Can a vibration energy change emit a photon?
When a molecule of CO2 absorbs a frequency of IR it will undergo a vibrational energy change. Will this still later emit a photon? I though that only electronic energy changes could emit photons of light (when the electron drops back down). A vibrational energy change is to do with the whole nuclei, not specific electrons moving between levels. Pretty much every explanation of global warming says CO2 re-emits IR. How is this possible? Responses are greatly appreciated.
Original post by splitter2017
When a molecule of CO2 absorbs a frequency of IR it will undergo a vibrational energy change. Will this still later emit a photon? I though that only electronic energy changes could emit photons of light (when the electron drops back down). A vibrational energy change is to do with the whole nuclei, not specific electrons moving between levels. Pretty much every explanation of global warming says CO2 re-emits IR. How is this possible? Responses are greatly appreciated.
You seem to think there there is a fundamental difference between IR radiation and visible radiation.
They are both forms of electromagnetic radiation, albeit with different wavelengths and hence frequencies.
They can both be described as "wavicles" having the character of both waves and particles (photons).
I understand that the EM spectrum is continuous and I am aware of wave-particle duality. I may have made it confusing by saying "light" ( I wasn't specifically referring to visible light but EM radiation/photons as a whole) my bad!
IR's energy corresponds to the vibrational energy change of a CO2 molecule, not the electronic excitation region.
The energy changes that can occur are ( in order of increasing energy)
Translational < Rotational < Vibrational < Electronic
When an electronic change occurs, an electron is excited to higher energy state (absorbing a photon) and later drops down releasing a photon.
So during a vibrational energy change ( a photon of IR is absorbed) and the molecule gains kinetic as a whole but there is no change in electrons energy levels.
Therefore how come when a CO2 molecule absorbs IR does it re-emit IR again? Unless this statement is not true- practically all websites explaining global warming say CO2 absorbs IR and then re-emits it.
I would have thought it absorbs IR and there was vibrational change (increasing kinetic energy) and so temperature of the atmosphere increases and it stops there - not sure how it can re-emit IR.
IR's energy corresponds to the vibrational energy change of a CO2 molecule, not the electronic excitation region.
The energy changes that can occur are ( in order of increasing energy)
Translational < Rotational < Vibrational < Electronic
When an electronic change occurs, an electron is excited to higher energy state (absorbing a photon) and later drops down releasing a photon.
So during a vibrational energy change ( a photon of IR is absorbed) and the molecule gains kinetic as a whole but there is no change in electrons energy levels.
Therefore how come when a CO2 molecule absorbs IR does it re-emit IR again? Unless this statement is not true- practically all websites explaining global warming say CO2 absorbs IR and then re-emits it.
I would have thought it absorbs IR and there was vibrational change (increasing kinetic energy) and so temperature of the atmosphere increases and it stops there - not sure how it can re-emit IR.
(edited 6 years ago)
Original post by splitter2017
So during a vibrational energy change ( a photon of IR is absorbed) and the molecule gains kinetic as a whole but there is no change in electrons energy levels.
All of the diagrams where you draw electrons moving around between energy levels are a simplification. There's no such thing as electrons changing energy levels in isolation within molecules.
Molecules can exist in different energy states depending on various factors. When you excite an electron with a UV photon that electron doesn't get excited, the molecule itself transitions to a higher energy state as every particle experiences a different potential than before. The same occurs when a molecule absorbs IR. If there is then an allowed transition to a lower energy state that involves emission of a photon, then it has a finite probability of occurring.
Original post by splitter2017
So during a vibrational energy change ( a photon of IR is absorbed) and the molecule gains kinetic as a whole but there is no change in electrons energy levels.
So during a vibrational energy change ( a photon of IR is absorbed) and the molecule gains kinetic as a whole but there is no change in electrons energy levels.
But there IS a change in energy levels.
The vibrational state moves to a higher quantum level. If it moves to a lower quantum vibrational state it must either emit IR radiation or it must lose the energy via another (forbidden) mechanism, such as tranlational energy during collision.
Thank you both, I believe I understand the mechanism now.
Whatever energy is gained must later be lost. The CO2 molecule will absorb IR and cause a vibrational change (IR to kinetic energy) and then the molecule will give up this extra energy by re-emitting IR and "stop" vibrating.
This is of course a simplified model.
I found an excellent article here if you were interested: https://scied.ucar.edu/carbon-dioxide-absorbs-and-re-emits-infrared-radiation.
Thanks all!
Whatever energy is gained must later be lost. The CO2 molecule will absorb IR and cause a vibrational change (IR to kinetic energy) and then the molecule will give up this extra energy by re-emitting IR and "stop" vibrating.
This is of course a simplified model.
I found an excellent article here if you were interested: https://scied.ucar.edu/carbon-dioxide-absorbs-and-re-emits-infrared-radiation.
Thanks all!
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