High-resolution vacuum ultraviolet photodynamic of the nitrogen dioxide dimer (NO2)2 and the stability of its cation

Abstract

We report a comprehensive vacuum ultraviolet (VUV) photoionization study of the nitrogen dioxide dimer (NO₂)₂ by using a state-of-the-art method of double imaging photoelectron photoion coincidence (i²PEPICO) spectroscopy at synchrotron SOLEIL. We find that the dimer cation N₂O₄⁺ from direct ionization of (NO₂)₂ is not stable and fully dissociates into the NO₂⁺ and NO₂ fragments. After identifying and separating the two different sources of NO₂⁺ cations observed in experiments, direct photoionization of the monomer NO₂ and dissociative photoionization of the co-existing dimer (NO₂)₂, the pure mass-selected threshold photoelectron spectrum (TPES) of the dimer (NO₂)₂ has been recorded without the need of subtraction. An upper limit of the adiabatic ionization energy of the dimer (AIE ≤ 9.59 ± 0.05 eV) and the appearance energy of the NO₂⁺ fragment (AE = 10.15 ± 0.05 eV) are also given. In addition, the state-selected dynamics of the five low-lying electronic states of the cation N₂O₄⁺ have been analyzed from the corresponding kinetic energy release distributions.