energy levels are labeled with the nuclear spin quantum number I and the nuclear magnetic quantum number m I .