Atomic, molecular, and optical physics
Typically, the theory and applications of emission, absorption, scattering of electromagnetic radiation (light) from excited atoms and molecules, analysis of spectroscopy, generation of lasers and masers, and the optical properties of matter in general, fall into these categories.
[4] Both subfields are primarily concerned with electronic structure and the dynamical processes by which these arrangements change.
Additionally to the electronic excitation states which are known from atoms, molecules are able to rotate and to vibrate.
From measuring rotational and vibrational spectra properties of molecules like the distance between the nuclei can be calculated.
[9][10] Researchers in optical physics use and develop light sources that span the electromagnetic spectrum from microwaves to X-rays.
The field includes the generation and detection of light, linear and nonlinear optical processes, and spectroscopy.
The applications of optical physics create advancements in communications, medicine, manufacturing, and even entertainment.
At this stage, it wasn't clear what atoms were - although they could be described and classified by their observable properties in bulk; summarized by the developing periodic table, by John Newlands and Dmitri Mendeleyev around the mid to late 19th century.
[13] Experiments including electromagnetic radiation and matter - such as the photoelectric effect, Compton effect, and spectra of sunlight the due to the unknown element of Helium, the limitation of the Bohr model to Hydrogen, and numerous other reasons, lead to an entirely new mathematical model of matter and light: quantum mechanics.
The theory was developed to attempt to provide an origin for the wavelength-dependent refractive index n of a material.
In this model, incident electromagnetic waves forced an electron bound to an atom to oscillate.
[16]: 4–8 Max Planck derived a formula to describe the electromagnetic field inside a box when in thermal equilibrium in 1900.
[16]: 4–8, 51–52 In 1911, Ernest Rutherford concluded, based on alpha particle scattering, that an atom has a central pointlike proton.
He also thought that an electron would be still attracted to the proton by Coulomb's law, which he had verified still held at small scales.
Niels Bohr, in 1913, combined the Rutherford model of the atom with the quantisation ideas of Planck.
[16]: 9–10 These results, based on a discrete set of specific standing waves, were inconsistent with the continuous classical oscillator model.
[16]: 8 Work by Albert Einstein in 1905 on the photoelectric effect led to the association of a light wave of frequency
[16]: 16 This semi-classical treatment is valid for most systems,[2]: 997 particular those under the action of high intensity laser fields.
These are naturally in a ground state but can be excited by the absorption of energy from light (photons), magnetic fields, or interaction with a colliding particle (typically other electrons).
However, if the lower state is in an inner shell, a phenomenon known as the Auger effect may take place where the energy is transferred to another bound electrons causing it to go into the continuum.
