Bimodal kinetic energy release in the unimolecular dissociation of energy-selected formic acid ion

Dissociation of HCOOH⁺ and DCOOH⁺ have been studied using threshold photoelectron-photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown diagrams were obtained over internal energies of the molecular ion less than 8 eV. Kinetic energy release distributions (KERDs) for HCO⁺ and DCO⁺ were extracted from their flight time distributions by using a multiple linear regression method. The breakdown curve of HCO⁺ not only exhibited several maxima but also the surprisal analysis of its KERD demonstrated a considerable deviation from a single statistical distribution. Bimodal KERDs were observed for the formation of HCO⁺ and DCO⁺. The large KER component results from dissociation to the products in the ground state, HCO⁺ (¹Σ⁺)+ṡOH (²Π_i). Average kinetic energy releases observed for this process are, however, about twice higher than those calculated by QET/RRKM. The small KER component shows average KERs remarkably smaller than the statistical expectations and this value remains constant over the energy range of 4.5 eV. The origin of this small KER component was ascribed to dissociation after fluorescent transition of initial vibronic states to the first excited state (1 ²A`). In contrast with these results average KERs measured for H₂O⁺ and HDO⁺ are in good agreement with the RRKM/QET calculations.