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 http://ugc-inno-nehu.com/links_from_web.html

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

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

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

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