Berenbak, B.

Butler, D.A.

Kleyn, A.W.

Stolte, S.

VU Research Portal

VU Research PortalElastic scattering in a reactive environment: NO on Ru(0001)-(1x1)H.Butler, D.A.; Berenbak, B.; Stolte, S.; Kleyn, A.W.published inPhysical Review Letters1997DOI (link to publisher)10.1103/PhysRevLett.78.4653document versionPublisher's PDF, also known as Version of recordLink to publication in VU Research Portalcitation for published version (APA)Butler, D. A., Berenbak, B., Stolte, S., & Kleyn, A. W. (1997). Elastic scattering in a reactive environment: NO onRu(0001)-(1x1)H. 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Dec. 2021VOLUME 78, NUMBER 24 P H Y S I C A L R E V I E W L E T T E R S 16 JUNE 1997erlandsngElastic Scattering in a Reactive Environment: NO onRus0001d-s1 3 1dHD. A. Butler,1 B. Berenbak,1,2 S. Stolte,2 and A. W. Kleyn1,*1FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands2Faculteit der Scheikunde, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Neth(Received 23 December 1996)Local variations in the reactivity of NO on the Rus0001d-s1 3 1dH surface have been probed witha supersonic molecular beam. The presence of adsorbed hydrogen atoms creates an elastic scatterichannel in the specular direction, concomitant with a suppression of the initial sticking probability.Argon is scattered in a qualitatively similar, but instrumentally limited, specular peak from the samesurface. The well characterized position of the hydrogen atoms allows the scattering to be related tocollisions at the threefold hollow sites within the surface unit cell. [S0031-9007(97)03398-X]PACS numbers: 82.65.My, 34.50.Lfabiseattotiioarooehwec6rher-fewisretoeeeion)toc1Oingy,mdsnalgyallcearOne of the recurrent questions in the field of gas-surfadynamics has been the lateral surface site dependeof the reaction probability. It is generally accepted ththe reactivity of semiconductor surfaces is dominatedhighly localized, dangling bonds. The saturation of thedangling bonds by hydrogen results in the remarkableertness of silicon surfaces treated with HF. In contraon metal surfaces, the electrons are delocalized, sugging a laterally rather uniform potential energy for a molcule above a surface. However, recent calculations hindicated a considerable lateral dependence of the potial and reactivity of a metal surface [1–3]. In orderinvestigate the importance of these effects in a reacenvironment, we have studied the catalytic NO reductreaction on Ru(0001), where a number of associativedissociative adsorption states exist.Measurements of the sticking probability alone agenerally unable to provide information on the positionthe adsorption sites within the surface unit cell. The minformative probe of the interaction dynamics is providby the angle and energy resolved measurement ofdirectly scattered molecules [4]. In this Letter we repothe remarkably elastic, specular scattering of NO froa Ru(0001) surface in which the adsorption channelbeen partially closed by the presence of hydrogen atoin the fcc threefold hollow sites.The experiments were performed in an ultrahigh vauum (UHV) scattering chamber equipped with a three stasupersonic molecular beam source [5]. The samplealigned and polished to within 0.1± of the (0001) crystalface and cleaned by repeated flashing to a surface tempture sTsd of 1500 K in2 3 1028 mbar of O2s gd. A singleflash to 1600 K in UHV was found to remove all thremaining oxygen and leave a clean ordered surface, astained by a Debye-Waller plot of the thermal helium refletivity which extrapolated toIyI0 ­ 1.0 at 0 K, whereI0 isthe incident He beam intensity. The Rus0001d-s1 3 1dHoverlayer was prepared by dosing5 3 1027 mbar of H2s gdfor 300 s atTs , 200 K. Sticking probability experimentswere performed using the technique of King and Wells [Angle resolved time-of-flight scattering experiments we0031-9007y97y78(24)y4653(4)$10.00cencetysen-t,est--veen-venndefstdthertmasmsc-geasera-cer--].eperformed with a 0.5% duty cycle chopped beam and tr sults extrapolated to the zero NO coverage limit. Duing these measurements the coverage never exceeded apercent of a monolayer.The behavior of NO on the Ru(0001) surfacedependent onTs. Below 200 K a strongly chemisorbedmolecular state is formedsEads ø 1.49 eVd, either ontop of a ruthenium