The data analysis becomes more demanding if there are competing parallel dissociation channels or if the dissociation at threshold is too slow to be observed on the time scale (several μs) of the experiment. In the first case, the slower dissociation channel will appear only at higher energies, an effect called competitive shift, whereas in the second, the resulting kinetic shift means that the fragmentation will only be observed at some excess energy, i.e. only when it is fast enough to take place on the experimental time scale. When several dissociation steps follow sequentially, the second step typically occurs at high excess energies: the system has much more internal energy than needed for breaking the weakest bond in the parent ion. Some of this excess energy is retained as internal energy of the fragment ion, some may be converted into the internal energy of the leaving neutral fragment (invisible to mass spectrometry) and the rest is released as kinetic energy, in that the fragments fly apart at some non-zero velocity.
More often than not, dissociative photoionization processes can be described within a statistical framework, similarly to the approach used in collision-induced dissociation experiments. If the ergodic hypothesis holds, the system will explore each region of the phase space with a probability according to its volume. A transition state (TS) can then be defined in the phase space, which connects the dissociating ion with the dissociation products, and the dissociation rates for the slow or competing dissociations can be expressed in terms of the TS phase space volume vs. the total phase space volume. The total phase space volume is calculated in a microcanonical ensemble using the known energy and the density of states of the dissociating ion. There are several approaches how to define the transition state, the most widely used being RRKM theory. The unimolecular dissociation rate curve as a function of energy, k(E), vanishes below the dissociative photoionization energy, E.
Statistical theory can also be used in the microcanonical formalism to describe the excess energy partitioning in sequential dissociation steps, as proposed by Klots for a canonical ensemble. Such a statistical approach was used for more than a hundred systems to determine accurate dissociative photoionization onsets, and derive thermochemical information from them.
Furthermore, algorithms based on probabilistic Bayesian analyses are known to considerably reduce systematic biases induced by false coincidences. The intensity of these false coincidences can big strong enough to appear as a separate peaks in the signal and complicate the analysis of the spectra.