University of Ottawa NMR Facility Web Site

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Tuesday, November 20, 2007

Excitation Profiles

The range of frequency over which NMR resonances are excited depends on the duration of the monochromatic radio frequency pulse applied to the sample. Very long pulses will excite a very narrow range of frequencies whereas very short pulses will excite very wide frequency ranges. For example, presaturation pulses used for solvent suppression, are typically several seconds long while non-selective pulses are typically micro seconds in duration. It is essential to use very short pulses to provide even excitation over large spectral widths. The shape of the excitation profile is related to the Fourier transform of the pulse.

3 comments:

Liming Wang said...

Dear Glenn

thanks a lot for these posters which are of great helpful for new beginners like me. may i ask the reason why a relative longer pulse can only excite a narrow width? thank you very much for your time.

Glenn Facey said...

Liming,
In simple terms, the excitation profile of a pulse is the Fourier transform of the pulse. Short monochromatic pulses transform to broad excitation profiles whereas long monochromatic pulses transform to narrow excitation profiles. In the limit, an infinitely long pulses will excite only a single frequency.
Glenn

Anonymous said...

GLEN,
YOU ARE A HELP TO MILLIONS OF PEOPLE AND YOU SHOULD THINK THIS EVERYDAY BEFORE GETTING UP OF THE BED.
THANK YOU!
Vindana