Mark D. Alvey

NH3 adsorption on Ni(110) and the production of the NH2 species by electron irradiation

Abstract The adsorption of NH 3 on Ni(110) has been examined using electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and thermal desorption spectrometry (TDS). At ∼ 85 K the NH 3 molecule enters into a series of chemisorption and physisorption states whose structures have been partially characterized by means of ESDIAD and LEED. Upon heating, these NH 3 states desorb without dissociation; for adsorption below 300 K there is essentially no thermal decomposition. The ammonia adiayer was found to be extremely sensitive to electron irradiation effects. Evidence was found to support the irradiation induced conversion of NH 3 (ads) to an amido intermediate, nh 2(ads) . The NH 2 adsorbs with its C 2v axis normal to the surface and its NH bonds aligned along the [001] and [001] directions. In the absence of further electron irradiation the nh 2(ads) species is stable to 375 K whereupon it dissociates to N (ads) and H 2(g) . The remaining N (ads) desorbs near 750 K with significant attractive N…N interaction. No evidence is found for an imido intermediate, nh (ads) . nh 2(ads) also undergoes a disproportionation/recombination reaction upon heating to produce an additional NH 3 desorption state. A significant isotope effect for NH versus ND scission, sensitive to the adsorption state of the ammonia, is found to occur upon electron irradiation.

Enhancement of the ESDIAD method for imaging bond directionality in chemisorbed species

Abstract The ESDIAD method for imaging adsorbate bond directions by photographic observation of positive ion angular distributions during electron stimulated desorption suffers from inherent low contrast due to background effects. The use of a digital acquisition system designed to overcome this difficulty in ESDIAD measurement is presented. Measurements on a Ni(110) single crystal substrate show the presence of a significant background signal due to soft X-ray generation by electron impact. By subtraction of the background signal, a significant enhancement of positive ion signal-to-noise ratio is achieved in ESDIAD, converting the ESDIAD method into a high contrast, high resolution surface measurement technique. Quantitative studies of the soft X-ray background have shown it to be linearly dependent on electron current density and electron energy, with no change in angular shape. These properties permit an accurate background subtraction procedure to be employed to significant;y enhance the capability of the ESDIAD method.

Interaction between NH3 and CO on the Ni(111) and (110) surfaces: A study by ESDIAD

The interaction between adsorbed NH3 and adsorbed CO molecules on two Ni single crystal planes has been investigated using ESDIAD and temperature programmed desorption (TPD). Interactions have been observed on both surfaces which influence the ESDIAD patterns of both adsorbed species. Evidence for long distance azimuthal orientation interactions of NH3 with CO on Ni(110) is observed, whereas shorter distance interactions are observed on Ni(111). In the case of the short distance CO…NH3 interactions on Ni(111), a tipping of the C3v axis of NH3 away from the normal is seen. The role of the substrate crystal structure is shown to be important in determining the character of the intermolecular interactions on the two surfaces.

Angular distributions of H2 thermal desorption: Coverage dependence on Ni(111)

Temperature programmed desorption measurements for H2/Ni(111) yield two desorption states, β1(T=290 K) and β2(T=370 K) for saturation H coverage. The two states are found to have distinctly different angular distributions. β2‐H2 desorption is strongly focused along the surface normal, while β1‐H2 desorbs diffusely. The angular distribution of desorbing β1‐H2 is very close to that for the CO/Ni(111) system which has a cos θ angular distribution. The different angular distributions can be explained by a model involving a coverage‐dependent location of an activation energy barrier on the potential energy surface which describes the interaction of hydrogen with Ni(111).Temperature programmed desorption measurements for H2/Ni(111) yield two desorption states, β1(T=290 K) and β2(T=370 K) for saturation H coverage. The two states are found to have distinctly different angular distributions. β2‐H2 desorption is strongly focused along the surface normal, while β1‐H2 desorbs diffusely. The angular distribution of desorbing β1‐H2 is very close to that for the CO/Ni(111) system which has a cos θ angular distribution. The different angular distributions can be explained by a model involving a coverage‐dependent location of an activation energy barrier on the potential energy surface which describes the interaction of hydrogen with Ni(111).

