NOO Peroxy Isomer Exposed with Velocity-Map Imaging

Letter pubs.acs.org/JPCL

NOO Peroxy Isomer Exposed with Velocity-Map Imaging B. A. Laws, S. J. Cavanagh, B. R. Lewis, and S. T. Gibson* Research School of Physics and Engineering, The Australian National University, Canberra ACT 2601, Australia S Supporting Information *

ABSTRACT: The chemistry of NO2, a key atmospheric trace gas, has historically been interpreted in terms of the C2v isomer ONO, with the peroxy isomer NOO only postulated to be stable. In this work, a velocity-map-imaged photoelectron spectrum of the nitrite anion, NO2−, reveals energetic-electron structure that may only occur by photodetachment from the NOO−(X̃ 1A′) isomer. This measurement defines NOO(X̃ 2A′) bond frequencies and an electron affinity of only 335(30) cm−1, which, supported by ab initio calculations, confirm the first observation of this important reactive species.

providing the first direct spectroscopic measurement of NOO(X̃ 2A′). Details of the experimental apparatus and image analysis are given in the Supporting Information and ref 22. NO2− ions are produced by passing pure N2O gas through a pulsed-valve, 4pin electric discharge, with a backing pressure of ∼2 atm. Absolute eKE calibration of photoelectron spectra is established from separate measurements of known species, including NO2−,20 O−,22 O2−,23 and NO−,24 covering the same radial area of the detector as the NO2− measurement. The eKE full width at half-maximum (fwhm) resolution varies from 40−15 cm−1, depending on the eKE (see the Supporting Information). A velocity-map image of the photoelectrons collected from 518.92 nm (2.39 eV) photodetachment of mass-isolated NO2− is shown in Figure 1a. The image reveals some surprising, high eKE structure, more energetic than could be produced from C2v NO2− photodetachment with an EA of 2.273(5) eV.20 The strong central ring structure is readily assigned to ONO(X̃ 2A1) ← ONO−(X̃ 1A1) + hν vibrational electron detachment transitions, with a positive anisotropy parameter (defined in eq 2). (A detailed analysis of the ONO− photodetachment spectrum will be discussed elsewhere.) The photoelectron spectrum extracted from the velocity-map image (radial intensity distribution) is shown in Figure 1b. It has structures due to electron detachment from ONO− (from VMI center), O− (the VMI middle ring), and NOO− (edge of VMI). The photoelectron spectrum of mass-isolated NO− recorded under identical experimental conditions is also shown for comparison. For the more energetic electrons, which cannot originate from ONO−, the middle-image ring has

N

itrogen dioxide, NO2, is a molecule of significance in atmospheric chemistry. As a toxic gas and prominent component of photochemical smog, high concentrations in the atmosphere can lead to various health risks in humans, including decreased lung function and respiratory problems.1 As a result, neutral NO2 has been the feature of numerous studies.2−10 However, little consideration has been given to the possibility of different isomers of NO2 and their potential impact on atmospheric chemistry. The NOO peroxy isomer was first discussed by Clyne and Thrush,11 who postulated that NOO could play an important intermediate role in the formation of NO in the atmosphere through the reaction N + O2 → NOO → NO + O

(1)

Such a pathway for forming NO may have a large effect on nitric oxide cooling in the D-region of the ionosphere.12,13 Various experimental and computational studies have attempted to confirm the existence of NOO; however, currently there is still debate on whether NOO represents a stable minimum on the NO2 potential energy surface or is merely a transient reaction intermediate.14−16 In experimental studies, some total photoelectron-yield measurements have indicated a longwavelength photoelectron tail, which has been posited as evidence of the peroxy radical, having a lower electron affinity (EA) than that of C2v NO2.17,18 The presence of the tail is sensitive to anion production methods, being unobserved in some of the later measurements, precluding confirmation that this observed tail is a consequence of peroxy NOO.19−21 In this work, we report on a velocity-map imaging (VMI) measurement of the photodetachment of NO2−, produced in a pulsed-valve electric discharge of pure N2O. The image of the electron detachment reveals high electron kinetic-energy (eKE) structure that cannot be produced from electron detachment of C2v NO2−. Analysis identifies that the additional structure is due to photodetachment of a stable peroxy isomer, NOO−(X̃ 1A′), © XXXX American Chemical Society

Received: August 18, 2017 Accepted: August 30, 2017 Published: August 30, 2017 4397

DOI: 10.1021/acs.jpclett.7b02183 J. Phys. Chem. Lett. 2017, 8, 4397−4401

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The Journal of Physical Chemistry Letters

