Spectroscopy observing universe

The natural linewidth comes from the lifetime, r, of the upper state of a spontaneous transition, which is related to the Einstein A coefficient so that r = A l faster transitions have shorter lifetimes and vice versa, and similarly an allowed transition will have a short lifetime for the upper state whereas forbidden transitions will have a long lifetime. The lifetime consideration is very important in the laboratory where transitions have to occur on the timescale of the experiment, otherwise they are not observed. Hence in the laboratory allowed transitions are observed and in general (but not specifically) forbidden transitions are not seen. For astronomy this does not matter. So what if a forbidden transition has a lifetime of 30 million years - the Universe is 15 billion years old - if you wait long enough it will happen. The rules of spectroscopy need to be understood but in space anything goes ... 

Much of the electromagnetic spectrum has been used to investigate the structure of matter in the laboratory but the atmospheric windows restrict astronomical observations from Earth. Irritating as this is for astronomers on the ground, the chemical structure of the atmosphere and the radiation that it traps is important to the origins of life on Earth. The light that does get through the atmosphere, however, when analysed with all of the tools of spectroscopy, tells the molecular story of chemistry in distant places around the Universe. 

Fortunately, in favorable cases enhancement mechanisms operate which increase the signal from the interface by a factor of 105 — 106, so that spectra of good quality can be observed - hence the name surface-enhanced Raman spectroscopy (SERS). However, these mechanisms seem to operate only on metals with broad free-electron-like bands, in particular on the sp metals copper, silver and gold. Furthermore, the electrodes must be roughened on a microscopic scale. These conditions severely limit the applicability of Raman spectroscopy to electrochemical interfaces. Nevertheless, SERS is a fascinating phenomenon, and though not universally applicable, it can yield valuable information on many interesting systems, and its usefulness is expected to increase as instrumentation and preparation techniques improve. 

It is noteworthy that the neutron work in the merging region, which demonstrated the statistical independence of a- and j8-relaxations, also opened a new approach for a better understanding of results from dielectric spectroscopy on polymers. For the dielectric response such an approach was in fact proposed by G. Wilhams a long time ago [200] and only recently has been quantitatively tested [133,201-203]. As for the density fluctuations that are seen by the neutrons, it is assumed that the polarization is partially relaxed via local motions, which conform to the jS-relaxation. While the dipoles are participating in these motions, they are surrounded by temporary local environments. The decaying from these local environments is what we call the a-process. This causes the subsequent total relaxation of the polarization. Note that as the atoms in the density fluctuations, all dipoles participate at the same time in both relaxation processes. An important success of this attempt was its application to PB dielectric results [133] allowing the isolation of the a-relaxation contribution from that of the j0-processes in the dielectric response. Only in this way could the universality of the a-process be proven for dielectric results - the deduced temperature dependence of the timescale for the a-relaxation follows that observed for the structural relaxation (dynamic structure factor at Q ax) and also for the timescale associated with the viscosity (see Fig. 4.8). This feature remains masked if one identifies the main peak of the dielectric susceptibility with the a-relaxation. 

The motivation is the same as that which lannched spectroscopy in such a spectacular way over the past thirty years, that is, to test the theory of nucleosynthesis and the chemical evolntion of galaxies. Cosmologists are delighted to point ont that today we may assess chemical abundances in the remote Universe by pure observation, whereas even ten years ago, such a feat remained ont of reach. 

From the time of the first direct observation of a NMR signal in 1957 (128) until about 1968, the use of NMR spectroscopy was in the hands of the few specialists who were prepared to build their own spectrometers. During this period, attention was focused on the Present address Department of Chemistry, The University, Sheffield, England. 

As a result, cuvettes for Raman spectroscopy should be carefully selected. They may, due to their impurities, add a background to the spectrum of the sample. In addition, all cuvette materials produce their own Raman spectra, which have to be considered, when the Raman spectra of the sample are evaluated. Fig. 3.5-17 a shows a Raman spectrum of a typical optical glass BK7, Fig. 3.5-17 b that of quartz glass suprasil, and Fig. 3.5-17 c of sapphire. Suprasil is a synthetic quartz which does not normally contain impurities. Therefore, Suprasil of ESR quality is highly recommended as Raman cuvette material. Also, sapphire is a good cuvette material, as it is very hard, inert, has a good thermal conductance, and shows only weak but sharp Raman lines (Porto and Krishnan, 1967). It is used for the production of the universal Raman cell (Schrader, 1987). The sharp Raman lines of sapphire observed in the spectra of the sample may be subtracted from the spectrum or used as internal standard for quantitative analyses (Mattioli et al, 1991). 

The application of IR spectroscopy to catalysis and surface chemistry was later developed in the fifties by Eischens and coworkers at Texaco laboratories (Beacon, New York) in the USA [7] and, almost simultaneously, by Sheppard and Yates at Cambridge University in the UK [8]. Mapes and Eischens published the spectra of ammonia chemisorbed on a silica-alumina cracking catalyst in 1954 [6], showing the presence of Lewis acid sites and also the likely presence of Br0nsted acid sites. Eischens, Francis and Pliskin published the IR spectra of carbon monoxide adsorbed on nickel and its oxide in 1956 [9]. Later they presented the results of an IR study of the catalyzed oxidation of CO on nickel at the First International Congress on Catalysis, held in Philadelphia in 1956 [10]. Eischens and Pliskin also published a quite extensive review on the subject of Infrared spectra of adsorbed molecules in Advances in Catalysis in 1958, where data on hydrocarbons, CO, ammonia and water adsorbed on metals, oxides and minerals were reviewed [11]. These papers evidence clearly the two tendencies observed in subsequent spectroscopic research in the field of catalysis. They are the use of probes to test the surface chemistry of solids and the use of spectroscopy to reveal the mechanism of the surface reactions. They used an in situ cell where the catalyst sample was... 

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