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Lok Yiu Wu, [Maksymilian J. Roman](https://orcid.org/0000-0002-5113-9470), [Brianna R. Heazlewood](https://orcid.org/0000-0003-2073-4004), [Mitsunori Kurahashi](https://orcid.org/0000-0002-3802-4513)

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[Steric effects in the adsorption of O<sub>2</sub> on a Cu(111) surface](https://mdr.nims.go.jp/datasets/2afc0e72-818a-40f8-87f2-554bc80ccbf0)

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Steric effects in the adsorption of O2 on a Cu(111) surfaceThis journal is © the Owner Societies 2025 Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 |  5701Cite this: Phys. Chem. Chem. Phys.,2025, 27, 5701Steric effects in the adsorption of O2 on a Cu(111)surface†Lok Yiu Wu,‡ab Maksymilian J. Roman, *a Brianna R. Heazlewood a andMitsunori Kurahashi *cProbing the stereodynamics of a gas–surface interaction is a useful tool to investigate the mechanismsresponsible for adsorption. Experimental results are provided on the adsorption of alignment-controlledO2 interacting with a Cu(111) surface for the first time, across a range of incident energies (65–550 meV)and angles of incidence (0–601). Molecules of O2 in a supersonic beam are prepared in a single spin-rotational state, and aligned with a Cu(111) surface so that the rotational angular momentum of O2 iseither parallel or perpendicular to the surface. A strong steric effect is observed, where the initial stickingprobability is higher in the case of a ’side-on’ collision, with measurable adsorption appearing at normalincident energies of 100 meV. The onset of sticking occurs at incident energies of approximately200 meV in the case of an ’end-on’ collision. The results also indicate that the adsorption of O2 onCu(111) is predominantly due to an activated process in the energy range probed, corroborating previousexperimental and theoretical results.IntroductionAs an abundant and reactive component of our atmosphere,molecular oxygen is responsible for many important reactionsthat occur on exposed surfaces. The corrosion of metal surfaceshas been a long-standing issue, and many strategies have beendeveloped in a bid to minimise corrosion—indeed, copperutensils were plated with tin as long ago as Roman times.1More recently, there has been significant and sustained interest incopper due to its widespread use in modern electronic compo-nents and as an inexpensive active metal catalyst. In order toefficiently and effectively protect copper surfaces from corrosion,and to improve the design of Cu-based catalysts more broadly,reactions between O2 and copper must be better understood.The adsorption of oxygen on a Cu(111) surface has beenextensively studied using a variety of spectroscopic techniquesthat can probe gas–surface interactions. Dissociative adsorp-tion can proceed through two pathways, known as ‘direct’ and‘indirect’ processes. In the direct process, the O2 molecule inthe gas phase dissociates on impact and each O atom formsa bond with the surface. As bond-breaking of the moleculeoccurs, this is often an activated process, with the translationalor internal energy of the molecule providing the energy neededto overcome the dissociation activation barrier. In the indirectcase, the O2 molecule is first physisorbed onto the surface, andtwo competing mechanisms can then occur: desorption of theO2 molecule back into the gas phase, or dissociation into twoadsorbed O atoms on the surface. The indirect process is alsoknown as a trapping-mediated process, as the molecule istrapped onto the surface for a period of time.Ultraviolet photoelectron spectroscopy (UPS) studies haveshown that adsorption of oxygen is dissociative at 300 K—theO(1s) spectrum of oxygen adsorbed on a Cu(111) surface showsa single peak at 300 K due to the atomic species, while at 100 K,the spectrum shows an additional higher binding energy peakdue to the molecular species.2 In high resolution electronenergy loss (HREEL) spectra recorded at 100 K, it was foundthat molecular oxygen could bind to Cu(111) surfaces in twoways (the bridged and bound on top peroxo species), withatomic oxygen also present.3 The presence of molecular oxygenis in agreement with a theoretical study by Ramos et al. con-ducted using density functional theory (DFT) with a semi-localexchange–correlation functional and quasi-classical trajectory(QCT) calculations, where oxygen adsorption was found tobe non-dissociative at low surface temperatures (110 K) as theO2 molecule can be trapped in a stable adsorption well afterovercoming an initial energy barrier.4 A late barrier thena Department of Physics, University of Liverpool, Oxford Street, Liverpool, L69 7ZE,UK. E-mail: m.j.roman@liverpool.ac.ukb Physical and Theoretical Chemistry Laboratory, University of Oxford, South ParksRoad, Oxford, OX1 3QZ, UKc National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047,Japan. E-mail: kurahashi.mitsunori@nims.go.jp† Electronic supplementary information (ESI) available: Further details on themolecular beam properties, data analysis procedure, and additional experimentalresults. See DOI: https://doi.org/10.1039/d4cp04595e‡ Current address: School of Chemistry, University of Birmingham, EdgbastonB15 2TT, UK.Received 5th December 2024,Accepted 10th February 2025DOI: 