Short pulses in NMR - what is their shape?

Question

In the Wikipedia article Nuclear magnetic resonance, section Fourier-transform spectroscopy, it says the following:

Fourier-transform spectroscopy Most applications of NMR involve full NMR spectra, that is, the intensity of the NMR signal as a function of frequency. Early attempts to acquire the NMR spectrum more efficiently than simple CW methods involved illuminating the target simultaneously with more than one frequency. A revolution in NMR occurred when short radio-frequency pulses began to be used, with a frequency centered at the middle of the NMR spectrum. In simple terms, a short pulse of a given "carrier" frequency "contains" a range of frequencies centered about the carrier frequency, with the range of excitation (bandwidth) being inversely proportional to the pulse duration, i.e. the Fourier transform of a short pulse contains contributions from all the frequencies in the neighborhood of the principal frequency. The restricted range of the NMR frequencies made it relatively easy to use short (1 - 100 microsecond) radio frequency pulses to excite the entire NMR spectrum.

My 1st question is: what is the general shape of these pulses? Are they simply Gaussian-like pulses, or is it a monochromatic wave of given frequency emitted for a short time?

My second question is: what does the author mean by short pulse of "given carrier" frequency?

As a bonus, I would also enjoy some hints about the other assertions in the quoted text.

Answer 1

Modern NMR spectrometers can make RF pulses with any envelope, and can also create other kinds of RF modulation. However, for simple spectroscopy applications it is usually sufficient to use square pulses. A typical application would be proton NMR at 500 MHz (about 11 Tesla). Proton spectra cover a chemical shift range of about 10 ppm, so the width of the spectrum will be 5 kHz. A typical RF excitation pulse will have its carrier frequency tuned to the center of the spectrum, and a square envelope lasting about 10 us. This will uniformly excite the entire spectrum.