Sensitivity Enhancement Schemes in MQMAS Experiments

Interconversion of single- and triple-quantum coherences was first suggested by Vega and Naor, who demonstrated how application of amplitude-modulated pulses, having a modulation frequency equal to the 13/2) 11/2) satellite transition frequency t i/2,3/25 

It is not surprising, therefore, that upon the introduction of the MQMAS experiment, frequency and amplitude modulations were suggested to replace the hard pulses applied for the transfer of MQ SQ coherences. and FAM methods have been 

Both FAM and DFS methods yield relatively undistorted lineshapes. Prior to the introduction of FAM and DFS, Wu et suggested to replace both excitation and conversion pulses by long RF irradiation pulses lasting for quarter of the rotor period, a scheme that was entitled RIACT. Such pulses have been shown to induce adiabatic transitions between different orders of transitions in an efficient way. RIACT results in enhancements similar to those of FAM and DFS and, whilst it provides uniform excitations for a large range of Vq values, it results in ill-defined lineshapes.  

In the following, we will primarily deal with FAM pulses. The hard pulses are denoted as Ph and soft RF pulses as Ps which act selectively on the central-transition SQC. In all the figures and discussions below we denote groups of pulses with an overall 180° phase shift by an over-bar. 

Two cases need attention Improving the excitation and conversion efficiency of MQC in 2D MQMAS experiments and implementing appropriate modulation techniques to 

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