Molecular Vibration and Translational Kinetic Energy in a Gas

[Wilko]

Mods, I hope I'm not breaching any guidelines here, haven't had any responses in General Physicsand thought perhaps the question might be more at home here.

I understand that CO2 molecules absorb infrared at 2.7, 4.3 and 15 microns, this makes them become vibrationally excited (rocking, stretching, bending, I don't know all the modes).

I have a few questions from this point:

1. When the CO2 molecule re-emits that energy is it obliged to do so at wavelengths similar to its absorption spectrum; I had assumed so but I don't know for certain that this is the case despite googling the hell out of it. CO2 does not behave in anyway like a blackbody when it re-radiates, correct?

2. In a gas, can the vibrational energy be passed from the CO2 molecule to other molecules during collisions, or can it only pass on as radiation? I understand there's a lattice effect in solids, but I don't think its relevant in a gas. Can molecular vibration 'turn into' translational kinetic energy?

3. Assuming that the CO2 molecule re-radiates at 2.7, 4.3 and 15 microns, I imagine that H20 may 'feel' that radiation at 4.3 microns, but I guess what I'm really asking is, can vibrationally excited CO2 molecules, do work on the rest of the molecules in the gas? Or is the vibrational energy of a CO2 molecule limited to doing work on other CO2 molecules?

P.S the gas is not an ideal gas, it's air.

 

[eljose79]

Hello there,

Thank you for bringing your questions to this forum. I am happy to provide some insight and clarification on the topics you have raised.

1. When a CO2 molecule absorbs infrared radiation, it becomes vibrationally excited and eventually re-emits that energy. This re-emission can occur at different wavelengths, not just the ones at which it absorbed the energy. This is because the molecule can undergo different types of vibrational modes, resulting in a range of possible wavelengths for emission. Additionally, the re-emission can also occur as multiple photons, resulting in a broader spectrum of wavelengths. Therefore, the re-emission spectrum of CO2 is not limited to just the absorption spectrum.

2. In a gas, the vibrational energy of a CO2 molecule can be transferred to other molecules through collisions. This is known as collisional energy transfer. The vibrational energy can also be converted into translational kinetic energy during these collisions. This process helps to distribute the energy throughout the gas, resulting in a more uniform temperature.

3. Vibrationally excited CO2 molecules can indeed transfer their energy to other molecules in the gas, including water vapor. This can result in heating of the gas as a whole, as well as changes in the composition of the gas. However, the amount of energy transferred by a single CO2 molecule may be relatively small compared to the total energy of the gas, so its impact on the overall temperature may be limited.

I hope this helps to answer your questions. Keep exploring and asking questions – that's what science is all about!

 

[astrokreng]

I can provide some insights into the questions you have raised about molecular vibration and translational kinetic energy in a gas.

1. When a CO2 molecule absorbs infrared radiation, it becomes vibrationally excited. This means that the bonds between the atoms in the molecule start to vibrate, causing the molecule to have a higher energy level. When the molecule re-emits this energy, it can do so in different ways. One way is through radiation, where the molecule emits photons at specific wavelengths that correspond to its vibrational energy levels. This is known as the molecule's absorption spectrum. However, it is not obliged to re-emit the energy at the same wavelengths as its absorption spectrum. It can also transfer the energy to other molecules through collisions, as discussed in the next question.

2. In a gas, molecular vibration can indeed be transferred to other molecules through collisions. When a CO2 molecule collides with another molecule, it can transfer some of its vibrational energy to the other molecule, causing it to vibrate as well. This is known as collisional deactivation. Similarly, a molecule can also gain vibrational energy through collisions with a vibrating molecule. So, in a gas, molecular vibration can turn into translational kinetic energy through collisions.

3. In a gas, the vibrational energy of a molecule can be transferred to other molecules through collisions, as mentioned above. This means that a vibrationally excited CO2 molecule can indeed do work on other molecules in the gas, including other CO2 molecules and molecules of different gases such as H2O. This can lead to an increase in the overall temperature of the gas, as the molecules gain more kinetic energy through collisions.

It is important to note that these processes are not limited to ideal gases, as they can occur in any gas, including air. The behavior of gases is governed by the laws of thermodynamics, which apply to all gases, ideal or non-ideal.

In summary, molecular vibration and translational kinetic energy are intricately linked in a gas, and energy can be transferred between molecules through collisions. The absorption and emission of infrared radiation by CO2 molecules is just one aspect of this complex system. I hope this helps clarify some of your questions.