Chapter 4. Chemical Reaction Dynamics at Surfaces

Chapter 4. Chemical Reaction Dynamics at Surfaces

Chapter 4. Chemical Reaction Dynamics at SurfacesAcademic and Research Staff

Professor Sylvia T. Ceyer, Dr. Kenneth B. Laughlin, Dr. Kevin J. Maynard

Graduate StudentsJohn D. Beckerle, David J. Gladstone,Schulberg, Qingyun Yang

Andrew D. Johnson, Marianne McGonigal, Michelle

Undergraduate StudentGerald Cain

4.1 Chemical ReactionDynamics on SemiconductorSurfaces

Sponsor

Joint Services Electronics ProgramContract DAAL03-89-C-0001

Project Staff

Professor Sylvia T. Ceyer, Dr. KennethDavid J. Gladstone, MarianneMichelle Schulberg, Gerald Cain

B. Laughlin,McGonigal,

The etching of semiconductor materials inhalocarbon plasma environments is acomplex chemical process. The plasma isused to produce species that are highly reac-tive with the semiconductor surface.Because many different reactive species,including radicals, ions, and highly excitedneutral molecules are produced in a plasma,the mechanism of the etching reaction is dif-ficult to probe. Our program is aimed atusing molecular beam reactive scatteringtechniques to systematically examine the roleof each of these species in the etchingprocess.

The initial goal of this project was tomeasure the barrier to F2 dissociative chem-isorption on Si(100). Contrary to popularbelief, we found essentially no barrier to thedissociation of F2 on the unfluorinatedsurface. The dissociative chemisorption of F2on Si(100) proceeds with unit probability fortranslational energies of the incident F2 mole-cules as low as 0.5 kcal/mol. However, thereis a substantial barrier to dissociation abovefluorine coverages of 0.5 monolayer. Higher

fluorine coverages sufficient to produce theetch product SiF4 require a higher transla-tional energy of the F2 molecule. We haveobserved efficient etching of Si(100) at 300K for an incident kinetic energy of F2 Of 15kcal/mol. The angular and translationalenergy distributions of the SiF4 - as meas-ured in our newly constructed scatteringapparatus consisting of two beams incidenton a surface and a rotatable, triplydifferentially pumped quadrupole mass spec-trometer - indicate that the last step of thereaction is the thermal desorption of theproduct SiF4 molecule.

We have established that silicon can beetched without the use of molecules incidentwith energies of hundreds of electron volts,such as those present in plasma etchingenvironments. Molecular beam techniquesutilize molecules with energies only slightlyhigher than thermal (<1 eV), and thereforedo not introduce radiation damage or defectsinto the Si lattice, which is a typical result ofplasma etching. We are continuing our workon this system to understand the lack of F2energy dependence on the formation of asecond reaction product, SiF2.

We have also recently observed a new kindof mechanism for dissociative chemisorption.In the limit of zero fluorine coverage, weobserved that the Si surface strips one of thefluorine atoms from the incident F2 molecule,leaving the other fluorine atom to scatter intothe gas phase. Although this strippingreaction is analogous to the well-knownstripping reactions in gas phase chemicalreaction dynamics, this is the first observa-tion of its kind from a surface.


Publications

Ceyer, S.T., D.J. Gladstone, M. McGonigaland M.T. Schulberg, "Molecular Beams:Probes of the Dynamics of Reactions onSurfaces." in Physical Methods of Chem-istry. 2nd ed. Eds. B.W. Rossiter, J.F.Hamilton amd R.C. Baetzold. New York:Wiley, 1990. In press.

Gladstone, D.J., M.T. Schulberg, K.B.Laughlin, M. McGonigal and S.T. Ceyer."Design of a Power Supply for ResistiveHeating of Semiconductor Crystals."

Gladstone, D.J. Reaction Dynamics ofFluorine with Silicon (100): Design of aMolecular Beam Surface Reactive Scat-tering Chamber. Ph.D. diss. Departmentof Chemistry, MIT, 1989.

Laughlin, K.B., D.J. Gladstone, M.McGonigal and S.T. Ceyer. "Dynamics ofthe Reaction of F2 with Si(100)." Paperpresented at the Dynamics of Gas-SurfaceInteractions, Gordon Conference, August,1989.

