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I keep getting myself lost when thinking about saturation pulses, a pulse in which there is no magnetisation in the z direction. Now I know a 90 degree pulse rotates polarisation to xy however to make sure we have have got all the magnetisation we do a train of 90 degree pulses. The problem comes if you do two 90 degrees pulse right after each other it is similar to a 180 pulse - a few degrees. So we shall assume we rotate about x. do we have to time each pulse so that the xy magnetisation is pointing in x (so the rotation does nothing)and any z magnetisation will get rotated as normal. The problem with this is that the magnetisation will de phase over time so does that mean there is also a 180 echo when the xy magnetisation is in the y to keep the overall phase? Is there something i'm missing? Does the classical idea still work or do i have to look at quantumly and think about emission and stimulation probabilities? |
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I assume you are discussing a saturation pulse train and not a solvent supression-presat pulse. When I set up this experiment for T1 measurement in solids, one needs to set the 90 pulse (5us) separation to a 1/4 rotor period ~40-70us depending on rotor speed (this is to destroy the possibility of phase coherence from MAS, I am not sure how this choice is made for liquids). In this way, ~100 pulses will beat the magnetization along z to zero. The idea of this experiment is to at t=0 have Mz=0 and to measure the recovery of Mz to thermal equilibrium. The reason is that this way is faster than inversion recovery. I have never observed an echo from this method of T1 measurement. It sounds like you need to adjust the pulse spacing to find your null @ t=0. |
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From a pure technical standpoint, a single 90 pulse is not a "saturation pulse", even though common parlance refers to a 90 pulse as such. Saturation is steady state spin state produced by a pulse whose duration is many times the spin lattice relaxation time constant. A single resonant pulse whose amplitude is such that w1^2T1T2 is much greater than unity, and whose length is say 10*T1, can saturate the magnetization. The questions you raise are due to confusion between the transient spin response and the steady state response. Surely, a "train" of 90 pulses, that is, many pulses of short separation, with produce saturation. Yes echos will occur, and since the RF is inhomogeneous, one will have RF and Bo inhomogeneities contribute to the decay of the transient approach to the steady state. |
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Now that I am logged in, I can make another point. Because pulsed NMR is the modern method, one no longer hears much about the steady state solution of the Bloch Equations, which in the days of CW NMR were very important and not glossed over. Yet, your question brings to the front, that the basics are still important. No, you do not need a quantum perspective to answer your question. The so called "classic" Bloch Equations are all you need. |
