Applied Surface Science 666 (2024) 160340

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A Blyholder mechanism in the chemisorption of N2 O on Ni(111) – studied
with Auger-photoelectron coincidence spectroscopy
Fredrik O.L. Johansson a,b ,∗, Lucas M. Cornetta a,c , Elin Berggren a , Artem Kuklin a ,Yi-Chen Weng a , Swarnshikha Sinha d,e , Danilo Kühn d , Alexander Föhlisch d,e , Hans Ågren a ,Andreas Lindblad aaDivision of X-ray Photon Science, Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala, 751 20, Sweden
Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, 4 pl. Jussieu, Paris, F-75005, FrancecInstituto de Física da Universidade de São Paulo, Universidade de São Paulo, São Paulo, BrazildInstitute Methods and Instrumentation for Synchrotron Radiation Research PS-ISRR, Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Straße
15, Berlin, 12489, Germanye Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, Potsdam, 14476, Germanyb

ARTICLEINFO

Keywords:
Auger-photoelectron coincidence spectroscopy
Blyholder model
Heterogeneous catalysis

ABSTRACTIn heterogeneous catalysis the surface-adsorbate bond strength is critical for the function of the system. Here we
study a series consisting of multilayer, bilayer and monolayer N2 O on Ni(111) and employ Auger-photoelectron
coincidence spectroscopy (APECS) to study the interaction between the molecule and the substrate directly. We
observe intensity in the nitrogen Auger spectra that arise from the interaction between molecule and surface(not observed in free molecules) whereas the oxygen spectra are thickness-independent. Since the two nitrogenatoms of N2 O are chemically inequivalent we can assign the intensity present in the bilayer and monolayercases to orbitals centered on the terminal nitrogen which is closest to the Ni(111) surface. Using ab initio,
molecular dynamics and solid-state density functional theory calculations we infer a Blyholder model of thesurface bond as consisting of donation from the terminal nitrogen lone-pair valence orbital with back-donationfrom the metal into the unoccupied orbitals on that nitrogen. This coincidence technique can readily be used to
study substrate–adsorbate interactions directly with chemical and orbital specificity — this opens up prospects
to study fundamental steps of molecular adsorption and heterogeneous catalysis with unprecedented detail.

1. Introduction
The interaction between metal surfaces and molecules constitutes achallenging and important area of study not the least for applications
in catalysis. The oxides of nitrogen (NOx ) play a particularly importantrole as they are products from combustion and thus contribute to theanthropogenic concentration in the atmosphere of these gases. The
main source of NOx is from surface emissions of nitrous oxide, N2 O,
which also is the third most significant greenhouse gas [1]. N2 O as anozone-depleting agent is predicted to remain the main driver of ozone
depletion during the 21st century [2].
N2 O is a triatomic molecule with the configuration N⋯N⋯O thatis linear in the isolated case with the terminal (NT ) and central (NC )nitrogen atoms being chemically inequivalent. The NT –NC bond is
1.126 Å and the NC –O bond is 1.191 Å. [3] N2 O is an obvious candidate
for heterogeneous catalysis since it is thermodynamically unstable withrespect to N2 and O2 (standard enthalpy of formation 𝛥𝑓 𝐻 −◦ at 82
kJ/mol) but with a high activation energy barrier [4].
Catalytic conversion of N2 O into non-pollutant components is anactive challenge for the surface science community [5,6] and thedevelopment of catalytic converters in combustion engines (reviewed
by [7]) is a success story for the field. If new fuels (e.g. NH3 as analternative to H2 [8]) or energy conversion systems like fuel cells [9]are to be employed the problem of removing pollutants remain.
Nitrous oxide has been extensively studied in its own regard using
electron spectroscopy. Since the NT and NC have different chemicalsurrounding, both core level and unoccupied orbitals can be studied,with electrons [10] and tunable X-rays (e.g. from synchrotrons) toobtain information about site specific excitations into different orbitals
in the electronic structure [11–13].
An interesting application for this is that N2 O decomposition on Ni
surfaces can be used to measure Ni particle sizes in catalyst via pulse
titration of the gas over the catalyst [14].

∗ Corresponding author at: Division of X-ray Photon Science, Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala, 751 20, Sweden.

E-mail address: fredrik.johansson@physics.uu.se (F.O.L. Johansson).
https://doi.org/10.1016/j.apsusc.2024.160340
Received 7 March 2024; Received in revised form 17 May 2024; Accepted 19 May 2024
Available online 21 May 2024
0169-4332/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Applied Surface Science 666 (2024) 160340

F.O.L. Johansson et al.

The adsorption of N2 O on various metal surfaces has been extensively studied as reviewed by Zeigarnik [15]. From early studies
on Ni [16] to molecular orientation after physisorption on Ni(111)
surface [3], and further: monolayers on Ag [17] and ab initio modelingof the self-assembly on these surfaces [18]. Similar to NO2 and N2 ,the decomposition of N2 O has also been studied on various surfaces
including Ni(110) [19]. Kodama et al. have also shown that N2 O does
not decompose on fcc(111) faces, but on the vicinal (755) surface ofNi(111) it occurs while step-sites are available and when those are
saturated by O, one of the decomposition products, further adsorption
leave the N2 O intact [20].
The chemisorption bond of molecules on metal surfaces may insome instances be conceptualized by the Blyholder model, where donation of HOMO (highest occupied molecular orbital) electron densityfrom the molecule to the surface is compensated by a back-donation ofelectron density from the surface into the LUMO (lowest unoccupied
molecular orbital) to keep the system charge neutral. A classical example is CO chemisorption on Pt(111) where the 2𝜋 and 5𝜎 orbitals are
involved in the back-donation and donation [21].
The electronic ground state of N2 O is()
1𝜎 2𝜎 2 3𝜎 2 4𝜎 2 5𝜎 2 6𝜎 2 1𝜋 4 7𝜎 2 2𝜋 4 1 𝛴 +2with 1𝜎, 2𝜎 and 3𝜎 corresponding to localized O 1𝑠, NC 1𝑠 and NT 1𝑠
respectively.
The Auger spectrum of free N2 O was analyzed already in 1976 by
Connor et al. using restricted Hartree–Fock and limited configurationinteraction wave functions coupled to the one-center model for Auger
intensities [22]. These authors could grossly assign features to localization of orbitals and to the spread of singlet and triplet multiplet
states.
The non-equivalent N atoms also make for ideal studies with coincidence spectroscopy. With studies including the dissociation of N2 O
using electron–ion coincidences [23], site-specific fragmentation [24],multi ion–electron coincidences [25], and site-selective Auger electron
spectra recorded using photoelectron-Auger electron coincidence spectroscopy (APECS) [26]. All of the mentioned coincidence studies were
performed in the gas phase and to the best of our knowledge no APECS
studies have to date been published on adsorbed N2 O.

