Dynamics of physisorbed monolayer of methane on Ag(1 1 1)

Surface Science 593 (2005) 195–201

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Dynamics of physisorbed monolayerof methane on Ag(111)

M. Sakurai a,*, C. Yamada b

a Department of Physics, Kobe University, Rokkodai 1-1, Nada, Kobe 657-8501, Japanb Institute for Laser Science, University of Electro-Communications Chofu, Tokyo 182-8585, Japan

Available online 18 July 2005

Abstract

The adsorption, desorption and vibrational excitation dynamics of methane monolayer physisorbed on a Ag(111)surface was studied by LEED and HREELS measurements. For LEED observation, electron stimulated desorption ofadsorbed CH4 was obvious even at a minimized incident electron current of 1 nA, and the result suggests the possibilityof leftover of electron stimulated reaction on the substrate that cannot be identified by LEED nor desorb at raised tem-perature. Mode dependence of the width of energy loss peak was observed in the HREEL spectra. The excitation of thevibrational mode with sharper width is mainly associated with dipole scattering mechanism.� 2005 Elsevier B.V. All rights reserved.

Keywords: DIET; EELS; LEED; Physical adsorption; Silver; Crystallization; Solid–gas interfaces; Single crystal surfaces

1. Introduction

The adsorption systems of simple moleculesincluding rare gases physisorbed or condensed oncryogenic substrates have been studied with vari-ous kinds of approaches such as thermodyna-mics measurements, electron or neutron diffractiontechniques, and electron spectroscopy. The crystal-line monolayer of physisorbed molecules has been

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reservdoi:10.1016/j.susc.2005.06.061

* Corresponding author. Tel.: +81 78 803 5648; fax: +81 78803 5770.

E-mail address: [email protected] (M. Sakurai).

studied extensively on graphite substrate [1–3]. Formetal substrates, especially rare gas layers havebeen studied [1,4,5]. Among metal substrates, aAg(111) surface has been often used for physi-sorption systems due to its inert nature. On aAg(111) substrate, rare gas atoms adsorb inlayer-by-layer manner, which means that the iso-therm curve around the formation of the first layershows a sharp riser from almost zero coverage tofull coverage at a specific pressure. The rare gasmonolayer is known to form a hexagonal closepacked structure in azimuthally commensuraterelation with the substrate lattice as proved byLEED observations. We have observed the LEED

ed.

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196 M. Sakurai, C. Yamada / Surface Science 593 (2005) 195–201

pattern of CH4 monolayer physisorbed onAg(111) that also shows hexagonal structure [6].Physisorption system of simple molecules has

also provided specific topic on electron scatteringresonance phenomena [7–11]. The weak interac-tion between adsorbed molecules and substratemodifies the scattering potential for shape reso-nance resulting in the peak shift in the cross-sec-tion of vibrational excitation by electrons. Wehave observed another type of resonance in thevibrational excitation cross-section of CH4 physi-sorbed on Ag(111) that is possibly explainedby a surface resonance effect in the crystal ofadsorbed monolayer [12].In the present paper, we made further experi-

ments on LEED and HREELS measurements forthe monolayer of CH4 physisorbed on Ag(111)with improved experimental conditions, and madediscussions on the several experimental aspects.

2. Experimental

The experimental apparatus is based on anultrahigh vacuum chamber evacuated by a tita-nium sublimation pump with a liquid nitrogenshroud, a sputter-ion pump (230 ‘/s) and a turbomolecular pump (450 ‘/s). The chamber comprisesa sample manipulating system with temperaturecontrol, a sputter-ion gun, a LEED optics(Omicron, SPECTALEED) and a high-resolu-tion electron-energy-loss spectrometer (HREELS;LK Technologies, ELS5000). Twofold magneticshields are installed in the vacuum chamber; outerone covers the LEED and EELS stages and innerone covers only the EELS stage. The samplemanipulating system consists of XYZ-translator(Vacuum Generators), a differentially pumpedrotational platform (Thermionics, RNN250) anda cryogenic closed-cycle refrigerator (IWATANI,Cryo Mini). The rotational platform is evacuatedby a small turbo-molecular pump (70 ‘/s) and ascroll pump. The substrate is mounted on a sampleholder that is connected to the cold end of therefrigerator via. a high-purity (6 N) copper rodand insulating sapphire crystal plates. A resistiveheater and Au–0.07%Fe vs. Chromel thermocou-ple are attached on the sample holder for the