10.1039/d4cp04595ersc.li/pccpPCCPPAPEROpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0000-0002-5113-9470https://orcid.org/0000-0003-2073-4004https://orcid.org/0000-0002-3802-4513http://crossmark.crossref.org/dialog/?doi=10.1039/d4cp04595e&domain=pdf&date_stamp=2025-02-27https://doi.org/10.1039/d4cp04595ehttps://rsc.li/pccphttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595ehttps://pubs.rsc.org/en/journals/journal/CPhttps://pubs.rsc.org/en/journals/journal/CP?issueid=CP0270115702 |  Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 This journal is © the Owner Societies 2025emerges for the dissociation of O2 into O atoms. The heights ofthe early and late barriers were found to depend on the impactsite of O2 on the Cu(111) surface—with the lowest energyentrance channel being the top-bridge-top (t-b-t) configuration,and the lowest energy exit channel involving the bridge-fcc-bridge (b-fcc-b) configuration. As such, the minimum energypathway for the combined process is for the molecule toapproach and adsorb in a t-b-t configuration, then to rearrangeand adopt a b-fcc-b geometry prior to dissociation. Anotherapproach has been taken recently by van Bree and Kroes, usinga screened hybrid density functional and QCT calculationsto investigate the dissociation dynamics of O2 on Cu(111).5Similar to Ramos et al., the potential energy surfaces (PESs)show that the exit barrier is high for the lowest energy entrancebarrier, and so the minimum energy pathway requires a rear-rangement of the O2 molecule on the surface.Experimentally measured sticking probabilities are one wayto probe the dynamics occurring in gas–surface interactions.As the quantum state (including orientation), angle of inci-dence, and velocity of the incoming beam all influence how O2molecules interact with Cu(111) surfaces, molecular beams area useful tool for gaining insight into this process—providing amechanism for exerting control over the distribution of inci-dent energies and the rovibrational states initially populated inthe oxygen beam. Previous studies have used the King andWells method6 (described in further detail in the Experimentalsection) to measure the sticking coefficient of O2 on Cu(111). A2012 study by Minniti et al. involving molecular beams foundthat the sticking coefficient increases exponentially with inci-dent energy, up to 400 meV.7 This is in qualitative agreementwith a more recent molecular beam study conducted by Zhanget al., which showed the same trend, but reported highersticking coefficients.8 The study by Zhang et al. measured thesticking coefficients as a function of incident energy (100 to400 meV), angle of incidence, surface temperature (90 K to670 K), and coverage.8 They identified an activated adsorptionprocess with an activation barrier of approximately 100 meV athigher surface temperatures, in agreement with entrance viathe t-b-t channel in the previous theoretical study by Ramoset al.4 However, at the lowest surface temperature probed in thestudy (90 K), although no signs of trapping were detected, thepossibility of a molecular precursor state could not be excluded.While the cumulative knowledge arising from these studieshas enhanced our understanding of the sticking of O2 on aCu(111) surface, several questions still remain. For instance,the sticking probabilities obtained from theory and experi-ment, and indeed between the two experiments reported thusfar, while qualitatively consistent, lack quantitative agreement.There has been a call for more experimental measurements to beundertaken to resolve these points of difference.5 In addition,the effect of the molecular alignment of O2 with respect toa Cu(111) surface has not been studied experimentally withmolecular beams. The PES is predicted to have a stronganisotropy9—the activation barrier for dissociation is higherfor an end-on collision, where the molecular axis is perpendi-cular to the surface, compared to a side-on collision. In additionto the activated processes that appear in the high energy regime,questions also remain regarding the contribution of processesthat occur in the low energy regime, such as trapping-mediatedadsorption or steering.10,11Here, experimental measurements are presented that allowus to begin answering these questions. In its ground (3Sg�)electronic state, molecular oxygen is paramagnetic and hencepossesses a magnetic dipole moment due to the presence of theunpaired electrons. As it is best described by Hund’s case (b),the total angular momentum J is due to the spin-rotationcoupling of the molecule. By selecting a single spin-rotationalstate of O2 and controlling its quantisation axis direction usingexternal magnetic fields, measurements of steric effects in theadsorption of O2 on a Cu(111) surface are experimentallyprobed and reported here for the first time.ExperimentalThe experimental setup has been previously described indetail,12 with the key features set out briefly here. A schematicdiagram of the apparatus is shown in Fig. 1. A continuous beamof O2 molecules seeded in He is generated via a supersonicexpansion, with the ratio of O2 : He varied (and in some cases,with the nozzle heated) to create beams with mean transla-tional energies of 65–548 meV. A series of hexapolar magnetsfocus target O2 molecules in the (J,MJ) = (2,2) state around acentre stop and into the reaction chamber. Here J is the totalangular momentum quantum number and MJ is the projectionof the total angular momentum J onto