Vacuum ultraviolet photochemistry of sulfuric acid vapor: a combined experimental and theoretical study.

We present a vacuum ultraviolet (VUV) photoionization study of the gas-phase sulfuric acid (H 2 SO 4 ) molecule in the 11-14 eV energy range by using the method of synchrotron radiation-based double imaging photoelectron photoion coincidence (i 2 PEPICO) spectroscopy complemented with accurate theoretical calculations. The slow photoelectron spectrum (SPES) of H 2 SO 4 has been acquired and the three electronic states of H 2 SO 4 + , X 2 A, A 2 A and B 2 A have been populated and assigned. The adiabatic ionization energy of the H 2 SO 4 molecule towards the X 2 A cationic ground state is measured at 11.684 ± 0.006 eV, in accordance with high-level calculated findings. With increasing photon energies, the H 2 SO 4 + cation dissociates into HSO 3 + and OH fragments and their adiabatic appearance energy is measured at 13.498 ± 0.007 eV. Then, the enthalpies of formation for the species involved in the photoionization and dissociative photoionization have been determined through a thermochemical cycle.