Selective Spectroscopy

There are a large number of cases where the spectra of luminescence centers remain broad up to helium temperatures. In certain cases, this is explained by a strong electron-phonon interaction, but more often the inhomogeneous broadening, connected with several types of the same center presence, causes this. In such cases it is possible to simplify the spectrum by selective excitation of specific centers. 

---

Osad ko, Selective Spectroscopy of Chromophore Doped Polymers and Glasses. Vol. 114, pp. 123-186. 

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

In the following sections (Sections 5.5.2.2 and 5.5.2.3), two approaches to spatially resolving chemical conversion within a reactor are demonstrated (a) n-dimensional Chemical Shift Imaging (CSI), and (b) volume selective spectroscopy. 

Figure 5.5.8 shows the volume selective spectroscopy pulse sequence used. This pulse sequence combines elements of NMR spectroscopy and MRI pulse sequences three slice selective rf pulses are applied in three orthogonal directions to obtain 1H spectra from pre-determined local volumes within the sample [30]. In this particular application, spectra were recorded from local volumes of dimension 1.5 mm x 1.5 mm x 0.5 mm within the fixed bed the data acquisition time for each spectrum was 3 min. Figure 5.5.9(a) shows the local volumes selected within slice 3 of the bed, as identified in Figure 5.5.7. In Figure 5.5.9(b) the 1H spectra recorded from within these volumes are shown data are presented only for the range of... 

Fig. 5.5.8 The volume selective spectroscopy rf pulse sequence used to acquire the data shown in Figures 5.5.9-5.5.11. Magnetic field gradients applied alongthex, y and zdirections enable localized spectra to be recorded from...

Fig. 5.5.9 Volume selective spectroscopy within the fixed-bed reactor, (a) The location of the ten selected volumes within image slice 3 (see Figure 5.5.7) are identified, (b) It is clearly seen that the chemical shifts of peak B obtained from the spectra recorded from the selected volumes occur at significantly different values...

Bonsma S, Purchase R, lezowski S, Gallus J, Konz F, Volker S (2005) Green and red fluorescent proteins Photo- and thermally induced dynamics probed by site-selective spectroscopy and hole burning. Chemphyschem 6 838-849... 

In this pulse sequence the attributes of NMR spectroscopy and MR imaging are again combined. In this case three slice-selective r.f. pulses are applied in three orthogonal directions to obtain H spectra from predetermined local volumes within the sample.27 A typical volume-selective spectroscopy pulse sequence is shown in... 

Fig. 9 A volume-selective spectroscopy r.f. pulse sequence. Magnetic field gradients applied along x-, y-, and z-directions enable recording of localized spectra from pre-defined local volume elements within the sample.

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. 

Various compounds have been analyzed by site-selection spectroscopy. An interesting application is the identification of loci of damage in DNA resulting from reactions with carcinogens such as polycyclic aromatic hydrocarbons. 

SITE SELECTIVE SPECTROSCOPY AND EXCITED STATE ABSORPTION... 

---