A Discharge Flow-Photoionization Mass Spectrometric Study of the NO3(2A2') Radical: Photoionization Spectrum, Adiabatic Ionization Energy, and Ground State Symmetry

Abstract

The photoion efficiency spectrum of the nitrate radical (NO3) was measured over the region , I = 90-104 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. NO3 was generated by the reaction of fluorine atoms with nitric acid. A value of 12.57 f 0.03 eV was obtained for the adiabatic ionization energy (IE) of NO3 from photoion thresholds, corresponding to the N03+('A1') -N03(2A~') transition. These direct ionization measurements are the first to be reported for the NO3 radical. Relative energetics and optimized geometries for ground state NO3 and several states of NO3+ were determined in both D3h and CzV symmetries using multiconfiguration self-consistent-field calculations. The results of the present study strongly suggest that the neutral ground state of NO3 has D3h symmetry. This conclusion is based on the following observations: (1) the experimental photoion threshold exhibits a large, steep initial step with no evidence of significant structure, indicating that the neutral and cation must share the same symmetry, and (2) the theoretical evidence is unambiguous that the cation symmetry is D3h. A value of AfH0298(N03+) 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 HN03 are discussed.

Novel gaseous transient species: Generation and characterization

“…The removal of an electron from the HOMO (1a′ 2 ) will leave the cation in the singlet state 1 A′ 1 , whereas, removing an electron from each of the other valence shell orbitals will result in both triplet and singlet states of the cation. The PES experimentally obtained 12.55 eV is well in agreement with the previous PIMS determined 12.57 eV [87] . From the expanded spectrum of the NO 3 radical, it is clearly seen that the second band at 13.18 eV displays vibrational fine structure of 1050 ± 60 cm −1 , which shows that the ionization originates from a strongly bonded state 3 E″.…”

“…NO 2 + O 3 → NO 3 + O 2 (15) K 1 (T = 298 K) = 3.2×10 −7 cm 3 molecules −1 s −1 For the past few years, the geometry and neutral ground electronic state has been a subject of controversy, various spectroscopic studies conclude that D 3h symmetry is preferred for the neutral ground state, while other theoretical calculations predict that C 2v structure is the lowest in energy [86] . The first experimental work was carried out by Monks and co-workers [87] in which they measured the photoion efficiency spectrum of NO 3 in the region of 90-104 nm using a discharge flowphotoionization mass spectroscopy (PIMS). They concluded that the lowest adiabatic ionization potential I a is 12.57 eV, corresponding to the NO 3 ( 2 A′ 2 ) → NO 3 + ( 1 A′ 1 ) transition with D 3h symmetry.…”

Theoretical study of the photoelectron spectrum of NO3 and the excited states of NO3+. I. Electronic spectrum

“…Monks et al also find that the cation ground state ''unambiguously'' has a D 3h equilibrium geometry and calculate a second stationary point at C 2v configuration to be 1.13 eV higher in energy. This is in blatant disagreement with calculations by Miller and Francisco 38 and our very recent findings that the NO 3 ϩ equilibrium structure at CASSCF and MRCI level of theory is indeed of C 2v symmetry with one very short and two considerably longer bonds, 39 which is not even qualitatively in agreement with the calculations of Monks et al 32 Finally, they determined the first excited triplet and singlet cationic states to be ͉ 3 A 2 Ј͘ and ͉ 1 EЈ͘ with excitation energies of 0.37 and 0.39 eV, respectively, which means that they obtained not only the wrong states but also far too low transition energies. They acknowledged that a treatment of dynamic electron correlation would be necessary to improve the CASSCF energies.…”

“…In 1994, Monks et al published the first photoionization experiments and determined the first principal ionization potential at 12.57 eV by discharge flow photoionization mass spectrometry. 32 They also deduced from their results that the ground-state equilibrium geometries for both NO 3 34,35 In fact, the first principal IP of both studies was in close agreement, but Wang et al obtained several higher IPs which were not found by Monks et al…”

Dissociative electron impact ionization of N2O5

“…On energetic reasons there is only the reaction N 2 O 5 + e → NO 3 + + NO 2 + 2e, AE = 13.53 eV (5) which may contribute to the production of this ion via ionisation of N 2 O 5 at threshold and the calculated appearance energy is in good agreement with the measured value taking into account the involved error bars of the thermo-chemical data and the present determination. Moreover, it is interesting to note that the adiabatic ionization energy for the nitrate radical NO 3 as determined to be 12.57 ± 0.03 eV by Monks et al [21] using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source is well below the present appearance energy, thus excluding contamination of the N 2 O 5 target with the nitrate radical. It should be noted that the structure and the atmospheric implications of the existence of the NO 3 + cation has been extensively studied and discussed recently [22].…”