Understanding Secondary Echoes in Spins Rephase NMR

  • Thread starter restfull
  • Start date
  • Tags
    Nmr
In summary, when a 90 degree pulse is followed by a 180 degree pulse, the spins may rephase and generate secondary echoes. This is due to the spin environment being flipped by the 180 pulse, which causes the spins to catch up and rephase.
  • #1
restfull
7
0
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?
 
Physics news on Phys.org
  • #2
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.
 
Last edited:
  • #3
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.
 
  • #4
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
 

Related to Understanding Secondary Echoes in Spins Rephase NMR

What is the principle behind Spins Rephase NMR?

The principle behind Spins Rephase NMR is based on the phenomenon of nuclear magnetic resonance (NMR). In NMR, a sample is placed in a strong magnetic field, causing the nuclei of the atoms in the sample to align with the field. When radiofrequency pulses are applied, the nuclei absorb and emit energy, which can be detected and used to create a spectrum.

What are secondary echoes in Spins Rephase NMR?

Secondary echoes are additional signals that appear in the NMR spectrum after the primary echo. These echoes are caused by interactions between the spins of the nuclei in the sample and can provide valuable information about the molecular structure and dynamics of the sample.

How are secondary echoes different from primary echoes?

Primary echoes are the first signals that appear in an NMR spectrum after the application of a radiofrequency pulse. They are caused by the rephasing of the spins in the sample after the initial pulse. Secondary echoes, on the other hand, are caused by further interactions between the spins after the primary echo and can provide more detailed information about the sample.

How can understanding secondary echoes improve NMR experiments?

By understanding secondary echoes, scientists can use them to extract more information from NMR spectra. Secondary echoes can provide information about molecular dynamics, such as the rate of molecular motion, and can also reveal structural features that are not visible in the primary echoes. This can lead to more accurate and detailed analyses of samples.

What are some applications of understanding secondary echoes in Spins Rephase NMR?

Understanding secondary echoes in Spins Rephase NMR has a wide range of applications in various fields, including chemistry, biochemistry, and materials science. Some examples include studying protein dynamics and interactions, analyzing the structure of polymers, and characterizing the composition of complex mixtures.

Similar threads

  • Classical Physics
Replies
24
Views
3K
Replies
13
Views
2K
  • Quantum Physics
Replies
2
Views
833
  • Atomic and Condensed Matter
Replies
4
Views
4K
  • Quantum Physics
Replies
5
Views
3K
  • Quantum Physics
Replies
1
Views
3K
  • Special and General Relativity
2
Replies
51
Views
2K
  • Biology and Medical
Replies
14
Views
13K
  • Sci-Fi Writing and World Building
Replies
9
Views
2K
Replies
7
Views
3K
Back
Top