the quantisation axis.Previous characterisation studies have established that trans-mitted O2 molecules are nearly 100% quantum state-selected.13Linear motion feedthroughs are connected to the seven selec-tion hexapoles, allowing each of these hexapoles to be inde-pendently retracted or positioned on the beam axis, dependingon the beam velocity. The centre stop removes non-targetparticles that are travelling along the central beam propagationaxis, such as the He carrier gas. After traversing the hexapolesand centre stop, O2 molecules enter the spin flipper region.In this instance, the spin-flipper is operated in a non-flip mode,and simply maintains a quantisation axis through to the mainreaction chamber, where the O2 molecules impinge on theCu(111) surface. Gas composition analysis conducted in themain chamber confirms that close to 100% of the species thatsuccessfully traverse the hexapoles are O2 molecules.The Cu(111) surface is first prepared in the sample preparationchamber, where it undergoes argon ion sputter cleaning andannealing at 773 K, with the crystal structure confirmed by lowenergy electron diffraction (LEED) measurements. Once cleaned,the sample is transferred to the main reaction chamber, where itis mounted on a three-dimensional (x,y,z) linear and angular (y)translation stage with stepper motors. The sample has a tempera-ture of 305–313 K following annealing and transfer.The main reaction chamber is encapsulated by three sets ofHelmholtz coils, allowing a magnetic field to be applied to thechamber. By defining the quantisation axis of the magnetic field,Paper PCCPOpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595eThis journal is © the Owner Societies 2025 Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 |  5703the alignment of the O2 molecules with respect to the Cu(111)surface can be controlled, as shown in Fig. 2. The axis of the end-over-end rotation of the O2 molecule aligns with the direction ofthe external magnetic field. By altering the magnetic field direc-tion, three different interaction configurations can be achieved:a helicopter alignment (H), where the rotation axis is alongthe x-axis; and two cartwheel orientations (Cy, Cz), where theO2 molecule rotates around the y- and z-axes, respectively. Here,the x-axis is defined as the direction parallel to the surfacenormal. In the helicopter alignment, the O2 internuclear axis ispredominantly parallel to the surface. For the cartwheelgeometries, the O2 internuclear axis rotates parallel and perpendi-cular to the surface, through a continuous range of mixedintermediate orientations.The sticking probability of O2 on the Cu(111) surface isestablished using the King and Wells method,6 with beam flagsregulating the exposure of the molecular beam to the surfaceand an ion gauge serving as the detector. A gate valve betweenthe beam preparation chamber and the main reaction chamberis first opened, with an inert beam flag present in front of thesurface, allowing the background for the case where no O2sticking occurs to be determined. The beam flag in front of thesample is then retracted, allowing O2 molecules to impinge onthe surface. For each measurement, an alternating controlsignal is used to switch the molecules between two alignments,allowing the sticking probability of two geometries ([H vs. Cz] or[Cy vs. Cz]) to be obtained in the same measurement. Eachmeasurement is repeated seven times, with a fresh part of thesurface exposed to the beam each time (achieved using thetranslation stage). The ion gauge signal is recorded throughoutthe repeat measurements. Species with residence time shorterthan the timescale of the experiment do not contribute to thedecrease in the ion gauge signal. Sticking coefficients aremeasured for a range of incident beam energies and angles,by varying the O2 : He ratio and rotating the surface relative tothe molecular beam axis.When sticking probability measurements are completed, thesurface is removed from the reaction chamber and a beamcomposition analysis is performed using a residual gas analyser(RGA). Following the RGA measurements, a second gate valve isopened (see Fig. 1), allowing the O2 molecular beam to passthrough the main reaction chamber and into the analysischamber. There, the O2 molecules pass through a series ofanalyser hexapoles and into a quadrupole mass spectrometerFig. 2 Schematic representation of the different geometries O2 can adoptwhen colliding with a Cu(111) surface. The red arrows indicate the rotation axesfor the three geometries: helicopter (H), cartwheel-y (Cy), and cartwheel-z (Cz).Fig. 1 Schematic diagram of the experimental apparatus. Oxygen molecules are supersonically expanded through the nozzle (on the right hand side ofthe figure) and subsequently pass through a skimmer, a pre-determined number of hexapoles, are focused around a centre stop, enter the spin flipperregion (operated in spin-preserving mode), and then reach the Cu surface (labelled S). Hexapoles that can be positioned in or out of the beam by linearmotion feedthroughs are shown in blue, with the hexapoles that are fixed in position shown in green. A flag (F) is placed directly in front of the surface toblock the passage of the beam, with O2 molecules only permitted to pass through when measurements are recorded. A residual gas analyser (RGA) andquadrupole mass spectrometer (QMS) are present to analyse the