Surface reactivity of highly vibrationally excited molecules prepared by pulsed laserexcitation: CH 4 (2ν 3 ) on Ni(100)Mathieu P. Schmid, Plinio Maroni, Rainer D. Beck, and Thomas R. Rizzo Citation: The Journal of Chemical Physics 117, 8603 (2002); doi: 10.1063/1.1519860 View online: http://dx.doi.org/10.1063/1.1519860 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/117/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Alignment dependent chemisorption of vibrationally excited CH4(ν3) on Ni(100), Ni(110), and Ni(111) J. Chem. Phys. 135, 224703 (2011); 10.1063/1.3665136 Dissociative chemisorption of methane on Ni(100): Threshold energy from CH 4 (2ν 3 ) eigenstate-resolvedsticking measurements J. Chem. Phys. 119, 6407 (2003); 10.1063/1.1613935 Assessing a microcanonical theory of gas-surface reactivity: Applicability to thermal equilibrium, nonequilibrium,and eigenstate-resolved dissociation of methane on Ni(100) J. Chem. Phys. 118, 9762 (2003); 10.1063/1.1570393 Semirigid vibrating rotor target model for CH 4 dissociation on a Ni(111) surface J. Chem. Phys. 117, 7698 (2002); 10.1063/1.1509062 Reactivity of vibrationally excited methane on nickel surfaces J. Chem. Phys. 115, 5611 (2001); 10.1063/1.1398075

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Surface reactivity of highly vibrationally excited molecules preparedby pulsed laser excitation: CH 4 „2n3… on Ni „100…

Mathieu P. Schmid, Plinio Maroni, Rainer D. Beck,a) and Thomas R. RizzoLaboratoire Chimie Physique Mole´culaire, Ecole Polytechnique Fe´derale de Lausanne, CH-1015 Lausanne,Switzerland

~Received 8 August 2002; accepted 17 September 2002!

We report state resolved sticking coefficients for highly vibrationally excited CH4 on Ni~100! atwell-defined kinetic energies in the range of 12–72 kJ/mol. Incident methane molecules areprepared by pulsed laser radiation in single rovibrational levels of the first overtone of theantisymmetric stretch (2n3) at 6004.69 cm21 and collided at normal incidence with a clean Ni~100!single crystal. We find that the vibrational excitation enhances the reaction probability by a factor100 at an incident translational energy of 72 kJ/mol, but this enhancement increases to more than 4orders of magnitude at low kinetic energy. Despite this large increase in the sticking coefficient,vibrational energy in 2n3 appears to be about 80% as effective as an equivalent amount oftranslational energy in promoting the chemisorption reaction. ©2002 American Institute ofPhysics. @DOI: 10.1063/1.1519860#

INTRODUCTION

Quantum state resolved measurements of reaction crosssections are central to understanding chemical reaction dy-namics at the most detailed level. While such experimentsare common for unimolecular and bimolecular reactions inthe gas phase, measurements of state resolved sticking coef-ficients for gas–surface reactions have largely been limitedto H2 and its isotopomers.1,2 Recently, laser preparation ofvibrationally excited methane in molecular beams has en-abled state specific studies of methane chemisorption,3,4 areaction that has long served as a prototype system for theactivated dissociative chemisorption of a polyatomicmolecule.5–12 Juurlink et al.13 have reported rovibrationalstate resolved sticking coefficientsS0(n3 , J, Et) of methaneon Ni~100! and Higginset al. have measured the stickingcoefficient for CH4 exited to the first overtone 2n3 onPt~111! at a kinetic energy of 5.4 kJ/mol4. In contrast tothese experiments, which used continuous laser excitationand continuous molecular beams, we prepare vibrationallyexcited methane for state specific measurements by pulsedlaser radiation. Pulsed excitation provides access to overtoneand combination bands at higher vibrational energy as wellas to Raman active vibrational statesvia stimulated Ramanscattering. We present here the first measurements of thestate-resolved sticking coefficientsS0(2n3 , J52, Et) ofCH4 on Ni~100!.

