AD-A17l 648 STRUCTURE OP MOLECULAR ADSORBATES ON SURFACES(U) 1/1PITTSBURGH UNIV PA SURFACE SCIENCE CENTER J T YATES98 AUG 86 AFOSR-RR-86-8979 AFOSR-82-ei33

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17. COSATI CODES 11S. SUBJECT TERMS lComa on. roe ,wuf Wmegery mand identify by dioach numlop)F IGLD GROUP SUBOR11. -Chemisorption, Bond Directionality at Surfaces,

Electron Stimulated Desorption, ESDIAD, Digital

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(see attached)

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AFOSR-TR- 8 6 - U 9 18 SECOMPLETED PROJECT SUMMARY

1. TITLE: "Structure of Molecular Adsorbates on Surfaces"

2. PRINCIPAL INVESTIGATOR: John T. Yates, Jr.Department of ChemistryUniversity of PittsburghPittsburgh, PA 15260

3. INCLUSIVE DATES: 3/15/82 - 4/14/86Approved for public release;

4. CONTRACT/GRANT NUMBER: 82-0133 distributioni-ilimited.• . Accession For

5. COSTS AND FY SOURCE: FY-82 - $148.360FY-83 - $113,471 NTIS GRA&IFY-84 -. $121,434 DTIC TAB

Ullannouncea UFY-85 - $109,341 JUstificod [

$492,606Justification-$492,606

6. SENIOR RESEARCH PERSONNEL: Dr. Craig Klauber EvDr. Miles Dresser ibution/Dr. Anne-Marie Lanzillottolabillty Codes

Avail and/or--7. JUNIOR RESEARCH PERSONNEL: Mr. Mark Alvey Dit Special

Mr. Kurt Kolasinski

8. PUBLICATIONS:

1. "Evidence for Chemisorption Site Selection Based on anElectron Donor Mechanism," C. Klauber, M. D. Alvey, J. T.Yates, Jr., Chem. Phys. Letters 106, 477 (1984).

INSPECTED2. "NH3 Adsorption on Ni(ll0) and the Production of the NH 2

Species by Electron Irradiation," C. Klauber, M. D. Alvey andJ. T. Yates, Jr., Surface Science 154, 139 (1985).

3. "Summary Abstract: "Surface Bonding of the NH 3 and NH2Species to Ni(1l0)," M. D. Alvey, C. Klauber and J. T. Yates,Jr., J. Vac. Sci. & Technol A 3(3), 1631 (1985).

4. "ESDIAD Studies of the Structure of Species Chemisorbed onNi(110). The Surface Bonding of NH 3 , NH 2 and CO," J. T.Yates, Jr., C. Klauber and M. D. Alvey, Proceedings of SecondInternational Workshop, Desorption Induced by ElectronicTransitions DIET-II, Schloss Elmau, Bavaria, October 15-17,1984; Eds. W. Brenig and D. Menzel, Springer-Verlag, Berlin,1985.

5. "A Coverage-Induced Tilting of CO Molecules Adsorbed onNi(il0),- J. Lee, H. Metiu, R. M. Martin, J. Arias, C.Hanrahan, C. Klauber, M. D. Alvey, and J. T. Yates, Jr.,Surface Science, 159, L460 (1985).

06 10 16 U31

COMPLETED PROJECT SUMMARY Page Two.

PUBLICATIONS (continued)

6. "Metastable Angular Distributions from Electron StimulatedDesorption," Mark D. Alvey, Miles J. Dresser, and J. T.Yates, Jr., Phys. Rev. Lett. 56(4), 367 (1986).

7. "Conformational Changes in Chemisorbed CO on Ni(110) Due toMolecular Interactions: An ESDIAD Study," Mark D. Alvey,Miles J. Dresser, and J. T. Yates, Jr., Surface Science 165,447 (1986).

