Scattering of Small Molecules by Surfaces

Transcription

1 Scattering of Small Molecules by Surfaces J. R. Manson Clemson University Guoqing Fan Wayne Hayes Haile Ambaye Iryna Moroz Ileana Iftimia, Jinze Dai, Andre Muis Hongwei Zhang, Judson Powers Work supported by the NSF and DOE

2 Judson Powers Anil Kota Fan Guoqing Jason Brown Wayne Hayes Iryna Moroz Haile Ambaye

3 Scattering of Molecules from Surfaces.1 ev < < 5 ev 4 amu < m < 3 amu

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5 CH 4 /Pt(111) Angular Distributions CH 4 /Pt(111) T S = 827 K; θ f = 9 o θ i TOF CH 4 /Pt(111) T S = 827 K 1 = 5 mev a) 1 = 5 mev a) Intensiyt [arb. units].5.5 D = 4 mev = 19 mev b) Intensity [arb. units].5.5 = 19 mev v R =17 m/s L =.454 m D = 4 mev b) Scattering Angle [deg.] Time of flight [µs] Data: T. Kondo, T. Sasaki, S. Yamamoto, J. Chem. Phys. 116, 7673 (22)

6 Basic Theory Approximations: Classically allowed trajectories Rapid collision Linear coupling with displacement

7 Classical limit: atom-surface scattering (multiphonon transfer)

8 Flat surface potential Weakly corrugated surface Strongly corrugated; discrete, isolated atoms

9 Elastic diffraction Atom-surface scattering: limiting cases Single phonon transfer Multiphonon scattering

10 Molecule-surface scattering: Classical translational and rotational motion

11 Molecule-surface scattering: Classical translation and rotation, semiclassical internal mode excitation

12 Low energy ionsurface scattering K + /Cu(1)<1> = 154 ev ; θ i = θ f = 45 ; T S = 675 K J. Powers, J. R. Manson, C. E. Sosolik, J. R. Hampton, A. C. Lavery and B. H. Cooper, Phys. Rev. B7, (24)

13 General properties of the classical scattering intensity

14 Single scattering trajectory, three different incident energies Single and multiple scattering Single and multiple scattering trajectories at 99 mev.

15 Classical Scattering Intensity: General Properties

16 High? energy He scattering: He/Cu(1), > 1 mev M. Bertino, W. Silvestri and J. R. Manson, J. Chem. Phys. 18, 1239 (1998).

17 High? energy He scattering: He/Cu(1), Ei > 1 mev Both energy and temperature dependence of intensity agree with the flat surface model

18 Molecule-surface scattering: Classical translation and rotation, semiclassical internal mode excitation

19 Form factor τ Parameters = fi VJ M v R~ c R ω R 1 1 Discussion h 2 k iz m k fz 2 1 m / s < vr < 2m / s 1 Value is unimportant s Physisorption potential well: Square well of depth D Averaging over molecular orientation

20 CH 4 /LiF(1) 1.5 = 5 mev θ i + θ f = 9 deg. T = 3 K a) Expt. Data v R = 15 m/s 1.5 a) D = mev Expt. Data T = 3 K θ ι = 3 deg. v R = 15 m/s θ f = 6 deg. = 5 mev Intensity [arb. units].5.5 = 35 mev = 19 mev b) c) Intensity [arb. units] b) c) = 35 mev = 19 mev i Scattering Angle [deg.] Time of flight [ µs] T. Kondo, T. Tomii, T. Hiraoka, T. Ikeuchi, S. Yagyu and S. Yamamoto, J. Chem. Phys. 112, 994 (2). T. Tomii, T. Kondo, T. Hiraoka, T. Ikeuchi, S. Yagyu and S. Yamamoto, J. Chem. Phys. 112, 952 (2) Calcs.: I. Moroz and J. R. Manson, PRB 69, 2546 (24).

21 CH 4 /LiF(1) Incident angle dependence.5 1 D = mev θ i =5deg. Expt. data = 35 mev T = 3 K v R = 15 m/s a) [arb. units] Intensity θ i =4deg. θ i =37.5deg. θ i =3deg. d) Time of flight [µs] b) c) T. Tomii, T. Kondo, T. Hiraoka, T. Ikeuchi, S. Yagyu and S. Yamamoto, J. Chem. Phys. 112, 952 (2)

22 .5 a) CH 4 /LiF(1) Physisorption potential well 1 D = mev D = 25 mev D = 65 mev Expt. Data T = 3 K v R = 15 m/s θ ι = 5 deg. θ f = 4 deg. = 5 mev Intensity [a arb. units] b) c) = 35 mev i.5 = 19 mev Time of flight [ µs] T. Tomii, T. Kondo, T. Hiraoka, T. Ikeuchi, S. Yagyu and S. Yamamoto, J. Chem. Phys. 112, 952 (2)

