Hard pulse is a high power pulse for short duration. It non selectively excites large bandwidth of frequencies. it is designed to excites the whole range of chemical shifts of one type of nuclei(example 1H). Soft pulse is a long pulse with low power. This is also called shaped pulse. Different shapes of the RF profile can be chosen to obtain homogeneous frequency response in the region of interest. This is used for selective excitation of small bandwidth (few Hz) while leaving other resonances unaltered.

Hi,

A radio-frequency pulse is used in NMR to excite a desired band-width (bw), such that the duration of the pulse (pulse width, pw) dictates the bw of excitation. The free induction decay signal resulting from such a pulse is obtained by discrete sampling of the points as a function of time. The rate of sampling decides the spectral/sweep width (sw) of the spectrum.

In the cases where the bw of excitation is greater than the sw, then it is termed as a hard pulse. This is the most common type of pulse that is employed in NMR. For example, a 10 us long rectangular (rect) pulse would excite a bw of 1/(4*10us), bw = 250 kHz. Assuming that it is a pulse that excites protons, then the desired spectral width would be say 10-15 kHz. In this case the bw >> sw, thus making this a hard pulse.

Soft or shaped pulses are employed for selective excitation of the spectral width, where the bw < total sw. That is, the pulse is used to excite only one portion of the entire window being probe. For example, pre-saturation rect pulse is applied (to suppress only the water resonance) say for ~500ms would excite a bw of ~0.5 Hz. In order to achieve better selectivity (such that the resonances out of the bw experiences minimum perturbation from the pulse) and use minimum possible power to achieve the execution of the pulse, the shape (amplitude and phase) is selected/derived and optimized depending on the purpose it has to solve. I would like to draw your attention to this and also several publications from Ray Freeman if you need more info.

Waj