Molecular physics

Like atomic physics, it relies on a combination of classical and quantum mechanics to describe interactions between electromagnetic radiation and matter.

Experiments in the field often rely heavily on techniques borrowed from atomic physics, such as spectroscopy and scattering.

This picture of a molecule is based on the idea that nucleons are much heavier than electrons, so will move much less in response to the same force.

This is the case for most low-lying molecular energy states, and corresponds to transitions in the visible and ultraviolet regions of the electromagnetic spectrum.

Vibrational energy levels refer to motion of the nuclei about their equilibrium positions in the molecule.

[1] Actual molecular spectra also show transitions which simultaneously couple electronic, vibrational, and rotational states.

X-ray diffraction allows determination of internuclear spacing directly, especially for molecules containing heavy elements.

A thermally excited segment of protein alpha helix. In addition to electronic quantum states, molecules have internal degrees of freedom corresponding to rotational and vibrational motion. At appreciable temperatures, many of these new motional modes are excited, resulting in constant motion as seen above.
Motion associated with rotational and vibrational energy levels within a molecule. Different rotational and vibrational levels correspond to different rates of rotation or oscillation. The example shown here is a simple diatomic molecule, but the principle is similar for larger and more complicated structures.