temperature control. The system enables XYZ-translation, rotation around vertical axis and tem-perature control of the sample over the range of20–700 K. A gas handling system was also usedfor the introduction of Ar or CH4.The ultimate pressure less than 1 · 10�8 Pa was

obtained after bakeout procedure for 24–48 h. TheX-ray-aligned and chemically polished Ag(111)substrate was cleaned by standard proceduresinvolving low-energy Ar-ion bombardment andheating cycles, which was repeated until clear dif-fraction spots could be observed. The layer ofCH4 physisorbed on the Ag(111) substrate wasformed under adsorption equilibrium conditionsat the substrate temperature ranging from 20 to50 K. The LEED patterns of CH4 monolayer wererecorded by a liquid N2 cooled CCD camera(Princeton Instruments).The HREEL spectra were measured after the

exposure of substrate to CH4 with various incidentelectron energies ranging from 4 to 20 eV. Theoverall resolution was 5–8 meV, which was limitedby the vibration of the sample caused by the move-ment of the refrigerator. The amount of exposurewas 5–10 Langmuir with the CH4 pressure in therange of 10�6 Pa. The spectra were recorded at al-most specular reflection condition with the glanc-ing angle of 67�.

3. Results and discussions

Fig. 1 shows LEED patterns for various inci-dent electron energies. The diffraction spots corre-sponding to CH4 crystal were observable at theprimary energies between 35 and 100 eV. The cir-cle in each photograph indicates one of the hexag-onally distributed diffraction spots of CH4 crystal.Outer lattice with threefold symmetry comes fromsubstrate Ag(111). Double diffraction spots arealso remarkable, which appear between Ag(111)spots like 3 · 3 structure because the lattice con-stant of CH4 crystal is 3/2 of Ag(111)�s lattice.The pattern with best contrast where diffractionspots from CH4 crystal is most clear was obtainedat around 56 eV, possibly because the diffractioncondition for the second order reciprocal latticevector become effective around this energy. Thus

Fig. 1. LEED patterns of physisorbed CH4 on Ag(111) at various incident electron energies observed under 2D condensationconditions with the substrate temperature of 37 K and ambient CH4 pressure of 5 · 10�6 Pa. The circle in each photograph indicatesone of the hexagonally distributed diffraction spots of CH4 crystal. The rectangle corresponds to the region of interest picked up fordetailing intensity profile shown in Fig. 3.

M. Sakurai, C. Yamada / Surface Science 593 (2005) 195–201 197

further observation was carried out with this inci-dent energy. The LEED patterns were observedfor various adsorption conditions of the ambientCH4 pressure and substrate temperature. All theLEED patterns with CH4 diffraction spots havethe same lattice constant, however, according tothe analysis of spot intensities, the coverage ofCH4 saturates at less than monolayer under spe-cific adsorption conditions, which is not the casefor the growth scheme of rare gases. The obtainedresults suggest the existence of an intermediateadsorption phase with the coverage less than unitybetween the 2D gas phase and monolayer phase

[6]. Fig. 2 illustrates the difference of the adsorp-tion scheme between CH4 and rare gases monolay-ers, where the formation of island like structure ofCH4 micro crystals with the same lattice constantas that of monolayer is assumed at the intermedi-ate phase (Fig. 2(b)). Some kind of rare gas formsffiffiffi

3p

structure commensurate with substrate latticeas shown in Fig. 2(e) before reaching saturationcoverage.For the LEED observation of physisorbed layer

of CH4, electron stimulated desorption is a criticalobstacle. We monitored the variation of spotintensities of CH4 and substrate Ag under the