Fig. 1. (a) shows the evolution of the N 1𝑠 core level while heating the sample. At
650 s the thermocouple readout was 39 K and at 800 s 42 K with all molecules
desorbed, further discussed in the text. The bottom two panels shows 3 typical spectraof N 1𝑠 (b) and O 1𝑠 (c) of multilayer, bilayer and monolayer coverage. With dashedlines highlighting the binding energy shifts of the different coverages and red arrows
the binding energy of dissociated O and N.

Herein we present the first APECS study on adsorbed N2 O on
Ni(111). This particular surface was chosen since the weak chemisorption leaves the molecular adsorbate intact [15]. Together with ab initio
calculations we characterize the surface-adsorbate bond mechanism indetail.

2.2. Spectroscopic methods
The measurements were performed using the Coincidence Spectroscopy for Chemical Analysis (CoESCA) endstation [27] at the UE52PGM beamline [28] at the BESSY II electron storage ring operated by
the Helmholtz-Zentrum Berlin für Materialien und Energie.
For all measurements a photon energy of 700 eV was used andfor the coincidence measurements an exit slit of 100 𝜇m, resulting ina photon bandwidth of 100 meV. The CoESCA endstation is equippedwith two angular resolving time-of-flight spectrometers (type ARTOF
by Scienta Omicron). For the desorption and coincidence measurements
the two ARTOFs were recording different kinetic energy regions. The
N 1𝑠/N KVV coincidence measurements at all N2 O coverages and theO 1𝑠/O KVV coincidence measurements at monolayer N2 O coverage
were done by measuring the Auger electrons with the ARTOF1 spectrometer (ARTOF10k) with a lens mode covering an energy window
of 7% of the center kinetic energy and by measuring the photoelectrons with a ARTOF2 EW spectrometer with a lens mode covering anenergy window of 4% of the center kinetic energy. The multilayer andbilayer N2 O O 1𝑠/O KVV coincidence measurements were performed
during a second beamtime and in-between these sessions the ARTOF1
spectrometer was replaced with a second ARTOF2 EW, again using a
7% energy window. A consequence of addition of a second wide angle

2. Materials and methods
2.1. Samples and preparation
The Ni(111) crystal (99.995% MaTecK Gmbh) was cleaned withrepeated cycles of argon sputtering (1.5 kV, 10 mA, 5 ⋅ 10−6 mbar)
for 5–10 min each followed by annealing to 1070 K for 3 min. The
surface was then checked for contamination using X-ray photoelectron
spectroscopy (XPS) revealing a clean surface with O 1𝑠 and C 1𝑠
contributions within the noise of a survey spectrum. After cooling the
crystal using liquid helium, N2 O (5N, Messer) was dosed using a leakvalve into the experimental chamber. The thermocouple readout during
deposition was 31 K (with the temperature on the sample probably
higher as discussed in the results below) with N2 O at a pressure of1 ⋅ 10−7 mbar. For the thick film a total of 100L N2 O was dosed and forthe intermediate and monolayer films 5L was dosed prior to heating todesorb all but the desired thickness, details in the text below.
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F.O.L. Johansson et al.
Table 1
Summary of the coincidence statistics for the different APECS measurements, showing,number of true coincidence counts, the true to accidental ratio and the count rate. Notethat the count rate is considerably larger for the multi- and bilayer O runs, this is fromtwo different experiments where the experimental set-up was upgraded in-between as
detailed in the experimental section.
True counts

True/Acc

Countrate (𝑠−1 )NitrogenMultilayer
Bilayer
Monolayer

24 59584 149155 890

0.71
1.96
1.04

4.3
3.5
4.4

OxygenMultilayer
Bilayer
Monolayer

84 58264 73557 182

0.56
0.85
1.13

15.7
12.2
1.7

orbitals. The convergence of the wave-function was based on a stateaverage procedure considering a configuration interaction between thelowest 10 states.
For the calculations of the Auger transition intensities we firstly
note that these are governed by two-electron Hamiltonian elementsthat couple to the continuum function of the outgoing Auger electron.
As the precise determination of the Auger electron continuum functionin the anisotropic molecular potential is a quite daunting task only a
few methods have been put forward at this level of theory and applied
for the smallest free molecules. Therefore a more practical approach
adopted here is to consider the local nature of Auger transitions anduse so-called one-center models [44,45], where two-electron coupling
to the continuum is replaced by atomic continuum integrals weighted
by the local on-atom atomic orbital expansion coefficients [46] of theinvolved (two) molecular orbitals (MOs) for the atom in question.
Providing a quick inspection of the spectra, one can utilize an evensimpler model based on orbital population, and only qualitatively
consider the variation of the coupling to the continuum. In this view,combinations of orbital populations can guide the Auger spectrum evenfor surface adsorbates. It is notable that there are both singlet andtriplet spin couplings for the two-hole Auger final states, but that apropensity rule [44,45] favors the singlets (owing, respectively, to constructive and destructive interference between Coulomb and exchange
continuum integrals). Finally, we note that the two hole states are
subject to ’’Coulomb explosion’’ leading to dissociative final states andbroad features, possibly except for one or two of the lowest lying
final two-hole states [47]. It is to be noted that second quantization
derived intensity expressions for Auger transitions between core ionizedand double hole final states in a non-orthogonal basis contains many
terms [48], but that the common Wentzel one-term ansatz [49] is still
almost always assumed in actual calculations. That raises the questionof which orbital basis is to be preferred. We have here in the below
the presentation of the spectra using the one-center intensity model,
assuming both ground and core hole optimized orbitals, bearing in
mind that rather significant differences may arise.