atom, in a twofold bridging oin a threefold hollow site, depending on the surfaccoverage [7,8]. Above 200 K, NO can dissociate inNsadsd and Osadsd adatoms, given the availability of freesites. Above 450 K, molecular NOsadsd desorbs from thesurface. Finally, Nsadsd and Osadsd associatively desorb asN2 and O2 at ,500 and,1500 K, respectively.The result of scattering NO from Ru(0001), in thzero coverage limit, is shown in Fig. 1. Because of thhigh chemisorption probability very few molecules arscattered. Those that are form a broad angular distribut[full width at half maximumsFWHMd > 37±], similarto that observed in the NO-Ag(111) [9] and Pt(111[10] systems. However, on Ru(0001) it is possiblereduce the surface reactivity by blocking all the fcthreefold hollow sites with hydrogen atoms [11]. Figurehows the final energy and angular distributions of Nmolecules scattered directly from the Rus0001d-s1 31dH surface. Strikingly, the angular distribution (toppanel) now forms a sharp peak in the specular scatterdirection sFWHM ­ 8±d and can be approximated by acosnsQf 2 Qspecd function wheren ­ 380 6 170, farhigher than any previously reported value for a heavreactive molecule. Comparison with the direct beaprofile reveals a slightly asymmetric broadening towarsupraspecular directions, consistent with some rotatioexcitation at the expense of normal translational enersEi cos2 Qid [12]. The final energy of the scatteredmolecules (bottom panel) reveals the surprisingly smenergy transfer in the collision.EfyEi increases atmore grazing angles of incidence and at lower incidenenergies, so that atQi ­ 60±, Ei ­ 0.34 eV, andQi ­70±, Ei ­ 2.1 eV, molecules scatter along the speculdirection in a truly elastic collisionsEfyEi ­ 1.0d.© 1997 The American Physical Society 4653VOLUME 78, NUMBER 24 P H Y S I C A L R E V I E W L E T T E R S 16 JUNE 1997soeyrat-eofeec-sa-andre-n,byFIG. 1. Top panel: angular distribution of NO moleculescattered from the Ru(0001) (diamonds,Ei ­ 1.56 eV,Ts ­ 650 K, Qi ­ 60± along the k1120l direction) andRus0001d-s1 3 1dH (circles, Ei ­ 2.1 eV, Ts ­ 140 K,Qi ­ 60± along the k1120l direction) surfaces, normalizedto the direct beam intensity. Direct beam profile is showfor comparison. Bottom panel: final energy distributionsNO scattered from Rus0001d-s1 3 1dH. Ei ­ 0.34 eV (opensymbols) and 2.1 eV (filled symbols),Ts ­ 140 K, Qi ­ 50±(squares), 60± (circles), and 70± (triangles) along the closepackedk1120l direction.In order to measure the effect of Hsadsd on the surfacereactivity, the initial sticking probability (zero coveraglimit) of NO sS0d was measured on both the clean and hdrogen covered surfaces (Fig. 2). On the clean surfaceS0is high s.0.7d at all incidence energies. These data weobtained at a surface temperature of 650 K, where thesorption of NO is dissociative. However,S0 was found tobe independent of the surface temperature and adsorpinto the molecular state at 140 K follows the same curvThe effect of Hsadsd on the Rus0001d-s1 3 1dH surface isto suppressS0 but still leaves a significant reaction probability sS0 ­ 0.17 0.47d. Because of the desorption temperature of hydrogen from Ru(0001) (350 K [13]) thesdata could be measured only at low temperatures, whNO adsorption is entirely molecular.Figure 3 shows the thermally programmed desorpti(TPD) spectra for masses 2 (H2) and 30 (NO) from theRus0001d-H 1 NO overlayer formed by the adsorption o4654nf-ed-ione.-erenFIG. 2. Initial sticking probability of NO on Ru(0001) (Ts ­650 K, filled circles) and Rus0001d-s1 3 1dH (Ts ­ 140 K,open circles) as a function ofEi . Qi ­ 0±.NO on Rus0001d-s1 3 1dH. The only other species ob-served was mass 28 (N2) formed from the well knowndecomposition of NO on Ru(0001) [8]. The associativdesorption of H2 from the Rus0001d-s1 3 1dH overlayerand the desorption of NO from the Rus0001d-s2 3 2dNOoverlayer accurately reproduce previously reported sptra [8,13]. When NO is adsorbed on the Rus0001d-s1 31dH surface the H2 desorption peak narrows and shiftfrom 350 to 325 K, with a shoulder at higher tempertures, characteristic of compressed Rus0001d-nH overlay-ers observed by Jachimowski [13]s1.0 , n , 1.4d. Incontrast, in the presence of Hsadsd, the NO desorptionpeak remains completely unchanged in both shapepeak position. The area under the desorption peaksmains unchanged for both H2 and NO, demonstratingthat hydrogen is not desorbed during NO adsorptionor is the number of NO adsorption sites reducedFIG. 