181* NMR pulse, atomic description

[dilberto]

Hi, long time user, first time poster.

A while back someone posted a great question
https://www.physicsforums.com/showthread.php?t=33584

To paraphrase, can you explain on a spin-by-spin level exactly how a 181* pulse is possible?

You always hear this really intuitive explanation of NMR pulses as a net imbalance of low energy state spins and as you excite them with RF energy, they go to high energy state. This causes the net imbalance to tip from 0*, to 180*. However, this explanation seems to only describe up to 180* pulses. The answers from 2004 trend towards bulk response and arm waving as soon as the questioner really presses on a 181* pulse.

So what's going on? Is the common explanation actually lacking in completeness. This seems like such a great little thought experiment. Thanks for any insight.

 

[ZapperZ]

I don't think it was "arm waving" at all, considering the medium that one has to use to do such explanation (public forum). I thought the last 2 explanations by clint and Mike H was quite detailed. They tried to explain what is meant by the bulk magnetization, which is what we measure.

For a "long time user" (you officially joined yesterday), you had to dig up a very old thread to make your first post?

Zz.