composition and velocity of the beam, respectively. Gate valves are represented bycircles with crosses. The preparation chamber is not shown in this schematic.PCCP PaperOpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595e5704 |  Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 This journal is © the Owner Societies 2025(QMS) where time-of-flight (ToF) profiles are obtained by mon-itoring the time-resolved O2 QMS signal while switching thespin flipper mode as described in previous studies.14ResultsThe translational energy distributions of the different O2 beamshave been recorded as ToF profiles and are provided in the ESI†(see Fig. S1). From these distributions, we can define the mostprobable incident energy for each set of conditions, denoted asEi. In configurations where the copper surface is rotated by anangle of incidence (yi), the normal component of the incidentenergy, En, is calculated. The range of incident beam energiesused in this study, alongside the corresponding experimentalconditions, are set out in Table S1.†An example of a King and Wells measurement is provided inFig. 3, recorded with the O2 beam travelling at Ei = 150 meV andat yi = 01. The change in the ion gauge current (reflecting thechange of pressure inside the experimental chamber) isrecorded during the King and Wells measurement. When thegate valve is opened, the molecular beam is admitted into themain reaction chamber and the pressure gauge signal rapidlyincreases. After the signal has been given time to equilibrate,the flag is retracted, exposing the Cu(111) surface to themolecular beam and resulting in a short, sharp spike in thesignal due to outgassing caused by the motion of the flag.The peak of the signal spike can be seen to occur at t0 = 7.3 s,corresponding to when the surface is first fully exposed to themolecular beam. After the initial spike, the ion gauge signalfalls again, as oxygen adsorbs on the surface—thereby remov-ing it from the gas phase. The magnitude of this decrease insignal is dependent on the initial sticking probability, S0, for agiven set of experimental conditions (Ei, yi, oxygen alignment).Further measurements are provided in the ESI† showing thesignal recorded under different experimental conditions.The control signal with which the magnetic field is switchedis indicated in Fig. 3 in blue, where the resulting alternationbetween the helicopter and cartwheel geometries is shown. Thecoverage of oxygen on the surface increases steadily over the timetaken to record a complete measurement, decreasing the num-ber of available adsorption sites. This is reflected in the averagesignal recorded after the surface is exposed increasing with time,and reducing the differences in the interactions of the differentO2 geometries. After approximately 10 seconds of exposure, theflag is raised and the Cu(111) surface is blocked from themolecular beam, accompanied by a second spike in the iongauge signal. Following the spike, the signal can be seen toreturn to approximately the same equilibrium level as it was afterthe gate valve was opened and before the surface was exposed.For experimental conditions that yield large S0 values—cor-responding to high initial sticking probabilities, as typicallyseen for high Ei and low yi—the presence of steric effects in theadsorption of O2 on Cu(111) is immediately evident in the Kingand Wells data. For example, as can be seen in Fig. 3, alternatingbetween the helicopter and cartwheel geometries results indistinctly different measured ion gauge signals; the adsorptionis lower for the cartwheel alignment, resulting in a higher iongauge signal (as more O2 remains in the gas phase). These initialobservations can be confirmed quantitatively by measuring theinitial sticking probability, S0, values for each alignment andconfiguration. The data from a single King and Wells measure-ment can be separated into two sets, one for each O2 geometry.A masking function (corresponding to the times at which theexternal magnetic fields are switched, cf. the block function inFig. 3) can then be used to parse the data. While the magneticfield direction can be switched effectively instantaneously, thesystem takes a non-trivial amount of time to respond to thischange due to the time constant of the vacuum system. As set outin the ESI,† care is taken to ensure that this delay is considered inthe data analysis by excluding the first few data points followingthe magnetic field switch. Once parsed, the selected data pointsare fitted (using non-linear least squares) to an exponentialfunction, from which the value of the ion gauge signal at t0,corresponding to the time when the surface is exposed, isextracted. The S0 value for each trace is established from the ratioof the ion gauge signal at t0 to the baseline (i.e., the signal beforethe Cu(111) surface is exposed to the beam). The baseline value isobtained from the mean of 180 data points between ca. 2.5 and 7 sin Fig. 3. The offset value of the ion gauge signal (i.e., the signalbefore the gate valve is opened to the molecular beam) must alsobe taken into account. The offset value is obtained from the meanof 95 data points before ca. 2.4 s in Fig. 3.In cases of low adsorption probability (particularly at largeyi, low Ei, or both), the exponential function does not provide areliable fit to the data and a linear fit is used instead. In the fewinstances where