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101.
Electron stimulated desorption (ESD) experiments using a time-of-flight pulse counting method are reported for molecular CO chemisorbed on the Cr(110) surface at 90 K. Consistent with previous qualitative observations, negligible CO+ and O+ desorption signals were measured from the 1CO overlayer which saturates at 1/4 monolayer. For θCO > 0.25, a terminally-bonded (2CO) binding mode is populated in addition to the existing ∝1CO binding mode and the ion yield increases sharply. For 2CO, both O+ and CO+ ions are observed; the CO+ ions desorb with characteristically lower kinetic energies than O+ ions. Near saturation coverages of CO(ads), an observed decrease in the O+ yield is attributed to adsorbate-adsorbate interactions which reduce the ion desorption probability, as seen in ESD studies of terminally-bonded CO on other metals. These results are considered in the context of two possible models proposed for the 1CO binding state and related ESD observations for CO chemisorbed on Fe(001) and potassium-promoted Ru(001). 相似文献
102.
Bernard Collins Theodore O. Poehler Wayne A. Bryden 《Photochemistry and photobiology》1995,62(3):557-560
Electron paramagnetic resonance is used to detect the formation of free radicals caused by exposure to ultraviolet radiation in chemically untreated rabbit skin. A fast jump in EPR signal level, occurring over a few seconds, is observed immediately after a skin sample is exposed to UV. This is followed by a slower increase toward an elevated steady-state signal over a period of hours as the skin is continuously exposed to a UV light source. Upon cessation of UV light exposure, EPR signal levels undergo an abrupt drop followed by a slower decay toward natural levels. Elevated free radical concentrations following UV exposure are found to persist for several hours in whole skin. These results are consistent with time-resolved EPR measurements of photoinduced radicals in various natural melanins. 相似文献
103.
Theodore Voronov 《Communications in Mathematical Physics》2002,228(1):1-16
We introduce and study the complex of “stable forms” on supermanifolds. Stable forms on a supermanifold M are represented by Lagrangians of “copaths” (formal systems of equations, which may or may not specify actual surfaces) on
M×ℝ
D
. Changes of D give rise to stability isomorphisms. The resulting (direct limit) {Cartan-de Rham} complex made of stable forms extends both
in positive and negative degree. Its positive half is isomorphic to the complex of forms defined as Lagrangians of paths,
studied earlier. Including the negative half is crucial, in particular, for homotopy invariance. For stable forms we introduce
(non-obvious) analogs of exterior multiplication by covectors and contraction with vectors and find the anticommutation relations
that they obey. Remarkably, the version of the Clifford algebra so obtained is based on the super anticommutators rather than
the commutators
and (before stabilization) it includes some central element σ. An analog of Cartan's homotopy identity is proved, which also
contains this “stability operator”σ.
Received: 3 January 2000 / Accepted: 15 September 2001 相似文献
104.
105.
Within the class of Tychonoff spaces, and within the class of topological groups, most of the natural questions concerning ‘productive closure’ of the subclasses of countably compact and pseudocompact spaces are answered by the following three well-known results: (1) [ZFC] There is a countably compact Tychonoff space X such that X × X is not pseudocompact; (2) [ZFC] The product of any set of pseudocompact topological groups is pseudocompact; and (3) [ZFC+ MA] There are countably compact topological groups G0, G1 such that G0 × G1 is not countably compact.In this paper we consider the question of ‘productive closure” in the intermediate class of homogeneous spaces. Our principal result, whose proof leans heavily on a simple, elegant result of V.V. Uspenski?, is this: In ZFC there are pseudocompact, homogeneous spaces X0, X1 such that X0 × X1 is not pseudocompact; if in addition MA is assumed, the spaces Xi may be chosen countably compact.Our construction yields an unexpected corollary in a different direction: Every compact space embeds as a retract in a countably compact, homogeneous space. Thus for every cardinal number α there is a countably compact, homogeneous space whose Souslin number exceeds α. 相似文献
106.
Let \(\dot y\) =f(y,t) withy(t 0)=y 0 possess a solutiony(t) fort≧t 0. Sett n=t 0+nh, n=1, 2,.... Lety 0 denote the approximate solution ofy(t n) defined by the composite multistep method with \(\dot y_n \) =f(y n ,t n ) andN=1, 2,.... It is conjectured that the method is stiffly stable with orderp=l for alll≧1 and shown to be so forl=1,..., 25. The method is intrinsically efficient in thatl future approximate solution values are established simultaneously in an iterative solution process with only one function evaluation per iteration for each of thel future time points. Step and order control are easily implemented, in that the approximate solution at only one past point appears in each component multistep formula of the method and in that the local truncation error for the first component multistep formula of the method is easily evaluated as $$T^{[l]} = \frac{h}{{t_{Nl} - t_{(N - 1)l - 1} }}\{ y_{Nl}^{PRED} - y_{Nl} \} ,$$ wherey Nl PRED denotes the value att Nl of the Lagrange interpolating polynomial passing through the pointsy (N?1)l+j att (N?1)l+j withj=?1, 0,...,l ? 1. 相似文献
107.
The interaction of Eu(fod)3 with organic salts is studied. 相似文献
108.
109.
Evidence is presented for a local interaction between CO and Na adsorbed on Ru(001). For low coverages of Na (θNa ? 0.15 ML) and saturation coverages of CO at 80 K. a fraction of the CO molecules undergo a substantial change in bonding configuration: molecular CO bound perpendicular to the Ru(001) surface changes to an “inclined” configuration in the presence of low coverages of Na. 相似文献
110.
The ESDIAD method (electron stimulated desorption ion angular distributions) has been combined with LEED (low energy electron diffraction) in a study of the adsorption of NO on Ni(111). For adsorption at 80 K, NO appears to be bonded with its molecular axis perpendicular to the Ni(111) surface at all coverages. Heating the 80 K layer leads to a striking structural change which we interpret as the formation of inclined or bent NO in the range 120 ? T ? 250 K. Upon adsorption at 150 K, only the bent form of NO is present at low coverages; at higher coverages at 150 K, the perpendicular form appears, in agreement with recent electron energy loss spectroscopy (EELS) data of Lehwald, Yates, and Ibach. When NO is coadsorbed with p(2 × 2) oxygen, the perpendicular form of NO dominates at all coverages and temperatures studied. Dissociated NO adsorbed at steps and defect sites on Ni(111) produces a welldefined hexagonal ESDIAD pattern. 相似文献