What are Ground and Excited States?

When talking about energy levels, the lowest level is called the ground state. At room temperature, everything (electrons, vibrations, etc.) is in the ground state. If the atoms or molecules can absorb energy, they can be promoted into a higher energy level, called an excited state. Be careful, though. There are many excited states, so it is possible to absorb energy to promote from an excited state to an even higher energy excited state.