there were very low adsorption probabilities,neither (exponential nor linear) fitting approach provided aFig. 3 King and Wells measurement recorded for Ei = 150 meV, yi = 01 isprovided in red, with key features indicated on the figure. The controlsignal, responsible for switching the magnetic field (and therefore thealignment of the O2 molecules), is shown above the measurement trace inblue, where ‘‘1’’ corresponds to the helicopter alignment and ‘‘0’’ to thecartwheel alignment. The black dashed lines are exponential fits used todetermine the S0 values. The time of 0 s refers to the start of the controlprogramme, with relative times used in the data analysis.Paper PCCPOpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595eThis journal is © the Owner Societies 2025 Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 |  5705good representation of the data. In such cases, the S0 value wastaken as the average of the data points included in thatparticular measurement. Using the same procedure for mea-surements where the O2 geometry is switched between Cy andCz (see Fig. S7 for an example, ESI†), no measurable differencesare found in the behaviour of the two alignments; the samevalues of S0 are obtained for the same beam conditions. Wenote here that the S0(Cz) values measured from these experi-ments are on average higher by approximately 12% comparedto the S0(Cz) values measured in the experiments alternatingbetween the H and the Cz alignments. Therefore, the S0(Cz)values presented in the text may be slightly undervalued.The quantified steric effect can be seen in Fig. 4, where theinitial sticking probability for the parallel alignment [S0(8)] and theperpendicular alignment [S0(>)] are shown as a function of thenormal component of the incident energy, En. As the horizontallyaligned O2 keeps the molecular axis mostly parallel to the plane ofthe surface, S0(H) can be used to quantify S0(8). Although theperpendicular alignment is not directly probed, adsorption of adistribution of alignments around the perpendicular one (heresimply referred to as S0(>)) for close-packed (111) surfaces can beobtained from the relationship set out in eqn (1):S0(>) = 2S0(C) � S0(H) (1)thanks to structural symmetry,10 as the cartwheel reflects theaverage of parallel and perpendicular alignments of O2. For aclose-packed surface such as Cu(111), where the extent ofcorrugation is small in comparison to the size of an incidentO2 molecule, only the normal component of the incident energyis expected to contribute to the adsorption process (assuming itis an activated mechanism).For both parallel and perpendicular configurations, the valueof S0 increases with increasing En, following an S-shaped curve(see Fig. 4). Focusing first on the parallel alignment, a measur-able adsorption appears at around En = 100 meV. The S0(8)values start to plateau above 400 meV, reaching a maximumvalue of 0.89 at the highest experimental incident energy con-sidered in this work (En = 548 meV). The steric effect can be seenin the apparent translation of the S0 curve from parallel toperpendicular, where the shape of the distribution is consistentbut the onset energy is approximately 100 meV higher for theS0(>) dataset. Measurable adsorption for the perpendicularalignment appears at roughly 200 meV, with the maximummeasured S0(>) value of 0.83 obtained at the onset of the highenergy plateau (which is not fully observed for this alignment, asit extends beyond the highest achievable incident energy). Forboth alignments, normal energy scaling (NES) applies as the datafor all yi can be seen to lie on the same curve when plottedagainst En. The simulations by van Bree and Kroes5 havepredicted that the NES holds well at high En where S0 4 0.1while deviation from it occurs at low En where S0 o 0.1. Ourresults are thought to be consistent with the simulation since themeasured S0 values are mostly 4 0.1.The S0 plots are typically fitted using a sigmoidal function,such as the function set out in eqn (2).S Enð Þ ¼A2� 1þ tanhEn � Ecd� �� �(2)Here, A is a scaling factor, Ec refers to the critical energy atwhich the sticking probability is half of the maximum, and dis the width parameter. This functional form was originallyproposed by Harris15 for measurements of the dissociativeadsorption of H2 on copper surfaces, having also been recentlyemployed by Zhang et al.8 The fits provided in Fig. 4 are thoseobtained using eqn (2), resulting in the parameters provided inTable 1. Following the precedent set by others in the field, wehave used eqn (2) to provide a reliable, albeit simple, repre-sentation of the experimental data. This approach facilitates astraightforward comparison between different sets of experi-mental measurements.The presence of a high energy plateau, and the valuesobtained for the A parameter, indicate that saturation in O2adsorption on Cu(111) occurs at S0 o 1. This is qualitativelyconsistent with the results of a previous theoretical study byRamos et al.,4 where a similar saturation trend was predicted—although they expected the plateau to occur at a lower S0 valueof approximately 0.65. Recent theoretical work by van Bree andKroes also reported a plateau region, again occurring at a lowerFig. 