# Resonance condition for absorption of energy

In NMR, for example, why it is not possible to excite nuclei from one spin state to another using energy of arbitrary frequency?

## 1 Answer

This is indeed the basis of modern Fourier transform NMR (the technique that all modern spectrometers use).

The molecule is irradiated with a range of frequencies and the response detected. Application of some clever maths (the Fourier transform) then allows the individual signals to be observed.

You can think of this a bit like bashing every key on a piano and recording the noise. Application of Fourier transform would then allow you to get back the sound for the individual keys.