We demonstrate, both theoretically and experimentally, that the excitation
efficiency of triple-quantum coherences in 3QMAS experiments of spin 5/2 sy
stems can be enhanced by manipulating the population of the external satell
ites prior to hard pulse excitation of the 3Q coherence. Theoretically, a 6
7% or 33% increase can be achieved by respectively inverting or saturating
the outer satellites. Experimentally an increase of approximately 25% is ob
tained. The manipulation of the external satellites is achieved using time-
dependent amplitude-modulated pulses. It is demonstrated for the mineral an
dalusite that combining enhancements in 3Q-excition and 3Q --> 1Q conversio
n leads to 3QMAS spectra with well-defined line shapes for sites experienci
ng strongly different quadrupolar interactions. (C) 2001 Elsevier Science B
.V. All rights reserved.