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Chemistry · Glossary

What is Radiofrequency pulse, flip angle?

Also known as: radiofrequency pulse · flip angle

Definition 8.8 University Chemistry — Year 3 · Chapter 8 — NMR in Depth: Pulses, Relaxation and 2D

A radiofrequency pulse is a short burst of radio waves at νrf≈ν0\nu_{\mathrm{rf}} \approx \nu_0, whose magnetic field B1B_1, fixed in the rotating frame along x′x', turns M\mathbf M about x′x'. The angle turned is the flip angle θ=γB1tp\theta = \gamma B_1t_p for a pulse of duration tpt_p: a 90∘90^\circ pulse brings M\mathbf M into the x′y′x'y' plane, a 180∘180^\circ pulse inverts it.

The magnetisation in the rotating frame. A 90 pulse about x' turns M from z to y' (with > 0 the rotation follows the left-hand rule about B_1, the usual convention); a 180 pulse inverts it.
The magnetisation in the rotating frame. A 90∘90^\circ pulse about x′x' turns M\mathbf M from zz to y′y' (with γ>0\gamma > 0 the rotation follows the left-hand rule about B1\mathbf B_1, the usual convention); a 180∘180^\circ pulse inverts it.
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