McGonigal, M., M.T. Schulberg, D.J.Gladstone, K.B. Laughlin, S.T. Ceyer."Reactions of F2 with Si(100)." Paper pre-sented at Materials Research Society,Boston, December, 1989.

McGonigal, M. Reactive Chemisorption ofMolecular Fluorine on Si(100). Ph.D.diss. Department of Chemistry, MIT, 1989.

Schulberg, M.T., M. McGonigal, D.J.Gladstone, K.B. Laughlin, S.T. Ceyer."The Etching of Si(100) with a MolecularBeam of F2." Paper presented at theAmerican Vacuum Society, Boston,October, 1989.

4.2 Collision InducedDissociative Chemisorption ofCH4 on Ni(111) by Inert GasAtoms: The Mechanism forChemistry with a Hammer

Sponsors

MIT Energy Laboratory - Synthetic Fuels CenterNational Science Foundation

Grant CHE 85-08734Petroleum Research Fund

Contract 19014-AC5

Project Staff

Professor Sylvia T. Ceyer, John D. Beckerle,Andrew D. Johnson, Qingyun Yang

We observed that the dissociation of CH4physisorbed on Ni(111) at 46 K is inducedby the impact of incident inert gas atoms.We studied the dynamics and mechanism ofthis new process - collision induceddissociative chemisorption - using molecularbeam techniques coupled with ultrahighvacuum electron spectroscopies. The abso-lute cross section for collision induced disso-ciation is measured over a wide range ofkinetic energies (28-109 kcal/mol) and inci-dent angles of Ne, Ar and Kr atom beams.The cross section displays a complexdependence on the energy of the impinginginert gas atom, characteristic of neither totalnor normal energy scaling. Quantitativereproduction of the complex dependence ofthe cross section on the Ar and Ne incidentenergy by a two-step, dynamical modelestablishes the mechanism for collisioninduced dissociation.

Collision induced dissociation occurs by theimpulsive transfer of kinetic energy upon col-lision of Ar or Ne with CH4, followed by thetranslationally activated dissociative chem-isorption of the CH4 upon its subsequent col-lision with the Ni surface. The dependenceof the probability of activated dissociation onthe resultant CH4 normal energy, derivedfrom the fit of the model to the experimentalcross section, is in excellent agreement withthe results of a previous study of thetranslationally activated dissociative chem-isorption of CH4 on Ni(111). We haveshown that collision induced activation andtranslational activation are consistent

36 RLE Progress Report Number 132

mechanisms for providing energy to CH4,surmounting the barrier to dissociative chem-isorption.

Publications

Beckerle, J.D., A.D. Johnson, Q.Y. Yang andS.T. Ceyer. "Collision Induced Dissocia-tive Chemisorption of CH4 on Ni(111) byInert Gas Atoms: The Mechanism forChemistry with a Hammer." J. Chem.Phys. 91: 5756 (1989).

Ceyer, S.T. "Translational and CollisionInduced Activation of CH4 on Ni(111):Phenomena Connecting UHV SurfaceScience to High Pressure HeterogeneousCatalysis." Langmuir 6:82 (1990).

Ceyer, S.T. "The Mechanism for CH4 Disso-ciation and the Synthesis of C6 H6 fromCH4 on Ni(111)." Paper presented at thePacifichem '89 Conference, AmericanChemical Society, Honolulu, Hawaii,December, 1989.

Ceyer, S.T. "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers."Paper presented at the National Sympo-sium on Frontiers of Science, NationalAcademy of Science, Irvine, CA March,1989.

Ceyer, S.T. "Dynamicstive ChemisorptionDesorption of CH4presented at theAmerican ChemicaDallas, April, 1989.

of Activated Dissocia-and Collision Inducedon Ni(111)." PaperDebye Symposium,

il Society Meeting,

Ceyer, S.T. "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers."Paper presented at the Ninth InternationalSummer Institute in Surface Science, Uni-versity of Wisconsin at Milwaukee,August, 1989.

Ceyer, S.T. "Dynamics of Collision InducedDissociation and Desorption of CH4 onNi(111)." American Institute Chem. Eng.Meeting, San Francisco, November, 1989.


