Contains reports on three research projects.Joint Services Electronics Program (Contract DAAL03-86-K-0002)Joint Services Electronics Program (Contract DAAL03-89-C-0001)National Science Foundation (Grant CHE 85-08734)MIT Energy Laboratory - Synthetic Fuels CenterPetroleum Research Fund (Contract 19014-AC5

Beckerle, John D.

Ceyer, Sylvia T.

Gladstone, David J.

Johnson, Andrew D.

Laughlin, Kenneth B.

McGonigal, Marianne

Schulberg, Michelle T.

Yang, Qingyun

DSpace@MIT

Chapter 4. Chemical Reaction Dynamics at SurfacesChapter 4. Chemical Reaction Dynamics at SurfacesAcademic and Research StaffProfessor Sylvia T. Ceyer, Dr. Kenneth B. LaughlinGraduate StudentsJohn D. Beckerle, David J. Gladstone, Andrew D. Johnson, Marianne McGonigal, MichelleSchulberg, Qingyun Yang4.1 Chemical ReactionDynamics on SemiconductorSurfacesSponsorJoint Services Electronics Program(Contracts DAAL03-86-K-0002and DAAL03-89-C-0001)Project StaffProfessor Sylvia T. Ceyer, Dr. Kenneth B.Laughlin, David J. Gladstone, MarianneMcGonigal, Michelle SchulbergThe 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 in this environment. Ourprogram is aimed at systematically examiningthe role of each of these species in theetching process with molecular beam reactivescattering techniques.While investigating the role of translationallyexcited F2 molecules in the etching ofSi(100), we were surprised to find that, evenwith F2 translational energies as low as 0.5kcal/mole, F2 reacts with a clean Si(100)with unit probability to form a fluorinatedlayer. However, its reactivity stops at fluorinecoverages above approximately a half of amonolayer. The translational energy of the F2molecule must be raised by molecular beamtechniques to achieve further fluorination ofthe surface to provide enough fluorine toproduce the etch product SiF 4. Prior to thisdiscovery, F2 had never been observed toetch silicon. Therefore, we have shown thatsilicon can be efficiently etched by molecularbeam techniques without using a highenergy plasma. Unlike plasma techniques,the molecular beam techniques do not causeradiation damage or introduce defects intothe silicon.4.2 Collision InducedDissociative Chemisorption:Chemistry with a HammerSponsorsNational Science Foundation(Grant CHE 85-08734)MIT Energy Laboratory -Synthetic Fuels CenterPetroleum Research Fund(Contract 19014-AC5)Project StaffProfessor Sylvia T. Ceyer, John D. Beckerle,Andrew D. Johnson, Qingyun YangWe have now completed thorough investi-gations of our newly discovered mechanismfor dissociative chemisorption - collisioninduced dissociative chemisorption. In thisprocess, the dissociation of CH4, molecularlyadsorbed on a Ni(111) surface at 46 K, isinduced by the impact of incident Ar atoms.The impact of the Ar atoms deforms thephysisorbed CH4 into the configuration forthe transition state that leads to dissociation.Detailed measurements have shown that theabsolute cross section for collision induceddissociative chemisorption exhibits a compli-Chapter 4. Chemical Reaction Dynamics at Surfacescated dependence on the normal componentof the kinetic energy of the Ar atoms. Thestrict adherence to normal kinetic energyscaling observed in the case of translationalactivation of methane is not observed in col-lision induced activation. We have shownthat the breakdown in normal energy scalingin the Ar kinetic energy is expected becauseof the range of impact parameters whichcontribute to the cross section for dissocia-tion.For example, the Ar atom can collide withthe adsorbed methane molecule with animpact parameter as large as the hard spherecollision diameter or as small as zero in ahead-on collision. The effectiveness of thecollision of the Ar atom with the methane intransferring energy and thereby promotingdeformation that leads to dissociationdepends on the type of collision or impactparameter. A head-on collision transfersmuch more energy than a glancing collision.Once the collision has occurred, the Ar atomis reflected from the surface, and the rest ofthe CH4-surface collision is the same as inthe case of translational activation. Themethane molecule