Fig. 1.

Pulse sequences for refocusing inhomogeneous and dipolar broadening. a The combined Hahn and solid echo sequence CHASE-5 with cycle time t_c = 3t_π + τ_evol consisting of the total gate time 3t_π and total free evolution time τ_evol = 6τ (where τ ≫ t_π is the free evolution time between two pulses). b Extension to CHASE-10, which is less sensitive to finite pulse durations t_π > 0. c Using symmetry considerations, a further optimised sequence CHASE-20 is constructed. d The longest sequence CHASE-34 with a total gate time 20t_π has the best refocusing capability for t_π → 0. The radio frequency (rf) carrier phases are φ_x = 0, φ_−x = π for ±x rotations around the ${ê}_{\mathrm{x}}$ axis and φ_y = π/2, φ_−y = 3π/2 for ±y rotations around ${ê}_{\mathrm{y}}$. Narrow pulses indicate π/2-rotations with pulse time t_π/2 and broad pulses correspondingly represent π-rotations with duration t_π