3. Left panel: TPD spectra of H2 and NO fromRus0001d-s1 3 1dH (dashed lines), Rus0001d-s2 3 2dNO(dotted lines), and Rus0001d-s1 3 1dH exposed to an NO beam(solid lines). Heating rate5 K s21. Right panel: Normalizedscattered helium intensity from Rus0001d-s2 3 2dNO andRus0001d-H 1 NO, Ei ­ 65 meV, Qi ­ 60± along the closepackedk1120l direction,Ts ­ 140 K. Labels show diffractionpeak indices.VOLUME 78, NUMBER 24 P H Y S I C A L R E V I E W L E T T E R S 16 JUNE 1997mwsrnollrHFanoguihlas.nsesns1)sol islertiseaegyceelytheendthegee-e-ingthe presence of hydrogen. These results suggest thathe region exposed to the beams,4 mm diamd, a pureRus0001d-s2 3 2dNO phase is created, surrounded byregion of compressed hydrogen adatoms outside the beareas,10 mm diamd.Figure 3 also shows the diffraction of a helium beaby the Rus0001d-s2 3 2dNO and Rus0001d-H 1 NO sur-faces. Both surfaces show almost identical diffractiopeaks, indicating that NO adsorbs in the sames2 3 2dstructure on the hydrogen precovered surface as onclean Ru(0001) surface. This conclusion is supportedthe observation of as2 3 2d low energy electron diffrac-tion (LEED) pattern on both surfaces. The long rangorder of the Rus0001d-s2 3 2dNO overlayer seen and thehigh saturation coverage (0.75 monolayers [7,8]) shothat NO adsorption cannot occur only at surface defectOur discussion will concentrate on whether the expemental results are consistent with a local site dependeof the molecule-surface interaction. The suppressionS0 and the increase of direct (single collision) scatteing suggest that part of the surface is rendered inertNO by the presence of Hsadsd. If H sadsd were to passi-vate the entire unit cell, then,30% of the H sites in theRus0001d-s1 3 1dH surface would need to be vacant taccount for the remaining sticking probability. This is extremely unlikely on such a well defined surface unit ceBy occupying the Ru bonding orbitals in the fcc threefohollow sites, Hsadsd seems to create an activation barriefor NO adsorption which is strongly site dependent. Athe site occupied by the hydrogen atom this barrier is cosiderable, creating a local plateau in the potential enesurface, resulting in the narrow specular reflection. Elswhere in the unit cell a smaller barrier appears to cauthe increase ofS0 with energy as observed for the poisoning of palladium by sulfur [14].Recent theoretical studies on the dissociation of2have shown that where strongly attractive geometriesclose to repulsive ones, the local potential gradient w“steer” the molecule into an adsorption site [15,16]. Thresult is a tendency forS0 to approach unity at lowenergies where the molecule is traveling slowly, andchaotic scattering of those molecules not adsorbed.the heavy NO molecule steering reorients the moleculeading to enhanced sticking on Pt(111) [17]. To whextent steering leads to the remarkably narrow angudistribution requires further study.In order to compare the behavior of NO with ainert, physisorbed particle, argon was also scattered frthe hydrogen covered ruthenium surface and the anresolved results are shown in Fig. 4 for four differenincidence angles. A sharp peak is seen at the specangle, superimposed on a broad background, simto observations on the 2H-W(100) surface [18]. Thspecular peak, which increases in intensity at mograzing angles of incidence, is even sharper than tobserved for NO and is essentially limited by the angut inaamnthebyes.i-ceofr-to-l.drtn-gye-se-lieilleaorle,tlarmletlarlarereatrFIG. 