How do nuclei in the higher energy state return to the lower state?
1 Answer
Nuclei in the higher energy state return to the lower state by spin-lattice and spin-spin relaxation processes.
Spin-Lattice Relaxation
The nuclei in an NMR experiment are in a sample called the lattice.
Nuclei in the lattice are rotating and vibrating. This creates a complex magnetic field called the lattice field.
Some of the components of the lattice field will be equal in frequency and phase to the Larmor frequency of the nuclei of interest.
These components can interact with nuclei in the higher energy state. This causes them to lose energy and return to the lower state
The energy that a nucleus loses increases the amount of vibration and rotation within the lattice. The temperature of the sample increases by a tiny amount.
Spin-Spin Relaxation
Spin-spin relaxation is the exchange of energy between neighbouring nuclei that have identical precessional frequencies but different energy levels.
One nucleus will gain energy, and the second nucleus will lose energy.
There is no net change in the number of nuclei in each state. But the average lifetime of a nucleus in the excited state decreases. This leads to line broadening.
