Molybdenum-100, ground-state

Fig. 2. X-ray absorption near-edge structure (XANES) of reference compounds with various molybdenum valence states and mine tailings samples. The Mo K-edge excitation potential for molybdenum in the ground state (Mo°) is at 20000 eV. The Mo K-edge excitation potential increases with increasing valence state. 

More complex iron-sulfur clusters are also known to exist. These include the iron-molybdenum cofactor of nitrogenase (Thornely and Lowe, 1984) and probably larger clusters in which the only metal is iron (Hagen, 1987). They are characterized by highly anisotropic EPR spectra from S > ground states the nitrogenase cluster, for example is S = j and has EPR features near g = 4 and g = 2. 

All the monomeric molybdenum(III) complexes are paramagnetic, and in Table l5 the magnetic moments of some typical examples are given. For six-coordinate compounds the values lie between 3.53 and 3.86 BM, with most in the region 3.7 to 3.8 BM as predicted for octahedral d3 complexes.37 The value of 1.73 BM in ELt[Mo(CN)7] 2H20 is consistent with a spin-doublet ground state d3 system.3S... 

In addition to the above parameters, determined from the microwave data, the metal Fermi contact interaction constants in 97 MoN and 53 CrN were determined from the optical spectra [72] to be -508 and -179.8 MHz respectively. The element molybdenum belongs to the 4d transition metal row, which we will discuss further in due course. For the CrN molecule in its ground state, ab initio calculations [73] and molecular orbital considerations [74] suggest that the dominant electron configuration may be written... 

For the majority of elements commonly determined in water by AAS, an air—acetylene flame (2300°C) is sufficient for their atomisation. However, a number of elements are refractory and they require a hotter flame to promote their atomisation. Because of this, a nitrous oxide—acetylene flame (3000° C) is used for the determination of these elements. Refractory elements routinely determined in water are aluminium, barium, beryllium, chromium and molybdenum. Chromium shows different absorbances for chromium(III) and chromium(VI) in an air-acetylene flame [15] but use of a nitrous oxide-acetylene flame overcomes this. Barium, being an alkaline earth metal, ionises in a nitrous oxide—acetylene flame, giving reduced absorption of radiation by ground state atoms, however in this case an ionisation suppressor such as potassium should be added to samples, standards and blanks. 

Initially the molybdenum fluxes in [55] measured via atomic Mo-lines (379.8 nm, 386.4 nm and 390.3 nm) led to unreasonably high flux values. This was the case because for the calculation of the respective excitation rates the formula of van Regemorter was used [56], as these lines are optically coupled to the ground state (resonance lines). Therefore, experiments were performed to measure the excitation and ionization rates directly both in a linear plasma machine [57] and in a crossed beam experiment with a thermal molybdenum emitter [58], The results can be seen in Fig. 6.17 and show that the experimental values for excitation are about a factor of 5 larger than those from the van Regemorter formula, which leads to a reduction of the flux by the same order according to the smaller S/XB. More refined R-Matrix calculations have later confirmed the same factor and are included in Fig. 6.3. 

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