spectrometer is a considerable increase in the coincidence count rate
as seen in Table 1.
During a APECS measurement, events when both spectrometers
record an electron within a given time window, are recorded as acoincidence event. This includes both true and accidental coincidences.
Accidental coincidence events are when electrons from different ionization events hit the detectors in the same time window. The true
events are extracted from the total coincidence counts by subtracting
the accidental coincidences, as described in detail in [27,29]. In all
coincidence spectra below, only the true coincidence events are plotted.
The statistics for the coincidence measurements are presented inTable 1. The binding energies were calibrated using the Fermi levelof the Ni(111) crystal as a reference and the kinetic energies were
calibrated using the Fermi level plus the work function on Ni(111),
5.23 eV [30]. For details on the APECS technique at the CoESCA stationsee [27,29,31,32].
2.3. Theoretical methods
Density functional theory (DFT) calculations were carried out using
projector augmented-wave (PAW) formalism [33] as implemented inthe Vienna ab initio simulation package (VASP) [34,35]. The revised
Perdew–Burke–Ernzerhof (rPBE) [36] and Heyd–Scuseria–Ernzerhof(HSE06) [37] exchange–correlation functionals with the Grimme D3
correction [38] to weak disperse interactions were utilized to optimize the geometry and calculate electronic structure. Mostly the rPBE
functional was used unless otherwise specified. The plane wave kineticenergy cutoff of 450 eV was used. The first Brillouin zone was sampled
by the Monkhorst–Pack scheme [39] with 12 × 12 × 1 k-points. The
convergence tolerances of the force and electronic minimizations were10−4 eV/Å and 10−6 eV, respectively. A vacuum region of > 10 Å was
set to avoid artificial interactions between images. Atomic structures
were visualized using The Visualization for Electronic and Structural
Analysis (VESTA) software [40] and the ’’vaspkit’’ code was employedfor postprocessing the results [41].
In addition to solid state calculations we have carried out ab initio
calculations on N2 O and N2 O-Ni using the MOLCAS program suite [42],with the purpose of having a theoretical characterization of the backdonation. We focused first on the electronic structure of the ground state
considering a simple model, where for N2 O-Ni the internal geometryof the N2 O molecule was kept as it gas phase equilibrium geometry,whilst the distance between the NT and the Ni atom was 1.87 Å.
The calculations was addressed at the RASPT2 level of theory [43],together with the ANO-RCC-VQZP basis set. The design of the active
space was carried with respect to energy convergence and occupation
numbers. The active space finally used for N2 O was composed by six
𝑎1 orbitals, one 𝑎2 orbital, four 𝑏1 and four 𝑏2 orbitals. Such active
space was chosen like (6𝜎)(7𝜎)(2𝜋)(3𝜋)(4𝜋) orbitals of N2 O and the(3𝑑)(4𝑠)(4𝑝𝑧 ) atomic orbitals of Ni (Fig. 3), totaling 22 electrons in 15

3. Results and discussion
3.1. XPS and coverage
The N 1𝑠 core level photoelectron spectrum of N2 O consists of twocontributions, one at higher binding energy from the NC atom and one
at lower binding energy from the NT [11] . As can be seen in themultilayer spectra, blue spectrum in Fig. 1b, the contributions are well
separated and the binding energy positions and peak widths fit wellwith earlier studies [16]. The multilayer O 1𝑠 primarily consists of one
component as can be seen in Fig. 1c with a small contribution at lower
binding energies that arises from dissociated O at the surface [3,20,29].
The presence of dissociated O also suggests a presence of dissociated N
which could explain the shoulder seen at 401 eV binding energy for
all coverages in the N 1𝑠 spectra in Fig. 1 [3,50]. The two dissociatedspecies are marked with red arrows in Fig. 1. At minute coverages N2 O
is known to dissociate at steps at vicinal Ni(755), a process that is
terminated when step-sites are saturated with O [20]. The dissociation
observed here occurs at defects on our Ni(111) crystal and the corelevel spectra suggest that it is in small quantities. As noted, the binding
energy position of the dissociated N is well separated from those of
N2 O [50]. The KLL Auger spectra from N2 and N2 O will overlap in our
kinetic energy region of interest; the main feature of N2 Auger spectrum
may be found around 360 eV kinetic energy [51] and being a minor
contribution to the total spectrum and indistinguishable. This and thecontribution to the O Auger spectrum are discussed in detail below.
In order to investigate the interaction between the substrate andthe N2 O, thinner films were produced by heating the sample to control
desorption. After exposing the cooled Ni surface to 5L of N2 O, it was
slowly heated using resistive heating while simultaneously recording3

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F.O.L. Johansson et al.

Fig. 2. Panels a–f show the N coincidence maps and integrated spectra from the regions marked with colored lines in the coincidences maps. Panel a also shows a comparison to
the coincidence spectra from gas-phase N2 O, the gas-phase data is taken from Ref. [26], note the top energy axis for the gas-phase reference. Panels g–l shows the corresponding
O coincidence data.

the N 1𝑠 and O 1𝑠 photoelectron spectra using the two ARTOF spectrometers. Monitoring the N 1𝑠 spectra while heating, as seen in Fig. 1a,shows a clear point of the multilayer desorption with a sharp decreasein the peak intensities followed by a small shift (𝛥 = 0.52 eV) and theappearance of a doublet peak for each N atom, the green spectrum inthe middle panel of Fig. 1. With further heating one peak of the doublet
disappears with the other remaining unchanged, the orange spectrum
in the middle panel of Fig. 1, until all molecules are desorbed and no
N signal can be seen. The O 1𝑠 spectrum shows a similar trend as seenin the bottom panel of Fig. 1.
A similar behavior is discussed by Väterlein et al. [3] where their
thermal desorption spectra revealed multilayer desorption at 70 K,
monolayer desorption at 92 K and a third species which they attributed
to a dilute monolayer that desorbed at 100 K containing 10% of amonolayer. They also claim no significant binding energy shift for themonolayer and dilute monolayer. This is similar to our results withthree distinct steps in the desorption process but we observe a binding
energy shift between the two lower coverage cases and that the low
coverage case consists of 50% of the intermediate case. However, weobserve this with significantly lower temperature read-out indicatingthat the actual sample temperature in our experiment is higher than
the read-out, probably due to the location of the thermocouple. We
assign the intermediate state with the doublet peak structure (green in
Fig. 1) as bilayer coverage whereas the single peak structure (orange in
Fig. 1) is assigned to monolayer coverage, with a molecular coverage of𝜃 = 5∕9 determined from calculations as described below. This would
follow a Frank–Van der Merwe layer-by-layer growth structure.
3.2. Auger-Photoelectron Coincidence Spectroscopy
3.2.1. Multilayer films
In Fig. 2d the N 1𝑠/N KVV coincidence map of a multilayer N2 O filmon Ni(111) can be seen. The map is dominated by two distinct intensity
regions distributed in binding energy that are marked with blue (406–411 eV) and green (402–406 eV) lines. Looking at the binding energy
positions for these regions the intensity comes from the two different
N atoms in the molecule and they have seemingly different Auger
spectra. By integrating the region around the binding energies for thetwo different N atoms the Auger spectra for each can be extracted.
4

