Decomposition of Secondary-Ion Kinetic Energy Distributions into Three Characteristic Components in MeV-SIMS

Kinetic energy distributions (KEDs) of secondary ions encode information about ion formation in secondary-ion mass spectrometry (SIMS), yet are typically interpreted on a per-ion basis without a unifying framework. In the MeV-SIMS regime, where sputtering is governed by electronic excitation within a transient ion track, the diversity of KED shapes reflects a complex and heterogeneous emission environment. Within this paper, we analyze an extensive data set of KEDs for atomic and molecular ions, including protonated, deprotonated, and cationized species of various organic molecules, using non-negative matrix factorization (NMF). We show how the data set of measured samples can be well described by three characteristic components. Dimensionality analysis demonstrates that three components provide the minimal description of the data, and the result is robust with respect to two different reconstructions of KEDs from yield–reflectron-voltage measurements. Mapping ion-specific weights in this three-component space can reveal systematic trends in KED fingerprints. Protonated ions are confined to the positive-energy regimes, whereas deprotonated ions exhibit a significant contribution from the negative-energy component, indicating additional reorganization processes. Atomic ions display a mass-dependent redistribution between the two positive-energy regimes, while the KED of cationized molecular ions is well aligned with the KED of the corresponding cation, especially for alkali metals. The three-component representation, therefore, provides a unified description of secondary-ion KEDs and is most likely consistent with emission under varying excitation conditions within the ion track. These results establish a direct link between kinetic energy distributions and ion formation pathways, offering a general framework for interpreting SIMS data across different ion classes and molecular systems.