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

Xiaofeng Tang *a, Gustavo A. Garcia *b and Laurent Nahon b
aLaboratory of Atmospheric Physico-Chemistry, Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Hefei, Anhui 230031, China. E-mail: tangxf@aiofm.ac.cn
bSynchrotron SOLEIL, L’Orme des Merisiers, St. Aubin, BP 48, Gif sur Yvette 91192, France. E-mail: gustavo.garcia@synchrotron-soleil.fr

Received 30th June 2020 , Accepted 11th September 2020

First published on 11th September 2020


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.


1. Introduction

Nitrogen dioxide (NO2) as a key reactive agent of atmospheric chemistry plays an essential role in the formation of secondary pollutants such as ozone and secondary organic aerosols in the troposphere, and has attracted a great deal of attention in the past.1,2 It has been long known that NO2 coexists with its dimer, dinitrogen tetroxide (NO2)2 or N2O4, with a monomer–dimer equilibrium in the gas phase, i.e. up to 80% of N2O4 in the NO2 sample at room temperature and atmospheric pressure.3 It is therefore necessary to consider the involvement of both species and even their reciprocal transformation in reactions. In particular, N2O4 is also an important reactive species in the atmosphere and is believed to be one major source of nitrous acid (HONO), a significant precursor of atmospheric OH radical.4,5 In addition, the absorption cross section of the dimer N2O4 is twice that of the monomer NO2 at shorter wavelengths in the ultraviolet-visible region.3

The accurate information of the dimer N2O4 including the spectroscopy, structure and thermochemistry are of considerable importance to model its chemistry in the atmosphere and deserve to be investigated in detail.6 Previously, the structure of the dimer N2O4 has been theoretically calculated by using ab initio methods and it has been shown that the most stable form of N2O4 can be viewed as a weakly bonded dimer with a D2h planar symmetric structure (sym-O2N–NO2) and a long N–N bond (1.756 Å), whereas the trans-asymmetric dimer (trans-ONO–NO2) has the second most stable structure with a higher energy of ∼7.7 kcal mol−1 than the sym-O2N–NO2.6–9Ab initio calculations by Jordan and Yoshioka predicted that the dimer cation N2O4+ also has a D2h local minimum structure with a barrier to fragment into NO2+ and NO2.10 The potential energy curves of the cation N2O4+ calculated by Li et al. show that its ground electronic state X2Ag is a bound state with a substantially elongated N–N bond (2.25 Å).11

The HeI photoelectron spectrum (PES) of N2O4 has been measured by several groups and mainly deduced from those of the NO2/N2O4 quasi-equilibrium mixture by reducing or subtracting the NO2 component at different temperatures, due to the impossibility to get purified N2O4.12–14 The clusters of NO2 in supersonic molecular beam were probed by photon or electron ionization mass spectrometry and an extensive oligomerization of NO2 beyond that of the dimer was observed.15,16 The synchrotron electron–ion velocity vector correlation experiments by Dowek et al. found that the dissociative ionization of N2O4 dominates over non-dissociative ionization.17 Despite many theoretical and experimental studies, fundamental properties of the dimer N2O4 such as the adiabatic ionization energy (AIE) are still not established or unknown, due to experimental challenges which include the coexistence of NO2/N2O4, difficult to overcome due to their very close energies,12–14 the lack of stability of the parent N2O4+, and the weak Franck–Condon factors (FCFs) for direct ionization to the ground state of N2O4+, due to the N–N bond elongation upon ionization.

In this work, we present a comprehensive vacuum ultraviolet (VUV) photoionization study of the dimer N2O4 by using the state-of-the-art method of double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy.18 We find that the dimer cation N2O4+ from direct ionization of N2O4 is not stable and fully dissociates into the NO2+ and NO2 fragments. In particular, after identifying two different sources of the observed NO2+ cation, photoionization (PI) of the neutral monomer NO2 and dissociative photoionization of the dimer N2O4, the mass-selected threshold photoelectron spectrum (MS-TPES) of pure N2O4 is acquired in coincidence, without interferences from other species. An upper limit of the AIE of the dimer N2O4 is indirectly determined via known thermochemical values and from the appearance energy (AE) of the NO2+ fragment measured in this work. In addition, five electronic states of the cation N2O4+ have been prepared and their state-specific dissociation mechanisms are discussed.

