PC IV Grenzflächen WS 2011/12. Adsorptions- und Desorptionsdynamik

Transcription

1 Adsorptions- und Desorptionsdynamik

2 Gas-Surface Dynamics Pd(111) / He 300 K direct scattering sticking Pd(111) / CO 300 K trapping desorption T. Engel & G. Ertl, ~1980

3 He Beugung rekonstruierte Au(111) Oberfläche PAIRING ROW BUCKLED PAIRING ROW RUASI HEXAGONAL z BUCKLED PAIRING ROW QUASI HEXAGONAL ANGLE 8 S -T T Manninen et al., 1982

4 Nicht-aktivierte Adsorption Energy Chemisorption well Distance from Surface, z (a) (b) (c) (d) Physisorption well Nonactivated adsorption

5 W Ei Ei/W Kleyn, Prog. Surf. Sci. 54, 407 (1997)

6 PC IV Grenzflächen WS 2011/12 (4.13) Setting E=O in (4.13), we obtain the Debye-Wailer exponent (4.14) which shows that the elastic line is attenuated if the driving force is strong, or if M, is also a small M, gives a large small (cf (4.8) a strong force is equivalent to large M 2, p in (4.2)). Indeed, as explained by Ham's (86,1321, the collision becomes more classical as E increases. If E < cph,quantum features such as an elastic peak are evident and the scattering is dominated by excitations of single phonons, while for E > ~~h multiphonon excitations dominate and the collision is well represented classically. This transition is elastisch illustrated in figure 8 for the case of an H atom scattering from an exponential wall.! g* 0I l&=0'5e inelastisch Figure 8. Results for the probability of an atom losing an amount of energy E when it has an initial kinetic energy 8, according to the forced oscillator model. Values chosen correspond to the collision between a hydrogen atom and an atom in a Cu surface in iks ground vibrational state, For an initial kinetic energy of 0.5 ev, in addition to the inelastic peak centred at 0.03 ev there is an elastic component to the scattered distribution. For a greater initial kinetic energy (2.0 ey) the loss to the substrate is far in excess of the phonon band width, and appears as a Gaussian, centred on the classical value obtained using the binary collision model (equation 4.2) John Harris, 1991 To obtain these results, we have had to assume some form for!he forcef(t). The most common approach is to use the trajectory approximation in which the motion of the projectile is solved for a static lattice and the resulting trajectory is used to evaluate the force from the interaction potential [ This is clearly unsatisfactory for

7 Aktivated Adsorption WJ FIG. 3.-The interaction of a molecule and a metal. from Meld Lennard-Jones, 1932

8 Since evaporation and condensation are in general thermodynamically reversible phenomena, the mechanism of evaporation must be the exact reverse of that of condensation, even down to the smallest detail I. Langmuir, 1916 Mikroskopische Reversibilität: einzelne Trajektorie Detailed Balance: Reaktionsgeschwindigkeiten

9 Clausing Rules Der 2. Hauptsatz bedingt, dass die Verteilung der Teilchen, die die Oberfläche verlassen, durch cos θ beschrieben wird. Das Prinzip des detailierten Gleichgewichts verlangt, dass für jede Richtung und jede Geschwindigkeit gleich viele Teilchen emittiert werden, wie einfallen. Jede wohldefinierte kristalline Oberfläche hat einen Haftkoeffizienten kleiner als 1, der vom Winkel und der Geschwindigkeit abhängen mag.

10 Detailed Balance T S j in j out T g Wenn TS = Tg, dann müssen allen Teilchenflüssen jin, entgegengerichtete Teilchenflüsse jout gegenüberstehen

11 Gasphase Velocity [m/s] Sticking Adsorption Velocity [m/s] Streuung Velocity [m/s] im Gleichgewicht Desorption Velocity [m/s]

12 Dissoziative Adsorption H/Mg Nørskov Lundqvist, 1981

13 Übergangsmetalle d band s band d band s band E F E F Position d-band centre, εd [ev]

14 RAB A R BC A

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16 IC4, i i c to Surface + I Early barrier Reagent -A 11 Trans O B t Reagent o 2.5 V stb 0~~~~~~~~~~1 Z \ l VD Surface + 11 Late barrier Reagent 2.5 DTrans Fig. 17. (A) and (B) show specimen trajectories with (A) largely reagent translation (T = 9, V = 0) and (B) largely reagent vibration (T = 1.5 and V = 14.5) on an "early barrier" surface, that is, type I. (C) and (D) show trajectories with (C) largely reagent vibration (T = 1.5 and V = 7.5) and (D) largely reagent translation (T = 16.0, V = 0). All energies are in kilocalories per mole; reagent vibration is relative to E(v = 0); the partidces A, B, and C have equal mass. Positive and negative vibrational phases are shown with solid and broken lines, respectively. Barrier heights on both surfaces I and II are 7 kcal/mole; contours are labeled in kilocalories per mole. [Courtesy of The Journal ofchemical Pysis (90)] O i o2 0. i J. C.Polanyi, Science 236, 680 (1987)

17 H 2 /Cu

18 H 2 /Cu: Desorption N = exp E kt S E trans ads E a des E a T [K] TSubstrat 925 Trot 1020 Tvib 1820 Etrans 3360 estimated from Michelsen, et al., JCP 98, 8294 (1993)

19 H 2 /Cu: Isotopeneffekt