8. "Enhancement of the ESDIAD Method for Imaging BondDirectionality in Chemisorbed Species," M. J. Dresser, M. D.Alvey, and J. T. Yates, Jr., Surface Science, 169, 91(1986).

9. "An Enhanced ESDIAD Method for Imaging Molecular Orientationsof Adsorbed 3pecies," M. J. Dresser, M. D. Alvey, and J. T.Yates, Jr., J. Vac. Sci. & Technol., A 4(3), 1446 (1986).

\ 9. ABSTRACT OF OBJECTIVES AND ACCOMPLISHMENTS:

---This project had as its primary objective the extension ofthe ESDIAD (electron stimulated desorption ion angular distribu-tion) method to the study of interesting molecular adsorbatesstructure on single crystal surfaces. A second objective was todevelop a digital ESDIAD apparatus capable of more accurate studyof ion angular distributions from the ESDIAD phenomena. The fun-damental fact that positive ions are ejected from adsorbatesalong the fragmentation bond direction was employed in all ofthese studies The following basic facts were determined in the9 papers publish r- s contract. ,

a. NH 3 , an electron donor molecule, seeks the electroposi-tive Ni bonding sites on the corrugated Ni(ll0) crystal.

b. The NH 3 molecule is not subject to axial rotationalbarriers greater than about 8 meV on these sites, i.e.,the molecule shows no preferential N-H bonding azimuth,and seems to rotate freely above 30 K.

c. NH 3 (a) can be quantitatively converted to NH 2 (a) brelectron bombardment with a cross section of 1.6 U for55 eV electrons.

d. The NH 2 (a) species is locked to the Ni(ll0) lattice withits plane in the (001)and (001) directions.

10

COMPLETED PROJECT SUMMARY Page Three.

9. ABSTRACT OF OBJECTIVES AND ACCOMPLISHMENTS (Continued)

e. NH 2 (a) thermally decomposes, forming N(a) + H(a) atabout 375 K. NH 3 (a) does not thermally decompose, butdesorbs as the intact species.

f. CO chemisorbs on atop Ni sites on Ni(ll0) forming a ver-tical C-O bond for coverages below 0.75 CO/Ni. Abovethis coverage, CO-..CO repulsions cause the C-O bondaxis to tip, alternately left and right by ± 190. Thistransition corresponds to the production of a crowdedplgl LEED structure.

g. Metastable Quenching Spectroscopy, involving He* probeatoms, confirms the result in statement f above, and thetilting results in increased exposure of the 2w orbitalto the incoming He atoms.

h. The digital ESDIAD apparatus may be employed foraccurate analysis of ion trajectories. A troublesome X-ray background effect, present in all ESDIAD measure-ments heretofore, can be eliminated using the digitalmethod.

i. The digital ESDIAD apparAtus has been used to make thefirst angular distributi6n measurements of electroni-cally excited neutral species produced by electron bom-bardment. It was found that CO* (a 3w likely) issharply-peaked in its emission direction like the posi-tive ions in ESD.

- rhe studies-listed above, and considered together,NIaveaffirmed the utility of digital ESDIAD to deliver informationabout the detailed orientation of chemical bonds at surfaces, andabout the molecular dynamics of surface species on singlecrystals.

10. INDIVIDUAL ABSTRACTS OF WORK PUBLISHED

1. "Evidence for Chemisorption Site Selection Based on anElectron Donor Mechanism"

ABSTRACT

The orientation of NH 3 chemisorbed on the Ni(ll0) surfaceindicates that an electron donor type adsorbate will chooseelectropositive ridge atom sites in preference to available sitesof higher coordination within the surface valleys.

iP

COMPLETED PROJECT SUMMARY Page Four.

