Synchronised resonance

To obtain a high-resolution spectrum free from the unwanted CT CT peak, the third method proposed uses half-rotor synchronisation, instead of full-rotor period synchronisation, during [40]. The increment in (or kt for split-ti experiments) is set to half the rotor period, thereby doubling the isotropic spectral width (Fi=2Vj., where Vj. is the MAS rate). This means that two sets of STn CT resonances will be observed, a centreband and a spinning sideband. In principle, the latter will be well resolved from the CT CT diagonal peaks, which in general do not display any sidebands in Compared with a full-rotor synchronisation, the S/N ratio is reduced by a factor of two and the experimental time is doubled. As an example. Fig. 13a shows the STMAS spectrum of zeolite scolecite. 

In laser vaporisation experiments, generating a plume , the laser s frequency may be synchronised with the resonance line of the element (analyte) to be analysed. The basic principles are (i) absorption of the radiation by the analyte (LAAS laser atomic absorption spectrometry) (ii) fluorescence (LIE, laser-induced fluorescence LEAFS) or (Hi) production of ionisation products (ions and electrons). LIF is an analytical method of high precision that is suitable for the measurement of diatomic species in the plume. Excitation spectroscopy or laser-excited fluorescence is not concerned with the spectral composition of the fluorescence but with how the overall intensity of emission varies with the wavelength of excitation. 

---