EXPERIMENT

Our state resolved sticking coefficient measurements areperformed in a molecular beam/surface science apparatus de-

signed to study the surface interactions of laser-excited mol-ecules. We summarize the most important features here andgive a detailed description in a forthcoming publication.14

The apparatus consists of a triply differentially pumped mo-lecular beam source connected to an ultra-high vacuum sur-face science chamber~base pressure 6310211 mbar). Wegenerate a pulsed molecular beam at 20 Hz by expandingmixtures of methane~99.9995% purity! and hydrogen~99.9999% purity! through a temperature-controlled solenoidvalve with an open time of 300ms. The beam pulses passthrough a 1 mmdiameter skimmer into the second differen-tial pumping region where their duration is reduced to 40msby a chopper wheel rotating at 200 Hz. After a further dif-ferential pumping stage, the beam pulses enter the UHVchamber through a 1 mmdiameter aperture, traverse a laserbeam alignment tool and impinge at normal incidence onto asingle crystal surface. The alignment tool serves to overlapthe molecular beam with the laser beam, which is focused toa line by a cylindrical lens and crosses at 90°. The length ofthe line focus and the duration of the gas pulses are matchedso that depending on the speed of the beam, between 33%and 100% of the methane molecules are exposed to the ex-citation laser. We produce infrared laser pulses, tunable in thevicinity of 1.7 mm (6000 cm21), by generating the differ-ence frequency between the fundamental output of an injec-tion seeded, single mode Nd-YAG laser and a narrow banddye laser in a lithium niobate crystal. We amplify the result-ing 1–2 mJ pulses of tunable IR to 140 mJ/pulse in a twostage optical parametric amplifier using KTP crystals. To en-sure that the infrared laser is tuned into resonance with theselected rovibrational transition of methane during deposi-tion, we split off a small portion of the IR beam and docavity ring-down spectroscopy~CRDS! in a separate expan-sion chamber. We also use the CRDS spectra of CH4 in the

a!Author to whom correspondence should be addressed. Electronic mail,Rainer.Beck@epfl.ch

JOURNAL OF CHEMICAL PHYSICS VOLUME 117, NUMBER 19 15 NOVEMBER 2002

86030021-9606/2002/117(19)/8603/4/$19.00 © 2002 American Institute of Physics

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jet expansion to obtain information on the rotational levelpopulation needed for the sticking coefficient determination.

The molecular beam pulses impinge on a 10 mm diam-eter Ni~100! single crystal surface oriented to within 0.1°.The surface is cleaned in the UHV chamber by Ar1

sputtering/annealing cycles followed by oxidation and reduc-tion treatments until impurities can no longer be detected byAuger electron spectroscopy~AES!. We measure the transla-tional energy,Et , of the methane molecules in the beam bytime-of-flight using the chopper wheel and an on-axis quad-rupole mass spectrometer. We adjustEt by varying the seedratio of CH4 in H2 and by small changes in the nozzle tem-perature. To determine absolute sticking coefficients formethane, we perform a timed exposure of the clean Ni~100!crystal to the molecular beam, measuring both the incidentdose of CH4 molecules per unit area as well as the resultingsurface density of carbon reaction products. Initial stickingcoefficientsS0 are calculated as the ratio between the surfacedensity of carbon and the incident dose per unit area in thelimit of low coverage~5%–10% ML!. We determine the fluxof methane@molecules s21 cm22# incident on the surfacefrom the methane partial pressure rise due to the molecularbeam monitored by a calibrated quadrupole mass spectrom-eter together with the beam cross section at the surface,which is measured by AES to be 0.03 cm2. The primaryreaction products of the direct methane chemisorption onNi~100! are adsorbed methyl and hydrogen according to15

CH4~g!→CH3~ads!1H~ads!.

At the surface temperature of 475 K, at which we performthe deposition, the methyl radicals quickly dehydrogenateand the hydrogen leaves the surface by recombinative de-sorption of H2 .16 The carbon atoms remain on the nickelsurface and are detected by AES. We repeat the depositionseveral times at different positions on the Ni~100! surface forimproved statistics of the sticking coefficient measurements.After the depositions are completed, we move the surface infront of the CMA Auger system and quantify the C coverage,

recording C and Ni Auger signals at typically 80 pointsacross the surface in an automated computer controlled scan.

Figure 1 shows an example of such an Auger line scanacross the Ni~100! surface on which two depositions wereperformed with~left! and without~right! laser excitation. Forthe laser-on deposition,;14% of the incident CH4 mol-ecules were excited with two quanta of asymmetric C–Hstretch excitation via the 2n3R(1) transition at6026.21 cm21. The two ‘‘carbon spots’’ produced by local-ized carbon deposition from the incident molecular beamshow the footprint of the molecular beam on the Ni~100!crystal surface. The observed C/Ni Auger signal ratio~left-hand scale! is calibrated in terms of C coverage~right-handscale! using ethylene exposures which lead to a known satu-ration coverage of 0.5 ML carbon on the Ni~100! surface.17

A comparison of the carbon spots, normalized to theincident dose of methane, shows the increase in the stickingcoefficient averaged over all states populated with and with-out laser excitation,S0

laser-on and S0laser-off, respectively. As

Juurlink et al. have shown,3 the state-resolved sticking coef-ficient can be calculated from the difference betweenlaser-on and laser-off measurements, taking into account thefractional populationf exc of the laser-excited state and add-ing the sticking coefficient for the vibrational ground statev50:

S02n35

S0laser-on2S0

laser-off

f exc1S0

v50. ~1!