4. Angular distribution of Ar scattered from theRus0001d-s1 3 1dH surface. Ei ­ 65 meV, Ts ­ 140 K,Qi ­ 40± (circles), 50± (squares), 60± (triangles), and 70±(crosses) along the close packedk1120l direction. Angularprofile of the direct beam (70±) is shown for comparison.profile of the direct beamsFWHM ­ 2±d. Like NO,this is consistent with two distinct scattering regimeThe specular component arises from laterally flat sectioof the potential, while the diffuse component originatlargely from inelastic collisions on the corrugated sectioof the potential, as seen in calculations of Ar on Ag(11[19]. For NO these inelastic collisions lead to sticking,scattering from the corrugated section of the potentianot observed.We will now turn our attention to the most remarkabobservation, that the collision of NO with the inert paof the unit cell can be elastic. The simplest modelfor the collision of two hard spheres, with no attractivinteraction. In this caseEfyEi ­ 0.74 for particles ofmass 30 (NO) and 101 (Ru). This rises to 0.86 forsurface mass ofM ­ 202, 0.90 for M ­ 303, and 0.94for M ­ 505. Including an attractive molecule-surfacinteraction in the model would only increase the enertransfer in the collision. Hence, even in the absenof an attractive well,EfyEi can approach unity onlywhen the mass of the surface scatterer becomes extremlarge. This model neglects the quantized nature ofsurface phonons. However, the phonon energies extform 0 meV upwards and have been measured onRus0001d-s1 3 1dH surface at only 21 meV [20], easilyaccessible at the incidence energies used here.A more likely explanation is related to the stiff bindinand high coordination of hydrogen adsorbed in the thrfold hollow site with which an NO molecule collides. Thhigh vibrational frequency of Hsadsd perpendicular to theRu(0001) surface (n' ­ 3.4 3 1013 Hz [20]) correspondsto a period of 29 fs. In this time the motion of an NO molecule along the surface normalsEi ­ 0.34 eV, Qi ­ 60±dwill be only 0.22 Å. Hence the H-Ru3 vibration will adjustadiabatically to the perturbation caused by the NO, leadto the elastic scattering seen in the experiments.4655VOLUME 78, NUMBER 24 P H Y S I C A L R E V I E W L E T T E R S 16 JUNE 1997tomtcrnifc..t.rf.rf..erf...l-In conclusion, we are able to distinguish two distinct and contrasting interactions of NO within theRus0001d-s1 3 1dH unit cell. Directly above Hsadsd, theNO experiences a weak physisorption well, similarthat of argon. Such an interaction will be smooth anflat leading to the specular, elastic scattering of NO frothe hydrogen filled threefold hollow site. Collisions aother impact sites within the unit cell will be inelasticand occur on a more corrugated potential energy surfaThis leads to efficient sticking (in the case of NO, whea deep chemisorption well exists) and diffuse scatteri(in the case of argon). The adsorption of NO resultsa migration of Hsadsd, forming compressed overlayers ohydrogen adatoms.The authors thank Alan Luntz for stimulating discussions and Joost Frenken for a critical reading of the manscript. The work was carried out as part of the researprogram of the FOM and was funded by the NWO and ainstitutional fellowship from the Human Capital Mobilityprogramme of the EU (ERBCHBGCT 940701) (D. A. B.)*Electronic address: kleyn@amolf.nl[1] B. Hammer, M. Scheffler, K. W. Jacobsen, and J. KNørskov, Phys. Rev. Lett.73, 1400 (1994).[2] J. A. White, D. M. Bird, M. C. Payne, and I. Stich, PhysRev. Lett.73, 1404 (1994).[3] S. Wilke, M. H. Cohen, and M. Scheffler, Phys. Rev. Let77, 1560 (1996).4656-de.egn-u-hn..[4] S. Holloway, M. Karikorpi, and J. W. Gadzuk, Nucl.Instrum. Methods Phys. Res., Sect. B27, 37 (1987).[5] A. Raukema, R. J. Dirksen, and A. W. Kleyn, J. ChemPhys.103, 6217 (1995).[6] D. A. King and M. G. Wells, Surf. Sci.29, 454 (1972).[7] E. Umbach, S. Kulkarni, P. Feulner, and D. Menzel, SuSci. 88, 65 (1979).[8] P. Feulner, S. Kulkarni, E. Umbach, and D. Menzel, SuSci. 99, 489 (1980).[9] E. W. Kuipers, M. G. Tenner, M. E. M. Spruit, and A. WKleyn, Surf. Sci.189/190, 669 (1987).[10] A. E. Wiskerke and A. W. Kleyn, J. Phys. Condens. Matt7, 5195 (1995).[11] M. Lindroos, H. Pfnür, P. Feulner, and D. Menzel, SurSci. 180, 237 (1987).[12] A. W. Kleyn, Surf. Rev. Lett.1, 157 (1994).[13] T. A. Jachimowski, B. Meng, D. F. Johnson, and W. HWeinberg, J. Vac. Sci. Technol. A13, 1564 (1995).[14] S. Wilke and M. Scheffler, Phys. Rev. Lett.76, 3380(1996).[15] A. Gross, S. Wilke, and M. Scheffler, Phys. Rev. Lett.75,2718 (1995).[16] M. Kay, G. R. Darling, S. Holloway, J. A. White, andD. M. Bird, Chem. Phys. Lett.245, 311 (1995).[17] R. J. W. E. Lahaye, S. Stolte, S. Holloway, and A. WKleyn, J. Chem. Phys.104, 8301 (1996).[18] E. K. Schweizer and C. T. Rettner, Phys. Rev. Lett.62,3085 (1989).[19] R. J. W. E. Lahaye, A. W. Kleyn, S. Stolte, and S. Holoway, Surf. Sci.338, 169 (1995).[20] H. Shi and K. Jacobi, Surf. Sci.313, 289 (1994).

