A Wahrhaftig diagram (named after Austin L. Wahrhaftig) illustrates the relative contributions in unimolecular ion decomposition of direct fragmentation and fragmentation following rearrangement. The x-axis of the diagram represents the internal energy of the ion. The lower part of the diagram shows the logarithm of the rate constant k for unimolecular dissociation whereas the upper portion of the diagram indicates the probability of forming a particular product ion. The green trace in the lower part of the diagram indicates the rate of the rearrangement reaction given by
and the blue trace indicates the direct cleavage reaction
A rate constant of 10 s is sufficiently fast for ion decomposition within the ion source of a typical mass spectrometer. Ions with rate constants less than 10 s and greater than approximately 10 s (lifetimes between 10 and 10 s) have a high probability of decomposing in the mass spectrometer between the ion source and the detector. These rate constants are indicated in the Wahrhaftig diagram by the log k = 5 and log k = 6 dashed lines.
Indicated on the rate constant plot are the reaction critical energy (also called the activation energy) for the formation of AD, E(AD) and AB, E(AB). These represent the minimum internal energy of ABCD required to form the respective product ions:  the difference in the zero point energy of ABCD and that of the activated complex.
When the internal energy of ABCD is greater than E(AD), the ions are metastable (indicated by m); this occurs near log k > 5. A metastable ion has sufficient internal energy to dissociate prior to detection. The energy E(AD) is defined as the internal energy of ABCD that results in an equal probability that ABCDand AD leave the ion source, which occurs at near log k = 6. When the precursor ion has an internal energy equal to E(AB), the rates of formation of AD and AB are equal.