The organometallic chemistry of the Ru(001) surface

The organometallic chemistry of the hexagonally close-packed Ru(001) surface has been studied using electron energy loss spectroscopy and thermal desorption mass spectrometry. The molecules that have been studied are acetylene, formamide and ammonia. The chemistry of acetylene and formamide has also been investigated in the presence of coadsorbed hydrogen and oxygen adatoms.

Acetylene is adsorbed molecularly on Ru(001) below approximately 230 K, with rehybridization of the molecule to nearly sp^3 occurring. The principal decomposition products at higher temperatures are ethylidyne (CCH_3) and acetylide (CCH) between 230 and 350 K, and methylidyne (CH) and surface carbon at higher temperatures. Some methylidyne is stable to approximately 700 K. The preadsorption of hydrogen does not alter the decomposition products of acetylene, but reduces the saturation coverage and also leads to the formation of a small amount of ethylene (via an η^2-CHCH_2 species) which desorbs molecularly near 175 K. Preadsorbed oxygen also reduces the saturation coverage of acetylene but has virtually no effect on the nature of the molecularly chemisorbed acetylene. It does, however, lead to the formation of an sp^2-hybridized vinylidene (CCH_2) species in the decomposition of acetylene, in addition to the decomposition products that are formed on the clean surface. There is no molecular desorption of chemisorbed acetylene from clean Ru(001), hydrogen-presaturated Ru(001), or oxygen-presaturated Ru(001).

The adsorption and decomposition of formamide has been studied on clean Ru(001), hydrogen-presaturated Ru(001), and Ru(001)-p(1x2)-O (oxygen adatom coverage = 0.5). On clean Ru(001), the adsorption of low coverages of formamide at 80 K results in CH bond cleavage and rehybridization of the carbonyl double bond to produce an η^2 (C,O)-NH_2CO species. This species is stable to approximately 250 K at which point it decomposes to yield a mixture of coadsorbed carbon monoxide, ammonia, an NH species and hydrogen adatoms. The decomposition of NH to hydrogen and nitrogen adatoms occurs between 350 and 400 K, and the thermal desorption products are NH_3 (-315 K), H_2 (-420 K), CO (-480 K) and N_2 (-770 K). At higher formamide coverages, some formamide is adsorbed molecularly at 80 K, leading both to molecular desorption and to the formation of a new surface intermediate between 300 and 375 K that is identified tentatively as η^1(N)-NCHO. On Ru(001)- p(1x2)-O and hydrogen-presaturated Ru(001), formamide adsorbs molecularly at 80 K in an η^1(O)- NH_2CHO configuration. On the oxygen-precovered surface, the molecularly adsorbed formamide undergoes competing desorption and decomposition, resulting in the formation of an η^2(N,O)-NHCHO species (analogous to a bidentate formate) at approximately 265 K. This species decomposes near 420 K with the evolution of CO and H_2 into the gas phase. On the hydrogen precovered surface, the Η^1(O)-NH_2CHO converts below 200 K to η^2(C,O)-NH_2CHO and η^2(C,O)-NH^2CO, with some molecular desorption occurring also at high coverage. The η^2(C,O)-bonded species decompose in a manner similar to the decomposition of η^2(C,O)-NH_2CO on the clean surface, although the formation of ammonia is not detected.

Ammonia adsorbs reversibly on Ru(001) at 80 K, with negligible dissociation occurring as the surface is annealed The EEL spectra of ammonia on Ru(001) are very similar to those of ammonia on other metal surfaces. Off-specular EEL spectra of chemisorbed ammonia allow the v(Ru-NH_3) and ρ(NH_3) vibrational loss features to be resolved near 340 and 625 cm^(-1), respectively. The intense δ_g (NH_3) loss feature shifts downward in frequency with increasing ammonia coverage, from approximately 1160 cm^(-1) in the low coverage limit to 1070 cm^(-1) at saturation. In coordination compounds of ammonia, the frequency of this mode shifts downward with decreasing charge on the metal atom, and its downshift on Ru(001) can be correlated with the large work function decrease that the surface has previously been shown to undergo when ammonia is adsorbed. The EELS data are consistent with ammonia adsorption in on-top sites. Second-layer and multilayer ammonia on Ru(001) have also been characterized vibrationally, and the results are similar to those obtained for other metal surfaces.

Steady surface wave pattern in a shear flow

The subject under investigation concerns the steady surface wave patterns created by small concentrated disturbances acting on a non-uniform flow of a heavy fluid. The initial value problem of a point disturbance in a primary flow having an arbitrary velocity distribution (U(y), 0, 0) in a direction parallel to the undisturbed free surface is formulated. A geometric optics method and the classical integral transformation method are employed as two different methods of solution for this problem. Whenever necessary, the special case of linear shear (i.e. U(y) = 1+ϵy)) is chosen for the purpose of facilitating the final integration of the solution.

The asymptotic form of the solution obtained by the method of integral transforms agrees with the leading terms of the solution obtained by geometric optics when the latter is expanded in powers of small ϵ r.

The overall effect of the shear is to confine the wave field on the downstream side of the disturbance to a region which is smaller than the wave region in the case of uniform flows. If U(y) vanishes, and changes sign at a critical plane y = y_cr (e.g. ϵy_cr = -1 for the case of linear shear), then the boundary of this asymmetric wave field approaches this critical vertical plane. On this boundary the wave crests are all perpendicular to the x-axis, indicating that waves are reflected at this boundary.

Inside the wave field, as in the case of a point disturbance in a uniform primary flow, there exist two wave systems. The loci of constant phases (such as the crests or troughs) of these wave systems are not symmetric with respect to the x-axis. The geometric optics method and the integral transform method yield the same result of these loci for the special case of U(y) = U_o(1 + ϵy) and for large Kr (ϵr ˂˂ 1 ˂˂ Kr).

An expression for the variation of the amplitude of the waves in the wave field is obtained by the integral transform method. This is in the form of an expansion in small ϵr. The zeroth order is identical to the expression for the uniform stream case and is thus not applicable near the boundary of the wave region because it becomes infinite in that neighborhood. Throughout this investigation the viscous terms in the equations of motion are neglected, a reasonable assumption which can be justified when the wavelengths of the resulting waves are sufficiently large.