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Glow in the dark things are intriguing. I have often wondered why glow in the dark wall stickers stay alight for certain amounts of time after being exposed to certain amount of daylight. My research lead me to find out that the colour emitted from these materials depends on what light they were exposed to in the first place. That the time that they luminesce for depends on the intensity of the light absorbed beforehand and that quantum jumps between energy levels result in certain colours being emitted.
What is it?
Photoluminescence is the process by which light is emitted from a substance due to exposure to a source of light. In the case of photoluminescence this light being from the Ultra-violet, visible or Infrared part of the electromagnetic spectrum. Photo luminescence is a specific type of luminescence.
Luminescence is the emission of light due to excited electrons not due to heating. It is light that can be emitted at cool temperature. It is also known as "cold light."  As opposed to incandescence which is light that can be formed from heating something. There are lots of different kinds of luminescence such as electro, thermo, chemi, and photoluminescence. They differ in the way electrons are excited. Electrons can be excited by electric impulses, elevated temperatures or chemical reactions. The excitation of the electrons means that they move from there ground state in an atom (low energy level) to an excited, higher state (higher energy level). Then due to the electrical attractions in the atom the electron then moves back down to the ground state therefore losing energy. This energy is lost as photons of light which is the luminescence you see in materials. The energy used to excite the electron is always more than or equal to the energy of the emitted photon.
Why is light emitted?
Isolated atoms have specific energy levels. When a molecule absorbs radiation and therefore energy its electrons are excited and move up to different higher energy orbitals. In absorption the energy from the radiation has to match the change in energy of the two energy levels. 9
Picture 1 - An image to show an electron moving up and down energy levels and the absorption and emission of a photon. In emissions the photon emitted has a specific wavelength and frequency depending on the change in energy when the electron moves from its excited state to its ground state. Using the equation E=hf you can work out the energy of a photon if you have its frequency. If you only have the wavelength you can then work out the frequency using Î»=c/f and then you can put the frequency into E=hf to find the energy. Electrons make quantum jumps between allowed energy levels. A downwards jump emits a photon whose energy is given by E=hf=Einitial + Efinal. http://farm5.staticflickr.com/4052/4706021793_6b29efdcdb_z.jpg
On the quantum jump down the electron may not go straight to the ground state, it may take a few steps until it reaches the ground level. If this happens then the atom will emit a photon specific to each step of the way.  A bigger jump means a smaller photon wavelength and a smaller jump means a bigger photon wavelength. Therefore the jump "size" and the wavelength are inversely proportional. As each element has a unique set of energy levels it will give of a unique set of wavelengths and therefore a unique spectrum. This spectrum can then be used to identify the element6 . An emission spectrum is a series of coloured lines on a black background whereas an absorption spectrum is a series of black lines on a coloured back ground.7 The series of lines on the emission spectra correspond to the distinct energies of the photons emitted. The dark lines on the absorption spectra are the missing photon frequencies when the photons have been absorbed by a material.
Florescence is a type of photo luminescence. Florescence mostly uses light from the ultra violet part of the spectrum. You can get this light from UV lamps or "black light".
Ultra violet radiation also known as UV is then divided into smaller groups long-wave, mid-wave, shortwave and extreme ultra violet.  In florescent materials the photon is usually emitted within milli-seconds of absorption.
Picture 2 this is an image of the electromagnetic spectrum ranging from the infrared to UV. The numbers on the side is the wavelength in Nano meters.Long wave ultra violet, also known as LW UV, is present in sunlight. It is the lowest frequency ultraviolet wave with the longest wavelength and therefore the lowest energy. Energy gets less as you go towards the infrared area of the picture shown. LW UV is the most commonly used UV because it is closest to the visible light area of the spectrum so when absorbed the atom emits a photon of less energy and this means you can get a colour visible to the naked eye. LW UV is used in black light blue florescent light bulbs as it can pass through transparent materials such as glass and plastic easily and it is readily available and therefore cheap.