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F.O.L. Johansson et al.

These spectra can be seen in Fig. 2a where the blue and green
spectra are the integrated region marked with the blue and green lines
in the map below. The blue corresponds to NC and the green to NT .
The NT Auger spectrum contains more distinct features compared to the
NC . The Auger spectra of both N atoms are similar to that of gas-phase
N2 O measured in coincidence by Bolognesi et al. [26] which is alsoplotted in Fig. 2a for comparison, note the separate kinetic energy axis
for the gas phase spectra. The main features of the gas phase spectra
are reproduced in the thick film albeit with a shift induced from thesolid structure and by the substrate. The two energy axes are aligned
so that the main Auger feature at 373.5 eV for both solid and gas end
up at the same position in the figure. With these aligned it can be seenthat other Auger feature also show some small shifts but are generally
well reproduced.
Using a mean free path (𝜆) of 11 Å (calculated with the TPP-2M
method [52]) for both photo- and Auger electrons (close to the samekinetic energy for this experiment) the effective mean free path for
the APECS experiment can be calculated to 5.5 Å or about 2 monolayers [53]. Since the film is thicker than this, the APECS measurement isof pure solid N2 O and does not probe the interface with the Ni surface.
The O 1𝑠/O KVV coincidence map of the multilayer film can be seenin Fig. 2j. It is dominated by 4 features in the 536–538 eV binding
energy range, noticeably these exhibit a slight tilt. This correlation of
the kinetic energies of the Auger- and photoelectron pairs can only
be revealed in a coincidence measurement. While the well known
energy sharing between an Auger and a photoelectron (e.g. [54]) leads
to intensity stripes at constant kinetic energy sum, here we observe
an opposite trend, where the stripes are rather at constant kinetic
energy difference. The tilt is most apparent in the multilayer caseand completely absent in the monolayer case (Fig. 2l). This could be
explained by a change in the dielectric function with different film
thicknesses resulting in different binding energies for spectra fromdifferent depths [55].

3.3. Theory - N2 O Auger
The Auger coincidence spectra, Fig. 2, show some salient features
spreading over an interval of about 15 eV in kinetic energy. It isclear that for both bi- and monolayer N2 O there is a shift of intensity
towards higher kinetic energies for NT compared to the NC spectra.
The former shows an additional weaker structure towards the highenergy end for NT not at all represented for NC . It is notable that
the N Auger spectrum of free N2 O [22], which is a composite of NT
and NC transitions, show three main bands in this energy interval, but
shifted towards lower kinetic energies by a few eV than the spectra in
Fig. 2. The results of the RASSCF/PT2 calculations in terms of leading
configuration and energy of the outer Auger states are given in Table 2.
Here the energy of the outermost state (2𝜋 −2 3 𝛴 − ) is used as a reference
point. By inspecting the core separated spectra in Fig. 2d-f it is clearthat the outermost band (at high Auger electron kinetic energies) of
the composite N spectrum covering 0–6 eV is dominated by Augertransitions connecting to the NT . The separated NT spectrum shows twobands in this region. The computational assignment is here clear — theoutermost of these two bands refers to terminally localized 2𝜋 and 7𝜎
orbitals, while the inner one of the two also receives contributions fromthe more central N localized 1𝜋 orbital, n.b. in combination with 2𝜋
MO so generating the 1𝜋 −1 , 2𝜋 −1 1 𝛱 and 3 𝛱 eigenstates. We note, as
is the general case for molecular valence Auger spectra, that there is agreat deal of breakdown of the molecular orbital picture in analyzing
the spectrum [47,56]. Thus only for the outer MOs (1𝜋, 7𝜎, 2𝜋) a oneto-one assignment can be made between two MOs and (spin-coupled)
single configurational eigenstates. Here a division between inner–inner,
inner–outer and outer–outer valence regions is appropriately based on
the inclusion of the atomic 2s or 2p into the MOs. It is often thecase that inner–inner valence states suffer a complete breakdown, theinner–outer a partial breakdown, while only the outer–outer valence
states may receive an orbital assignment including only two MOs [47].
This means that both orbital localization and the breakdown effects
come into play when analyzing the N2 O Auger spectrum. Thus goingto lower Auger energies and the second, NC , dominated broad band thebreakdown effects become progressively more important.

3.2.2. Bi- & monolayers
The bi- and monolayer coverages were produced by stopping theheating of a thick film (5L exposure) when the desired spectral features
were observed and subsequently the sample was cooled down to basetemperature again for the duration of the measurements. APECS measurements were then performed on these two coverages. The resulting
APECS maps and integrated region spectra can be seen in Fig. 2 forN 1𝑠/N KVV and O 1𝑠/O KVV for the few- and monolayer coverages.
Comparing the N coincidence maps in Fig. 2 for the different
coverages the shift seen in the XPS spectrum from the desorption is
also seen in the Auger although much larger, ≈ 2.5 eV. Particularly
the NT Auger spectra are comparatively different from the multilayer
and gas phase spectra. The monolayer spectrum contains sharp features
whereas the bilayer case contains both the features of the monolayerinteraction with the substrate and the interaction with the molecularlayers above it. As discussed previously, the signal in the bilayer caseis dominated by the top-most layers owing to the shallow information
depth in APECS.
The O 1𝑠/O KVV coincidence map, similar to the N 1𝑠/N KVV case,contains two different binding energy separated Auger spectra where
the one at high binding energy has a distinct structure similar to thatseen in the N spectra. The Auger spectra from the 534–537 eV binding
energy resemble those of the electron–ion coincidence gas phase Auger
spectrum [24]. The low binding energy part contains a single broad
peak in the Auger kinetic energy and binding energy region where we
expect dissociated O to be present [3,20,29].
The most striking feature in the monolayer case is the intensity
found above 380 eV kinetic energy for NT . It is also present in thebilayer spectra but with lower intensity as expected since the signal
from the monolayer is damped by the presence of the molecular layer
on top of it and it is not observed in the thick film case (Fig. 2j).

