A DISCHARGE FLOW-PHOTOIONIZATION MASS-SPECTROMETRIC STUDY OF THE NO3((2)A(2)') RADICAL - PHOTOIONIZATION SPECTRUM, ADIABATIC IONIZATION-ENERGY, AND GROUND-STATE SYMMETRY

The photoion efficiency spectrum of the nitrate radical (NO3) was meas ured over the region lambda = 90-104 nm by using a discharge flow-phot oionization mass spectrometer apparatus coupled to a synchrotron radia tion source. NO3 was generated by the reaction of fluorine atoms with nitric acid. A value of 12.57 +/- 0.03 eV was obtained for the adiabat ic ionization energy (IE) of NO3 from photoion thresholds, correspondi ng to the NO3+((1)A(1)') <-- NO3((2)A(2)') transition. These direct io nization measurements are the first to be reported for the NO3 radical . Relative energetics and optimized geometries for ground state NO3 an d several states of NO3+ were determined in both D-3h and C-2V symmetr ies using multiconfiguration self-consistent-field calculations. The r esults of the present study strongly suggest that the neutral ground s tate of NO3 has D-3h symmetry. This conclusion is based on the followi ng observations: (1) the experimental photoion threshold exhibits a la rge, steep initial step with no evidence of significant structure, ind icating that the neutral and cation must share the same symmetry, and (2) the theoretical evidence is unambiguous that the cation symmetry i s D-3h A value of Delta(f)H(298)degrees(NO3+) and the proton affinity of NO3 are also derived. A brief comparison is made of the ionization energies of NO, NO2, and NO3, and some new results on the dissociative ionization of HNO3 are discussed.