23 CH 4 /Pt(111) T S = 827 K; = 19 and 5 mev θ I = 45 Angular Distributions CH 4 /Pt(111) T S = 827 K; θ f = 9 o θ i TOF CH 4 /Pt(111) T S = 827 K 1 = 5 mev a) 1 = 5 mev a) Intensiyt [arb. units].5.5 D = 4 mev = 19 mev b) Intensity [arb. units].5.5 = 19 mev v R =17 m/s L =.454 m D = 4 mev b) Scattering Angle [deg.] Time of flight [µs] T. Kondo, T. Sasaki, S. Yamamoto, J. Chem. Phys. 116, 7673 (22)

24 CH 4 /Pt(111).5 = 19 mev T = 827 K a) 1.5 T = 7 K b) θ i =45 Intensity [arb. units] T = 6 K T = 5 K T = 4 K Scattering Angle [deg.] c) d) e) Intensity [arb. units] T = 7 K T = 6 K T = 5 K T = 4 K = 19 mev a) b) c) d) Time of flight [µs]

25 NO/Ag(111) NO/Ag(111): Average final translational energy Vs. θ i. Ts=4 K; T r =35 K; v R =1m/s; θ i =θ f <E f >/ Expt.; =.1 ev =.3 ev =.5 ev =.9 ev most probable =.1 ev =.3 ev =.5 ev =.9 ev θ i (deg.) Data: C.T. Rettner, J. Kimman, and D.J. Auerbach, J.Chem.Phys. 94,734(1991)

26 N 2 /Ru(1) N 2 /Ru(1): Average fractional normal energy vs rotational en Ts=61K; T r =2K; v R =1m/s; M s =2.3M Ru. 1 <E fn >/n < E s >/n =2.4 ev; θ i = o = 1.5 ev; θ i = 19 o Expt.( =2.4eV; θ i = o ; J.Chem.Phys.118,(23)112) Expt.( =1.5eV; θ i =19 o ) E x (ev) N 2 /Ru(1); Average final parallel energy. θ i =19 o ; T s =61 K; T r =2 K; v R =1 m/s. Expt. Theory; M s =M Ru Theory; M s =2.3M Ru (ev) E rot (mev) Expt.: H.Mortensen, E. Jensen, L. Diekhoener, A. Baurichter, A. C. Luntz, and V. V. Petrunin, J. Chem. Phys. 118, 112 (23). Data: H. Mortensen, E. Jensen, L. Diekhöner, A. Baurichter, A. C. Luntz and V. V. Petrunin, J. Chem. Phys. 118, 112 (23).

27 Molecular Rotational Excitation Ln[I] NO/Pt: Rotational energy resolved intensity. = 352 mev; v R = 1 m/s; θ i = θ f = 2 o ; T r = 9 K. T s =11 K T s =17 K T s =23 K T s =3 K Final rotational energy (mev) M. K. Ainsworth, V. Fiorin, M. R. S. McCoustra and M. A. Chesters, Surf. Sci , 79 (1999). Ln(I) NO/Ge(111)/C: Rotational energy resolved intensity. T r =49 K; v R =1m/s a) NO/Ge(111); T s =8 K; θ i =θ f =5 o =86 mev =225 mev =825 mev b) NO/C; T s =17 K; θ i =θ f =3 o 1 1 E =87 mev i Final rotational energy(mev) A. Mödl, H. Robota, J. Segner, W. Vielhaber, M. C. Lin and G. Ertl, J. Chem. Phys. 83, 48 (1985); Ertl et al., Chem. Phys. Lett. 9, 225 (1982).

28 Molecular Rotational Excitation A. W. Kleyn, B. A. C. Luntz and D. J. Auerbach, Phys. Rev. Lett. 47, 1169 (1981). C. T. Rettner, J. Kimman and D. J. Auerbach, J. Chem. Phys. 94, 734 (1991).

29 Effect of initial state rotational distribution on observed scattered rotational spectrum. Ln[I] N 2 /Cu(11): Rotational energy resolved intensity. =64 mev; v R =1 m/s; θ i =θ f = o. =64 mev j=2 2 j=1 j=7 4 Jennifer L. Siders and G. O. Sitz, J. Chem. Phys. 11, 6264 (1994) E f R (mev)

30 NO/Ag(111): Excitation probability vs n. Ts=76K; T r =35K; v R =1m/s; θ i =θ f =15 o Molecular Internal Vibrational State Excitation Probability Probability Theory Expt. Theory; α=1 Theory; α=2 Theory; α=3 Probab bility (%) N-O Stretch Mode NO/Ag(111): Excitation probability vs. Expt. Theory (mev) n (mev) Data: C. T. Rettner, F. Fabre, J. Kimman and D. J. Auerbach, Phys. Rev. Lett. 55, 194 (1985). D. M. Newns, Surf. Sci. 171, 6 (1986).

