Nuclear magnetic resonance selective excitation

Nuclear magnetic resonance spectroscopy is a technique that, based on the magnetic properties of nuclei, reveals information on the position of specific atoms within molecules. Other spectroscopic methods are based on the detection of fluorescence and phosphorescence (forms of light emission due to the selective excitation of atoms by previously absorbed electromagnetic radiation, rather than to the temperature of the emitter) to unveil information about the nature and the relative amount specific atoms in matter. 

Stott K, Stonehouse J, Keeler J, Hwang T-L, Shaka AJ. Excitation sculpting in high-resolution nuclear magnetic resonance spectroscopy application to selective NOE experiments. J. Am. Chem. Soc. 1995 117 4199-4200. 

The addition of a chemical species with a large dielectric constant to induce desired microwave effects in matrices devoid of such substances, or lacking substances with significantly different dielectric constants, can be compared, on a conceptual basis, to cross-polarisation experiments carried out in nuclear magnetic resonance spectroscopy (see Chapter 6). In that case, a nucleus that relaxes relatively rapidly is excited selectively and allowed to transfer that excitation energy to neighbouring nuclei with low or relatively lower relaxation rate (e.g., nuclei being cross-polarised to nuclei). 

Morris G A and Freeman R 1978 Selective excitation in Fourier transform nuclear magnetic resonance J. Magn. Reson. 29 433-62... 

The selection rules help to predict the probability of a transition but are not always strictly followed. If the transition obeys the rules it is allowed, otherwise it is forbidden. A molecule can become excited in a variety of ways, corresponding to absorption in different regions of the spectrum. Thus certain properties of the radiation that emerges from the sample are measured. The fraction of the incident radiation absorbed or dissipated by the sample is measured in optical (ultraviolet and visible) absorption spectroscopy and some modes of nuclear magnetic resonance spectrometry (NMR). Because the relative positions of the energy levels depend characteristically on the molecular structure, absorption spectra provide subtle tools for structural investigation. 

Nuclear excitation and nuclear resonant scattering with synchrotron radiation have opened new fields in Mossbauer spectroscopy and have quite different aspects with the spectroscopy using a radioactive source. For example, as shown in Fig. 1.10, when the high brilliant radiation pulse passed through the resonant material and excite collectively the assemblies of the resonance nuclei in time shorter than the lifetime of the nuclear excited state, the nuclear excitons are formed and their coherent radiation decay occurs within much shorter period compared with an usual spontaneous emission with natural lifetime. This is called as speed-up of the nuclear de-excitation. The other de-excitations of the nuclei through the incoherent channels like electron emission by internal conversion process are suppressed. Synchrotron radiation is linearly polarized and the excitation and the de-excitation of the nuclear levels obey to the selection rule of magnetic dipole (Ml) transition for the Fe resonance. As shown in Fig. 1.10, the coherent de-excitation of nuclear levels creates a quantum beat Q given by... 

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