We determinef exc experimentally from the laser power de-pendence of the carbon coverage. The power dependence isfitted to the functional form describing the saturation of theselected transition and the excited fraction is determined as afitting parameter. Both the fit of the experimental data andcalculations using the known Einstein coefficient for themethane overtone transition18 confirm that we can com-pletely saturate the overtone transition and transfer close to62.5% of the irradiated methane molecules fromv850, J851 to the excited state withv352 andJ52.

FIG. 1. Auger analysis of two carbon spots on the Ni~100! surface, resultingfrom local carbon deposition due to chemisorption of CH4 on a molecularbeam with 52 kJ/mol of incident kinetic energy. For the laser-on spot~left!14% of the incident CH4 molecules were prepared with 6004.69 cm21 ofvibrational energy by pulsed laser excitation via the 2n3R(1) transition. Forthe laser-off spot~right!, a 20-time-higher incident dose of methane wasdeposited to produce a detectable carbon spot.

FIG. 2. Sticking coefficients as a function of incident kinetic energy~normalincidence! for dissociative chemisorption of CH4 on Ni~100!. ~Squares!Laser-off data giving an upper limit forS0(v50); ~Circles! State resolvedsticking coefficients for 2n3 , J52. Dashed lines show fitted sticking func-tions using a model described in the text. Error bars are 95% confidence ofconvolute uncertainties.

8604 J. Chem. Phys., Vol. 117, No. 19, 15 November 2002 Schmid et al.

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RESULTS AND DISCUSSION

Figure 2 shows our experimental results for the state-resolved sticking coefficientS0

2n3 and the laser-off dataS0

laser-offas function of incident translation energyEt ~for nor-mal incidence!. We take the dataS0

laser-off as a close upperlimit for S0

v50 , based on the absence of any observed nozzletemperature dependence ofS0

laser-off in the temperature range~375–425 K! of our experiments. We find a dramatic in-crease in the sticking coefficient upon 2n3 excitation thatdecreases with increasingEt . At the lowest kinetic energyfor which we measuredS0

2n3 ~12 kJ/mol!, the laser-off stick-ing coefficientS0

laser-off is too small to be measured accuratelyin our experiment. From an extrapolation based on an expo-nential dependence ofS0

laser-off on the normal kineticenergy,3,7 we estimateS0

laser-off ~12 kJ/mol! to be of the orderof 231028. Our measured value forS0

2n3 ~12 kJ/mol! of 531024 implies an enhancement of more than 4 orders ofmagnitude upon excitation of 2n3 . This enhancement factordecreases to 100 at 72 kJ/mol. Figure 2 also shows that atenergies above 60 kJ/mol,S0

2n3 appears to saturate at a valueof a few percent, indicating that even at high kinetic energyonly a small fraction of the vibrationally excited CH4 mol-ecules undergo chemisorption.

In comparing the relative efficacy of vibrational andtranslational energy, one must take into account that as thetotal energy (translational1 internal) approaches and exceedsthe average barrier height, the reaction probability is ex-pected to level off. This causes the observed vibrational en-hancement to be a function of the incident translational en-ergy. The translational energy dependence of the stickingcoefficient has been modeled by several authors usingS-shaped sticking functions.1,2,7 Luntz has recently discussedthe physical origin of the sticking curves in terms of a dis-tribution of barrier heights reflecting the multidimensionalityof the dissociative chemisorption for polyatomicmolecules.19 We can compare the effect of translational andvibrational energy on the sticking coefficient by fitting stateresolved sticking functions to our experimental data. We usethe analytical form proposed by Luntz,

S0~E,n!5A~n!

2 S 11erfFE2E0~n!

W~n! G D , ~2!

which contains the parametersA(n), E0(n), andW(n) de-scribing the asymptotic value ofS0(n) at high translationalenergy, the centerpoint energy whereS0(n) has risen to 50%of the asymptote, and the steepness of the rise, respectively.We first fit our laser-off data to determineA(0), E0(0), andW(0). Then we fit theS0

2n3 data, settingA(2n3)5A(0),W(2n3)5W(0) and determineE0(2n3) by the fit. The re-sults of these fits to our data are shown in Fig. 2 as dashedlines. Although not all parameters are uniquely determinedby the fit, the difference between the centerpoint energiesE0(n50)2E0(2n3) depends only weakly on the choice ofA(0) andW(0), indicating that it allows a meaningful com-parison between translational and vibrational activation inthe form of a vibrational efficacy defined by

bv~2n3!5E0~v50!2E0~2n3!

h•2n3. ~3!