31 Conclusions Classical Atom and Ion Scattering from Surfaces Theory of Molecule-Surface Scattering Classical translation and rotation Semiclassical treatment of internal modes Correct angular and linear momentum conservation C 2 H 2 /LiF(1); CH 4 /LiF(1); CH 4 /Pt(111); O 2 /Al(111); NO/Ge(111); NO/Ag(111); N 2 /Ru(1); N 2 /Cu(11) Extensions: gas-surface interactions, adsorption-desorption, description of thermal energy transfer New Physical Information: Surface corrugation, Physisorption well depths, Rotational temperatures, Effective surface mass and collective effects, Surface segregation, Accommodation and energy transfer, Simple Baule approximations for energy losses are not accurate. Useful theory for describing molecule-surface scattering

32 Judson Powers Anil Kota Fan Guoqing Jason Brown Wayne Hayes Iryna Moroz Haile Ambaye

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35 Flat surface potential Weakly corrugated surface Strongly corrugated; discrete, isolated atoms

36 Temperature Dependence and Surface Corrugation Intensity vs Time of Flight Ar/In(liquid), =42kJ/mol, θ i =θ f =55, T s =436K, Ω/4π=.44 Intensity (arb. units) I DF S IB data single collisions all double collisions single and all double collisions Maxwell Boltzmann Distribution Time of Flight (µsec) Data: W. Ronk, D. Kowalski, M. Manning, and G. Nathanson, JCP 14, 4842 (1996). Most Probable Intens sity (arb. units) Ar/(liquid Ga and In), =42 kj/mol, θ i =θ f = Peak Height vs T s Gallium Indium Theory T 1/2 T 3/ T s (K)

37 Determination of Surface Corrugation Flat surface potential Weakly corrugated surface A simple measure of temperature dependence of most-probable intensities, when compared with classical theories, produces mean square corrugation heights. Examples: Ar/Ga, =42 kj/mol; h RMS.5 Å Ar/Ga, =95 kj/mol; h RMS.8 Å Ar/In, Ei=42 kj/mol; h RMS.3 Å Ar/In, Ei=95 kj/mol; h RMS.5 Å Θ f = θ i = 55 Strongly corrugated; discrete, isolated atoms

38 Classical Scattering Intensity: General Properties

39 Multiple scattering General properties of the single collision classical scattering intensity

40 Clemson University South Carolina

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43 General properties of the classical scattering intensity

44 Single scattering trajectory

45 Correlation speed v R h ˆ = k 2 α k β r eα Q ν eβ Q ν n ων Q + Q R R B S α = 1 β = 1 Q, ν ων ( ) r r r r r r ˆ ˆ v k T N Q (, ) (, )[2 ( ( )) 1]( ) e β r ( Q, ν ) r r r k = k k f n( ω ) = r ˆ R R = R i h / k BT e ω Phonon mode polarization vector 1 S Momentum transfer 1 2

46 C 2 H 2 /LiF(1) (a) (b) (c) Ei=11 (a), 275 (b) and 618 mev (c). T fr (K) Rotational temperature versus T Zhang Dai mf=1 Data mf= T (mev) Expt.: T. W. Francisco, N. Camilone and R. E. Miller, Phys. Rev. Lett. 77, 142 (1996). Theory: Ileana Iftimia and J. R. Manson, Phys. Rev. Lett. 87, 9321 (21). 2 log (I) T = 95 mev exp. data mf=1 mf=3 T = 242 mev T = 244 mev T = 618 mev E f R (mev)

47 CH 4 /LiF(1) Rotation and Internal Mode Excitation ν 4 = mev, α = 1 ν 2 = 19.2 mev, α = 1 a) log(i) = 5 mev = 35 mev = 19 mev = 75 mev ion probability Creati ν 2 = 19.2 mev, α = 2 ν 2 = 19.2 mev, α = 1 b) Final rotational energy E fr [mev] Incident translational energy [mev] Effective Rotational Temperature = 75 mev, T R-effective = 212 K (Miller et al. T R-expt = 24 K)

48 Molecular Beam-Surface Scattering Apparatus Steve Sibener, University of Chicago