2. Experimental setup

The experiments were performed with the DELICIOUS III double imaging photoelectron photoion coincidence (i2PEPICO) spectrometer on the DESIRS undulator beamline at synchrotron SOLEIL, France.18–20 Briefly, the NO2 clusters were generated by supersonic expansion of NO2 gas seeded in helium (1 atm), collimated with two skimmers (1 mm diameter, Beam Dynamics Inc.), and entered into the ionization chamber of the SAPHIRS molecular beam endstation. Synchrotron photons emitted from a variable polarization undulator were dispersed by a 6.65 m normal incidence monochromator with a 200 l mm−1 grating and focused onto a 200/70 μm (H/V) spot at the ionization region. The entrance and exit slits of the monochromator were set at 30 μm, providing an energy resolution of 3 meV. A gas filter located upstream of the beamline was filled with Ar (0.3 mbar) to eliminate high-order harmonics and provide an absolute photon energy calibration by using the resonant absorption lines of Ar.21 The i2PEPICO spectrometer installed in the ionization chamber of SAPHIRS is composed of an electron velocity map imaging device and a modified Wiley–McLaren TOF 3D-momemtum ion imaging analyzer operated in coincidence.18 The coincidence scheme can yield electron images and then photoelectron spectra via an Abel inversion algorithm,22 correlated to a definite ion mass and momentum. In turn, the ionization signal can be presented as a function of the correlated ion kinetic energy and electron binding energy for any given ion m/z, leading to electron and ion kinetic energy correlation diagrams.

3. Results and discussion

Synchrotron radiation photoionization time-of-flight (TOF) mass spectra were measured and their integrated spectrum in the 10–15 eV energy range is presented in Fig. 1(a). Four peaks at m/z = 30, 46, 92 and 138 are visible and assigned as NO+, NO2+, N2O4+ and (NO2)3+, respectively. The peak associated to the NO+ cation is Doppler broadened and hence attributed to the known dissociative PI of NO2.23 The NO2+ region presents two contributions centered at m/z = 46, one from direct PI of NO2 in the form of an intense and narrow peak and another one as a broad pedestal indicative of a large kinetic energy release upon dissociative PI. As depicted in the inset of Fig. 1(a), ray tracing simulations show that the narrow width of the direct PI peak corresponds to a very low translational temperature, estimated at 2 K.
image file: d0cp03495a-f1.tif
Fig. 1 (a) Photoionization TOF mass spectrum integrated in the 10–15 eV energy range. A zoom on the m/z = 44–48 region is provided in the inset, where two contributions centered at m/z 46 are visible, a narrow peak on a broad pedestal, which are modelled with ray-tracing simulations assuming different translational energies. The ion images corresponding to (b) the m/z = 45–47 range (KE(NO2+) ≤ 0.34 eV), (c) the narrow peak (KE(NO2+) ≤ 0.0015 eV) and (d) the wide pedestal (KE(NO2+) > 0.0015 eV). The image intensity projections along the y-axis are marked with white dashed lines.

This is corroborated by the corresponding coincident ion images displayed in Fig. 1(b–d), where direct and dissociative ionization processes can now be distinguished by the size of their spatial footprint on the detector, i.e. small and large, respectively. Note that the image of Fig. 1(d) is acquired in coincidence from the correlation to the wide dissociative PI pedestal only in the mass spectrum. In addition, the information in Fig. 1(a) and (b) is combined to yield the full 3D ion momentum distribution in the present coincidence scheme.18,19

