I posed this thought experiment elsewhere, so the tone may seem a bit odd. It's based on what I've been reading on physical chemistry, atmospheric radiation and thermal physics. Just wondering if my interpretation is on the money or if I've made any incorrect assumptions. "Time for another random thought experiment. Imagine a “balloon” filled with air, it's not essential but we can also say its a closed system in a vacuum to make it simpler. The balloon is a spherical membrane completely transparent to infrared radiation at 4.3 microns (one of the absorption peaks for CO2, Sodium Chloride windows are transparent to IR as it turns out.) Then you illuminate the “balloon” with infrared radiation at 4.3 microns. What happens then? Glad you asked….A CO2 molecule inside the balloon will absorb an IR photon, become vibrationally excited, undergo molecular collisions, gain a small amount energy in collisions, lose a small amount of energy in collisions, and then emit an IR photon almost identical to the one it absorbed, it’s a very small perturbation based on how much vibrational energy was gained from, or lost as translational energy during the collisions for this particular CO2 molecule we have been so intently following. I'm ignoring Doppler broadening and the like for simplicity's sake and because I'm mainly interested in the interaction between translational and vibrational energy. As it turns out, the vibrational energy is much larger than the energy involved in collisions, so it’s quite hard for the collisions to affect vibration even if they could interact directly. On average though, over many molecules, many absorption/emission cycles and many collisions the amount of translational kinetic energy stays the same, and the amount of energy present as radiation stays the same. The radiation does get a little ‘messier’ (spectral broadening) but it never undergoes wholesale conversion into translational kinetic energy. PHEW! So the average translational energy of the molecules in the gas stays the same, even when we illuminate it with infrared radiation. That’s important because average translational kinetic energy is what we’re defining when we talk about the temperature of a gas, particularly in the real world. Vibrational energy is a temporary form of internal energy that isn’t really relevant when we talk about the macroscopic properties of a gas like Temperature, Pressure and Volume; those qualities are very specifically derived from average translational kinetic energy of the molecules in the gas. SO, if we illuminate our “balloon” with infrared radiation at 4.3 microns, we would expect any increase in the temperature of the air inside to manifest itself as an increase in pressure, which we could measure as a change in the volume of the balloon (offsetting the pressure increase somewhat, I do realize) So, can the balloon expand by illuminating it with infrared at 4.3 microns? Based on what I’ve read and understand, I can’t see how." Fun stuff, anyone got some extra insight to offer?