Fig. 2. Illustration for the monolayer growth of physisorbed CH4 (a), (b), and (c) compared with the growth scheme of rare gasmonolayer (d), (e), and (f) starting from 2D gas phase.


irradiation of electrons. Fig. 3 shows the variationof profiles of both diffraction peaks with differenttotal irradiation time at an electron current of1 nA. The spot intensity of CH4 decreases and thatof Ag increases proportionally with the irradiationtime due to the desorption of CH4. The profile(peak width) also becomes broader by the irradia-tion. If we assume the size of electron beam in therange of 0.3–1 mm on the sample, the desorptionyield is estimated at as high as 0.1–1.The LEED pattern observed at a raised sub-

strate temperature, where CH4 molecules are com-pletely flushed away and no longer adsorb, showedthat of clean Ag surface. If CH4 is adsorbed againunder monolayer adsorption conditions, however,the LEED spot intensity of CH4 crystal at the pre-viously observed position became weaker thanthat of the original intensity for monolayer cover-age. When the hitting position of LEED electronbeam was moved to a �virgin� location, the inten-sity showed that of monolayer coverage. Thisimplies that electron irradiation on a surface cov-ered with physisorbed CH4 molecules induces not

only desorption but some reaction that leads to adissociation of CH4 molecule and creation ofchemically adsorbed species (possibly atomichydrogen), which cannot be detected by LEEDobservation but interferes with the formation ofsingle crystal of CH4 monolayer. The dissociationreaction of CH4 on a cryogenic surface stimulatedby an irradiation of UV light has been reported[13], which supports our assumption.Fig. 4 shows HREEL spectra of a CH4 physi-

sorbed layer with various incident electron ener-gies. Peaks for three kinds of vibrational modesof CH4 are observable in the figure. Among thevibrational modes of CH4 in the gas phase, infra-red active modes are m3 and m4 modes, and onlym4 mode is Raman inactive. Humps around the en-ergy loss of 90 meV are ascribed to ghost signal.Symmetric stretch mode (m1) does not appear dueto the optical selection rule. Similar measurementhas been done before [14], however, the resolutionand substrate temperature is much improved at thepresent work. The improved resolution revealedthe difference of peak width between m4 mode

Fig. 3. LEED spot intensity profiles of CH4 physisorbed onAg(111) after specific period of electron irradiation: (a) beforeirradiation, (b) 30 s, (c) 90 s, and (d) 150 s at the current of1 nA. The intensity of diffraction spot from CH4 crystaldecreases with irradiation time, while diffraction intensity ofsubstrate Ag crystal increases due to the desorption of CH4overlayer.

0 100 200 300 400

Inte

nsity

[arb

.uni

ts]

CH4/Ag(111)

20

1001×

14

9

7

6

4

Incident Electron Energy (Ei) [eV]

Energy Loss [meV]

ν1ν2 ν3

ν4

Exposure : 5LIncident angle : 67o

Fig. 4. Electron energy loss spectra from physisorbed CH4 onAg(111) for various incident electron energies measured at thesubstrate temperature of 20 K. Dashed lines are drawn on thevibrational energies for corresponding modes in gas phase.


and m2 and m3 modes. Tailed structure for m2 and m3modes reminds us rotational coupling spectra,however, the tailing is much more gradual thancalculated with the population of rotational stateat the present substrate temperature, then rota-tional coupling cannot explain the tailed structure.Similar tailing in a vibrational loss peak of over-tones has been observed under shape-resonancescattering conditions [8]. For the present experi-ment, however, the tailing structure was observedfor all of incident energies indicated in Fig. 4. Thusthe reason for the tailed structure is not under-standable at present.The loss peak intensities of three vibrational

modes are plotted as a function of incident elec-tron energy in Fig. 5. The incident energy depen-dence (IV plot) for m4 mode shows a dip causedby surface resonance corresponding to the creationof a beam diffracted by the Ag(111) lattice

propagating along the surface. Since dipole scat-tering is more affected by the surface resonance ef-fect than the impact scattering, dipole scattering isdominant for the excitation of m4 mode while theother dipole active m3 mode does not show the spe-cific feature of dipole scattering. The distinction ofvibrational modes based on presence or absence ofthe tailing of vibrational loss peaks coincides withthat on the dip in the IV plots. Thus it would bestated that the distinction comes not simply fromthe optical selection rules.Incident energy dependence of the intensity

of specularly reflected electrons (IV curve) frommethane monolayer has been measured by scan-ning primary energy with a different apparatusfrom those used for the present experiments[12,15]. The examples of IV curve for two differentambient CH4 pressures are shown in Fig. 6 [15].The intensity drop could be ascribed to a surfaceresonance that arises when an electron is diffractedalong the surface by the first order reciprocal rod

