Transition from:

The transition from the spin state of lower energy to the one of higher energy is brought up with the help of a radiation in the radiofrequency region. The transition called spin flip occurs when the radio frequency and the precessional frequencies are matched. This condition is called resonance. Once excited, the spins relax down to the lower state through radiationless processes called relaxation.  There are two types of relaxations namely, spin-lattice relaxation and spin-spin relaxation. In another method called FT-NMR the nuclei in a magnetic field are subjected to a pulse of intense radiofrequency radiation of a few microseconds duration. This causes all the nuclei in the sample to absorb the radiofrequency and get excited simultaneously. The relaxation is characterized by free induction decay (FID) which is detected and Fourier transformed to obtain a signal in the frequency domain.