Fine structure alkali

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

In light alkali atoms, Li and Na, the fine structure splitting of a low state is typically much larger than the radiative decay rate but smaller than the interval between adjacent states. In zero field the eigenstates are the spin orbit coupled tsjnij states in which and s are coupled. However, in very small fields and s are decoupled, and the spin may be ignored. From this point on all our previous analysis of spinless atoms applies. How the passage from the coupled to the uncoupled states occurs depends on how rapidly the field is applied. It is typically a simple variant of the question of how the m states evolve into Stark states. When... 

The fine structure intervals of the alkali atoms often fall in the 1-10 MHz range, in which case the transition between spin orbit and uncoupled states can be made either diabatically or adiabatically. Jeys et al.16 have observed the transition from an adiabatic to a diabatic passage from the coupled fine structure states to the uncoupled states. With a pulsed laser, they excited Na atoms from the 3p1/2 state to the 34d3/2 state with o polarized light, which leads to 25% my = 1/2 atoms and... 

Level crossing spectroscopy has been used by Fredriksson and Svanberg44 to measure the fine structure intervals of several alkali atoms. Level crossing spectroscopy, the Hanle effect, and quantum beat spectroscopy are intimately related. In the above description of quantum beat spectroscopy we implicitly assumed the beat frequency to be high compared to the radiative decay rate T. We show schematically in Fig. 16.11(a) the fluorescent beat signals obtained by... 

To put the alkali fine structure intervals in perspective it is useful to compare them to the hydrogenic intervals. For H the energy of Eq. (16.4) is valid if45... 

B. Excitation Transfer between Fine-Structure States in Alkali Atoms... 

Most, though not all, studies of collisional excitation transfer in alkali atoms have dealt with the 2P1/2 and 2Pa/i resonance substates. The experimental procedure usually involves the excitation of one 2P fine-structure state... 

When an excited alkali atom collides with a ground-state alkali atom of a different species, electronic excitation energy may be transferred either between fine-structure states of a single atom or from an excited state of one atom to that of another, as represented by the following typical equations ... 

Studies of mixing between fine-structure states in alkali atoms, induced in collisions with ground-state noble-gas atoms, have been carried out by several authors. The process may be represented by the equation... 

Cross Sections Q for Mixing between Alkali Resonance Fine-Structure States, Induced in Collisions with Noble-Gas Atoms (See Also Fig. 4.6) ... 

Studies of excitation transfer induced in collisions with molecules were not limited to fine-structure states of alkali atoms. Excitation transfer from the 621>3/2 to the 6 2 >5/2 state in thallium was investigated in a Hanle experiment... 

Cross Sections for Mixing between Alkali and between Mercury Fine-Structure States The Values in Brackets Are Theoretical, the Mercury Cross Sections Are Relative the Ratios Qi/Q Are Experimental... 

The solvent often exerts a profound influence on the quality and shape of the spectrum. For example, many aromatic chromophores display vibrational fine structure in non-polar solvents, whereas in more polar solvents this fine structure is absent due to solute-solvent interaction effects. A classic case is phenol and related compounds which have different spectra in cyclohexane and in neutral aqueous solution. In aqueous solutions, the pH exerts a profound effect on ionisable chromophores due to the differing extent of conjugation in the ionised and the non-ionised chromophore. In phenolic compounds, for example, addition of alkali to two pH units above the pKa leads to the classical red or bathochromic shift to longer wavelength, a loss of any fine structure, and an increase in molar absorptivity (hyper chromic... 

In this Chapter, we focus on alkali-aluminum surface alloys where the geometrical structure has been determined in detail. As can be seen from Table 1, which contains a list of the adsorbed phases formed by adsorption of alkali metals on aluminium surfaces, this limitation is not a serious restriction, since studies exist for a quite a number of low index aluminum surfaces and alkali metals. Although most of the structures of the phases listed in Table 1 have been determined by low energy electron diffraction (LEED), the crucial, first observation of substitutional adsorption for alkali-aluminium systems was made in a combined surface extended x-ray fine structure (SEXAFS) and density functional theory (DFT) study of the Al(l 11)—(v 3 x 3)7 30°—Na phase formed by adsorption of 1/3 ML Na at room temperature by Schmalz et al [7] in 1991. The structure of the Al(lll)-(4 x 4)—Na phase was also determined by SEXAFS. 

E.E.Nikitin, Nonadiabatic transitions between fine-structure components of alkalies in atomic collisions, Optika i Spektr. 19,161 (1965)... 

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