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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 infraredand transitions between rotational quantum states are typically in the microwaveregion 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.
| Interaction of radiation with matter |
| Fluorescence |
Eli Priyatna 29 Apr, 2011--
Source: http://basistik.blogspot.com/2011/04/make-animated-physics-with-java_6517.html
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