Quantum Processes

Quantum properties dominate the fields of atomic and molecular physics. Radiation is quantized such that for a given frequency of radiation, there can be only one value of quantum energy for the photons of that radiation. The energy levels of atoms and molecules can have only certain quantized values. Transitions between these quantized states occur by the photon processes absorption, emission, and stimulated emission. All of these processes require that the photon energy given by the Planck relationship is equal to the energy separation of the participating pair of quantum energy states.

Absorption and Emission

Taking the electron transitions associated with visible and ultraviolet interactions with matter as an example, absorption of a photon will occur only when the quantum energy of the photon precisely matches the energy gap between the initial and final states. In the interaction of radiation with matter, if there is no pair of energy states such that the photon energy can elevate the system from the lower to the upper state, then the matter will be transparent to that radiation.

Energy levels associated with molecules, atoms and nuclei are in general discrete, quantized energy levels and transitions between those levels typically involve the absorption or emission of photons. Electron energy levels have been used as the example here, but quantized energy levels for molecular vibration and rotation also exist. Transitions between vibrational quantum states typically occur in the infrared and transitions between rotational quantum states are typically in the microwave region of the electromagnetic spectrum.

It is possible for excited electrons in atoms and molecules to have some other kind of interaction which lowers their energy before they can make a downward transition. In that case they would emit a photon of lower energy and longer wavelength. This process is called fluorescence if it happens essentially instantaneously. It is also possible that material can hold onto the energy of excitation for a long time, gradually making downward transitions with emission. This delayed emission is called phosphorescence. Since by the Einstein A and B coefficients we know that the probabilities for absorption and emission are the same, the existence of phosphorescence would imply that some interaction took place quickly after the initial absorption that placed the electrons in a much more stable and long-lived state so that they could not immediately drop back down.

Stimulated Emission

Like absorption and emission, stimulated emission requires that the photon energy given by the Planck relationship be equal to the energy separation of the participating pair of quantum energy states.

Fluorescence

If a high energy photon is absorbed by an atom or molecule by exciting an electron and then quickly undergoes some interaction with the crystal lattice or some collisional process, the excited electron can be transferred to a lower quantum state. If the electron then makes a transition back to its original state, then the photon emitted will be of lower photon energy and longer wavelength.

There are many useful applications of fluorescence. The common "fluorescent lighting" makes use of the fact that certain phosphors will fluoresce at numerous wavelengths in the visible light range when they are bombarded with light in the ultraviolet. Since the mercury vapor in fluorescent lights has strong emission in the ultraviolet when electrically excited, that UV light can cause the phosphor coatings on the tubes to fluoresce in the visible, producing a light source much more efficient than incandescent lights.

Many minerals fluoresce and you can see a visible glow when you illuminate them with ultraviolet light. Fluorescence can be a useful tool for the study of minerals. With some systems you can do extensive fluorescence spectroscopy by bombarding with high energy photons and measuring the spectrum of light produced by fluorescence. Often xrays are used as a source and the photons emitted by fluorescence in the UV and visible are studied.

Many types of butterflies are fluorescent and will glow in the visible when illuminated with ultraviolet light. Fluorescence can cause objects to appear brighter if short wavelength light is absorbed and they give the light back by fluorescence at longer wavelengths. Some kinds of whitening agents for clothing make use of this principle - the detergent may have a fluroescent agent in it which picks up the short wavelength part of sunlight and reemits it by fluorescence in the longer wavelength visible range.