Echo Sequences

From NMR Wiki

Since the signals in solid state NMR are often very broad, the spins dephase quickly, allowing only a short time for observation. The spectrometers dead time can therefore become a significant problem. To overcome this, a second pulse phased 90° relative to the original excitation, can be used to trigger rephasing of the spins. The time between these two pulses should (in theory) be equal to the dead time, so that the spins are rephased once aquesition begins.

Usually τ is chosen to be bigger than DT, to make sure that the signal maximum of the echo is observed. The FID is then left shifted until the signal maximum is at the beginning.

Spin Echo

The spin-echo (or hahn-echo) pulse sequence is useful for lines broadened by chemical shift anisotropy or heteronuclear dipole-dipole couplings.

Solid Echo

The solid-echo (or quadropole-echo) pulse sequence is used for lines broadened by quadropole coupling or homonuclear dipole-dipole coupling.