Evidence for chemisorption site selection based on an electron-donor mechanism

Abstract The orientation of NH3 chemisorbed on the Ni(110) surface indicates that an electron-donor-type adsorbate will choose electropositive ridge atom si

Alkali sensitization of H+ electron stimulated desorption from H adsorbed on Ni(111)

It is shown that alkali adatoms coadsorbed in the presence of adsorbed hydrogen on Ni(111) can cause a large increase in the cross section for H+ emission during electron stimulated desorption. This phenomenon was investigated using the digital ESDIAD (electron stimulated desorption ion angular distribution) technique as well as by temperature programmed desorption (TPD). H+ ions produced by electron impact on alkali–hydrogen complexes are ejected in sharp normally oriented ion angular distributions. The sensitization of the H(a)→e−H+ process occurs with Li, Na, and K but not as strongly with Rb and Cs. Attractive interactions exist in the adsorbed layer between the hydrogen and the alkali adatoms. A model is discussed involving the formation of ‘‘alkali–hydride‐like’’ surface complexes, with Hδ− located on top of alkali atom sites.

The effect of potassium on the molecular orientation of CO chemisorbed on Ni(111): An esdiad study

Abstract Using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) techniques, we have determined that coadsorbed potassium systematically quenches the O + ion yield from CO on the Ni(111) surface for 1000 eV electron excitation energies. The quenching appears to be a short range K-CO interaction; 3 or 4 CO molecules are affected for each K atom adsorbed on the surface. The quenching effect of K on CO indicates that a significant electronic perturbation of CO is caused by its local interaction with K. This effect prevents ESDIAD observation of the K-quenched CO species. In addition, the CO molecules that are not quenched at a potassium coverage of 0.02 K/Ni exhibit a normally oriented C-O bond similar to that found for CO adsorbed on a K-free Ni(111) surface.

The epitaxial formation of adsorbed multilayers as studied by ESDIAD: NH3 adsorption on top of chemisorbed CO on nickel crystal surfaces

The epitaxial growth of an adsorbed layer of NH 3 on top of chemisorbed CO on Ni(111) and Ni(110) surfaces was studied using ESDIAD. A strong interaction yielding an activation energy for NH 3 desorption of ∼12 kcal/mol was observed. This interaction, possibly involving hydrogen bonding, between the adsorbed NH 3 and adsorbed CO causes a tilting of the NH 3 molecules on the CO-covered Ni surfaces. For the NH 3 /CO/Ni(110) system, the two-fold symmetry of the underlying Ni substrate is transmitted through the CO spacer layer to the NH 3 overlayer. This symmetry transfer was not observed for the NH 3 /CO/Ni(111) system at the current resolution of our ESDIAD detection system.

Ion angular distribution of species desorbed from single crystal surfaces by electron impact

Abstract The measurement of the angular distribution of desorbing positive ions produced by electron impact desorption (ESDIAD) is of fundamental importance in understanding molecular structure in the chemisorbed layer. In this short review, two applications of ESDIAD to structural problems in the adsorbed layer will be described. Examples of strong chemisorption and weaker physical adsorption effects will be discussed. In addition, interactions between adsorbed species, leading to changes in bonding geometry will be described. The apparatus used for this work allows digitized acquisition of ion angular distributions in the absence of background effects due to soft X-ray emission stimulated by electron impact.

Molecular orientation on metal surfaces by electrostatic interactions: The adsorption of cyclopentene on a stepped (221) silver surface

Step defects at metal surfaces are known to cause a local dipole at the surface, thereby creating an enhanced surface electric field in their vicinity. Using the electron stimulated desorption ion angular distribution (ESDIAD) technique, cyclopentene is observed to be ordered by the interaction of its permanent dipole moment with the electrostatic field at the steps on the Ag (221) surface. This ordering is not seen for cyclopentene on Ag (111). The experimental results agree with estimates of the interactional energy of the dipole with the electric field at the steps, and the direction of orientation is consistent with calculations of the sign of the cyclopentene dipole moment.Step defects at metal surfaces are known to cause a local dipole at the surface, thereby creating an enhanced surface electric field in their vicinity. Using the electron stimulated desorption ion angular distribution (ESDIAD) technique, cyclopentene is observed to be ordered by the interaction of its permanent dipole moment with the electrostatic field at the steps on the Ag (221) surface. This ordering is not seen for cyclopentene on Ag (111). The experimental results agree with estimates of the interactional energy of the dipole with the electric field at the steps, and the direction of orientation is consistent with calculations of the sign of the cyclopentene dipole moment.