2. "NH3 Adsorption on Ni(l0) and the Production of theNH2 Species by Electron Irradiation

ABSTRACT

The adsorption of NH 3 on Ni(l0) has been examined usingelectron stimulated desorption ion angular distribution (ESDIAD),low energy electron diffraction (LEED) and thermal desorptionspectrometry (TDS). At -85 K the NH 3 molecule enters into aseries of chemisorption and physisorption states whose structureshave been partially characterized by means of ESDIAD and LEED.Upon heating, these NH3 states desorb without dissociation; foradsorption below 300 K there is essentially no thermal decom-position. The ammonia adlayer was found to be extremely sen-sitive to electron irradiation effects. Evidence was found tosupport the irradiation induced conversion of NH3(ads) to anamido intermediate, NH2(ads). The NH 2 adsorbs with its C2v axisnormal to the surface and its N-H 'bonds aligned along the [0011and [001] directions. In the absence of further electronirradiation the NH2(ads) species is stable to 375 K whereupon itdissociates to N(ads) and H2(g). The remaining N(ads) desorbswith significant attractive N*N interaction. No evidence isfound for an imido intermediate, NH(ads). NH2(ads) alsoundergoes a disproportionation/recombination reaction uponheating to produce an additional NH3 desorption state. A signi-ficant isotope effect for N-H versus N-D scission, sensitive tothe adsorption state of the ammonih, is found to occur uponelectron irradiation.

3. "Summary Abstract: Surface Bonding of the NH 3 and NH 2

Species to Ni(ll0)"

ABSTRACT

Using electron impact, we have converted NH3(ads) on theNi(ll0) surface. Below 300 K, NH 3 is bound nondissociatively toNi ridge atoms on Ni(110) by electron donation from lone pair Nelectrons. Upon electron bombardment, the process NHA(ads)e-(55 eV) NH21ads) + H(ads) occurs quantitatively wit a crosssection oi 1.6 A2. ESDIAD results indicate that the initial NH3species is oriented normally to the (110) surface with no azi-muthal ordering down to 29 K. In contrast, NH 2 is azimuthallyordered at 300 K with the plane of the species perpendicular tothe rows of Ni atoms on the surface; this bonding orientation isconsistent with the molecular orbital structure of NH 2 . Thisspecies is stable to 375 K where it decomposes to N(ads) andH2 (g).

COMPLETED PROJECT SUMMARY Page Five.

4. "ESDIAD Studies of the Structure of Species Chemisorbedon Ni(ll0). The Surface Bonding of NH 3 , NH 2 and CO"

ABSTRACT

NH 3 is chemisorbed at 90 K as an undissociated molecularspecies on Ni(l10). Previous reports by others of the thermaldissociation of this molecule on Ni surfaces are probably due toinadvertent electron bombardment which efficiently degradesNH 3 (ads) to NH 2 (ads). Our results taken together suggest thatdetailed local structural information about chemisorbed speciesis able to be obtained through the use of the ESDIAD method. Thequalitative observation of the symmetry of ESD-generated positiveion fragments from chemisorbed surface species provides basicinsight into the details of species surface structure which maybe used to assign chemisorption site locations with accuracy infavorable cases. Correlation with other surface measurementtechniques is particularly valuable in assigning surface struc-tures.

5. nA Coverage-Induced Tilting of CO Molecules Adsorbed on

Ni(110)0

ABSTRACT

By means of the combined use of metastable quenchingspectroscopy (MOS), thermal desorption spectrometry (TDS) andelectron stimulated desorption ion angular distribution (ESDIAD),evidence has been found for a coverage dependent reversiblebonding geometry transition for CO chemisorbed on the Ni(ll0)surface. Over the temperature range of -90 - 120 K the CO chemi-sorption axis tilts from its surface normal orientation below 0 =

0.75 CO/Ni to an off-normal direction above that criticalcoverage.

6. "Metastable Angular Distributions from Electron

Stimulated Desorption"

ABSTRACT

We have, for the first time, measured by electron stimulateddesorption the angular distribution of electronically excited(metastable) species from CO adsorbed on Ni(1l0). The angulardistribution of metastable desorption is strongly peaked in thenormal direction, as is also found for ionic desorption products.This indicates that the metastable is repelled along the direc-tion of the bond being broken. We also observe an asymmetry inthe azimuthal angular distribution which is correlated with thesubstrate crystal structure. The metastable exhibits a maximumin its yield versus coverage.

