Careers in atomic physics were responsible for the advent of quantum mechanics. Atomic physics is easier to define than molecular physics since it concerns only isolated atoms and ions, electron configurations, and the excitation of electrons by photons or collisions. The property of the nucleus is relevant in atomic physics only when considering hyperfine coupling. Atomic physics jobs are the foundation of plasma physics and atmospheric physics. Careers in atomic physics are dedicated to the study of matter-matter and light-matter on the scale of single atoms or structures containing a few atoms. This broader field is called AMO physics or atomic, molecular, and optical physics.
Molecular physics jobs are usually dedicated to the study of molecules-electrons, nuclei, and chemical bonding-made up of a few atoms in the gas phase. Various types of spectroscopy (electromagnetic, electron, mass, etc.) are the most important experimental methods in this field so you will find that careers in molecular physics often incorporate this type of work. Molecular physics jobs overlap with quantum chemistry, physical chemistry, and chemical physics. Quantum chemistry is the use of quantum mechanics and quantum field theory in chemistry. Physical chemistry is the application of thermodynamics, quantum mechanics, statistical mechanics, and kinetics to phenomena in chemical systems. Chemical physics investigates chemistry using the techniques of scientists working in atomic physics, molecular physics, and condensed-matter physics. Whereas chemical physics studies chemistry from the point of view of physics, physical chemistry studies the physical nature of chemistry.
Optical physics is the study of the generation of electromagnetic radiation and its interaction with matter. Since the devices of optical engineering (lenses, optical sensors, lasers, fiber optic communication systems, etc.) are used for research in optical physics, there is no hard and fast distinction between optical physics, optics, and optical engineering.
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