 Next: Photodissociation Up: Basic Processes in Earth's Previous: Basic Processes in Earth's As solar photons penetrate into the atmosphere some fraction of them will be absorbed. The size of this fraction depends on both the wavelength of the incident photon and the interacting molecule. The primary quantity used to described a molecule's tendency to absorb light is the absorption cross-section,
The photon absorption process can be represented by,
where h The first, photoionization, is the process where the energy level of a valence electron is raised into the continuum. Generally, kinetic energy is imparted to both the election and the ion. Wavelengths which are capable of ionizing an atom or molecule do not penetrate below the mesosphere. The second, photodissociation, or photolysis (described in section 2.3.2) results in the splitting of a molecule through the channeling of the absorbed photon energy into overcoming the binding force of a chemical bond. The third, fluorescence, is the process whereby the excited molecule releases its energy by radiating another photon. This new photon need not have the same energy, polarization, or direction as the incident photon. This is, in fact, a form of inelastic scattering but is not important for this work. The fourth, quenching, is the transfer of energy and momentum as the result of a collision with another molecule (M is N2 or O2). This energy generally ends up as kinetic energy. The fifth describes a chemical reaction. The absorbed photon energy is required to overcome the reaction activation energy.
Next: Photodissociation Up: Basic Processes in Earth's Previous: Basic Processes in Earth's Chris McLinden 1999-07-22
I'm reading an excellent book by Kauffmann, Discovering The Universe, in which he describes when electron jump to a higher level or a lower level. If they go to a higher level they emit a photon. If they go to a lower level they absorb a photon.It causes me to ask if photons exist as a seperate entity within all atoms or are they created at certain energy levels for the purpose of absorption, or emission, or do they exist omnipresently in the fabric of the universe? Anyway, where does the photon for absorbtion come from? Hope this isn't too stupid a question. It certainly isn't a stupid question, and really cuts to the heart of the question of interpreting quantum mechanics. The simplest answer is that when a photon is absorbed by an electron, it is completely destroyed. All its energy is imparted to the electron, which instantly jumps to a new energy level. The photon itself ceases to be. In the equations which govern this interaction, one side of the equation (for the initial state) has terms for both the electron and the photon, while the other side (representing the final state) has only one term: for the electron. The opposite happens when an electron emits a photon. The photon is not selected from a "well" of photons living in the atom; it is created instantaneously out of the vacuum. The electron in the high energy level is instantly converted into a lower energy-level electron and a photon. There is no in-between state where the photon is being constructed. It instantly pops into existance. So the question is: where does the photon come from? Strangely, it doesn't seem to come from anywhere. The universe must put the extra energy somewhere, and because electrons in atoms are electromagnetic phenomena, a photon is born with the required energy. In a weak-force interaction, say the decay of a neutron, that energy goes into a neutrino particle which is also instantaneously created. Each force has its own carrier particles, and knows how to make them. That's really all we can say about it. There are many interpretations of what this and other phenomena in quantum mechanics mean on a deeper level, and whole libraries worth of books which argue points of view on the matter. But my personal philosophy is that of the famous physicist Richard Feynman, who said: "Shut up and calculate." Edit by Michael Lam on February 10, 2016: The quote has been misattributed to Richard Feynman, and sometimes Paul Dirac, but evidentally comes from Cornell physicist David Mermin (source).
Once emitted photons are indivisible units. When a photon hits an electron,both moving in the same direction, the photon will be partially absorbed and the electron emits another photon with lower energy. This happens for example at linear particle accelerators. The energy from the photon partially goes over to the electron and the electron moves faster. If a photon hits the electron, both moving against each other, the electron gets decelerated and emits photon(s) with in sum more energy than the incoming photon has. There is a chance to imagine how the photon goes over to the electron. For this one have to think about how an electric field - and a magnetic field too - can be quantized. For this one need two different quanta, one of them is on the end of the electrons and the antiprotons electric fields and one quanta, which is on the end of the positrons and the protons electric fields. I see clearly the objection of well educated people. Physics claims the infinity of the electric field of electrons and the other charged particles. The postulation of such two quanta has some charme. From this quanta it's easy to design electric and magnetic field lines. And the most important point is that all the photons (of different energy) are made from this two quanta too. Photons always are composed from the equal number of both quanta. Negative charged particles have more of the negative quanta and this difference is equal for all electrons and antiprotons. The same difference but with majority for positive quanta positrons and protons have. Emitted and absorbed photons don't change the particles charge. But the quanta from the photons will be stored partially on the charged particles or these particles gave the quanta back in form of photons of lower energy. This concept allows to show, that accelerated particles have higher mass and their charge get shilded more and more. And this concept - under the presumption that field lines exist and this field lines are made from this two quanta in clusters with continuous changing numbers - allows to show that the attraction of the electron and the proton in the atom has discrete limits. And now please forget about this two quanta. They are only a Gedankenexperiment because until now the electromagnetic spectrum seems to be continuous and there is no evidence for quantisation. But it's an amazing imagination to see how photons do not disappear when they hit a electron but travel biggyback on charged particles. I hope not to be sunk. |
What can absorb a photon?
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