Photoluminescence excitation
Encyclopedia
Photoluminescence excitation (PLE) is a specific type of photoluminescence
Photoluminescence
Photoluminescence is a process in which a substance absorbs photons and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon...

and concerns the interaction between electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

and matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

.

In a quantum-mechanic description, matter is made of quantum systems (atoms and molecules). In quantum mechanics a quantum system is described as having "possible states". There are systems with a continuous range of possible states, and system with a discrete range of states. As an example, we can consider the electrons in an atom. Since the electrons of an atom are not free particles, but are "confined" in the system, the possible states of the electrons are discrete. To better explain what discrete means, we have to consider that each state is labelled by (one or more) parameters. In other words to individuate a state we specify the value of one or more parameters, such as energy, velocity, angular momentum, etc. A specific state will have a specific energy, a specific velocity, etc. When a particle is confined, it can be only in some states, with some specific values of energy, jumping from one to the other, and it can not be in a state with an intermediate value of the energy between those (forbidden states).

When the electromagnetic radiation interacts with a quantum system (an atom), it can give it an amount (a quantum) of energy, and the atom goes from a lower-energy state to a higher one (excited state). Then, there is the opposite phenomenon, where an atom goes from a higher-energy state to a lower one (relaxation) and emits a quantum of electromagnetic energy in the process.
The "quantum" or "packet" of energy of the electromagnetic radiation is called a photon.
The amount of energy carried by a photon is proportional to its frequency.

On the other hand, there is a relationship between the energy carried by a photon (i.e. its frequency) and the energy difference between the two states involved in the jump of the atom.

The photoluminescence is then the phenomenon where light is shone on a quantum system (e.g. an atom or a molecule), it is absorbed and the system is excited. then the system relaxes and re-emits another photon.

There are in general cases where a photon is absorbed, the system is excited in the corresponding excited state, then it relaxes in an intermediate lower state, with a "non radiative relaxation" (a relaxation that doesn't involve the emission of a photon, but e.g. involves the emission of vibrational energy) and then there is the emission of a photon with a lower energy than the absorbed one, because of the relaxation from the intermediate, lower energy state to the "ground state".

In this case we have a "non resonant" excitation-emission photoluminescence process.

PLE is referred to the resonant case, where a photon is absorbed, and a photon of the same energy (frequency) is emitted, during the subsequent relaxation process.