Nitrogen dioxide, a brown toxic gas formed in most combustion processes, is an abundant atmospheric pollutant and a key component of photochemical smog[69]. High concentra- tions of NO2 in the atmosphere may lead to various health risks in humans, including higher risks of lung cancer and heart failure[70]. Consequently, there are many regulations in place, particularly in areas of high traffic density, in an attempt to limit the levels of NO2 in the atmosphere[71].
Nitrogen dioxide, along with nitric oxide (NO), also play a vital role in determining the distribution of ozone in the Earth’s atmosphere[72]. These compounds may be found naturally in the stratosphere, and help cycle ozone to molecular oxygen[73]. However due primarily to emissions from diesel motor engines and thermal power plants, nitric oxides (NOx) are also present in the troposphere, where they act as very efficient green house gases[74,75]. As a result, trying to understand the photochemical and reactive properties of the neutral NO2 molecule has been the focus of many previous studies[76–83].
4.1.1 Ions and photochemistry
The literature on the NO−2 negative ion is much less complete, with most of the scientific discussion based on only a handful of experimental results. The nitrite anion (NO−2) is a very desirable focus for spectroscopy studies, due to the significant role it plays in the Earth’s atmosphere. Most reactions in the lower atmosphere occur with neutral start and end states, but often pass through ionic intermediates, making the study of these transient species vital in order to fully understand the processes occurring[46]. However, it is in the upper atmosphere, where photoionisation and photofragment processes become important drivers of the chemistry, that the importance of the NO−2 ion becomes more prevalent[73,82]. In this region, the high density of UV photons available to partake in photochemistry translates into a larger density of ions and radicals. Valuable information may be obtained from total photodetachment cross section experiments of the NO−2 ion, which has led multiple studies to try and measure the cross section behaviour as accurately as possible.
The first photodetachment measurement of NO−2 was reported in 1969 by Peter War- neck[84], where nitrogen dioxide ions were produced in a gas discharge, and subsequently photodetached using a mercury-xenon light source. The total number of photoelectrons
emitted was measured as a function of the photon energy, allowing for the cross section, and electron affinity of the ion (3.1(5) eV), to be determined.
In 1974 the Brauman[6] and Lineberger[5] groups both measured the total photode- tachment cross section of NO−2, using a Hg-Xe arc and dye laser light source respectively. Improved experimental set-ups allowed for a more accurate determination of the total cross section, with a smaller NO2 electron affinity of 2.36(1) eV defined, cf. 3.1(5) eV in the earlier study. In both the Brauman and Lineberger measurements, a long photoelectron tail was observed well beyond the electron affinity threshold, with both papers showing photodetachment at energies as low as 1.8 eV. These additional electrons were attributed to a possible peroxy isomer of NO2, NOO, a radical that was first postulated to exist by Clynne and Thrush in 1961[4]. The observation of long photoelectron tails provided the first experimental evidence that may support this suggestion. However, as the experiments only measured the total photoelectron yield, providing limited spectral information, they were unable to confirm this hypothesis. The possibility of a new isomer prompted further investigation, with Huber et al.[85] reporting on excited NO−2 photodetachment, followed by a wide-range cross section experiment by Woo et al.[86]. But neither of these studies found any evidence of a NOO isomer.
Using their zero-core-contribution model, Clodiuset al.[87]modelled the photodetach- ment cross section from a purely theoretical approach. This model was extended to predict differential cross sections and electron distribution anisotropies. It was suggested that the differing behaviour of s partial waves near threshold may produce an interesting result in the anisotropy curve, where the angular distribution would rapidly become isotropic at very low kinetic energies. However no experimental NO−2 anisotropy information was available to test this prediction.
In 1988 the Lineberger group[88] measured the first photoelectron spectrum of NO− 2, using an argon ion laser and a hemispherical analyser to record the energy of the detached electrons. This resolved multiple vibrational transitions, corresponding to the electronic ground state transition NO2X˜2A1 ← NO−2X˜1A1, and redefined the electron affinity at 2.273(5) eV. Despite the large number of transitions observed, all of the spectral structure may be completely described by simple overtone and combination progressions in two totally symmetric vibrational modes of the ground state. One year later, the Neumark group[89] measured the NO−
2 photoelectron spectrum using the third, fourth, and fifth harmonics (355, 266, and 213 nm) of a Nd:YAG laser. The additional photon energy allowed for transitions to the excited states ˜A2B2 and ˜C2A2 to be measured, providing the first direct observation of the dark ˜C2A2 electronic state, which is dipole forbidden in standard one-photon optical experiments. None of the above photoelectron experiments found any evidence of a peroxy NO2 isomer, which started to create doubt about it’s existence. Multiple theoretical studies attempted to solve this dilemma, with ab-initio calculations of Meredith et al.[90] providing estimates for the vibrational and electronic energy structure of possible peroxy and cyclic isomers. By calculating potential energy surfaces along the N + O2 reaction coordinate, Walch[91] predicted that NOO should form a very shallow minimum, with a small barrier of∼0.3 kcal/mol to dissociation. No further experimental studies have been able to find any evidence of the NOO isomer.
More recent ab-initio studies have been reported, including an investigation into the geometry of the NO−2 anion[92] and the conical intersection between the neutral ˜X2A
1 and ˜A2B2 potential energy surfaces[93]. Both of these studies use the experimental results of Lineberger and Neumark for comparison to their models, despite the experimental measurements occurring ∼30 years ago. This highlights both the quality of the original
§4.2 Quantum structure ofC2v NO2 57
experiments, but also the need for new higher resolution spectra that can provide more spectral detail.
As such, the photoelectron spectrum of NO−2 is reinvestigated using the HR-PEI spec- trometer. The use of velocity map imaging allows for both the energetic and angular information to be obtained over a large energy range with high resolution. This helps re- solve new spectral detail, and exposes spectroscopic signatures of the elusive NOO peroxy isomer.