A theoretical study on dissociative photoionization and photoionization cross-sections of a typical ketohydroperoxide in n-butane low-temperature oxidation

Ketohydroperoxides (KHP) are closely relevant to the low-temperature reactivity of hydrocarbons and their derivatives. Photoionization mass spectrometry (PIMS) technology has played a crucial role in detecting and quantifying KHP in recent years. However, the dissociative photoionization behavior at the near-threshold may bias the collection of KHP signals, resulting in a significant underestimation of the concentration. In the present study, the dissociative photoionization mechanism of 3-hydroperoxybutanal (3-KHP), the most abundant KHP in n-butane oxidation, was investigated by quantum chemical calculations. The results suggest that a two-step dehydration reaction channel with a low barrier accounts for the observation of fragments at the near-threshold. The appearance energy of possible fragments was obtained at a high theoretical level. The photoionization cross-sections (PICS) of 3-KHP at photon energies ranging from the threshold to 11 eV were obtained by theoretical calculations in this study. By comparing the calculated PICS with the photoionization efficiency curve measured in a previous study, we found that approximately 70% of the 3-KHP signals (m/z = 104) were converted into cation fragments (m/z = 29, 43, 71, etc.) at 10 eV. The theoretically predicted dissociative photoionization fragment information and total PICS were finally used to refine the quantification of 3-KHP in the previous experiment. Our study indicates that for highly reactive species such as KHP, theoretical investigations on the dissociative photoionization behavior and photoionization cross-section will substantially benefit the species quantification by PIMS.