3.3.1. Auger spectra of free N2 O
The ground state of free N2 O has populations of atomic 2𝑠 of +2.01
and +0.95 for NT and NC , respectively, whilst for the three 2𝑝 orbitals
we have +3.33 and +2.77. Besides, in Tables 3 and 4 we can also see theexpansion coefficients of the valence MOs, by atomic center. We notea rather strong shift of charge towards the end atoms at the expenseof charge for NT - this complies with the fact that the overall intensityof the Auger spectrum is stronger for NT than for NC for both the biand monolayer cases. The shift of intensity towards lower energies for
NT compared to NC can also be qualitatively understood from this table
as one roughly can divide the Auger spectrum into 2𝑝2𝑝 outer valence,2𝑠2𝑝 outer–inner valence and 2𝑠2𝑠 inner valence regimes [47], goingfrom high to low kinetic energies. Thus 2𝑝 dominance of an atom tend
to give intensity of the upper region. This is further accentuated by theatomic coupling to the continuum, with integrals that are significantly
stronger for 2𝑝2𝑝 than for 2𝑠2𝑝 or 2𝑠2𝑠 two-holes states of the N
atom [46]. Thus a rough inspection of atomic charges indicates that NT
spectra are stronger in total than NC spectra and that the NT spectra areshifted towards lower kinetic energies.
The MO populations will also guide the intensities in these regions.
It is clear that the 6𝜎, 1𝜋, 7𝜎, 2𝜋 orbitals are well separated fromremaining orbitals (a predicted 16 eV gap), and thus well define the’’outer valence’’. Their energy positions also comply well with the solidstate density of states analysis given in Fig. 8. That they dominate thelow kinetic energy parts is also proven by the MCSCF analysis of total
N2 O two-hole state energies in Table 2. As we see from this table allstates in a 7 eV span derive from combinations of the four outer valenceorbitals. Especially, the 2𝜋 −2 1 𝛥 and 2𝜋 −1 7𝜎 −1 1 𝛱 states in the lower
5Applied Surface Science 666 (2024) 160340

F.O.L. Johansson et al.

Fig. 3. Diagram of the active space used for describing the ground state of the N2 O+Ni idealized structure. The MOs are disposed according to the symmetry components of
the 𝐶2𝑣 point group, and they are arranged by orbital energy from left to right. The vertical dashed line divides the occupied orbitals (left) from the unoccupied orbitals (right).
Numbers in parenthesis denote natural occupation numbers.
Table 2
Relative energies, in eV, of the triplet and singlet
Auger states of N2 O. The reference value corresponds
to the ground state 3 𝛴 − of N2 O2+ .
State

Config.

𝛥𝐸 (eV)

3𝛴−

2𝜋 −2
2𝜋 −2
2𝜋 −2
7𝜎 −1 2𝜋 −1
7𝜎 −1 2𝜋 −1
1𝜋 −1 2𝜋 −1
1𝜋 −1 2𝜋 −1
1𝜋 −1 2𝜋 −1
1𝜋 −1 2𝜋 −1
6𝜎 −1 2𝜋 −1

0.00
1.25
1.73
2.29
3.59
4.93
5.44
5.58
5.80
6.94

1𝛥
1

+

𝛴
𝛱
1
𝛱
1 −
𝛴
3
𝛥
3𝛴+
3𝛴−
3
𝛱
3

Table 3
N2 O MO coefficients for the valence 𝜎 orbitals.

Fig. 4. Theoretical Auger spectrum obtained with the one-center approximation forboth single N2 O and N2 O+Ni systems, calculated at the NT 𝐾-edge (top), NC 𝐾-edge(middle) and O 𝐾-edge (bottom). The energies in the horizontal axis are calculatedusing the 3 𝛴 − state as the reference. The molecular symbols in the bottom plot are thesame for the other graphics.

sym. adpat. basis

7𝜎

6𝜎

5𝜎

4𝜎

NT 1𝑠
NT 2𝑠
NT 2𝑝𝑧−0.0347−0.7187+0.5775+0.0101+0.3908−0.0840+0.0074−0.5884−0.2517−0.0066+0.2890+0.1269

NT 1𝑠
NT 2𝑠
NT 2𝑝𝑧+0.0062+0.2044−0.2807+0.0118−0.3548−0.1808+0.0228−0.1563+0.2801−0.0613+0.3948+0.0533

O 1𝑠
O 2𝑠
O 2𝑝𝑧

+0.0038+0.1256+0.2717+0.0124+0.5912+0.5690−0.0021+0.5351−0.1261−0.0096+0.5775−0.1877

The orbital characterization is also becoming less relevant due to theabove mentioned MO breakdown effects. Furthermore, analyzing therole of the surface, see below, it is clearly the outer 2𝑝2𝑝 region whichmatters most.
3.3.2. Auger spectrum of N2 O-Ni
Going from free to surface adsorbed N2 O the most salient spectralfeature in the Auger spectra is thus the addition of a structure at
385 eV kinetic energy appearing in the NT spectrum but not for NC .
Evidence that this feature relates to backdonation is reflected in theelectronic structure of the ground state. Therefore, in order to havea meaningful description of the backdonation, we have considered an
idealized model of a system composed of one molecule of N2 O plus asingle Ni atom, all disposed in a linear structure.

