1. Nuclear Magnetic Resonance
ANIMATED ILLUSTRATIONS
MS Powerpoint Presentation Files
Uses Animation Schemes as available in MS XP or MS 2003 versions
A class room educational material
File-7 Transition Rates: Relaxation Time

2. A system of single-spin for description
There is a non-zero XY component always
The Larmor precession frequency depends on the strength of external field
For proton spin of ½, there are two allowed orientations so that the component along z-axis is either +1/2 or -1/2
hν=gβH
If a rotating magnetic field of relatively small magnitude is present in the perpendicular plane at frequency ν , then the resonance occurs and the spin undergoes a flipping transition to another orientation.
+1/2 ħ
Lower energy
-1/2 ħ
Upper energy
Photon energy absorbed; transition occurs
-1/2 ħ
+1/2 ħ
Radiation
Induced Transition or stimulaed transition

3. Degeneracy removed/Energy levels split
random
No external magnetic field. The energy levels are degenerate
The ensemble of spins, have equally distributed population between the two levels for the spin ½ protons
-1/2
+1/2
No net magnetization
On the application of field…..
Splitting is instantaneous & population redistribution requires more time called the relaxation time
-1/2
No radiations
are present
-1/2
+1/2
Not stimulated transitions: but spontaneous relaxation transitions
Magnetic field
+1/2
Degeneracy removed/Energy levels split
Thermal equilibrium Boltzmann distribution
Net magnetization along Z-direction & ZERO XY component

4. Effect of applied radiation
RF electro magnetic radiation tilts the magnetization away from the equilibrium Z-direction.
System inherently has the tendency always to return to equilibrium, Time independent Z- magnetization and zero XY component
When the radiation is effecting a tilt the relaxation process is inevitably present restoring the equilibrium situation !
Effect of applied radiation
Restoring relaxation
Steady state for detection

5. The rate of radiation induced forward process depends upon the strength of the RF field
High power E.M. at RF frequency applied only for short times compared to relaxation times
KF >> KR
KF << KR
Pulse technique
C.W. technique
The relaxation rate is an inherent system characteristic depends on thermodynamic parameters
Very small RFpower applied continuously
If the system has larger relaxation rates, then the relaxation time is small
Equilibrium restoration is faster and hence the steady state would be more towards the equilibrium state if the radiation induced rates are not up to the comparable relaxation rates
If High Power is applied for longer time durations in comparison with Relaxation times, then the spin system would tend to ‘saturate’. This most often encountered in CW settings unwittingly