Evidence for two different bonding mechanisms of Al on Si(111)

By means of the ab initio cluster-model approach, we present theoretical evidence for two different mechanisms of bonding of atomic Al to Si(111). On the atop site (T1) the interaction of atomic Al to Si(111) is characteristic of an ionic bond whereas interaction above the threefold eclipsed site (T4) leads to the formation of a typical covalent bond. Moreover, both sites have a similar interaction energy if electronic correlation effects are included. While the conclusions regarding the nature of the chemisorption bond in the two sites do not depend either on the cluster-model size, the kind of embedding hydrogen atoms used, or the quality of the wave function (Hartree-Fock or configuration interaction), the chemisorption energy depends strongly on the wave function used. In fact, inclusion of correlation energy is necessary to properly describe the interaction energies.

Performance of correlation functionals in ab initio chemisorption cluster-model calculations: Alkali metals on Si(111)

The performance of different correlation functionals has been tested for alkali metals, Li to Cs, interacting with cluster models simulating different active sites of the Si(111) surface. In all cases, the ab initio Hartree-Fock density has been obtained and used as a starting point. The electronic correlation energy is then introduced as an a posteriori correction to the Hartree-Fock energy using different correlation functionals. By making use of the ionic nature of the interaction and of different dissociation limits we have been able to prove that all functionals tested introduce the right correlation energy, although to a different extent. Hence, correlation functionals appear as an effective and easy way to introduce electronic correlation in the ab initio Hartree-Fock description of the chemisorption bond in complex systems where conventional configuration interaction techniques cannot be used. However, the calculated energies may differ by some tens of eV. Therefore, these methods can be employed to get a qualitative idea of how important correlation effects are, but they have some limitations if accurate binding energies are to be obtained.

A construção de si pela atividade de trabalho: a socialização profissional

A sociologia funcionalista das profissões distingue dois tipos muito diferentes de atividades de trabalho: as profissões e as ocupações (ambas no sentido que os ingleses lhes dão). Como apenas as primeiras (a dos médicos, advogados, engenheiros, professores...) são consideradas escolhas e áreas autônomas que permitem a construção de uma carreira, as segundas (que constituem a maioria) acabam sendo desvalorizadas. Os sociólogos interacionistas (como os de Chicago: Hughes, Becker, Strauss etc.) e críticos (marxianos, weberianos etc.) contestam essa posição e consideram que todas as atividades de trabalho poderiam se tornar "profissionais" (no sentido francês), desde que resultassem de uma socialização que permitisse a aquisição de competências e o reconhecimento (inclusive monetário) de todos os que exercem e compartilham uma mesma atividade. A comparação da socialização de clínicos gerais e de auxiliares de enfermagem na França ilustra esta tese, ao tomar o trabalho como um processo de construção e de reconhecimento de si.

Measurements of glial metabolic fluxes with C-11-acetate using positron emission and an adapted NMR-based metabolic modeling approach

Objectives: Acetate brain metabolism has the particularity to occur specifically in glial cells. Labeling studies, using acetate labeled either with 13C (NMR) or 11C (PET), are governed by the same biochemical reactions and thus follow the same mathematical principles. In this study, the objective was to adapt an NMR acetate brain metabolism model to analyse [1-11C]acetate infusion in rats. Methods: Brain acetate infusion experiments were modeled using a two-compartment model approach used in NMR.1-3 The [1-11C]acetate labeling study was done using a beta scintillator.4 The measured radioactive signal represents the time evolution of the sum of all labeled metabolites in the brain. Using a coincidence counter in parallel, an arterial input curve was measured. The 11C at position C-1 of acetate is metabolized in the first turn of the TCA cycle to the position 5 of glutamate (Figure 1A). Through the neurotransmission process, it is further transported to the position 5 of glutamine and the position 5 of neuronal glutamate. After the second turn of the TCA cycle, tracer from [1-11C]acetate (and also a part from glial [5-11C]glutamate) is transferred to glial [1-11C]glutamate and further to [1-11C]glutamine and neuronal glutamate through the neurotransmission cycle. Brain poster session: oxidative mechanisms S460 Journal of Cerebral Blood Flow & Metabolism (2009) 29, S455-S466 Results: The standard acetate two-pool PET model describes the system by a plasma pool and a tissue pool linked by rate constants. Experimental data are not fully described with only one tissue compartment (Figure 1B). The modified NMR model was fitted successfully to tissue time-activity curves from 6 single animals, by varying the glial mitochondrial fluxes and the neurotransmission flux Vnt. A glial composite rate constant Kgtg=Vgtg/[Ace]plasma was extracted. Considering an average acetate concentration in plasma of 1 mmol/g5 and the negligible additional amount injected, we found an average Vgtg = 0.08±0.02 (n = 6), in agreement with previous NMR measurements.1 The tissue time-activity curve is dominated by glial glutamate and later by glutamine (Figure 1B). Labeling of neuronal pools has a low influence, at least for the 20 mins of beta-probe acquisition. Based on the high diffusivity of CO2 across the blood-brain barrier; 11CO2 is not predominant in the total tissue curve, even if the brain CO2 pool is big compared with other metabolites, due to its strong dilution through unlabeled CO2 from neuronal metabolism and diffusion from plasma. Conclusion: The two-compartment model presented here is also able to fit data of positron emission experiments and to extract specific glial metabolic fluxes. 11C-labeled acetate presents an alternative for faster measurements of glial oxidative metabolism compared to NMR, potentially applicable to human PET imaging. However, to quantify the relative value of the TCA cycle flux compared to the transmitochondrial flux, the chemical sensitivity of NMR is required. PET and NMR are thus complementary.

Ya Adimou Chani ; Ya Adimou Chani (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et perc.]

[Traditions. Afrique du Nord. Algérie]

Ya Adimou Chani ; El Hamed El Rab El Hanine / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Billah Ya Ziar ; Billah Ya Ziar (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Allah Ya Ramani ; Allah Ya Ramani (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc.voc et instr.]

[Traditions. Afrique du Nord. Algérie]

Salouala Has Brouhi ; Salouala Has Brouhi (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Alyoum Has Brouhi ; Alyoum Has Brouhi (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Lailaha Ila Allah Adaien Rabi ; Lailaha Ila Allah Adaien Rabi (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Ksida De Si Thami ; Aloichame / Thatha El Ralpya, chant [acc. instr.]

[Traditions. Afrique du Nord. Maroc]