Question

Explain why we need a plane-polarized oscillating magnetic field for resonance energy absorption in NMR spectroscopy.

Answer 1

Answer:

NMR is a phenomenon that occurs when the nuclei of certain atoms are immersed in a static magnetic field and exposed to a second oscillating magnetic field. Some nuclei experience this phenomenon, and others do not, depending upon whether they possess a property called spin.

Nuclear magnetic resonance involves the immersion of nuclei in a magnetic field and then matching the frequency at which they are precessing with electromagnetic radiation of exactly the same frequency so that energy absorption can occur.

Transverse mechanical waves can usually oscillate in any orientation that is perpendicular to the direction of travel; from side to side or up and down, etc. They are said to be polarised when there is a plane containing both the line of the direction of propagation of the wave and another line along which the particles are vibrating (the displacement of the particles). Since longitudinal waves have the particles vibrating back and forth along the line of propagation, there is no plane specified by these two parallel lines, and so the waves cannot be polarised.

A single transverse mechanical wave is polarised, but for several waves, if the waves are polarised, then the oscillation is only in one specific orientation. These are linearly polarised waves (sometimes referred to as plane polarised waves). If they are unpolarised, then the oscillation is in many orientations.

For historical reasons, the plane of polarisation is the plane perpendicular to that containing the electric vector. Most of the time we do not need to know which is which as we are looking at effects in polarisation and not causes.