5 eV region. We note from Tables 3 and 4 that both 7𝜎 and 2𝜋 orbitals
have strong NT 2𝑝 contributions, and the strong composite NT band at
370–375 eV should be assigned to these two states. Correspondingly
the NC 2𝑝 contributions to these orbitals are substantially smaller, explaining the much lower intensity in this region. Going further down inthe spectra, into the 2𝑠2𝑝 region, there is a rather mixed representationof NT and NC in terms of atomic orbital composition, and we also
observe rather smeared out ’’look-alike’’ features in the two spectra.
6

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F.O.L. Johansson et al.

Fig. 5. (a) Experimental Auger spectrum for isolated N2 O, as a function of the electron kinetic energy, in eV, from Ref. [22]. (b) Experimental Auger spectrum for the monolayer,
as a function of the electron kinetic energy, in eV. (c) Detailed composition of the extra structure at 380–385 eV in panel (b), showing density of doubly charge states withenergies below 3 𝛴 − , colored according to the electronic wave function symmetry (𝐶2𝑣 point group). The spectral curves are obtained from a straight summation of the densityof states. (d) Theoretical Auger spectrum obtained for isolated N2 O, corresponding to the main structures of the experimental signal. Bars represent relative intensities according
to the one-center approximation. (e) Theoretical Auger spectrum obtained for N2 O+Ni, corresponding to the main structures of the experimental signal. Bars represent relative
intensities according to the one-center approximation. (f) Same as panel c, but the density of doubly charge states with energies below 3 𝛴 − is here divided into doubly ionizedNi (red) and singly ionized Ni plus singly ionized N2 O (blue). Note that, in contrast to panel c.
Table 4
N2 O MO coefficients for the valence 𝜋 orbitals. The 𝜋 MOs are 𝑥∕𝑦-doubly degenerate(𝑏1 ∕𝑏2 ).
sym. adpat. basis

2𝜋

1𝜋

NT 2𝑝𝑥 /2𝑝𝑦
NT 2𝑝𝑥 /2𝑝𝑦
O 2𝑝𝑥 /2𝑝𝑦

−0.5659−0.2162+0.7743+0.3449+0.5742+0.4849

Table 5
N2 O+Ni MO coefficients for the valence 𝜎 orbitals.

Within this linear model, we can understand the effect of the nickel
atom as (i) the influence on the Auger transitions corresponding to
the states of isolated N2 O and (ii) the introduction of new transitions(mainly due to the backbonding). For the first part, a partial Auger
spectra was calculated based on the one-center approximation (Fig. 4).
Here we look at the same final states of single N2 O, and we report the
spectra calculated for N2 O and for N2 O+Ni. The assignments of the
final states respect the labels of single N2 O molecule, also shown in
Table 2. It is worth pointing out that the lowest-lying 3 𝛴 − state has
no intensities in the one-center approximation. The reason is that the
Coulomb and exchange integrals cancel each other out in this case. For
all three core-holes we observe three main bands in this energy interval,
the first corresponding to the 1 𝛥 plus 1 𝛴 + states, and the second andthird related to the 1 𝛱 and 1 𝛴 − states. The triplet 𝛱 states havelower intensities as discussed above. Although it can be considered asa marginal effect, the presence of the nickel atom change the relative
intensity of these transitions as a response to the changes on the MO
coefficients, when compared to the isolated N2 O (Tables 5 and 6).
Our interpretation of the overall spectra is that (i) the most intense
peaks in the experimental spectrum (taking NT , for instance, the peaks
from 365–375 eV) correspond to the transitions on N2 O, principally
the ones linked to the singlet states as discussed above, and (ii) the
extra structure at higher kinetic energies corresponds to states involving ionization of the Nickel atom. We thus understand that this is aconsequence of the latter effect of the nickel atom (the introductionof new transitions). This interpretation is supported by our RASPT2
calculations for the doubly charged N2 O+Ni. In fact, the lowest-lying
doubly charged state lies below the 3 𝛴 − state by more than 13 eV, andcorresponds to a double ionization of the nickel atom. With our active
space, more than 300 roots were found in this interval, below the 3 𝛴 − ,sym. adpat. basis

N2 O 7𝜎

N2 O 6𝜎

Ni 3𝑑2+

Ni 3𝑑0NT 1𝑠
NT 2𝑠
NT 2𝑝𝑧−0.048−0.799+0.461−0.020−0.006+0.069

0.000
0.000
0.000

0.030
0.271
−0.045NT 1𝑠
NT 2𝑠
NT 2𝑝𝑧+0.027+0.430−0.222+0.059−0.214−0.363

0.000
0.000
0.000

−0.021−0.167
0.088

O 1𝑠
O 2𝑠
O 2𝑝𝑧

+0.000−0.036+0.028+0.015+0.039+0.657

0.000
0.000
0.000

−0.001−0.003−0.020

Ni 3𝑑2+
Ni 3𝑑0Ni 4𝑠+0.000−0.138−0.189+0.000−0.015−0.0370.997+0.000+0.000+0.000−0.929+0.086

Table 6
N2 O+Ni MO coefficients for the valence 𝜋 orbitals. The 𝜋 MOs are 𝑥∕𝑦-doubly
degenerate (𝑏1 ∕𝑏2 ).
sym. adpat. basis

N2 O 2𝜋𝑥∕𝑦

N2 O 1𝜋𝑥∕𝑦

Ni 3𝑑1+∕−NT 2𝑝𝑥∕𝑦
NT 2𝑝𝑥∕𝑦
O 2𝑝𝑥∕𝑦Ni 3𝑑1+∕−+0.628+0.034−0.706−0.044+0.386+0.609+0.412−0.030+0.011+0.002+0.005−0.995which was set as the reference energy. Besides, the density of statesshows a broad peak between −4 eV and −10 eV, which is in a very good
agreement with the experimental value of the extra structure (Fig. 5).
The local nature of the Auger transitions suggests that the doubly
ionized atomic nickel states have very low intensities, for they would
involve non-local electron decays. On the other hand, those states
involving singly ionization of the nickel atom together with singly
ionization of the N2 O molecule would have larger intensities, althoughsmaller than the main peaks. From Mulliken charge analysis, the states
were divided into the ones involving doubly charged Ni (Ni+2) andthe ones corresponding to singly ionized Ni plus singly ionized N2 O
(Fig. 5f). The blue solid lines stand for the states that would provide
more intense Auger signals, in comparison to the red line states. We
7

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F.O.L. Johansson et al.