Fig. 6. Incident energy dependence of specular beam intensityelastically scattered at the surface of CH4 physisorbed onAg(111) for two kinds of ambient CH4 pressure quoted from[14].

Fig. 5. The loss peak intensities of three vibrational modes andelastic intensities plotted as a function of incident electronenergy. Lowest column shows intensities of loss peak normal-ized with elastic counts.


of CH4 crystal, while the dip in Fig. 5 arises fromthe surface resonance related to the substrate crys-tal. The reason for the presence of double dip isunclear, but the pressure dependence of the dipstructure is probably related with the CH4 crystalstructure, since ambient pressure governs adsorp-tion condition as discussed for Fig. 2. In the pres-ent experiment, we could not observe a shaperesonance effect reported at 2.4 eV. In the physi-sorption system, the resonance structure becomesweakened or disappear due to the interactionbetween the adsorbed molecules and substrates,which may explain our experimental results.In summary, we have observed the structure

and vibrational excitation of CH4 physisorbed ona Ag(111) surface using LEED and HREELSapparatus. By the observation of LEED spotintensity, it is assumed that not only electron stim-ulated desorption but electron stimulated dissocia-tion take place for a physisorbed monolayer ofCH4. Mode dependence of the peak width is nota-

ble in the HREEL spectra, which correlates to IVfeature for vibrational loss peak at the surface res-onance related to the diffraction with the substratecrystal. On the other hand, the ambient pressuredependence of the line-shape of surface resonancerelated to physisorbed CH4 crystal suggests thecorrelation with the coverage of CH4 underadsorption equilibrium conditions.

References

[1] H. Ohtani, C.-T. Kao, M.A. Van Hove, G.A. Somorjai,Prog. Surf. Sci. 23 (1986) 155.

[2] H. You, S.C. Fain Jr., S. Satija, L. Passell, Phys. Rev. Lett.56 (1986) 244.

[3] J. Cui, S.C. Fain Jr., J. Vac. Sci. Technol. A 5 (1987) 710.


[4] H.M. Kramer, J. Suzanne, Surf. Sci. 54 (1976) 659.[5] M. Sakurai, T. Okano, Y. Tuzi, J. Vac. Soc. Jpn. 29 (1985)451 (in Japanese).

[6] M. Sakurai, F. Nakajima, M. Yasuda, T. Nanba, J. Vac.Soc. Jpn. 46 (2003) 294.

[7] J.E. Demuth, D. Schmeisser, P. Avouris, Phys. Rev. Lett.47 (1981) 1166.

[8] L. Sanche, M. Michaud, Phys. Rev. B 30 (1984) 6078.[9] M. Sakurai, T. Okano, Y. Tuzi, J. Vac. Sci. Technol. A 5(1987) 431.

[10] K. Jacobi, M. Bertolo, Phys. Rev. B 42 (1990) 3733.[11] R.E. Palmer, P.J. Rous, Rev. Mod. Phys. 64 (1992)

383.[12] M. Sakurai, T. Okano, Y. Tuzi, Vacuum 41 (1990) 234.[13] K. Watanabe, Y. Matsumoto, Surf. Sci. 454–456 (2000)

262.[14] M. Sakurai, T. Okano, Y. Tuzi, Jpn. J. Appl. Phys. 26

(1987) L1651.[15] M. Sakurai, Thesis, University of Tokyo, 1988 (in

Japanese).

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Dynamics of physisorbed monolayer of methane on Ag(1 1 1)