which molecules in the atmosphere absorb infrared radiation?
The only way to study the entire range of infrared (as well as gamma rays, xrays, UV) is to place telescopes in space well above the atmosphere. Only some (not all) of the IR radiation between 1 and 40 microns makes it to the Earth's surface. The rest is absorbed by our atmosphere primarily by water vapor. IR is also absorbed to a lesser degree by carbon dioxide and oxygen molecules.
Water vapor and certain gases in the atmosphere absorb infrared radiation, causing what is called the Greenhouse Effect. This infrared radiation comes from materials on the Earth's surface that have been heated by sunlight.