Synchrotron advantage

The advantages of SEXAFS/NEXAFS can be negated by the inconvenience of having to travel to synchrotron radiation centers to perform the experiments. This has led to attempts to exploit EXAFS-Iike phenomena in laboratory-based techniques, especially using electron beams. Despite doubts over the theory there appears to be good experimental evidence that electron energy loss fine structure (EELFS) yields structural information in an identical manner to EXAFS. However, few EELFS experiments have been performed, and the technique appears to be more raxing than SEXAFS. 

The diffraction mechanisms in XPD and AED are virtually identical this section will focus on only one of these techniques, with the understanding that any conclusions drawn apply equally to both methods, except where stated otherwise. XPD will be the technique discussed, given some of the advantages it has over AED, such as reduced sample degradation for ionic and organic materials, quantification of chemical states and, for conditions usually encountered at synchrotron radiation facilities, its dependence on the polarization of the X rays. For more details on the excitation process the reader is urged to review the relevant articles in the Encyclopedia and appropriate references in Fadley. ... 

The major advantage of time-resolved X-ray techniques, as compared to optical spectroscopy, is that their wavelength X as well as the pulse duration r can be chosen to fit the atomic scales. This is not the case for optical spectroscopy, where the wavelength X exceeds interatomic distances by three orders of magnitude at least. Unfortunately, X-ray techniques also have their drawbacks. They require large-scale instruments such as the synchrotron. Even much larger... 

The advantages of XAS are that it is element specific, can be used in situ, and yields both chemical and structural information. Also, the theory is very well developed. The main disadvantage is the need to use a synchrotron facility. 

Single-line sources are now available which cut down the number of resonance lines in a spectrum and thereby reduce the resolution problems considerably. Since many laboratories have access to electron and ion accelerators to produce the parent nuclides Co and Cu, the major experimental obstacles to Ni spectroscopy have been overcome and a good deal of successful work has been performed in recent years. Moreover, the development of synchrotron radiation instead of conventional Mossbauer sources is of additional advantage for future Mossbauer applications (see below). 

Ta foil Nuclear forward scattering of synchrotron radiation (NFS) at Ta resonance in Ta foil without and with applied magnetic field, point out advantages over conventional Ta Mossbauer spectroscopy... 

By using NFS, information on both rotational and translational dynamics can be extracted. In many cases, it would be favorable to obtain separate information about either rotational or translational mobility of the sensor molecule. In this respect, two other nuclear scattering techniques using synchrotron radiation are of advantage. Synchrotron radiation-based perturbed angular correlations (SRPAC) yields direct and quantitative evidence for rotational dynamics (see Sect. 9.8). NIS monitors the relative influence of intra- and inter-molecular forces via the vibrational density of states (DOS) which can be influenced by the onset of molecular rotation (see Sect. 9.9.5). 

Table 8.54 indicates the main characteristics of X-ray powder diffraction. Powder samples of lmg or greater can be analysed. The principal advantage is that substances (e.g. BaS04) can be identified directly, and not indirectly via their elements (e.g. Ba and S). This is particularly advantageous if both silica and silicates (kaolin, talc) are present. Detection limits are matrix dependent, but ca. 3% in a two-phase mixture, with synchrotron radiation ca. 0.1 %. 

XRD and LEED are laboratory techniques, although synchrotrons offer advantages for X-ray diffraction. EXAFS, on the other hand, is usually done at synchrotrons. This, and the fact that EXAFS data analysis is complicated and not always without ambiguity, have inhibited the widespread use of the technique in catalysis. 

Using synchrotron radiation as a source for XRD has considerable advantages. First,... 

Synchrotron radiation can also be used as an excitation source with the advantage of almost constant intensity versus wavelength over a very broad range, but the pulse width is in general of the order of hundreds of picosecond or not much less. There are only a few sources of this type in the world. 

Synchrotrons have the advantage of brighter beams, multiple wavelength availabihty, and a more focused beam, all of which can greatly improve the quality of data and may be essential for the experiments that are to be carried out. The synchrotrons I have worked at are staffed by excellent people however, they do assume a certain level of user competence and are probably not the place to go to learn how to diffract crystals. Synchrotrons will grant time to users in a peer-reviewed process and generally have quite rapid access times (usually booked per 24-h slot). 

---