Quantum mechanics/Magnetic fields and spin
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Contents
Some classical physics[edit]
If a 'classical' electron rotates around a point with angular momentum , the system will have a magnetic dipole moment (a vector perpendicular to the plane of rotation) proportional to the angular momentum:
(Eq. 1)
γ is called the gyromagnetic constant and M is the mass of the electron. In classical physics, a magnetic needle can be approximated as magnetic dipole moment and it has the tendency to align itself to a magnetic field . More quantitatively, the energy of a magnetic dipole moment in a field is (scalar product) and if we consider the field aligned along the z axis
(Eq. 2)
The hydrogen atom in a magnetic field[edit]
The angular momentum along the z direction for an electron in the hydrogen atom is L_{z} = mħ with m the magnetic quantum number. If a hydrogen atom is put where there is a magnetic field, there is an extra term of the energy, dependent on the quantum number m:
(Eq. 3)
The previously degenerate levels of orbital 2p_{1}, 2p_{0}, 2p_{+1} are split by the external magnetic field. This is called the Zeeman effect.
The Zeeman effect does not explain all the observations. You need another ingredient to explain the finer detail of the hydrogen spectrum.
The SternGerlach experiment[edit]
A beam of particles with same mass, same velocity and different magnetic dipole moments passing through a region with inhomogeneous magnetic field will follow a trajectory that depends on the magnetic dipole moment.
The hydrogen atom in the ground state does not have magnetic dipole moment due to the orbital n=1, l=0, m=0. Nevertheless it is found that two different trajectories are followed by hydrogen atoms: there is an additional, previously unknown, magnetic dipole moment.
It was postulated (Goudsmit and Ulhembeck) that the electron possesses an intrinsic angular moment and (being charged) an intrinsic magnetic dipole moment.
Spin[edit]
The intrinsic angular momentum of the electron is called spin. The operators can be defined in analogy to the angular momentum operators with the eigenfunction σ satisfying
(Eq. 4)
 s is the angular quantum number for the spin
(Eq. 5)
 m_{s} is the magnetic quantum number for the spin
However, as the experiment shows that there are only two possible values of magnetic dipole moments, we have to acknowledge that the only possible quantum numbers are:
s = ½ ; m_{s} = ±½
So there are only two possible spin wavefunctions for the electron, conventionally called spin up (or α) and spin down (or β).
Consequences of the existence of spin[edit]
The extra magnetic dipole affects the energy of the atom under magnetic field (that is how it was discovered) but the details are not essential if you are not specifically interested in magnetic properties.
The angular momentum of the electron and the angular momentum of the spin combine together in a nontrivial way (the total energy is not exactly the sum of the energies of the two rotors). This effect is called spinorbit coupling and it is essential to understand the details of the spectra of multielectron atoms. The spinorbit coupling is small and important only for heavy atoms.
The existence of spin changes completely the behavior of multielectron systems. It gives rise to the periodic table and all the phenomenology of chemical bonding and reactions. This is important for all chemists and will be investigated in the next lesson.
Exercise

Next: Lesson 11  Many electron systems