Greenhouse Gases?

How do greenhouse gases work, quantum mechanically speaking? How do they block infrared radiation?

• So how do greenhouse gases work? They allow visible light from the sun to pass through the atmosphere, but block infrared radiation (heat) from leaving it. I know greenhouse gases are some of the bigger molecules, so I think that has something to do with it. Basically, what makes carbon dioxide a greenhouse gas and nitrogen not a greenhouse gas? Also, anybody know the specific wavelength ranges that greenhouse gases (particularly carbon dioxide) block and those that it allows to pass through? tl;dr: how do greenhouse gases work

• Answer:

  The idea is simple. Every molecule is made up of atoms and these atoms can vibrate within that molecule (think of them like balls on springs). If they can vibrate at a frequency that corresponds to an infrared photon frequency, then that molecule will absorb that photon and re-emit it in a random direction. So that photon will end up bouncing around in the atmosphere more that it would have if it were not absorbed and passed straight through the atmosphere. Now you might wonder why some molecules absorb in the infrared and some do not. Well, everything absorbs at some wavelength. The frequency something absorbs at depends on how tightly bound the parts of the 'something' are. Atoms typically absorb in the visible or ultraviolet frequencies because the electrons on the atoms are very tightly bound so they vibrate at very high frequencies. Molecules are less tightly bound, so they absorb in the visible and infrared. To absorb in the 'green house' band, then they simply need to be bound with the right 'tightness'. Actually, the mass of the molecule is not the critical issue. Greenhouse gasses are not heavier than other gasses. It simply that they have some frequency at which they vibrate that happens to be in the infrared (and most molecules have tens of different frequencies at which they vibrate). CO2 is an example of a greenhouse gas, but methane (CH4) is a much more 'efficient' greenhouse gas and weighs quite a bit less than N2. In fact, water is one of the worst greenhouse gasses (H2O). How good a gas is at being a greenhouse gas simply depends on what frequencies it vibrates at, which depends on how massive the atoms in the molecule are and how strong the bond is.