Cross-polarization population transfer experiments

Recently, Asbrook and Wimperis[89] proposed a novel approach for combining SQ cross-polarization and MQMAS the SQ coherences created in the CP step are transferred directly to 3Q coherences, rather than via a population state. This results in a pulse sequence with a minimum number of coherence-transfer steps. Two types of experiments have been proposed, using either a z-filter or a reversed split-ti, where pure-phase hneshapes are recorded via acquisition of the whole-echo. In addition, the authors showed that the sensitivity of both experiments is considerably improved by incorporating FAM pulses in the SQ to MQ excitation step. 

One of the main goals of the crossed-beam experiment is to measure the internal energy AEvlh rol transferred to the molecule. In principle, this is possible in either of two ways. First, the scattered molecules could be detected and their product-state population analyzed. Infrared emission or absorption techniques may be considered, similar to those used in cell experiments.13 21 Although such studies would lead to the most detailed results (at least for polar molecules), under crossed-beam conditions they are impossible for intensity reasons, even if the possibility of measuring differential cross sections is renounced and the molecules in the scattering volume itself are detected. Detection via electronic molecular transitions may be invisaged. Unfortunately, the availability of tunable lasers limits this possibility to some exotic molecules such as alkali dimers. The future development of UV lasers could improve the situation. Hyper-Raman... 

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