English

Elastic scattering in a reactive environment: NO on Ru(0001)-(1x1)H.

D. A. Butler

B. Berenbak

S. Stolte

A. W. Kleyn

Crossref

Physical Review Letters

Elastic Scattering in a Reactive Environment: NO on<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Ru</mml:mi><mml:mo>(</mml:mo><mml:mn>0001</mml:mn><mml:mo>)</mml:mo><mml:mo>−</mml:mo><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:mo>)</mml:mo><mml:mi>H</mml:mi></mml:math>

DSpace at VU

VOLUME 78, NUMBER 24 P HY S I CA L REV I EW LE T T ER S 16 JUNE 1997Elastic Scattering in a Reactive Environment: NO on Rus0001d-s1 3 1dH
D.A. Butler,1 B. Berenbak,1,2 S. Stolte,2 and A.W. Kleyn1,*
1FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
2Faculteit der Scheikunde, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
(Received 23 December 1996)
Local variations in the reactivity of NO on the Rus0001d-s1 3 1dH surface have been probed with
a supersonic molecular beam. The presence of adsorbed hydrogen atoms creates an elastic scattering
channel in the specular direction, concomitant with a suppression of the initial sticking probability.
Argon is scattered in a qualitatively similar, but instrumentally limited, specular peak from the same
surface. The well characterized position of the hydrogen atoms allows the scattering to be related to
collisions at the threefold hollow sites within the surface unit cell. [S0031-9007(97)03398-X]
PACS numbers: 82.65.My, 34.50.LfOne of the recurrent questions in the field of gas-surface
dynamics has been the lateral surface site dependence
of the reaction probability. It is generally accepted that
the reactivity of semiconductor surfaces is dominated by
highly localized, dangling bonds. The saturation of these
dangling bonds by hydrogen results in the remarkable in-
ertness of silicon surfaces treated with HF. In contrast,
on metal surfaces, the electrons are delocalized, suggest-
ing a laterally rather uniform potential energy for a mole-
cule above a surface. However, recent calculations have
indicated a considerable lateral dependence of the poten-
tial and reactivity of a metal surface [1–3]. In order to
investigate the importance of these effects in a reactive
environment, we have studied the catalytic NO reduction
reaction on Ru(0001), where a number of associative and
dissociative adsorption states exist.
Measurements of the sticking probability alone are
generally unable to provide information on the position of
the adsorption sites within the surface unit cell. The most
informative probe of the interaction dynamics is provided
by the angle and energy resolved measurement of the
directly scattered molecules [4]. In this Letter we report
the remarkably elastic, specular scattering of NO from
a Ru(0001) surface in which the adsorption channel has
been partially closed by the presence of hydrogen atoms
in the fcc threefold hollow sites.
The experiments were performed in an ultrahigh vac-
uum (UHV) scattering chamber equipped with a three stage
supersonic molecular beam source [5]. The sample was
aligned and polished to within 0.1– of the (0001) crystal
face and cleaned by repeated flashing to a surface tempera-
ture sTsd of 1500 K in 2 3 1028 mbar of O2s gd. A single
flash to 1600 K in UHV was found to remove all the
remaining oxygen and leave a clean ordered surface, ascer-
tained by a Debye-Waller plot of the thermal helium reflec-
tivity which extrapolated to IyI0 ­ 1.0 at 0 K, where I0 is
the incident He beam intensity. The Rus0001d-s1 3 1dH
overlayer was prepared by dosing 5 3 1027 mbar of H2s gd
for 300 s at Ts , 200 K. Sticking probability experiments
were performed using the technique of King and Wells [6].
Angle resolved time-of-flight scattering experiments were0031-9007y97y78(24)y4653(4)$10.00performed with a 0.5% duty cycle chopped beam and the
results extrapolated to the zero NO coverage limit. Dur-
ing these measurements the coverage never exceeded a few
percent of a monolayer.
