Solid-State Emission Spectroscopy

At room temperature, solid [Pt(bph)(CO)2] 26 (H2bph=biphenyl) displays an intense and broad emission at 726 nm [33]. As described in earlier report [34], [Pt(bph)(CO)2] packs in a columnar structure with Pt-Pt distances of 3.24 A. Such close interatomic contacts in the chain packing of square planar platinum(II) complexes always impart red-shifts in their emission spectra. The emission maximum is red-shifted to 791 nm upon decreasing the temperature from 296 to 77 K, which is consistent with a decrease in the Pt-Pt separation observed by Connick [35] and Yersin [3a]. 

For the trinuclear complex [Pt3(C1 ANAN)3(/t3-dpmp)]3+ 16 [27], the orange 16(BF4)3 exhibits solid-state emission at Amax 630 nm which is reminiscent of i(nn ) excimeric emission. The luminescence of the red and brown-red forms of 16(004)3 appear at Amax 674 and 709 nm respectively, suggesting an increase in 3MMLCT character (Fig. 8). 

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The melting process of potassium fluorotantalate, K2TaF7, was investigated by IR emission spectroscopy using thick layers of the melt [356]. It should be mentioned that in some cases, if the temperature of the sample is high enough, the above method enables to obtain spectra of the material in solid state as well. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

Characterization of the solid phases Si, A1 and Na contents were determined by proton-induced Y-ray emission (PIGE) (11,57) or by high resolution solid state i—NMR spectroscopy (49,50,58)... 

Regarding the study of these complexes by various physical techniques, only IR spectroscopy has been widely used so far. Only a few X-ray structural, electronic absorption, and fluoresence emission spectral data are available. Other methods such as ESR (especially of Gd(III) complexes), NQR, and Mossbauer (especially of Eu-151) have not been seriously applied for the study of these complexes in the solid state. In solution, only conductance studies have attracted attention NMR, dipole moment, and electronic spectral studies are few in number. The lack of physical data limits our understanding of the structure and bonding in these complexes. In future, when more interest is evinced in applying various physical techniques to study these complexes, one may hope to come across more interesting and useful revelations. 

This book treats the most basic aspects to be initiated into the field of the optical spectroscopy of solids, so that a student with some background in quantum physics, optics, and solid state physics may be able to interpret simple optical spectra (absorption, reflectivity, emission, scattering, etc.) and learn about the main basic instrumentation used in this field. 

Field emission is a tunneling phenomenon in solids and is quantitatively explained by quantum mechanics. Also, field emission is often used as an auxiliary technique in STM experiments (see Part II). Furthermore, field-emission spectroscopy, as a vacuum-tunneling spectroscopy method (Plummer et al., 1975a), provides information about the electronic states of the tunneling tip. Details will be discussed in Chapter 4. For an understanding of the field-emission phenomenon, the article of Good and Muller (1956) in Handhuch der Physik is still useful. The following is a simplified analysis of the field-emission phenomenon based on a semiclassical method, or the Wentzel-Kramers-Brillouin (WKB) approximation (see Landau and Lifshitz, 1977). 

Localization versus itineracy and the degree of hybridization of 5 f states with orbitals of the actinide atom (especially 6 d) as well as with those of the ligand in compounds are central questions for the understanding of bonding in actinide solids. Photoelectron spectroscopy provides answers to these questions. In narrow band solids, like the actinides, the interpretation of results requires the use of band calculations in the itinerant picture, as well as models of final state emission in the atomie picture. 

The very accurate spectroscopy by the solid state detectors have led to the discovery of the line profile variations in Be stars on a time scale of several hours to day. Such variations are considered due to nonradial pulsations(NRPs)(see Baade 1987), of which nature is correlated with Be emission activity. Mass-ejection driven by NRPs like radial pulsation was suspected by Willson(1986). But the quasi-periodicity of mass-loss cannot be explained naturally by this mechanism. 

If high temperatures eventually lead to an almost equal population of the ground and excited states of spectroscopically active structure elements, their absorption and emission may be quite weak, particularly if relaxation processes between these states are slow. The spectroscopic methods covered in Table 16-1 are numerous and not equally suited for the study of solid state kinetics. The number of methods increases considerably if we include particle radiation (electrons, neutrons, protons, atoms, or ions). We note that the output radiation is not necessarily of the same type as the input radiation (e.g., in photoelectron spectroscopy). Therefore, we have to restrict this discussion to some relevant methods and examples which demonstrate the applicability of in-situ spectroscopy to kinetic investigations at high temperature. Let us begin with nuclear spectroscopies in which nuclear energy levels are probed. Later we will turn to those methods in which electronic states are involved (e.g., UV, VIS, and IR spectroscopies). 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

The chemical composition with respect to Si and metallic impurities (mainly Fe, Ca, Al) is generally determined by wet chemical methods in combination with standard spectroscopic techniques (AAS, AES, XRF) (Table 8) [224-226]. A precondition is the dissolution of the powder. Typical dissolving processes are fusion with sodium carbonate or mixtures of sodium carbonate and boric acid, with alkaline hydroxides [225, 226] and special acid treatments [225]. A more effective analysis based on optical emission spectroscopy allows the direct analysis of impurities in the solid state and requires no dissolution step [227]. 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

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