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 (H2SO4) 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 H2SO4 has been acquired and the three electronic states of H2SO4+, X(2)A, A(2)A and B(2)A have been populated and assigned. The adiabatic ionization energy of the H2SO4 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 H2SO4+ cation dissociates into HSO3+ 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.