The behavior of NO on the Ru(0001) surface is
dependent on Ts. Below 200 K a strongly chemisorbed
molecular state is formed sEads ø 1.49 eVd, either on
top of a ruthenium atom, in a twofold bridging or
in a threefold hollow site, depending on the surface
coverage [7,8]. Above 200 K, NO can dissociate into
Nsadsd and Osadsd adatoms, given the availability of free
sites. Above 450 K, molecular NOsadsd desorbs from the
surface. Finally, Nsadsd and Osadsd associatively desorb as
N2 and O2 at ,500 and ,1500 K, respectively.
The result of scattering NO from Ru(0001), in the
zero coverage limit, is shown in Fig. 1. Because of the
high chemisorption probability very few molecules are
scattered. Those that are form a broad angular distribution
[full width at half maximum sFWHMd > 37–], similar
to that observed in the NO-Ag(111) [9] and Pt(111)
[10] systems. However, on Ru(0001) it is possible to
reduce the surface reactivity by blocking all the fcc
threefold hollow sites with hydrogen atoms [11]. Figure 1
shows the final energy and angular distributions of NO
molecules scattered directly from the Rus0001d-s1 3
1dH surface. Strikingly, the angular distribution (top
panel) now forms a sharp peak in the specular scattering
direction sFWHM ­ 8–d and can be approximated by a
cosnsQf 2 Qspecd function where n ­ 380 6 170, far
higher than any previously reported value for a heavy,
reactive molecule. Comparison with the direct beam
profile reveals a slightly asymmetric broadening towards
supraspecular directions, consistent with some rotational
excitation at the expense of normal translational energy
sEi cos2 Qid [12]. The final energy of the scattered
molecules (bottom panel) reveals the surprisingly small
energy transfer in the collision. EfyEi increases at
more grazing angles of incidence and at lower incidence
energies, so that at Qi ­ 60–, Ei ­ 0.34 eV, and Qi ­
70–, Ei ­ 2.1 eV, molecules scatter along the specular
direction in a truly elastic collision sEfyEi ­ 1.0d.© 1997 The American Physical Society 4653
VOLUME 78, NUMBER 24 P HY S I CA L REV I EW LE T T ER S 16 JUNE 1997FIG. 1. Top panel: angular distribution of NO molecules
scattered from the Ru(0001) (diamonds, Ei ­ 1.56 eV,
Ts ­ 650 K, Qi ­ 60– along the k1120l direction) and
Rus0001d-s1 3 1dH (circles, Ei ­ 2.1 eV, Ts ­ 140 K,
Qi ­ 60– along the k1120l direction) surfaces, normalized
to the direct beam intensity. Direct beam profile is shown
for comparison. Bottom panel: final energy distributions of
NO scattered from Rus0001d-s1 3 1dH. Ei ­ 0.34 eV (open
symbols) and 2.1 eV (filled symbols), Ts ­ 140 K, Qi ­ 50–
(squares), 60– (circles), and 70– (triangles) along the close
packed k1120l direction.
In order to measure the effect of Hsadsd on the surface
reactivity, the initial sticking probability (zero coverage
limit) of NO sS0d was measured on both the clean and hy-
drogen covered surfaces (Fig. 2). On the clean surface S0
is high s.0.7d at all incidence energies. These data were
obtained at a surface temperature of 650 K, where the ad-
sorption of NO is dissociative. However, S0 was found to
be independent of the surface temperature and adsorption
into the molecular state at 140 K follows the same curve.
The effect of Hsadsd on the Rus0001d-s1 3 1dH surface is
to suppress S0 but still leaves a significant reaction proba-
bility sS0 ­ 0.17 0.47d. Because of the desorption tem-
perature of hydrogen from Ru(0001) (350 K [13]) these
data could be measured only at low temperatures, where
NO adsorption is entirely molecular.
Figure 3 shows the thermally programmed desorption
(TPD) spectra for masses 2 (H2) and 30 (NO) from the
Rus0001d-H 1 NO overlayer formed by the adsorption of4654FIG. 2. Initial sticking probability of NO on Ru(0001) (Ts ­
650 K, filled circles) and Rus0001d-s1 3 1dH (Ts ­ 140 K,
open circles) as a function of Ei . Qi ­ 0–.
NO on Rus0001d-s1 3 1dH. The only other species ob-
served was mass 28 (N2) formed from the well known