4 The S0 values for perpendicular (red) and parallel (blue) geometriesof O2 molecules encountering a Cu(111) surface are shown plotted againstthe normal component of the incident energy, En. The contributions ofdifferent experimental yi values can be seen from the use of differentsymbols. Error bars indicate the standard deviation obtained from therepeated King and Wells measurements. The dashed lines represent fits tothe experimental data, using the equation from Harris et al.15Table 1 Parameters obtained from fitting the experimental data, asdefined in eqn (2). Uncertainty ranges are established from the standarderror of the fit, calculated using the least squares fitting procedure in thePython lmfit packageO2 alignment A Ec (meV) d (meV)Parallel 0.86 � 0.02 245 � 5 108 � 7Perpendicular 0.85 � 0.04 347 � 8 105 � 10PCCP PaperOpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595e5706 |  Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 This journal is © the Owner Societies 2025S0 value than is observed experimentally.5 Interestingly, Ramoset al. also predicted a decrease in S0 to occur at higher energies,En 4 400 meV. This predicted fall in S0 was not observed in thestudy by Zhang et al.,8 who instead observed an onset ofsaturation around that En (similar to what is seen in this study).However, the experimental beam energies in the Zhang et al.study only reached 396 meV, and that may have prevented themfrom seeing a high energy S0 decrease. Importantly, we canconfirm that no such decrease in S0 is observed in the resultsreported here, where we extend the En range well beyond400 meV (up to 548 meV).DiscussionThe steric effectThe increased reactivity of the parallel alignment compared tothe perpendicular alignment, as shown in Fig. 4, has beenpreviously predicted by density functional calculations per-formed by Moritani et al.9 Previous theoretical studies foundthe minimum entrance barrier for parallel O2 impacting withthe t-b-t geometry to be 97 meV (Ramos et al.)4 and 280 meV(van Bree et al.).5 Although they are not directly equivalent,these values correlate well with the onset of measurable S0(H) atEn B 100 meV measured in this work. A preference for theparallel alignment has also been seen experimentally in previousstudies of O2 adsorption on Al(111)10; Si(100);16 Pt(111);17,18Ag(110);19 Cu(110);20 and Fe(110)/W(110), Ni(111)/W(110), Co(0001)/W(110).21 Among these prior studies, a comparison betweenthe findings reported in this work and those reported for the O2adsorption on an Al(111) surface may be appropriate, given thecommon surface structure. Trends in the sticking probabilities withrespect to the incident beam energy are very similar, following anS-shaped curve for both surfaces. This implies that a directactivated adsorption mechanism is occurring in both systems, forbeam energies 4 100 meV. The energy gap between the two O2geometries (parallel and perpendicular) is around 80 meV forAl(111), which is similar to the results reported here for Cu(111).The mechanism(s) responsible for O2 adsorption on Cu(111)Recently, experiments conducted using a supersonic beam ofrandomly aligned O2 molecules provided insights into the mecha-nism of O2 dissociative adsorption on a Cu(111) surface.8 At roomtemperature, it was established that the adsorption proceeds via adirect, activated mechanism, where only molecules with energiesabove the height of the barrier to adsorption (as established for themost favourable geometry of approach) will stick to the surface.The experimental reality is more complicated than can be expressedin a simplified single-barrier model, and there will be a distributionof energy barriers that depend on factors such as the surfacestructure, impact site, angle of incidence, and the orientation ofthe impinging molecule. Geometric factors can be treated by the’hole model’ developed by Karikorpi et al.22 Dynamical effects thatsteer an incident molecule into a more favourable alignment are alsoknown to affect the sticking probability, as has been seentheoretically.23,24 This distribution of adsorption barriers, and thepresence of an activated mechanism, is reflected in the slope of theS-curve of the sticking probability measurements presented in Fig. 4.Fig. 5(a) shows the low-energy O2 adsorption as a plot ofS0(H) against En (equivalent to a low energy fragment of theblue data in Fig. 4). In this low incident energy regime,contributions of a process mediated by a physisorbed precursorand/or by a steering effect which reorients an impingingmolecule to a more favourable geometry need to be considered.Fig. 5(b) shows the difference between the sticking of helicop-tering and cartwheeling O2 molecules. The quantity is propor-tional to the difference in the ion gauge signal observed whilealternating the O2 geometry (see Fig. 3), and tends to be lessaffected by the experimental conditions.An interesting finding, presented in Fig. 5(a), is that the S0values recorded at yi = 01 are consistently higher than thoseobtained at other angles of incidence. This trend can be seen forboth experimental alignments. While quantitatively accountingfor the increased adsorption is beyond the scope of this study,and would likely require comprehensive theoretical investigationalongside more in-depth experimental studies, some possibleexplanations can be proposed.Excitation of the internal states of the adsorbate have beenfound to affect the sticking probabilities—vibrational excitationFig. 