Frequency stepped adiabatic passage excitation of half-integer quadrupolar spin systems

The excitation of half-integer quadrupolar spins by frequency stepped pulses is explored in powdered samples, under both static and magic angle spinning (MAS) conditions. The effects of sweep rate, sweep range, RF field strength, rotation and quadrupole frequency are investigated by numerical simulations and experiments. Using excitation by a frequency stepped half-passage it is possible to generate undistorted, quantitatively interpretable central-transition powder spectra in static samples, with an improved sensitivity compared with short-pulse excitation. Under MAS the duration of the half-passage must be shorter than 1/2 rotor period to get simple results. The conditions required for a quantitatively interpretable frequency stepped half-passage are formulated and subsequently tested on a number of model compounds. The conditions to bring about a complete inversion of the spin system by an adiabatic full-passage are more stringent than for excitation, and cannot be realized in most practical cases. Selective excitation and inversion of a single transition is feasible, however.