decomposition of NO on Ru(0001) [8]. The associative
desorption of H2 from the Rus0001d-s1 3 1dH overlayer
and the desorption of NO from the Rus0001d-s2 3 2dNO
overlayer accurately reproduce previously reported spec-
tra [8,13]. When NO is adsorbed on the Rus0001d-s1 3
1dH surface the H2 desorption peak narrows and shifts
from 350 to 325 K, with a shoulder at higher tempera-
tures, characteristic of compressed Rus0001d-nH overlay-
ers observed by Jachimowski [13] s1.0 , n , 1.4d. In
contrast, in the presence of Hsadsd, the NO desorption
peak remains completely unchanged in both shape and
peak position. The area under the desorption peaks re-
mains unchanged for both H2 and NO, demonstrating
that hydrogen is not desorbed during NO adsorption,
nor is the number of NO adsorption sites reduced by
FIG. 3. Left panel: TPD spectra of H2 and NO from
Rus0001d-s1 3 1dH (dashed lines), Rus0001d-s2 3 2dNO
(dotted lines), and Rus0001d-s1 3 1dH exposed to an NO beam
(solid lines). Heating rate 5 K s21. Right panel: Normalized
scattered helium intensity from Rus0001d-s2 3 2dNO and
Rus0001d-H 1 NO, Ei ­ 65 meV, Qi ­ 60– along the close
packed k1120l direction, Ts ­ 140 K. Labels show diffraction
peak indices.
VOLUME 78, NUMBER 24 P HY S I CA L REV I EW LE T T ER S 16 JUNE 1997the presence of hydrogen. These results suggest that in
the region exposed to the beam s,4 mm diamd, a pure
Rus0001d-s2 3 2dNO phase is created, surrounded by a
region of compressed hydrogen adatoms outside the beam
area s,10 mm diamd.
Figure 3 also shows the diffraction of a helium beam
by the Rus0001d-s2 3 2dNO and Rus0001d-H 1 NO sur-
faces. Both surfaces show almost identical diffraction
peaks, indicating that NO adsorbs in the same s2 3 2d
structure on the hydrogen precovered surface as on the
clean Ru(0001) surface. This conclusion is supported by
the observation of a s2 3 2d low energy electron diffrac-
tion (LEED) pattern on both surfaces. The long range
order of the Rus0001d-s2 3 2dNO overlayer seen and the
high saturation coverage (0.75 monolayers [7,8]) shows
that NO adsorption cannot occur only at surface defects.
Our discussion will concentrate on whether the experi-
mental results are consistent with a local site dependence
of the molecule-surface interaction. The suppression of
S0 and the increase of direct (single collision) scatter-
ing suggest that part of the surface is rendered inert to
NO by the presence of Hsadsd. If Hsadsd were to passi-
vate the entire unit cell, then ,30% of the H sites in the
Rus0001d-s1 3 1dH surface would need to be vacant to
account for the remaining sticking probability. This is ex-
tremely unlikely on such a well defined surface unit cell.
By occupying the Ru bonding orbitals in the fcc threefold
hollow sites, Hsadsd seems to create an activation barrier
for NO adsorption which is strongly site dependent. At
the site occupied by the hydrogen atom this barrier is con-
siderable, creating a local plateau in the potential energy
surface, resulting in the narrow specular reflection. Else-
where in the unit cell a smaller barrier appears to cause
the increase of S0 with energy as observed for the poison-
ing of palladium by sulfur [14].
Recent theoretical studies on the dissociation of H2
have shown that where strongly attractive geometries lie
close to repulsive ones, the local potential gradient will
“steer” the molecule into an adsorption site [15,16]. The
result is a tendency for S0 to approach unity at low
energies where the molecule is traveling slowly, and a
chaotic scattering of those molecules not adsorbed. For
the heavy NO molecule steering reorients the molecule,
leading to enhanced sticking on Pt(111) [17]. To what
extent steering leads to the remarkably narrow angular
distribution requires further study.
In order to compare the behavior of NO with an
inert, physisorbed particle, argon was also scattered from
the hydrogen covered ruthenium surface and the angle
