I don't really understand the hyperfine structure. I understand, that the magnetic dipole moment of the nucleus interacts with the spin of an electron, causing a split between the energy levels of spin states (like the Zeeman effect?). However, I do not understand how/why is there an oscillating transition between these states?

I've read that a spin correlated radical pair will oscillate between triplet and singlet states due to the effects of the hyperfine interaction. Is this oscillation caused by absorption and re-emission of photons?

Also, how can an external magnetic field influence this oscillation?

I've tried to read a lot of literature on this online but unfortunately, I am not a physicist, I do not know enough physics or mathematics to understand a lot of the explanations. (I only know basic electrodynamics and chemistry). I'm just trying to get an intuitive model, an idea of what is happening. I need it to understand magnetic field effects on biological systems.

If someone could explain this to me (even in layman's terms), I'd be really glad. Thank you.

  • $\begingroup$ An interaction means that there is a new energy term in the Hamiltonian, which splits the previously degenerated states into multiplets. In a semiclassical picture the oscillations are simply the "beating" frequencies between the now distinct eigenfrequencies of these modes. If you go into the QED picture the atom now couples to the modes of the electromagnetic field, i.e. in addition to the atomic stated we also have photons (states of the EM field) to deal with, and if you like you can formulate that as absorption/emission of virtual and real photons, with the usual caveats. $\endgroup$ – CuriousOne Oct 28 '15 at 17:59
  • $\begingroup$ Thank you very much for your answer, @CuriousOne . However, I still do not really understand how can the system oscillate between states that have different energies associated with them. Where does the energy required to transit into a higher energy state "come from"? I can see that you are an expert on this topic. Could you please explain it in a much simpler way? As I stated abowe, I'm not familiar with quantum physics. $\endgroup$ – David Oct 30 '15 at 21:39
  • $\begingroup$ We are basically just running into the self-consistency limits of non-relativistic quantum mechanics by talking about "beating" etc.. A system doesn't have to be in an energy eigenstate. When it is not, then there is an oscillation between states, but as long as the system isn't coupled to a bigger system that it can shed energy to (the vacuum, a collection of other atoms, nuclear spins etc..) nothing else can happen physically (where would the energy go?). The cool thing is that this dynamic can take on a very complex structure, which we use in spin echo and quantum computing. $\endgroup$ – CuriousOne Oct 30 '15 at 22:21

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