5 Detailed plot of the low energy adsorption region for S0(H) isshown in (a). Green symbols indicate the values for normally incident O2,with blue symbols used for all other experimental yi. The differencebetween S0(H) and S0(C) is shown in (b). For both graphs, the symbolshapes indicate the angle of incidence, yi.Paper PCCPOpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595eThis journal is © the Owner Societies 2025 Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 |  5707has been shown to increase sticking probability in the directadsorption of O2 on Cu(100), Cu(110), Cu(111),9 and Al(111).25However, at room temperature, the population of excitedvibrational levels of O2 is negligibly small.9 As only the O2beams with high incident energy are produced with a heatednozzle, no vibrational excitation is expected for O2 at low En.While rotational excitation is also thought to influence stickingprobability (with an inverse relationship between the two), it isnot expected to play a role here. There may be some variation inthe O2 internal state distribution following supersonic expansion,from the use of different gas mixtures and nozzle temperatures,but the O2 beam is then state-selected by the hexapoles prior toreaching the surface. The focusing of the target (J,MJ) = (2,2) statearound the centre stop, and the findings from previous character-isation studies,13 suggest there is likely to be very little variation inthe internal state distribution of O2 molecules that reach theCu(111) surface. As such, internal excitation cannot account forthe angular dependence that is observed.An uptake process mediated via a trapped physisorbed statehas been observed previously in the adsorption of low-energy O2on hexagonally close-packed transition metal surfaces such asNi(111),26 Pt(111),27 and Ru(0001).28 The probability of trapping-mediated O2 adsorption on Ru(0001) and Pt(111) exhibits a weakyi dependence—if S0 is plotted against En, it shows higher valuesat smaller yi.27,28 Additionally, the trapping-mediated adsorptionof O2 is known to exhibit little alignment effect12 and Fig. 5(b)shows that the difference in the S0(yi = 01) values between thehelicopter and cartwheel alignments at En = 87 meV is very small.Thus, the combination of enhanced sticking probability andminimal alignment dependence at En = 87 meV and yi = 01 couldindicate the presence of a trapping-mediated mechanism at lowenergy conditions. It may also be possible that steering con-tributes to the low alignment dependence of S0 under theseconditions. A previous study by Zhang et al.8 had a minimum Eivalue of 216 meV, requiring their lowest En measurements to beperformed at high angles of incidence (as denoted by thesymbols in Fig. 6). Their approach makes it challenging toobserve subtle low-energy effects, and as such it has not beenpossible to compare our experimental findings quantitatively inthe low-energy regime. In a recent theoretical study, van Breeand Kroes’ QCT simulations determined negligible trapping ofO2 molecules, with a trapping probability of approximately 0.002following trajectory calculations propagated for 1 ns.5 Trappingoccurred only for oxygen molecules impacting the surface atEn o 50 meV, lower than the minimum En values studiedexperimentally. The authors did, however, note that their modeldid not include provisions for energy dissipation into the surfaceupon adsorbate collision, and as such may not simulate trappingprocesses accurately.To explore whether a trapping-mediated mechanism may bepresent, additional S0 measurements have been conducted atTsurf = 107 K, for both helicopter and cartwheel O2 alignmentsand at yi = 0, 30, and 451. The temperature dependence of theprobability of trapping-mediated adsorption is determined bythe dynamic competition between thermal desorption and dis-sociation of the trapped molecule. Lower desorption ratescorrespond to a higher probability of trapping-mediated adsorp-tion at lower temperatures.27 Measurements recorded at Tsurf =107 K yield S0 values that are only marginally larger than (andfalling within the uncertainty range of) the equivalent roomtemperature measurements. Further details are provided in theESI.† As such, we conclude that the presence of a low-energytrapping-mediated adsorption channel is consistent with theexperimental data, but cannot be confirmed at this stage.Comparison with prior workIn two prior experimental studies, S0 measurements were carriedout for randomly aligned O2 interacting with Cu(111).7,8 Somedifferences were reported in the adsorption probability for En 4200 meV in the two studies, and this was attributed to differ-ences in surface cleanliness and preparation. This discrepancyposes an issue for theoreticians trying to compare their simu-lated energy-dependent sticking probabilities to experimentalresults, as highlighted in the recent work by van Bree and Kroes.5To provide independent verification of S0 recorded without anycontrol over the molecular geometry [S0(R)], the helicopter[S0(H)] and cartwheel [S0(C)] alignment results presented in thiswork can be combined to give a ‘random’ distribution using therelationship set out in eqn (3).10,12S0ðRÞ ¼23S0ðCÞ þ13S0ðHÞ (3)The resulting S0(R) values are presented in Fig. 6, alongsidethe measurements reported by Zhang et al.8 and Minniti et al.7A fit of eqn (2) to the data of Zhang et al. is also included. Thefitting parameters are provided in Table 2, where they can becompared to the values established by Zhang et al.8 It should benoted that Zhang’s data were initially plotted as a function ofFig. 