Figure 1. Photodetachment of mass-isolated NO2− at 518.92 nm: (a) Velocity-map image of 1, 642, 417 photoelectrons, from 170,000 laser shots, with structure due to ONO− (center), O− (middle, intensity enhanced ×4), and NOO− (outer rings, ×7). (b) Extracted radial photoelectron spectrum, with transitions ONO (X̃ 2A1) ← ONO−(X̃ 1A1), O(3P2,1,0) ← O−(2P3/2,1/2), and NOO(X̃ 2A′) ← NOO−(X̃ 1A′) (enhanced ×7). The latter is compared with a separate VMI measurement of NO(X 2Π) ← NO−(X3Σ−) recorded under identical conditions (inset). (c) Expanded energy-scale NOO− spectrum, illustrating asymmetric band-profiles due to the unresolved 1n0220 progression.

the well known spectroscopic signature of O− photodetachment, with a characteristic negative anisotropy parameter, opposite to the sign for electrons detached from ONO−. The outer weak ripple pattern near the detector edge, also with negative anisotropy, looks superficial, as though it could be produced from a NO− photofragment. However, a more detailed spectroscopic characterization reveals significant differences from what would be expected in this case. Furthermore, dissociation energies of ONO− calculated from known electron affinities25 and heats of formation26 give: D0(ON···O−) = 3.932 eV, D0(O···NO−) = 5.367 eV. Therefore, at the photodetachment energy of 2.39 eV, both photofragment channels yielding O− and NO− from ONO− are closed, making observation of electrons detached from O− in the velocity-mapped image surprising. This discrepancy may be explained by the presence of a stable peroxy isomer of the NO2− anion, which has a much smaller NO···O− calculated bond dissociation energy, 0.3 eV.15 The photoelectron spectrum of O− photodetachment is well known.22 The present measurement resolves fine-structure transitions, which when fitted to Gaussian functions provide an anion fine-structure separation of 175(4) cm−1, in agreement with the accepted value, 177.13(5) cm−1,27 and a common fwhm of 91(2) cm−1. This value is larger than the typically ∼40 cm−1 measured around 518.92 nm using our VMI-spectrometer. The ratio of intensities (areas) 3 P2 ← 2 P3/2: 3 P2 ← 2 P1/2 , taking into account the geometric fine-structure intensity ratios,28 is also anomalous, giving equally populated anion fine-structure levels. Photodetachment measurement of isolated oxygen anions, produced from a pure molecular oxygen discharge using the same ion source, typically yields a source anion temperature of ∼200 K. Thus the process leading to O− formation is extremely nonthermal. It is clear from Figure 1b,c that the high-eKE structure from NOO− photodetachment is distinct from that produced by

NO−, despite both having similarly low EA and diatomic-like structure. The vibrational spacing differs substantially, with the NOO bond frequency of the main progression ωe1 = 1270(20) cm−1, ωexe1 = 1(1) cm−1, significantly less than that for neutral NO, which has ωe = 1904.1346(18) cm−1.29 The flat-topped NO vibrational features, a consequence of the ∼123 cm−1 spin−orbit splitting of the doublet X2Π NO ground state, also differ from the much sharper singlet-like peaks of NOO. A diatomic-like spectrum is characteristic of a polyatomic molecule containing a weak central bond, which has been observed in a microwave spectroscopic study of HOON.30 Close inspection of the peroxy spectrum, (an expanded energy scale of the NOO region is included in Figure 1) reveals structure present on the right-side of each of the dominant peaks, which may be assigned to a 1n0220 progression, with a bond frequency of ω2 ≈ 720(10) cm−1. There is also weak structure between the transitions, which is likely a signature of the ω2 mode 1n0210 progression. The strong dominance of the ω1 fundamental vibrational progression in the photoelectron spectrum can be explained by the weak central O−O bond, 1.376 Å, along with the NOO → NO + O dissociation pathway that lies along the ω2 stretching coordinate. Assignment of the NOO peroxy photoelectron spectrum is verified by ab initio calculations of the neutral and anion species using NWChem software.31 These calculations are based on the coupled-clusters method that includes singles, doubles, and perturbative connected triples (CCSDT), with an augmented cc-pVDZ basis set. For the NOO peroxy molecule, the calculated Cs ground-state geometry and normal-mode vibrational frequencies are in good agreement with previous calculations, at a similar level of theory.14,15 The normal normal-mode frequencies of the two observed modes in neutral NOO are ω1 (N−O stretch) ≈ 1390 cm−1 (cf 1116−1639 4398

