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Name: Ho
Status: student
Grade: other
Country: Malaysia
Date: Fall 2012


Question:
How does a material absorb light (photon)? If photon of a particular frequency excite an electron in a molecule to a higher energy level, can the molecule while in its higher energy state absorb another photon of the same frequency? Let us just say you have a molecule with 20 electrons, if there are 20 photons moving towards the molecule, does each photon excite one electron and will you end up having a molecule with 20 excited electrons? Will the molecule with 20 excited electrons absorb a 21st photon directed towards it, how? The reason I am asking all these questions is that I was taught that when light is transmitted through a material, say a solution copper(II) sulphate, the non blue photons excite the electron to a higher energy d orbital and hence the photon disappears. So you see only blue photon. But there is only a fixed number of electrons in a material which can be excited so should there be a fixed number of photons that the material can absorb?

Replies:
Ho, When a material absorbs a photon, there are two very common results, depending on how long the atom can hold the photon.

If the atom does not hold the photon energy long, it is usually released at the same frequency as the original photon but in a random direction. This is the disorganized process that gives paint its color. The light leaves the object in all directions but at the original color.

If the atom holds the photon energy long enough, the energy often passes into the entire material, working its way into random motion of the molecules. This is why shining light on an object can eventually warm it. Enough light can set an object on fire. The energy does not stay in the individual electron, or even the individual atom. The energy distributes throughout the material long before the atoms fill up with energy.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


Let me try to address at least a few of your questions: How does a material absorb light (photon)? From the other questions you seem to have a reasonable understanding. A photon promotes a molecule from one state to another -- generally meaning an electron moves into a different (higher energy) orbital. Of course, this applies to absorption of light with energies in the visible or higher energy spectrum. Lower energy photons may excite molecular vibrations or rotations. If photon of a particular frequency excite an electron in a molecule to a higher energy level, can the molecule while in its higher energy state absorb another photon of the same frequency? Let us just say you have a molecule with 20 electrons, if there are 20 photons moving towards the molecule, does each photon excite one electron and will you end up having a molecule with 20 excited electrons? Let us talk about molecular states not excited electrons. The light interacts with the molecule, not just the electron. The transition energy depends on the state the molecule is in. Once it has absorbed one photon it is in a new state so the possible absorption energies change. While there are some systems that can absorb multiple photons of the same wavelength this could not be assumed. Thus, once the molecule has absorbed a photon it becomes transparent to that wavelength of light and the next photons pass through. The absorber is "bleached", meaning it will not absorb more light until it loses the energy from the first absorption. Having said this, the excited molecule might well absorb another photon at a different wavelength where the energy matches the difference between the current state and some higher excitation state of the molecule. Will the molecule with 20 excited electrons absorb a 21st photon directed towards it, how? Only in special circumstances -- the next photon happens to match an absorption energy of the current state or it actually is high enough energy to strip the electron from the molecule altogether. The reason I am asking all these questions is that I was taught that when light is transmitted through a material, say a solution copper(II) sulphate, the non blue photons excite the electron to a higher energy d orbital and hence the photon disappears. So you see only blue photon. But there is only a fixed number of electrons in a material which can be excited so should there be a fixed number of photons that the material can absorb? Absorption is not the end of the story. The excited state will lose that energy -- by re-emission or by conversion to heat or a combination of both. How fast that loss of energy occurs depends on the specific molecular state and the environment it's in (e.g., is it in a solvent or high vacuum?). With a bright enough source you can saturate (bleach) the absorber (which is not a single molecule but a solution of MANY molecules) so it becomes transparent (all molecules in the path are excited). This can be used to produce very short laser pulses.

Greg Bradburn


Hi Ho,

There appears to be a couple of concepts here that are interacting so that I'm having difficulty understanding. Let us restate what you said and separate out the concepts.

Photons of (W)avelength, W1, come in and excite a CuII atom's electron. The electron absorbs the (E)nergy, E1 and rises to a higher energy level. After an amount of time(Tdecay), the excited electron drops to the lower energy level and releases energy, E2 at W2. E2 and W2 are at lower energy and wavelengths than the original wavelength. In this case, the blue photons are the release of absorbed energy, not energy that is left over. The CuII solution is actively giving off blue photons, not absorbing all photons except blue.

Photons of different energy and density may excite the same electron up several energy levels or just one. Those same photons may excite different electrons up several levels or to the same level. The Tdecay tells us this because some levels may transfer energy to another electron and delay the Tdecay. Tdecay varies with E1,W1 and E2,W2, from many seconds to nanoseconds.

In theory, your scenario may occur: All 20 electrons could be elevated, but that is not very likely with at least four means of transferring the E1 and Tdecays being variable. However, photon saturation of a sample is a real phenomenon. Consider Europium doped Strontium Aluminate, St3Al2O5: Eu. It can be excited to the point where the number of photons input is more than it can absorb, so it reflects the energy.

Fun question! Thanks, Peter E. Hughes, Ph.D Milford, NH


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