Enlightening salt: enthalpy of formation of gaseous sodium chloride by photoelectron photoion coincidence spectroscopy

Abstract

An in-vacuum flash pyrolysis microreactor was used to seed a molecular beam with gaseous sodium chloride. The vapour was ionised with tuneable synchrotron vacuum ultraviolet radiation to determine the 0 K appearance energy of the sodium cation from sodium chloride using photoelectron photoion coincidence spectroscopy. In addition to the Na+ and NaCl+ peaks, Na2Cl+ was also observed in the mass spectrum, as a tracer for gaseous (NaCl)2. Larger clusters were not observed. The breakdown diagram showed that the monomer underwent significant collisional cooling in the molecular beam, tentatively because of the increased stagnation pressure in the reactor. It also yielded an Na+ appearance energy of E 0 = 9.351 ± 0.007 eV. By the ion cycle, this leads to D 0(NaCl) = 406.4 ± 0.7 kJ mol−1 and Δf H o⦵ 0K(NaCl(g)) = −179.0 ± 1.0 kJ mol−1, which agree with but are better defined than the previous values. This enthalpy of formation is independent of the crystalline phase, and therefore confirms the literature sublimation enthalpy, as well.