6 The values of S0(R) established from eqn (3) are plotted as purple solidsymbols as a function of En, at several different angles of incidence.Experimental data reported by Zhang et al.8 (yellow hollow symbols) andMinniti et al.7 (green solid circles) are also provided to facilitate a comparisonbetween the experiments. The contributions of different experimental yivalues can be seen from the use of differently shaped symbols. The dashedlines indicate the fits from eqn (2) to data from this work and from Zhang et al.PCCP PaperOpen Access Article. Published on 12 February 2025. Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595e5708 |  Phys. Chem. Chem. Phys., 2025, 27, 5701–5709 This journal is © the Owner Societies 2025average energy, measured from their O2 beam ToF profiles. Tofacilitate a direct comparison to the work presented here, Zhang’sdata are presented as a function of the most probable beamenergies, established from the same ToF profiles. As such, thefitting parameters differ slightly from those quoted by Zhang et al.in their article. The agreement between our results and those ofZhang et al. is very good. While there are minor discrepancies atthe highest En values considered by Zhang et al., where themaximum beam energy was 396 meV, and in the low-energy range(close to the detection limit of the King and Wells method), theagreement in these regions is still qualitatively strong. Notably,Zhang et al. reported measurable adsorption appearing around100 meV,8 which matches the onset of adsorption of parallel-aligned O2 in this work.When comparing the parameters extracted from fits to thedata presented in this work, and those reported by Zhang et al.,the Ec values are consistent for both studies (see Table 2). Thevalue of the d parameter is higher in the data reported here,suggesting a broader distribution of adsorption barriers for ourexperimental conditions. Comparing the widths of the measuredvelocity distributions, derived from ToF measurements, is non-trivial due to differences in the two experimental setups. Forexample, in this work, ToF traces are recorded after the beamtraversed a second set of hexapoles, which can affect the shape ofthe derived velocity distribution. While the ToF measurementspermit comparison of the beam conditions within each setup,they do not allow for straightforward absolute comparisonsbetween the two different experimental setups. Surface tempera-ture and sample cleanliness could also impact the value of the dparameter, and again these conditions are hard to assess andcompare between two different experiments.Overall, the quantitative agreement in the Ec value, and thesimilarity in the shape of the S0(R) distribution as a function ofenergy, confirms that the findings reported here are in goodagreement with those measured by Zhang et al.8 and hence alsoinconsistent with those of Minniti et al.7ConclusionsThe sticking probability of an alignment-controlled O2 beaminteracting with a Cu(111) surface has been studied across arange of incident energies and angles of incidence. A strongsteric effect is observed, with the sticking of O2 with itsmolecular axis parallel to the Cu(111) surface occurring at anormal incident energy of approximately 100 meV, with theonset occurring B 100 meV later for O2 molecules aligned withtheir molecular axis perpendicular to the surface. In the energyregime probed, O2 adsorption on Cu(111) is predominantly anactivated direct dissociative process—consistent with previousmolecular beam and theoretical studies. The measurementsreported here provide the first experimental confirmation thatmolecular alignment plays an important role in O2 interactionswith Cu(111) surfaces. The findings both deepen our under-standing of the adsorption process and provide key bench-marking data for future theoretical studies.Author contributionsConceptualization and methodology: M. K.; performing experi-mental measurements: M. K., M. J. R., L. Y. W.; formal analysisand investigation: all authors; writing – original draft prepara-tion: M. J. R., L. Y. W.; writing – review and editing: all authors;funding acquisition: B. R. H., M. K.; supervision: B. R. H., M. K.Data availabilityData for this article, including the tabulated results under-pinning the figures in the main text and the ESI† are availableat https://doi.org/10.17638/datacat.liverpool.ac.uk/2948.Conflicts of interestThere are no conflicts to declare.AcknowledgementsB. R. H. is grateful to the European Commission (ERC StartingGrant project 948373) and the Leverhulme Trust (RPG-2022-264, PLP-2022-215) for funding. M. K. thanks JSPS KAKENHI(Grant Numbers 20H02623 and 24K01349) and Iketani Scienceand Technology Foundation for support. M. J. R. is grateful tothe School of Physical Sciences at the University of Liverpool fora Postdoctoral Development Award.References1 R. M. Burns and W. W. Bradley, Protective coatings for metals,Reinhold Publishing Corporation, New York, 1967.2 M. Rajumon, K. Prabhakaran and C. Rao, Surf. Sci., 1990,233, L237–L242.3 T. Sueyoshi, T. Sasaki and Y. Iwasawa, Surf. Sci., 1996, 365,310–318.4 M. Ramos, C. Dı́az, A. E. Martı́nez, H. F. Busnengo andF. 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Downloaded on 3/15/2025 8:01:43 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4cp04595e