Mid wave ultra violet, also known as MV UV is also present in sunlight. It is used by the body to produce vitamin D. MV UV can be nearly completely stopped by normal glass. Some UV radiation can be stopped by common glass because glass is made up of atoms and in atoms are electrons. We know that electrons can absorb radiation of specific frequencies. Electrons in common glass can absorb wavelengths of UV radiation but not wavelengths of visible light and therefore act as a filter and let the visible light through and not the UV.3
Short wave ultra violet, also known as SW UV is completely stopped by most types of glass and plastic. Therefore quartz or silica based glass is usually used when you need SW UV so the waves can pass through.
Picture 3- Florescent materials
Energy equation for the excitation in florescence. 
For florescence the electron in the ground state has the same spin as when it is excited to a higher state. S1 is the excited state electron, S0 is the ground state, h is Plank constant and f is the frequency of the photon.
S0 + hf -> S1
You can work out the florescent lifetime of a material. This means you can work out how long the atom stays in its excited state before emitting a photon. You can use the equation below to work out the florescent lifetime. Due to the relationship between intensity and time is exponential you can use the equation below.
It = I0e-t/T
T is the florescent lifetime, It is the intensity at time t and I0 is the intensity at time 0.8
Quantum yield. 
Quantum yield is another way of saying the rate of excited decay or the ratio of the number of photons re-emitted to the number of photons absorbed.
Quantum yield= photons re-emitted / photons absorbed
Another sort of luminescence is phosphorescence which is the delayed luminescence of a material. This is not the same as florescence as florescent materials emit their photons immediately after the radiation has been absorbed and phosphorescent materials can delay the emission of the photon for several hours4.
Picture 4- Jellyfish that glows in the dark due to phosphorescence.
Phosphorescence works when an atom absorbs a photon and therefore energy. This energy excites an electron and raises it to a higher energy level. Then, unlike florescence, the absorbed photon energy then encounters an intersystem crossing into a triplet state. A triplet state is a when the spin of an electron is reversed. Spin is the angular momentum of a particle. The intersystem crossing is when an electron leaves a molecule in the triplet state. The energy can then become trapped in the triplet state and cannot leave due to forbidden transitions. These transitions can still occur but are unlikely and improbable, this results in less transitions occurring in a certain amount of time. As a result of this a photon can be emitted hours after the exposure to light energy meaning the substances can store the energy and let it out over longer periods of time5. If the quantum yield is high enough then the substances can be used to make glow in the dark materials.
Energy equation for phosphorescence.
S0 + hf -> S1 -> T1 -> S0 + hf'
S0 is the ground state, S1 is the excited state and T1 is the triplet state.
Photoluminescent materials are uses frequently in everyday life. Most are man-made but some Photoluminescent materials occur naturally for example the jellyfish on page 3. Phosphorescent materials are used for glow in the dark materials such as safety hall signs or step markers, toys and decoration due to their long lasting glow. Florescence is also used to identify certain minerals and gemstones which have a distinct florescence under certain wavelengths of light.
Pic 5 -This image is a summary comparison of how the electrons move in both florescence and phosphorescence. http://en.wikipedia.org/wiki/Phosphorescence 5/6/12Photoluminescence is a special type of luminescence which can be divided into two categories florescence and phosphorescence. In both circumstances the molecules in the materials absorb electromagnetic radiation, usually in the ultra violet part of the spectrum, and this causes an electron from the ground state of an atom to move to a higher, excited state. Depending on the material, the electron stays for a certain amount of time in the excited state. In florescent materials the electron only stays there for milli-seconds and then descends down to the ground level emitting a photon of certain energy depending on the energy change between the levels moved. In phosphorescent materials the electron can stay in the excited state for hours and in that time the spin of the electron is reversed and it moves into a triplet orbital before quantum jumping back down to the ground state because of the attraction of the nucleus on the electron. As is jumps it emits a photon of specific energy and frequency. Both florescence and phosphorescence are shown in the picture to the right.