4.3 Collision InducedDesorption

SponsorsMIT Energy Laboratory - Synthetic Fuels CenterNational Science Foundation


Contract 19014-AC5

Project Staff

Professor Sylvia T. Ceyer, JohnAndrew D. Johnson, Qingyun Yang

D. Beckerle,

The desorption of CH4 physisorbed onNi(111) is observed to be induced by colli-sion with Ar atoms incident with energiesless than 2 eV. The absolute cross sectionfor collision induced desorption is measuredas a function of the kinetic energy and inci-dent angle of the Ar beam. The mechanismfor desorption is shown to involve a directand impulsive, bimolecular collision betweenAr and CH4. Molecular dynamics simulationsshow that the energy and incident angledependence of the desorption cross sectionare the consequence of two competingdynamical effects.

Publications

Beckerle, J.D., A.D. Johnson and S.T. Ceyer,"Observation and Mechanism of CollisionInduced Desorption: CH4 on Ni(111),"Phys. Rev. Lett. 62: 685 (1989).

Ceyer, S.T. "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers."Paper presented at the National Sympo-sium on Frontiers of Science, NationalAcademy of Science, Irvine, CA, March,1989.

Ceyer, S.T. "Dynamics of Activated Disso-ciative Chemisorption and CollisionInduced Desorption of CH4 on Ni(111)."Paper presented at the Debye Symposium,American Chemical Society Meeting,Dallas, April, 1989.

Ceyer, S.T. "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers."Paper presented at the Ninth InternationalSummer Institute in Surface Science, Uni-


versity of Wisconsin at Milwaukee,August, 1989.

Ceyer, S.T. "Dynamics of Collision InducedDissociation and Desorption of CH4 onNi(111)." American Institute Chem. Eng.Meeting, San Francisco, November, 1989.

Ceyer, S.T. "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers."Paper presented at Workshop onDynamics of Surface Reactions,Copenhagen, Denmark, May, 1989.

4.4 Synthesis of Benzene fromMethane

Sponsors



Contract 19014-AC5

Project Staff

Professor Sylvia T. Ceyer, Dr. KevinAndrew D. Johnson, Qingyun Yang

Maynard,

With the knowledge gained from our studiesof methane activation about the microscopicorigins for the pressure gap, we have beenable to develop a scheme for bypassing thehigh pressure requirement. This schemeinvolves raising the energy of the incidentmolecule or collisionally inducing dissocia-tion. In this way, we have been able toperform a high pressure reaction at low pres-sure: the synthesis of C6H6 from CH4. Inaddition, because this reaction is carried outat low pressure, we have been able to iden-tify the adsorbed intermediates by high resol-ution electron energy loss spectroscopy andto determine the mechanism of this reaction.

The synthesis is effected by exposing a mon-olayer of CH4 physisorbed on Ni(111) at 47K to a beam of Kr atoms. The collision of theincident Kr with the physisorbed CH4 distortsthe CH4 from its tetrahedral configuration,thereby lowering the barrier to dissociationinto an adsorbed methyl radical and anadsorbed hydrogen atom. As the surfacetemperature is raised to 230 K, all the

adsorbed CH3 dissociates to CH and the CHrecombines to form adsorbed C2H2 . Some ofthe C2H2 trimerizes to adsorbed C6H 6 and at410 K and 425 K, respectively, the atomicallyadsorbed hydrogen desorbs as H2 and someof the chemisorbed C6 H6 desorbs. The gasphase benzene is detected mass spectrome-trically in a thermal desorption experiment.Although the maximum thermal desorptionyield for benzene is 1.5 percent at a crystalheating rate of 17 K/S, the gas phasehydrocarbon selectivity of this synthesis forbenzene production is 100 percent. This isthe first observation of a reaction of CH4 toform a gas phase, higher hydrocarbon over ametallic catalyst at the low pressurescommensurate with a UHV environment.

Publications

Ceyer, S.T. "Bridging the Gap BetweenSurface Science and High Pressure Proc-esses." Langmuir Award Lecture presentedat the American Chemical Society, Miami,Florida, September, 1989.

Maynard, K.J., Q.Y. Yang, A.D. Johnson andS.T. Ceyer, "The Synthesis of C6 H6 fromCH 4." Paper presented at the AmericanVacuum Society, Boston, October, 1989.

Yang, Q.Y., A.D. Johnson, K.J. Maynard andS.T. Ceyer. "Synthesis of Benzene fromMethane over a Ni(111) Catalyst." J. Am.Chem. Soc. 111: 8748 (1989).