In accordance with previous results of photon or electron ionization mass spectrometry,15,16 a very weak signal can be visible at m/z = 92 in the mass spectrum of Fig. 1(a), associated with the N2O4+ cation. The m/z = 92 peak has a broad width and also correlates with a large ion image (see Fig. S1, ESI), indicating that these N2O4+ cations were not formed from direct PI of the neutral dimer N2O4, and should be from dissociative PI of a larger cluster, such as the trimer (NO2)3. Presently the largest cluster cation observed within the mass range of m/z ≤ 500 is (NO2)3+ at m/z = 138, with a higher intensity than N2O4+ in the mass spectrum. The potential presence of larger clusters can be a source of incertitude since they might also contribute to the NO2+ channel through dissociative PI. But, dissociative ionization of small clusters is dominated by evaporation of a single neutral monomer (NO2)n+ → (NO2)n−1+ + NO2, which is thermochemically more plausible due to the lower ionization energy of the clusters with respect to the monomer, so that dissociative PI of the solely dimer N2O4 contributes to the m/z = 46 channel.14 Indeed, the mass spectra and the ion images of NO2+ were also measured at different backing pressures, and the experimental results show that the width of the pedestal (the hot NO2+ fragment) and its image size do not change with pressures, which also confirm the above assumption.

Within our lower/better detection limit than that of the previous vector correlation experiments by Dowek et al.,17 corresponding to a dissociative PI/PI ratio above 200–3000 depending on the photon energy (see Fig. S2, ESI), no evidence of the nascent, translationally cold, dimer N2O4 is found in our data, that would appear as a sharp coincident image and narrow TOF peak. Note that the present lower detection limit is about two orders of magnitude larger than the one leading to dissociative PI/PI ≥ 20 as quoted by Dowek et al.17 Thus, we consider that the dimer cation N2O4+ is fully unstable and that it directly dissociates into the NO2+ and NO2 fragments.

The coincident signal as a function of electron kinetic energy (eKE) and photon energy was measured for the NO2+ kinetic energy ranges visible in Fig. 1, namely KE(NO2+) ≤ 0.34 eV (m/z = 45–47 range), KE(NO2+) ≤ 0.0015 eV (PI of the monomer NO2) and KE(NO2+) > 0.0015 eV (dissociative PI of the dimer N2O4), by scanning the synchrotron photon energy with a step size of 10 meV, and are shown in Fig. 2(a, c, e). In these images, direct photoionization transitions appear as diagonal lines with a unity slope, eKE = − IEth, where IEth is the ionization energy of a cationic state.24 The MS-TPES corresponding to each mass are obtained by the integration of threshold electrons (eKE ≤ 10 meV) and presented as black solid lines in Fig. 2, while summation over all electrons can yield photoionization efficiency curves (PIE), also shown as blue dotted lines.


image file: d0cp03495a-f2.tif
Fig. 2 Mass-selected electron signal as a function of electron kinetic energy (Ele KE) and photon energy, mass-selected TPES and PIE curves corresponding to (a and b) the m/z = 45–47 mass range, (c and d) the monomer NO2, and (e and f) the dimer N2O4 (hot NO2+), with 20 times magnified TPES data in red. The resonant absorption lines of Ar in the gas filter are marked with asterisks.

The threshold photoelectron signal is obtained when the photon energy is resonant with the ionization transition and thus can provide sensitive and structural information.24–26 As shown in Fig. 2(b), the MS-TPES of the m/z = 45–47 mass range has four bands in the energy range of 10–15 eV, which arise from a combination of the electronic bands of the NO2+ cation and the dimer cation N2O4+. Two electronic states of NO2+, X1Σ+g and a3B2, can be identified with vibrational structures and assigned in the MS-TPES of Fig. 2(d).23 Several peaks are also observed in the PIE of NO2+, and ascribed to the autoionization of the Rydberg states of NO2 converging towards the a3B2 limits. The higher energy states of the NO2+ cation, above the a3B2(020) vibrational level, are fully dissociative to produce the NO+ and O fragments.23