From our fit we obtainbv(2n3)'0.860.1, indicatingthat over the range of kinetic energies studied here, 2n3 vi-brational excitation is about 80% as efficient as translation inpromoting the chemisorption reaction.

It is of interest to compare our results forS0(2n3) withdata forS0(n3) obtained by Utzet al.20 for the same systemCH4 /Ni(100). They also observe a significant vibrationalenhancement of the sticking coefficient uponn3 excitation.The comparison of our data sets20 shows that the vibrationalenhancement in the sticking coefficient is larger for the firstquantumn3 of excitation than for the second, yielding avibrational efficacybv(n3) for the n3 fundamental of closeto unity20 compared to our value of 0.8 forbv(2n3). Theobserved decrease in vibrational efficacy for 2n3 comparedto n3 is in agreement with calculations of Carre´ andJackson12 where CH4 is modeled as quasidiatomic CH3– H.In their study of CH4(2n3) with Pt~111!, Higginset al.4 havemeasured a vibrational efficacy of 40% at a translational en-ergy of 5.4 kJ/mol and pointed out that a value different fromone is inconsistent with a purely statistical reactionmechanism.21 Our b value of 0.8 suggests that this is also thecase for reaction on the Ni~100! surface.

In the context of the comparison betweenbv(n3) andbv(2n3), it should be pointed out that the normal modes inmethane are coupled by anharmonic stretch–bend reso-nances and the vibrational eigenstates can be grouped intopolyads with a polyad quantum numberN5vb12vs . Thestate at 6004.69 cm21 which we prepare in our experimentsand label as 2n3 belongs to the 14 vibrational states of theN54 tetradecad. The widely used label 2n3 for this state isdue to the largest normal mode contribution when the wavefunction is written in a normal mode basis. Calculations bySibertet al.22 of vibrational states of CH4 up to 9000 cm21

obtain the leading coefficients for the 6004.69 cm21 state as73% 2n3 and 15% n11n3 . In comparison, the3019.49 cm21 state which is prepared in the experiments byUtz et al. is part of theN52 polyad~pentad! and contains96% n3 and 3%n21n4 . Because the states assigned ton3

and 2n3 involve slightly different normal mode composition,the observed difference inbv(n3) andbv(2n3) may includea contribution due to mode specificity. Calculations of Milotet al.23 and Halonenet al.24 suggest mode-specific vibra-tional efficacies for CH4 on Ni and predict bv(n1).bv(n3).bv(n2)'bv(n4). A more detailed discussion ofvibrational efficacies will be presented together with stickingcoefficient measurements for different vibrational states ofCH4 on Ni~100! in a forthcoming publication.25

SUMMARY AND CONCLUSIONS

In summary, we present the first state resolved stickingcoefficient measurements for CH4 excited to the 2n3 state onNi~100!. We find that vibrational energy placed in the over-tone of the antisymmetric C–H stretch dramatically enhancesthe sticking coefficient in agreement with earlier experimen-tal results forS0(n3) ~Refs. 3, 13! confirming theoreticalpredictions that the reaction coordinate contains a significant

8605J. Chem. Phys., Vol. 117, No. 19, 15 November 2002 Reactivity of vibrationally excited CH4 on NI(100)

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contribution due to C–H stretch motion.11 Our results alsoshow that even at high kinetic energy only a few percent ofthe 2n3 excited CH4 molecules react with the Ni~100! sur-face and that the majority appear to scatter without undergo-ing chemisorption. This low reactivity may be explained by anarrow range of approach orientations and/or reactive sur-face sites on Ni~100! for CH4 excited to 2n3 and could beinvestigated in detail in simulations of the reactive scatteringof vibrationally excited CH4 with the Ni~100!, where it ispossible to follow individual trajectories and analyze the ef-fect of orientation and impact site on the reactionprobability.11,12,23

ACKNOWLEDGMENTS

The authors would like to thank Professor A. L. Utz forvaluable discussions. Financial support by the Swiss Na-tional Science Foundation through Grant Nos. 2063591.00and 2000-59261.99 and the EPFL are gratefully acknowl-edged.

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8606 J. Chem. Phys., Vol. 117, No. 19, 15 November 2002 Schmid et al.

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