Fig. 6. Equilibrium geometry (a) and densities of states (b) of monolayer N2 O on Ni(111).

from the surface, while some of them change their positions from top
to hcp/fcc and demonstrate some bending compared to the molecules
adsorbed in the on-top position. At 300 K, we found that the number ofdesorbed molecules increased while the diversity in the geometries ofadsorbed molecules became greater. For example, some molecules areadsorbed in a ‘‘lying down’’ geometry by binding both O and N atoms
with Ni(111).
3.3.4. Density of states analysis
To investigate the density of states (DOS) in different bonding
configurations we compare the isolated molecule to those adsorbed
on-top and in a lying-down configuration. Fig. 8 from top to bottom
demonstrates DOS of isolated N2 O, N2 O adsorbed in top position(lowest in energy without taking into account ZPE, zero-point energy,
and entropy factors), and in lying down position with N2 O adsorbed by
O (top) and N (hcp) which is the second in energy, but only predicted
to be 0.05 eV higher compared to the top standing N2 O. We thus
have two bonding situations that give very different DOS and therefore
Auger spectra. Here we clearly notice states corresponding to the three
uppermost occupied orbitals - 1𝜋, 7𝜎, 2𝜋 - while above the 2𝜋 there
is a HOMO-LUMO gap to the unoccupied 3𝜋 orbital. Going upwards inenergy we see a gap to the higher lying orbitals, in a region represented
by the breakdown effects according to Table 2. This plot thus represents
a one-particle interpretation of the ionization spectrum of N2 O. Theenergy levels and localization, e.g. with the NT localization of thetwo uppermost states, are in accordance with the Hartree–Fock (HF)
analysis present in Table 5. The up and down plots represent spin-up
and spin-down DOS. They are indeed equal for free N2 O as expected
for a closed shell molecule. That result is also comparable well with theHartree–Fock results for free N2 O, and is not surprising as Koopmans
theorem (HF) and Janaks theorem (DFT) both equalize orbital energies
with ionization potentials to a large degree of accuracy. For isolated
molecules, it is thus quite clear that HOMO is localized on NT , while
LUMO is localized on both N atoms. Adsorption leads to interaction
with the Ni(111) surface and as a result prominent shift of states tolower energies caused by electron transfer from Ni to the molecule. At
the top position, HOMO is at −4.4 eV and LUMO is broadened between
1.5 and 4.4 eV due to proximity-induced effects. One can also see small
but non-zero spin polarization of N2 O states.
The electronic structure of N2 O adsorbed in the complex configuration reveals a significant broadening of orbitals and spin polarization
resulting from stronger interaction with the Ni(111) surface. This also
can be confirmed by charge transfer analyzed by the Bader approach.
The isolated N2 O reveals that the O atom gains 0.32e from the NC part.
Adsorption in the top position keeps the charge of O the same as in theisolated molecule while N gain 0.2e transferred from Ni(111). Complex
adsorption leads to significant charge transfer from the surface to both
N and O parts. For instance, O gains additionally 0.24 e as compared
to isolated N2 O, while 0.53 e is transferred from the metal surface to
N. This results in significant induced spin polarization of the molecule
as demonstrated in DOS.

Fig. 7. Snapshots of N2 O monolayer on Ni(111) after 3 ps of the AIMD simulation at
60 (top panel) and 300 K (bottom panel) within NVT thermostat and a time step of 1
fs.

noticed that the band denoted by the blue line is roughly centered
between −5 eV and −8 eV, which means 7 to 10 eV below the mostintense 1 𝛥 peak and agrees well with the experimental NT spectrum. In
Tables 5 and 6 we show the coefficients of the Ni 3𝑑 atomic orbitals in
the outer valence MOs. The Ni 3𝑑0 orbital in fact interacts with the
2𝑠 orbital of the NT . That led us to conclude that the extra peak isbuilt up principally of transitions involving Ni 3𝑑0−1 plus N2 O 2𝜋 −1 .
Furthermore, we can also use the same argument to explain why thisstructure cannot be clearly observed in the NC spectrum.
3.3.3. Adsorption geometry
To examine the monolayer electronic structure we have made abinitio molecular dynamics simulations (AIMD) of the monolayer anddensity of state calculations of isolated and adsorbed N2 O. The equilibrium geometry at 0 K revealed a 3 × 3 overlayer structure (5 N2 O
molecules per unit cell) with NT closest to the surface on-top of the Ni
atoms as seen in Fig. 6. The molecules demonstrate nearly the same
preferable orientation as in the case of a single molecule on-top of Ni(discussed below) with some small bending. Thus reproduce the single
molecule on Ni(111) by other authors [57]. In the adsorbed monolayer,
the HOMO (which is contributed only by the NT atoms bonded with thesurface) is located between −5.8 and −3.8 eV and LUMO (contributedby both atoms) is between 1 and 3 eV.
To reveal the geometry of N2 O monolayer on the Ni(111) surface at
finite temperature, we employed ab-initio molecular dynamics simulations (Fig. 7) at 60 and 300 K within NVT thermostat and a time stepof 1 fs. It was found that at 60 K, part of the molecules are desorbed
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F.O.L. Johansson et al.