4.5 The Structure andChemistry of CH3 and CHRadicals and Isotopic VariantsAdsorbed on Ni(111)

Sponsors



Contract 19014-AC5

Project Staff

Professor Sylvia T. Ceyer, Qingyun Yang, AndrewD. Johnson, Dr. Kevin Maynard


We present a detailed analysis of thevibrational spectra of CH3 , CH2D and CD3adsorbed on Ni(111) and of the products ofits reactions. Molecular beam techniqueseffect the synthesis of adsorbed methyl radi-cals from CH4 , CH3D or CD4. Because wecan measure these spectra at both higherresolution (30 cm-') and higher sensitivity(5 x 106 counts/sec), we have been able toobserve both new features and also to carryout a symmetry analysis. The CH3 radical isshown to be adsorbed with C3v symmetry ona threefold hollow site. The symmetric C-Hstretch mode of CH3 and the overtone of theasymmetric deformation mode are shown tobe in Fermi resonance. At temperaturesabove 150 K, CH3 dissociates to formadsorbed CH.

Confirmation for the assignment of the spec-trum to a CH species is found in the spec-trum measured after thermal decompositionof CH2D. This species decomposes to amixture of adsorbed CH and CD. The fre-quencies of the C-H and C-D stretch modesare identical to the frequencies of thesespecies produced from the decomposition ofCH3 and CD3. The adsorption site of the CHspecies is determined to be a threefoldhollow site and the geometry of the Ni3-C-His determined to be pyramidal. At temper-atures above 230 K, carbon-carbon bond for-mation between the CH species is observedto yield C2H2. Low coverages of C2H 2 areshown conclusively to dissociate to C2H andC2 species at 400 K in contrast to a literaturereport of C2H2 dissociation to adsorbed CH.At temperatures above 380 K, high cover-ages of C2H2 result in trimerization toadsorbed benzene. The relative stabilities ofthe C, species on Ni(111) are determined tobe CH2 < CH3 < CH.

Publications

Yang, Q.Y. The Chemistry of CH4 onNi(111). Ph.D. diss. Dept. of Chemistry,MIT, 1989.


4.6 High Resolution ElectronEnergy Loss Spectroscopy of Hon Ni(111)

Sponsors



Contract 19014-AC5

Project StaffProfessor Sylvia T. Ceyer, Qingyun Yang, AndrewD. Johnson, Dr. Kevin Maynard

A high resolution electron energy loss spec-trum of half of a monolayer of H(D)adsorbed on Ni(111) has been measuredwith sufficient intensity and resolution toallow seven vibrational loss features to beobserved: Vasy = 745, 790 cm-'; sym = 1085,1105 cm-'; 2Vasy = 1260, 1400 cm-';2vsym = 2180 cm-'. The assignments of thefundamentals are based on the measuredanharmonicity and angular distributions ofthe vibrational loss features.

The splitting of the vasy and Vsym modes isexplained by the local potential difference ofH adsorbed on the fcc and hcp threefoldsites. The absence of dispersion of thesemodes, as evidenced by measurements of thespectra at I of the Brillouin zone, indicatethat dynamical coupling between thehydrogen atoms on neighboring sites is notoperative to within the resolution of thisexperiment.

The splitting of the overtone of the asym-metric Ni-H stretch mode is due to a transi-tion of each of the two different hydrogenatoms to the second vibrationally excitedstate. The observation of two distinctvibrational frequencies corresponding to thehydrogen atoms adsorbed on two threefoldsites, whose structures differ only in thepresence of a Ni atom in the second layer,confirms the previously proposed structurefor the (2x2)2H overlayer. Since this is thefirst observation of any experimentally deter-mined distinction in the physical propertiesof hcp and fcc threefold sites on any metalsurface, these results provide a critical test oftheory.


Publications

Yang, Q.Y., A.D. Johnson and S.T. Ceyer,"High Resolution Electron Energy LossSpectroscopy of H on Ni(111)." Paperpresented at American Vacuum Society,Boston, October, 1989.

Yang, Q.Y. The Chemistry of CH4 onNi(111). Ph.D. diss. Dept. of Chemistry,MIT, 1989.

Professor Sylvia T. Ceyer with the molecular beam surface scattering apparatus.


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Chapter 4. Chemical Reaction Dynamics at Surfaces