with its newly acquiredenergy is accelerated into the surface,deforms and dissociates. The importantpoint is that only the energy of the Ar atomthat is actually transferred in the normaldirection to the methane molecule leads toovercoming the barrier. It is the dependenceof the energy transfer on impact parameterthat results in the breakdown in normalenergy scaling in the Ar kinetic energy.This picture has been shown to be correct bycomparing the experimental cross section fordissociation with that calculated from theprobability for dissociation integrated overthe impact parameter. The dissociation prob-ability at the energy acquired by the methanemolecule after its collision with the Ar atomis obtained from the translational activationresults. The energy transferred in the normaldirection to CH4 by collision with Ar is cal-culated from a hard sphere collision model.The cross section calculated from this proce-dure yields the observed dependence on thenormal component of the Ar atom energy.Therefore, dissociation of methane aftertranslational activation, and after collisioninduced activation are completely consistentwith each other. They are simply differentmethods for providing the energy to deformthe molecule, but the mechanism for disso-ciation of the methane is the same.Schematic of the molecular beam-surface scattering apparatus showing two beams aimed at a Si-surface and thetriply differentially pumped, rotatable quadrupole mass spectrometer as a detector of the scattered reaction product.36 RLE Progress Report Number 1314.3 Collision InducedDesorptionSponsorsNational Science Foundation(Grant CHE 85-08734)MIT Energy Laboratory -Synthetic Fuels CenterPetroleum Research Fund(Contract 1 9014-AC5)Project StaffProfessor Sylvia T. Ceyer, John D. Beckerle,Andrew D. Johnson, Qingyun YangIn competition with dissociation induced bythe impact of the Ar atom, collision induceddesorption also occurs. That is, once the Aratom transfers energy to the methane, themethane molecule collides with the surfaceand can rebound into the gas phase if thesite on which it is physisorbed or if its orien-tation are not energetically favorable for dis-sociation. Desorption induced by collisionsis roughly an order of magnitude more prob-able than collision induced dissociation.The dynamics of collision induced desorptionare studied by measuring the desorptioncross section as a function of the energy andincident angle of the Ar atoms. The absolutemagnitudes of the cross sections are aboutan order of magnitude smaller than the hardsphere, gas kinetic cross section for collisionbetween Ar and CH4. The magnitudes of thedesorption cross sections measured in theisolated CH4 molecule limit are identical tothose measured at a saturated CH4 monolayercoverage. These two observations imply thatonly direct, small impact parameter collisionsof the Ar atom with the physisorbed methaneare responsible for the desorption event.The desorption cross section at an incidentenergy of 51.8 kcal mol - 1 is observed toapproximately double as the incident angle isincreased from 0 degrees to 70 degrees.However, the magnitude of the increase inthe cross section is dependent on and islargest for high total energies of the incidentAr atom. This is the result of two competingeffects. As the incident angle is increased,the normal component of the kinetic energy,Chapter 4. Chemical Reaction Dynamics at Surfaceswhich is the component responsible fordesorption, decreases. This should lead to adecrease in the desorption cross section asthe incident angle is increased because thereis less energy in the normal direction to pushthe molecule away from the surface.However, as the incident angle increases, thecollision or geometrical cross section of anincoming Ar atom with the adsorbed mole-cule increases, thereby countering thedecrease in the energy in the normal direc-tion. The complicated dependencies of thedesorption cross section on energy and inci-dent angle are the result of these competingfactors. We have carried out classical trajec-tory calculations to confirm this picture.Perhaps more important