Fig. 9. Schematic view of the backdonation.

one sees that N2 O orbital structure is only minorly varied but that the
3d- 𝜋 ∗ backbonding band complex is significantly stabilized in energy.
3.4. Blyholder or not
So considering the standing position it seems that the excess intensity in the high kinetic energy part of the Auger spectrum of the
NT atom (in comparison with the NC atom) is due to charge transferfrom metal to the 𝜋 ⋆ of the NT atom. This is similar to the Blyholder
back–donation model for CO / N2 on transition metals, thus that in the
ground state, there is back donation from metal 3𝑑 to the (otherwise
empty) LUMO on the N atoms, something that strengthens the bondingof the adsorbate to the surface [21,58]. This back-donation has also
been suggested by Kiss et al. for the N2 O on Pt(111) system [59] andfrom DFT analysis on N2 O on Fe-doped graphene [60].
The kinetic energy position of this extra intensity has been observedof N2 O in the gas phase both as a shake-up from core ionization [61]and resonant Auger [62]. In Fig. 9 we present backdonation into the
LUMO from the nickel surface and the donation from the 7𝜎 lonepair orbital of the NT . This explains the chemisorbing nature of theN2 O/Ni(111) interaction and the extra intensity seen in APECS athigh kinetic energy for the thin coverages that is not present in themultilayer film.
As indicated by the density of states analysis there is a broad extra
structure around the Fermi level that can be interpreted as due to a
N2 O (3d- 𝜋 ∗ ) backbonding situation. It is also possible to gain insight
into the backdonation in the ground electronic structure from theidealized model considered above comprising a system composed of
one molecule of N2 O plus a single Ni atom, all disposed in a linear
structure, see further information in the computational section. Direct
analysis of the Mulliken charges of the ground state indicates a slighttotal charge transfer from the Ni atom to the N2 O molecule. In fact,
the total atomic charge on Ni is −0.05, and the charges of the NT , NC
and O atoms are, respectively, −0.27, +0.78 and −0.47. By large thisagrees well with the analysis of the DOS in Fig. 8. When considering
the electronic charge occupying only the 3𝑑 and 4𝑠 atomic orbitals of
Ni we obtained −9.59, against the expected −10 for the isolated Ni. On
top of that, the occupation number of the orbitals also presents valuable
information on the backdonation. First, the sum of the MO coefficientssquared associated with Ni atomic orbitals of the orbital labeled as 7𝜎- occupation number 1.99 - is 0.07, which is a way of quantifying the
hybridization of the 7𝜎 orbital. Besides, the hybridization between the
N2 O LUMO orbital (3𝜋) and the 3𝑑4𝑠 atomic orbitals of Ni produces
a 0.13 population on the 3𝜋 orbital (second and third lines of Fig. 3),
thus supporting the notion of a 3d- 𝜋 ∗ backbonding.

Fig. 8. (a) DOS of isolated N2 O (top), N2 O adsorbed in top position (middle) and N2 O
adsorbed in lying down position (bottom). (b) Core ionized (NT or NC ) N2 O densityof states, where the core hole potential is represented by the equivalent core (Z+1)
approximation (top panel core ionized on NT and bottom panel on NC ). The Fermi
level is set to 0 eV.

The DOS results indicate a shifted N2 O structure but with an additional broad structure appearing a few eV above the Fermi surface,replacing the LUMO level of free N2 O. This structure represents thesurface to N2 O (3d- 𝜋 ∗ ) backbonding situation with the broad feature
related to a splitting of the 3d band of Nickel by the N2 O bonding.
Here we see that the free N2 O DOS results are very much compromised
for the HOMO and LUMO levels, showing neither the preserved N2 O
structure nor the 3d- 𝜋 ∗ backbonding indicated by the standing up N2 O
case in the middle panel of Fig. 8a.
Finally, in Fig. 8b we show the case of standing N2 O on the surface
for core ionized (NT or NC ) N2 O density of states, where the core holepotential is represented by the equivalent core (Z+1) approximation.
As the core ionized state (CS) is the initial state of the Auger transition
it can be relevant to compare it with the ground state results — here
9Applied Surface Science 666 (2024) 160340

F.O.L. Johansson et al.

4. Conclusions

References

APECS represents an experimental method where backdonation insurface–adsorbate bond may be directly measured. Additionally since
we are able to distinguish the Auger spectra from chemically nonequivalent atoms, we can in cases when those are well separated determine
which orbital at which atom receives the backdonated electron density.
We have compared multilayer, bilayer and monolayer coverage of
N2 O on a Ni(111) surface where the multilayer film in many regards issimilar to a kinetic energy shifted and broadened gas-phase spectra andwe can see that the monolayer contains features not found in the gasphase. For the N coincidences this new intensity feature arises from aninteraction with the nickel substrate whereas in the O case the spectrum
is not dependent on coverage. The observed behavior is explained withcalculations that demonstrate that the N2 O is standing up with the NT
closest to the surface with a surface bond built up from donation andbackdonation of electron density involving the Ni surface and the NT
atom.
The APECS methodology outlined here suggests that the methodshould be applied to problems in catalysis where small molecules interact with a substrate to make a detailed assessment of how they interact.
In the present case for instance, we could identify the decomposition
products and differentiate them from intact species. The present work
also illustrates the insight into the electronic structure and dynamics
of the surface adsorption gained by combining APECS and theoreticalanalysis — solid state and quantum ab initio.

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CRediT authorship contribution statement
Fredrik O.L. Johansson: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis,
Conceptualization. Lucas M. Cornetta: Writing – review & editing,Writing – original draft, Visualization, Investigation, Formal analysis. Elin Berggren: Writing – review & editing, Investigation. Artem
Kuklin: Writing – original draft, Visualization, Investigation, Formal
analysis. Yi-Chen Weng: Writing – review & editing, Investigation.
Swarnshikha Sinha: Writing – review & editing, Investigation. Danilo
Kühn: Writing – review & editing, Methodology, Investigation. Alexander Föhlisch: Writing – review & editing, Methodology, Investigation.
Hans Ågren: Writing – review & editing, Writing – original draft,
Methodology, Investigation, Formal analysis. Andreas Lindblad: Writing – review & editing, Writing – original draft, Supervision, Investigation, Formal analysis, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared toinfluence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
F.O.L.J acknowledges support from the Swedish Research Council(Grant 2020-06409). L.M.C acknowledges the Brazilian funding agency
FAPESP, under processes nr. 2020/04822-9 and 2021/06527-7. A.K.
acknowledges Olle Engkvist Byggmästare Foundation for the support
under contract no. 212-0178. We thank the Helmholtz-Zentrum Berlinfür Materialien und Energie for the allocation of synchrotron radiation beamtime. The authors also acknowledge the Swedish National
Infrastructure for Computing (NAISS 2023-3-40 and NAISS 2024-573) at the National Supercomputer Centre of Linköping University(Sweden) partially funded by the Swedish Research Council throughgrant agreements no. 2022-06725 and no. 2018–05973.
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