The MS-TPES of the dimer N2O4 is obtained and presented in Fig. 2(f). Note that the contribution from the direct ionization of NO2 is effectively removed since there is no signal from this process correlated to ions having translational energies above 1.5 meV. Previous calculations show that the dimer has several structures and the symmetric N2O4 (O2N–NO2) is the most stable without barrier for its formation from two isolated NO2 monomers. The enthalpic barrier to form trans-N2O4 (ONO–NO2), the second most stable structure, is calculated to be 13.2 kcal mol−1, and the enthalpic barrier for N2O4 isomerizing to trans-N2O4 is 43.9 kcal mol−1.6,7 Presently the dimer N2O4 was formed in a cold molecular beam (∼2 K) and so only the symmetric N2O4 can be populated in the experiments. The symmetric dimer has a D2h symmetry and its X1Ag ground state has the electronic configuration (4b3u)2(1b1g)2(1au)2(4b2g)2(6ag)2.8 Ejection of an electron from these orbitals can lead to five electronic states of the cation N2O4+, (6ag)−1X2Ag, (4b2g)−1A2B2g, (1au)−1B2Au, (1b1g)−1C2B1g and (4b3u)−1D2B3u, three of which are easily attributed to individual experimental bands in the MS-TPES of Fig. 2(f), with vertical ionization energies for the X2Ag, A2B2g and B2Au, bands measured at 11.24, 12.35 and 13.02 eV respectively.12–14 The C2B1g and D2B3u states overlap within the fourth band, with a vertical ionization energy measured at 13.47 eV.

The overall shape of the present MS-TPES is similar to the previously-published PES.12–14 The AIE of N2O4 was reported at 10.8 eV by Frost et al. with HeI PES after subtraction of the NO2 contribution.14 However, due to the absence of cation information their reported value should actually be taken as their observed appearance energy of the NO2+ fragment ion from dissociative ionization of N2O4, which incidentally is much higher than the one reported in this work probably due to their large baseline, a likely consequence of their subtraction procedure.

From the MS-TPES of Fig. 2(f), the AE of the NO2+ fragment in the dissociative PI of the dimer N2O4 is measured at 10.15 ± 0.05 eV from the extrapolation of the observed linear onset. Assuming a barrierless, fast dissociation and the absence of kinetic shifts, a precise determination of the adiabatic appearance energy (AE0K) would also need to take into account the internal energy of the parent cation N2O4+, with contributions from the inefficient cooling of the vibrational modes in the molecular beam despite the low translational temperature (∼2 K). Nevertheless, our value for the AE is in good agreement with the thermochemical results, as shown in Fig. 3. Through a thermochemical cycle, the AE0K is calculated at 10.141 ± 0.003 eV, based on the AIE of NO2 (9.5862 ± 0.0025 eV)27 and the N–N bond energy of N2O4 (D0 = 0.555 ± 0.003 eV) obtained with the ATcT recommended enthalpies of formation for the N2O4 dimer ΔfH00K(N2O4) = 20.15 ± 0.14 kJ mol−1 and the NO2 monomer ΔfH00K(NO2) = 36.859 ± 0.065 kJ mol−1.28 Moreover, combining our measured AE of hot NO2+ with the calculated bond energy of the cation N2O4+, 0.56 eV,11 an upper limit of the AIE of the N2O4 dimer is provided at 9.59 ± 0.05 eV, much lower than the reported value of 10.8 eV by Frost et al.,14 but agreeing well with the calculated results of 9.581 eV (see Fig. 3). Note that there is no signal in our TPES between this value and the observed AE of the NO2+ fragment due to the very weak FCFs expected from the potential energy curves.


image file: d0cp03495a-f3.tif
Fig. 3 Calculated thermochemical cycle of the dimer N2O4 with the experimental AIE of NO2.27,28

Indeed, previous calculations show a significant structure change upon photoionization, and in particular the X2Ag state of N2O4+ has a substantially elongated N–N bond of 2.25 Å, much longer than that of 1.756 Å of the neutral X1Ag ground state.11 The potential energy curves of N2O4+ adapted from ref. 11 are presented in Fig. 4. Based on the Franck–Condon transition in photoionization, the N2O4+ cations should be firstly populated in the repulsive part of the potential energy curve of the X2Ag state and then directly dissociate to the NO2+ and NO2 fragments. This is a fast direct dissociation and most of the available energy will be released into kinetic energy. Note that, in the absence of a Franck–Condon simulation, the weak FCFs at threshold further complicate the precise determination of the AIE.


image file: d0cp03495a-f4.tif
Fig. 4 Potential energy curves of N2O4 and its cation N2O4+ along the O2N–NO2 coordinate, adapted from ref. 11 (reprinted with permission from AAAS). The purple arrow shows the Franck–Condon (F–C) region relevant for the ionization of N2O4.