COMPLETED PROJECT SUMMARY Page Six.

7. "Conformational Changes in Chemisorbed CO on Ni(ll0 Dueto Molecular Interactions: An ESDIAD Study"

ABSTRACT

The surface bonding geometry of CO chemisorbed at 83 K onthe Ni(1l0) surface was studied as a function of CO coverageusing the ESDIAD technique. A study of the 0+ angular distribu-tion from CO indicated that at CO coverages below about 0.75CO/Ni, a normal orientation of the C-O bond exists. Above thiscoverage, CO species begin to be tilted approximately 190 awayfrom the normal in a plane perpendicular to the Ni atom rows.The 0+ ion angular distribution evolves from a single normal beamto a split beam during this transformation. The yield of posi-tive ions from chemisorbed CO was studied as a function ofelectron energy. Since a sharp threshold for 0+ emission occursat the O(ls) core level energy, measurements above this thresholdare particularly effective in observing the conformational trans-formation. In addition, smaller yields of O++ and CO+ have beencharacterized as functions of the electron energy. The confor-mational behavior of CO has been correlated with thermal desorp-tion spectra and LEED behavior for the overlayer. The work wascarried out in a new ESDIAD analyzer which acquires digital ionangular distribution data. The new apparatus is able to elimi-nate a serious soft X-ray background present in all photographicESDIAD measurements. Coherent accumulation of ESDIAD data andsubsequent background subtraction yields excellent signal-to-noise ratios for ion angular distributions, and facilitates quan-titative analysis of ion beam orientations.

8. "Enhancement of the ESDIAD Method for Imaging Bond

Directionality in Chemisorbed Species"

ABSTRACT

The ESDIAD method for imaging adsorbate bond directions byphotographic observation of positive ion angular distributionsduring electron stimulated desorption suffers from inherent lowcontrast due to background effects. The use of a digitalacquisition system designed to overcome this difficulty in ESDIADmeasurement is presented. Measurements on a Ni(ll0) singlecrystal substrate show the presence of a significant backgroundsignal due to soft x-ray generation by electron impact. Bysubtraction of the background signal, a significant enhancementof positive ion signal-to-noise ratio is achieved in ESDIAD, con-verting the ESDIAD method into a high contrast, high resolutionsurface measurement technique. Quantitative studies of the softx-ray background have shown it to be linearly dependent onelectron current density and electron energy, with no change inangular shape. These properties permit an accurate backgroundsubtraction procedure to be employed to significantly enhance thecapability of the ESDIAD method.

COMPLETED PROJECT SUMMARY Page Seven.

9. "An Enhanced ESDIAD Method for Imaging MolecularOrientations of Adsorbed Species"

ABSTRACT

A striking improvement has been made in the quality of posi-tive ion angular distribution data obtained in electron-stimulated-desorption ion angular distribution (ESDIAD). Theimproved method has been used to study configurational changes inchemisorbed CO on Ni(110) which are induced by intermolecularinteractions at high coverage. CO molecules adsorbed with normalC-O bond orientations on ridge Ni sites tilt away from the normalby ± 190 in directions perpendicular to the ridges for CO covera-ges above 0.75 CO/Ni. Evidence for the C-O tilting has also beenstudied as a function of electron energy. This has demonstratedthat an O(ls) excitation process leads to intense and sharp 0+ESDIAD beams. The improved ESDIAO method involves the removal ofa soft x-ray background signal which originates from electronimpact on the metal surface, and is therefore present in all pho-tographic ESDIAD measurements made, to date. The angular distri-bution of the background effect is; independent of electron energyand current density. The resultant "pure ESDIAD" patterns showmore quantitative details in high and low signal regions than hasbeen seen before.

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