DOI: 10.1021/acs.jpclett.7b02183 J. Phys. Chem. Lett. 2017, 8, 4397−4401

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The Journal of Physical Chemistry Letters cm−114) and ω2 (O−O stretch) ≈ 640 cm−1 (cf 519−761 cm−114). A key piece of supporting evidence for the presence of NOO is the observation of photofragment O− in the spectrum shown in Figure 1. While fragmentation from C2v ONO is closed at 518.9 nm (D0(ON···O−) = 3.932 eV > hν = 2.39 eV), the CCSDT calculations in this work predict a bond energy for the peroxy isomer of D0(NO···O−) ≈ 0.13 eV. This is in good agreement with previous estimates of ∼0.3 eV15 and explains the presence of O− in the experimental photoelectron spectrum. For the neutral peroxy radical the D0(NO···O) bond energy is higher, calculated to be ∼1.1 eV. The electron affinity of NOO is established here in relation to the NO species. A full rotational band model fitted to the measured NO− photoelectron spectrum determines the EA for NO to be 240(10) cm−1, in agreement with the accepted value 210(40) cm−1.24 This position lies 6 cm−1 below the (0, 0) peak intensity maximum. Engelking’s symmetric top approximation32 predicts a similar shift for both NO and NOO, so applying the same 6 cm−1 correction to the NOO peak position in Figure 1 gives the EA of the peroxy NOO isomer as 335(30) cm−1 [0.042(4) eV]. This value is smaller than that evaluated from previous ab initio calculations;14 however, as they note, calculations are uncertain due to the importance of electron correlation. The previous experimental estimation of EA(NOO) ≈ 1.8 eV, obtained from extended photoelectron tails measured in cross section experiments,17,18 is likely due to observation of photoelectrons detached from the fragment anion O− (EA = 1.4611 eV) rather then the peroxy isomer itself, with the more energetic, lower intensity NOO electrons undetected. This is a problem that the VMI technique employed here overcomes. Information about the parent molecular orbital is encoded in the angular distribution of the emitted photoelectrons. The relative intensity of the image as a function of angle (θ), with respect to the axis of polarization, follows the relation33 I(θ , ϵ) =

σtotal(ϵ) [1 + β(ϵ)P2(cos θ)] 4π

with parameters AS and C = cos(δS + 1, S − 1), where δS + 1, S − 1 is the phase shift induced by interactions with the remaining neutral species. Molecules lack spherical symmetry, relaxing the S selection rule, to give an anisotropy parameter that is the sum over partial waves.36 It turns out that the atomic expression still applies quite well to molecular photodetachment23 in cases where the electron is ejected from a molecular orbital that possess dominant atomic character. Experimental electron anisotropy parameters determined from the transitions of NOO− for photoelectron spectrum measured at 518.92 nm are shown in Figure 2, together with

Figure 2. Anisotropy parameter for NO− (open symbols) and NOO− photodetachment (solid symbols). NO−, this work (red open circles); Siegel37 (green open squares); Khuseynov36 (cyan open diamonds). These measurements are adequately represented by the Hanstorp35 approximation for l = 2 detachment with the parameters A2 = 0.5(1), C = 0.94(1) (solid line). Detachment from the peroxy NOO− anion, this work (blue solid circles), is distinctly more isotropic than from NO−.

those for NO− determined at both 518.92 and 1064 nm. (The 1064 nm wavelength photoelectron spectrum, which resolves the ground-state NO fine structure, will be described in more detail elsewhere.) The S = 2 , A2 = 0.5(1), and C = 0.94(1) atomic Hanstorp model provides a good representation of the measured energy dependence for detachment from NO− without any need for the mixed atomic-character model because the mixing is small, with the 2π* HOMO being ∼98.5% d-like.36 While the anisotropy parameters measured for detachment of NOO− follow a similar trend to NO−, they indicate substantially less anisotropy than for detachment from NO−. This is a well known characteristic of clustered molecules.38,39 Images of the molecular orbitals of NOO− and ONO−, calculated using NWChem software,31 are shown in Figure 3. The different orbital structure between the two isomers explains the opposite sign in the anisotropy. Dividing the

(2)

where β is the eKE(ϵ)-dependent anisotropy parameter, with value −1 ≤ β ≤ 2, and P2(cos θ) is a second-order Legendre polynomial in cos θ. From the velocity-map image in Figure 1a, it is clear that electrons detached from O− and NOO− have a propensity to be ejected perpendicular to the laser polarization, opposite to those electrons detached from ONO−. At the 518.92 nm measurement wavelength, ONO− photodetachment has a positive anisotropy parameter, for example, βONO−(100200) = +1.66(5), while O− has a negative value, βO−(3P) = −0.78(3), which is typical for this wavelength.34 NOO− also has a negative anisotropy parameter. Quantum mechanically, the anisotropy parameter for atoms is defined, in the central-potential model, as a coherent sum of radial transition dipole matrix elements for the two possible partial waves (ΔS = ± 1) for the detached electron, at a given eKE.33 If the ratio of the magnitudes of the matrix elements is assumed to vary linearly with eKE, as AS ϵ, then an analytical expression (Hanstorp approximation35) for the anisotropy parameter results βS (ϵ) =

S(S − 1) + (S + 1)(S +

2)AS2 ϵ2

Figure 3. Orbital diagrams of the highest occupied molecular orbital in C2v ONO− and Cs NOO−, calculated using NWChem software. The ONO− orbital has a symmetry plane through the center of the N atom, whereas the NOO− orbital appears similar to a combination of a π* antibonding NO and an atomic p orbital.