The MS-TPES corresponding to the mass m/z = 92 has also been acquired and is presented in Fig. S1 (ESI). The envelope of the MS-TPES is totally different to that of Fig. 2(f), and the appearance energy of the N2O4+ fragment ion is measured at 10.63 ± 0.06 eV.

To reveal the dissociation dynamics of the N2O4+ cation at different electronic states, a photoelectron photoion coincidence (PEPICO) experiment has been performed at a fixed photon energy of = 14 eV, and the electron and ion kinetic energy correlation diagram of the NO2+ fragment is presented in Fig. 5(a), together with the PES. The correlation diagram exhibits fine structures and can be divided into four parts, correlating to the X2Ag, A2B2g, B2Au, and C2B1g/D2B3u electronic states of the N2O4+ cation, respectively.


image file: d0cp03495a-f5.tif
Fig. 5 (a) Electron and ion kinetic energy correlation diagram of the NO2+ fragment ion (KER > 0.0015 eV) recorded at fixed photon energy of = 14 eV, together with the PES marked as a white dashed line, and the total kinetic energy released (KER) in the dissociation of the (b) X2Ag, (c) A2B2g, (d) B2Au, and (e) C2B1g/D2B3u states of N2O4+.

The corresponding total kinetic energy released (KER) in the dissociation of state-selected N2O4+ cations are shown in Fig. 5(b–e) and exhibit different shapes, implying state-specific dissociation mechanisms of the N2O4+ cation. More concretely, for the X2Ag and A2B2g states, a large ratio of the available energy in the dissociation has been released into the kinetic energy and their total KER curves takes a contour of Gaussian function, indicating that their dissociations should be fast, such as direct dissociation (the X2Ag state) or predissociation (the A2B2g state), seen in the potential energy curves of Fig. 4.11 On the contrary, the dissociation of the C2B1g/D2B3u states are slow and their KER curve can be fitted with a Boltzmann function, while the total KER curve of the B2Au state has both Gaussian and Boltzmann characters.

4. Conclusions

In summary, the VUV photodynamics of the dimer N2O4 has been investigated in detail by using the high resolution method of i2PEPICO associated to the VUV DESIRS beamline. After separating the two different sources of the NO2+ cations observed in the mass spectra, photoionization of the monomer NO2 and dissociative photoionization of the co-existing dimer N2O4, the pure MS-TPES of N2O4 has been acquired, without interferences from other species. Up to five low-lying electronic states of the N2O4+ cation, X2Ag, A2B2g, B2Au, C2B1g and D2B3u, have been prepared and their state-specific dynamics are analyzed. The present experimental results directly demonstrate that the N2O4+ cation from direct ionization of N2O4 is unstable and fully dissociates into the NO2+ and NO2 fragments. The observed N2O4+ cations in the mass spectra are from dissociative PI of the trimer (NO2)3, not from photoionization of the dimer N2O4. In addition, the appearance energy of the NO2+ fragment ion in dissociative PI of the dimer N2O4 is determined at 10.15 ± 0.05 eV and an upper limit of the AIE of the dimer is provided at 9.59 ± 0.05 eV.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

X. T. thanks support from the National Natural Science Foundation of China (No. 91961123, 21773249, 91644109) and the International Partnership Program of Chinese Academy of Sciences (No. 116134KYSB20170048). The authors would like to thank Dr Serguei Patchkovskii for sending us the data of Fig. 4. The authors are also grateful to the SOLEIL staff for running the facility and providing beamtime under proposal 20140915.

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Footnote

Electronic supplementary information (ESI) available: The ion image of the m/z = 92 cation and its MS-TPES and PIE. The determination of the detection limit for the N2O4+ parent ion. See DOI: 10.1039/d0cp03495a

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