Understanding Secondary Echoes in Spins Rephase NMR

[restfull]

When a 90 degree pulse is followed by a 180 degree pulse, a spin echo may be seen at a time tau later...i understand this in general terms, as the direction of precessional drift has been flipped about its own axis by the 180 pulse, which means the spins catch up and rephase in an echo etc...

what i don't understand is why, under certain conditions, there are secondary echoes...as in...90 degree, 180 degree, echo, echo...wouldn't that mean that the spins rephase all by themselves without a 180 degree pulse? how does that work?

 

[ZapperZ]

It's been more than a dozen years since I last did pulsed NMR, so I'm very rusty. This means that you should not trust everything that I tell you.

If you look at the subsequent echos, they get smaller and smaller and smaller. Think of these an an "overshoot" of the spin phasing and dephasing. Remember that when you flip the spin with a 180 degree pulse, you are essentially dephasing the perpendicular component of the bulk magnetization - resulting in zero component in this plane. Yet, after that 180 degree pulse, you get a rephasing of the spins, causing an "echo" that first time around.

The individual spins will start to get randomize due to the spin environment and thermal fluctuations, but depending on your situation, not all of them will do that all at once. There will still be a few that will continue to coherently evolve without any randomization and will "overshoot" and rephase at a later time. This will continue until all the individual spins have decohered with the environment.

Zz.

Edit: P.S. Note that multiple posting is not allowed per our Guidelines.

 

[marcusl]

Restfull, did you read this in a book or observe it experimentally?

In general you get only a single echo. You can get a train of echoes by applying additional 180 pulses spaced by 2*tau, known as the Carr-Purcell-Mieboom-Gill or CPMG experiment.

It is possible to get multiple echoes from a single 90-180 pair through highly non-linear effects. These are especially prominent in samples producing very strong signals (e.g., large samples at high fields) that have long relaxation times. Maybe someone else can help with references here, I know longer remember the articles or name of the effect.

 

[zeta]

sorry I've only just joined, this is prob. a little late...
the pheno theory (in terms of magnetization) is in deville et al, PRB 19, 1979, 5666-5688 and I think there is a brief discussion in schlichter, but the appearance of multiple echos may be for a variety of reasons incl ferromagnetism