[polymerator]

Hi there,

I'm trying to figure out how it's possible to achieve pulses past 90 degrees in a pulsed NMR experiment.

My textbook ("Basic One- and Two-Dimensional NMR Spectroscopy, 3E" by Horst Friebolin) describes the resonance condition. It says that when irradiating the sample, the two populations will eventually reach saturation and then no net absorption will occur. This makes sense given the probabilities of absorption and emission are the same.

It then goes on to describe the pulsed NMR experiment. Here it raises a few questions that I've been wondering about.
Firstly, using this "linear alternating magnetic field along the x-direction" gives us a a steady magnetic component (B1) in the rotating coordinate system. This B1 component gives rise to the transverse magnetization during the duration of the pulse. This makes sense to me as well. However, why was this not addressed in the previous discussion of the resonance condition? Was it to be understood that in that experiment, the irradiating field irradiated from more or less all angles, so no transverse magnetization was achieved?
Secondly, if no further net absorption is achieved on saturation, how can the pulse go past 90 degrees meaning more nuclei in the upper energy beta level? (in this case: I = ½). What are we doing differently from before other than just making sure the irradiating field is only along the x-axis?

I realise that this might not be answered easily because of quantum mechanical aspects, but if you could at least give me an idea about what's going on, that would really be helpful to me  And maybe you could provide a link with an explanation...

Thanks in advance!