ESDIAD

Fig. VIII-13. LEED and ESDIAD on clean and oxygen-dosed Ni(lll) (a) LEED, elean surface (b) H ESDIAD of NH3 on Ni(Ill), the halo suggesting free rotation of the surface NH3 groups (c) ESDIAD after predosing with oxygen, then heated to 600 K and cooled before dosing with NH3—only well-ordered chemisorbed NH3 is now present. (From Ref. 93.)...

ESDIAD Electron-stimulated desorption ion angular distribution [150-152] A LEED-like pattern of ejected ions is observed Orientation of adsorbed species... 

The nature of reaction products and also the orientation of adsorbed species can be studied by atomic beam methods such as electron-stimulated desorption (ESD) [49,30], photon-stimulated desoiption (PDS) [51], and ESD ion angular distribution ESDIAD [51-54]. (Note Fig. VIII-13). There are molecular beam scattering experiments such... 

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

The ESDIAD pattern does, however, provide very usefril infomiation on the nature and synnnetry of an adsorbate. As an example, figure A1.7.13(a) shows the ESDIAD pattern of desorbed collected from a 0.25 ML coverage of PF on Ru(OOOl) [89]. The pattern displays a ring of emission, which indicates that the molecule adsorbs intact and is bonded tlirough the P end. It freely rotates about the P-Ru bond so that tlie emission occurs at all azimuthal angles, regardless of the substrate structure. In figure A1.7.13(b), the... 

Figure Al.7.13. ESDIAD patterns showing the angular distributions of F emitted from PF adsorbed on Ru (0001) under electron bombardment, (a) 0.25 ML coverage, (b) the same surface following electron beam damage.

Figure C3.2.19. In this ESDIAD experiment where ions are produced and collected (see text), an adsorbed acetate species is excited by an incoming electron. ions are emitted in tire direction of tire C-H bond in tire upward pointing -CH group in tire species. Circular symmetry of figure indicates tliat C-H bonds are spinning around tire vertical axis in tire acetate species. From Lee J G, Aimer J, Mocutta D, Denev S and dates J T Jr 2000 J. Chem. Phys. 112 335.

A, A , A ESD ESDIAD SIMS SNMS GDMS FARMS RBS LEIS ERDA NRA ... 

ESDIAD Electron-stimulated desorption ion angular distribution IPES Inverse photoemission spectroscopy... 

Electron Stimulated Desorption (ESD) and ESD Ion Angular Distribution (ESDIAD)... 

The most common ions observed as a result of electron-stimulated desorption are atomic (e. g., H, 0, E ), but molecular ions such as OH", CO", H20, and 02" can also be found in significant quantities after adsorption of H2O, CO, CO2, etc. Substrate metallic ions have never been observed, which means that ESD is not applicable to surface compositional analysis of solid materials. The most important application of ESD in the angularly resolved form ESDIAD is in determining the structure and mode of adsorption of adsorbed species. This is because the ejection of positive ions in ESD is not isotropic. Instead the ions are desorbed along specific directions only, characterized by the orientation of the molecular bonds that are broken by electron excitation. 

Electron spin resonance (esr), 22 132 for lignin characterization, 15 10 Electron-stimulated desorption-ion angular distribution (ESDIAD), 24 74 Electron transfer (ET), 9 376-381, 388 mechanisms of, 13 444 rate constant for, 13 447 Electron-transfer dynamics, in... 

Chemical state characterization IR and Raman Spectroscopy, FT-IR, EELS, XPS, SIMS, ESDIAD, ellipsometry, and laser fluorescence... 

Structural characterization LEED, HEED, RHEED, FIM, FEM, TED, XRD, HVEM, AEM, EXAFS, ISS, ion channeling, ESDIAD, UPS, electron channeling, SEXAFS, vibrational EELS, and Raman spectroscopy... 

Steps are usually the most stable chemisorption sites. A notable exception is NO/Pd(211), for which system Electron Stimulated Desorption (ESDIAD) and HREELS experiments have shown that the (111) terraces are occupied first [ 111, 112]. This intriguing result was explained recently by density functional theory calculations [113], which show that although the step edge is the most stable adsorption site for isolated molecules the mutual NO interactions reorganise the adsorbates during the adsorption process thus leading to non sequential site population of steps and terraces. 

A diatomic molecule has to be adsorbed parallel to the surface in order to dissociate. The more favourable adsorption complex for molecules like CO on a group 8-10 metal surface is that in which the molecular axis stands perpendicular to the surface. It has been demonstrated by ESDIAD (electron stimulated desorption, ion angular dependence) that the molecular axis vibrates with regard to the surface normal and that the amplitude of vibration increases with increasing temperature this is shown in Fig. 4.41. 

ESDIAD Electron Stimulated Desorption Ion Angular Distribution... 

Observation of the ion angular distribution after electron stimulated desorption of chemisorbed species (ESDIAD) can provide direct quantitative information on the orientation of adsorbed molecules on surfaces. Electrons incident on the surface can excite chemical bonds into non-bonding states, causing molecular decomposition. The excess energy can be converted into kinetic energy, which accelerates an ionic fragment of the molecule along the axis... 

---