resolved results are shown in Fig. 4 for four different
incidence angles. A sharp peak is seen at the specular
angle, superimposed on a broad background, similar
to observations on the 2H-W(100) surface [18]. The
specular peak, which increases in intensity at more
grazing angles of incidence, is even sharper than that
observed for NO and is essentially limited by the angularFIG. 4. Angular distribution of Ar scattered from the
Rus0001d-s1 3 1dH surface. Ei ­ 65 meV, Ts ­ 140 K,
Qi ­ 40– (circles), 50– (squares), 60– (triangles), and 70–
(crosses) along the close packed k1120l direction. Angular
profile of the direct beam (70–) is shown for comparison.
profile of the direct beam sFWHM ­ 2–d. Like NO,
this is consistent with two distinct scattering regimes.
The specular component arises from laterally flat sections
of the potential, while the diffuse component originates
largely from inelastic collisions on the corrugated sections
of the potential, as seen in calculations of Ar on Ag(111)
[19]. For NO these inelastic collisions lead to sticking, so
scattering from the corrugated section of the potential is
not observed.
We will now turn our attention to the most remarkable
observation, that the collision of NO with the inert part
of the unit cell can be elastic. The simplest model is
for the collision of two hard spheres, with no attractive
interaction. In this case EfyEi ­ 0.74 for particles of
mass 30 (NO) and 101 (Ru). This rises to 0.86 for a
surface mass of M ­ 202, 0.90 for M ­ 303, and 0.94
for M ­ 505. Including an attractive molecule-surface
interaction in the model would only increase the energy
transfer in the collision. Hence, even in the absence
of an attractive well, EfyEi can approach unity only
when the mass of the surface scatterer becomes extremely
large. This model neglects the quantized nature of the
surface phonons. However, the phonon energies extend
form 0 meV upwards and have been measured on the
Rus0001d-s1 3 1dH surface at only 21 meV [20], easily
accessible at the incidence energies used here.
A more likely explanation is related to the stiff binding
and high coordination of hydrogen adsorbed in the three-
fold hollow site with which an NO molecule collides. The
high vibrational frequency of Hsadsd perpendicular to the
Ru(0001) surface (n' ­ 3.4 3 1013 Hz [20]) corresponds
to a period of 29 fs. In this time the motion of an NOmole-
cule along the surface normal sEi ­ 0.34 eV, Qi ­ 60–d
will be only 0.22 Å. Hence the H-Ru3 vibration will adjust
adiabatically to the perturbation caused by the NO, leading
to the elastic scattering seen in the experiments.4655
VOLUME 78, NUMBER 24 P HY S I CA L REV I EW LE T T ER S 16 JUNE 1997In conclusion, we are able to distinguish two dis-
tinct and contrasting interactions of NO within the
Rus0001d-s1 3 1dH unit cell. Directly above Hsadsd, the
NO experiences a weak physisorption well, similar to
that of argon. Such an interaction will be smooth and
flat leading to the specular, elastic scattering of NO from
the hydrogen filled threefold hollow site. Collisions at
other impact sites within the unit cell will be inelastic
and occur on a more corrugated potential energy surface.
This leads to efficient sticking (in the case of NO, where
a deep chemisorption well exists) and diffuse scattering
(in the case of argon). The adsorption of NO results in
a migration of Hsadsd, forming compressed overlayers of
hydrogen adatoms.
The authors thank Alan Luntz for stimulating discus-
sions and Joost Frenken for a critical reading of the manu-
script. The work was carried out as part of the research
program of the FOM and was funded by the NWO and an
institutional fellowship from the Human Capital Mobility
programme of the EU (ERBCHBGCT 940701) (D. A. B.).
*Electronic address: kleyn@amolf.nl
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Elastic scattering in a reactive environment: NO on Ru(0001)-(1x1)H.

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