− 6S(S + 1)AS ϵC

3(2S + 1)[S + (S + 1)AS2 ϵ2] (3) 4399

DOI: 10.1021/acs.jpclett.7b02183 J. Phys. Chem. Lett. 2017, 8, 4397−4401

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The Journal of Physical Chemistry Letters HOMO of Cs peroxy NOO into two parts, it is seen that the orbital structure around the NO moiety appears similar to the standard diatomic π* antibonding orbital of NO− that would be expected if the terminal O was not present. This is consistent with the observed effect solvation of NO− has on the observed anisotropy,38 which, along with a relatively long O−O bond, suggests that there is only minor mixing between the NO and O segments. The minor character mixing also explains why the peroxy normal vibrational modes ω1 (N−O stretch) and ω2 (O−O stretch) remain largely uncoupled. Further validation of the peroxy spectral assignment may be obtained by examining the photophysical processes involved. The arrival time to the detector of each of the photofragments produced is measured relative to a baseline with the laser turned off (Figure 4). The laser interaction decreases the

NOO (fragmentation and detachment) is a similar order of magnitude to C2v NO2 above threshold, ∼7 × 10−18 cm2.17,18 During the experiment, 20 mJ laser pulses photodetach an average of 10 electrons per laser shot, with 12.5% from NOO−, 25% from O−, and 62.5% from ONO−. From the known photodetachment cross-sections of ONO− and O− at the detachment wavelength of λ = 518.9 nm (σO = 6.5 × 10−18 cm2, σONO = 1 × 10−19 cm218), the number density of each ion may be estimated using the Beer−Lambert law. Repeating this process for the peroxy ion, accounting for both fragmentation and detachment channels, gives an isomeric ratio of n NOO− ≈ 2% nONO− At 518.92 nm, the observed electron signal from C2v NO2− is suppressed by the Wigner threshold law, enhancing the apparent signal from NOO−. A large barrier to isomerization (∼1.3 eV14) makes interconversion unlikely. In conclusion, the NOO peroxy isomer of NO2 has been observed for the first time and characterized spectroscopically. This radical is much more reactive than the common isomer ONO due to its weak, long O−O bond. Previous work has predicted that the existence of NOO would have large implications in atmospheric science through reactions such as eq 1, which have an effect on the formation of NO in the Earth’s ionosphere.11 Similar reactions have also been observed for HOON, where photolysis of the weak O−O peroxy bond is involved in many radical reactions.30 These works suggest that any presence of peroxy NOO is likely to play a significant role in the chemistry of planetary atmospheric NO2.

Figure 4. Time-of-flight MCP voltage signal with the laser at 518.92 nm minus the baseline signal with the laser off. Four temporal signals are observed, the photoelectron signal, O− ions, NO2− ions, and the corresponding neutral molecules resulting from the ions that have electrons photodetached.

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number of NO2− ions, creating a photoelectron signal, a signal corresponding to the neutral species NO2, and the signal of O and O− ions. The O− signal is only observed when the laser is on, confirming that photon energy is required to break the peroxy O−O bond and that this isomer is in fact bound NOO− + hν → NO + O−

(4)

O− + hν → O + e−

(5)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02183. Detailed experimental methodology (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

As D0(NOO) ≈ 0.13 eV and at 519 nm hν ≈ 2.4 eV, ∼2.26 eV of excess energy from the first step will be distributed into translational and internal energy of the NO and O − photofragments. This excess energy explains the observed anomalous nonthermal population of the O− fine structure levels and is a possible explanation for the broadening observed, as noted earlier. This gives further evidence that the observed fragment O− results from dissociation of the much weaker peroxy NO−O− bond. As no other fragment signals are observed, eq 4 must be the only photofragment channel open, confirming that the highenergy electrons observed near the detector edge must originate from direct detachment of the peroxy NOO ion. Furthermore, the possibility of dissociative photodetachment occurring may be ruled out with a simple energy balance. Given the low source temperature of ∼200 K and the low measured binding energy of ∼0.042 eV, the only possible high eKE channel is NOO− + hν → NOO + e−

ASSOCIATED CONTENT

S Supporting Information *

ORCID

B. A. Laws: 0000-0001-8086-5482 S. T. Gibson: 0000-0002-3767-6114 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Australian Research Council Discovery Project Grant DP160102585. REFERENCES

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