than the physicsbehind these processes is the fact that theyhave been observed and what the knowledgeof their existence means for understandingthe complex environmentheterogeneous catalysis.chemistry and desorptioimportant roles in highreactions because, under tcatalyst is covered with Eadsorbate covered catalyof high pressureCollision inducedn probably playpressure catalytichese conditions, aidsorbate and the'st is continuallybombarded by gas phase molecules.With the observation of these processes, nomechanism for a high pressure reaction cannow be considered complete without anassessment of the role of collision inducedchemistry and desorption as potential majorsteps. These observations are now cause forreexamination of the mechanisms of catalyticreactions in which inert gas effects onreaction rates have been noted. Collisioninduced chemistry and desorption are addi-tional contributors to the pressure gap in thereactivity of heterogeneous catalysis. Theyare additional reasons why surface chemistryat high pressures is often very different fromsurface chemistry at low pressures.PublicationsBeckerle, J.D., A.D. Johnson, and S.T. Ceyer,"Collision Induced Dissociation andDesorption: CH4 and CO and Ni(111)," J.Vac. Sci. Technol. A6:903 (1988).Beckerle, J.D., A.D. Johnson, and S.T. Ceyer,"Observation and Mechanism of CollisionChapter 4. Chemical Reaction Dynamics at SurfacesInduced Desorption: CH4 on Ni(111),"Phys. Rev. Lett. 62:685 (1989).Beckerle, J.D., A.D. Johnson, Q.Y. Yang, andS.T. Ceyer, "Collision Induced Dissocia-tion and Desorption: CH4 on Ni(111)." InSolvay Conference on Surface Science,ed. F.W. deWette, 109. Berlin: Springer-Verlag, 1988.Beckerle, J.D., Q.Y. Yang, A.D. Johnson, andS.T. Ceyer, "The Adsorption of CO and 02.on Ni(111) at 8 K," Surf. Sci. 195:77(1988).Ceyer, S.T., "Dissociative Chemisorption:Dynamics and Mechanisms," Annu. Rev.Phys. Chem. 39:479 (1988).Ceyer, S.T., D.J. Gladstone, M. McGonigal,and M.T. Schulberg, "Molecular Beamsas Probes of Dynamics of Reactions onSurfaces." In Physical Methods of Chem-istry, eds. B.W. Rossiter, J.F. Hamiltonand R. C. Baetzold. New York: Wiley,1989, in press.Ceyer, S.T., M.B. Lee, Q.Y. Yang, J.D.Beckerle, and A.D. Johnson, "The Mech-anism for the Dissociation of Methane ona Nickel Catalyst," In MethaneConversion, eds. D. Bibby, C. Chang, R.Howe, S. Yurchak, 51. Amsterdam:Elsevier Science, 1988.Simonson, R.J., J.R. Wang, and S.T. Ceyer,"Spectroscopic Studies of the Adsorptionof C2 H2 and C2 H4 on a Rare Earth SingleCrystal II. Oxidized Gd(0001)," J. Phys.Chem. 92:1271 (1988).Yang, Q.Y., and S.T. Ceyer, "The Chemistryand Stability of Adsorbed Methyl Radi-cals," J. Vac. Sci. Technol. A6:851(1988).Conference Papers PresentedCeyer, S.T., "Dynamics ofwith a Surface: SplatsInternational SolvaySurface Science, Austin,17, 1987.Sticky Collisionsand Hammers,"Conference onTexas, DecemberCeyer, S.T., "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers,"MicroProcess Conference, Tokyo, Japan,July 4, 1988.Ceyer, S.T., "Collision Induced Dissociationand Desorption of CH4 on Ni(111),"American Chemical Society, Los Angeles,California, September 25, 1988.Ceyer, S.T., "Collision Induced Desorption ofCH 4 from Ni(111)," American VacuumSociety Meeting, Atlanta, Georgia,October 3, 1988.Ceyer, S.T., "Dynamics of Sticky Collisionswith a Surface: Splats and Hammers,"Royal Australian Chemical Institute Con-ference on Chemical Reaction Dynamics,Warburton, Victoria, Australia, November11, 1988.Ceyer, S.T., "Collision Induced Dissociationof CH4 and CO on Ni(111)," MaterialsResearch Society Meeting, Boston, Mass-achusetts, November 30, 1988.MIT ThesisBeckerle, J.D., Dynamics of MolecularAdsorption: I. Collision Induced Dissocia-tive Chemisorption and Desorption of CH 4on Ni(l 11) //. Adsorption of CO and 02on Ni(111) at 8 K. Ph.D. diss., Dept. ofChem., MIT, 1988.38 RLE Progress Report Number 131

Chemical Reaction Dynamics at Surfaces

http://dspace.mit.edu/bitstream/1721.1/57074/1/RLE_PR_131_01_01s_04.pdf

Chemical Reaction Dynamics at Surfaces

Abstract

Similar works

Full text

Available Versions

DSpace@MIT