Spectral line narrowing techniques

An identihcation of the nature of Cr luminescence in synthetic spinel by the line narrowing technique enabled to distinguish up to 25 different Cr + sites (Deren et al. 1996). Laser-induced time-resolved spectroscopy enables us to see typical for spinel emission of Cr ", while the different broadness of the spectral lines at different time windows demonstrates that different Cr " sites are present also in natural spinel (Fig. 4.51). 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Even if perfectly narrow spectral lines are not available, we may take a clue from this approach and measure a spectral line of known shape. De-convolution should then yield the instrument function. This technique does indeed work (Chapter 2, Section II.G.3). Some of the information needed to generate the known line shape can even be obtained directly from the observed data. 

The solute molecule is dissolved in the liquid crystal solvent at low concentration. A variety of nematic solvents are available, some of which are nematic at room temperature. Representative high-resolution proton NMR spectra are given in Figure 1. Because the solvent order depends on composition and temperature, it is important that temperature and composition gradients at the NMR probe be minimized if the narrow line widths of a few hertz are to be obtained. The spectra of Figure 1 show the rapid increase of spectral complexity with the number of nuclei. The spectra become almost continuous and uninterpretable at about 10 spins. Simplified proton NMR spectra can be obtained by partial deuterium substitution and decoupling.6 This has been described for cyclohexane, but has not been used extensively. Proton double resonance is also a useful experimental technique for the identification of spectral lines.6... 

The technique is applicable to proteins and nucleic acids in their solution state. A few protein structures have been solved by using solid-state NMR, which involves spinning the sample at thousands of revolutions per second. This gives narrow spectral lines that are similar to those obtained from proteins in solution, but the instruments and data analysis are much more complex than for solution-state NMR spectroscopy. 

Each hollow cathode emits the spectrum of metal used in the cathode. For this reason, a different HCL must be used for each different element to be determined. This is an inconvenience in practice and is the primary factor that makes AAS a technique for determining only one element at a time. The handicap is more than offset, however, by the advantage of the narrowness of the spectral lines and the specificity that results from these narrow lines. 

Densitometers use a variety of optical systems. The optics utilizes either a slit at a fixed position with a mechanism to slowly move the spectral plate across the slit, or a slit that moves across the spectral line with the photographic plate in a fixed position. Either technique permits scanning the spectral line. Slits must be sufficiently narrow so that, when centered on the image of the spectral line, no light enters the slit from alongside the line. Some densitometer slits are adjustable for length and width. Others are of fixed width, but a series of fixed slits is available for interchange. 

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