Heat Absorbing Gases
Selections RulesEven though water has 3 vibrational modes, we observe that the molecule can absorb heat energy to access only two of them. Only vibrations where the dipole moment changes occur. This is a general rule, called a selection rule, that a change in molecular dipole must occur in the vibration for the transition to the vibrational level to be allowed.
Carbon dioxide doesn't have a molecular dipole in its ground state. However, some CO2 vibrations produce a structure with a molecular dipole. Because of this, CO2 strongly absorbs infrared radiation.
Can you find the vibrations of CO2 that result in a change in the dipole moment?
IR SpectroscopyInfrared (IR) electromagnetic radiation is in the range necessary to molecular vibrations.
Electronic spectroscopy uses visible or ultraviolet (UV) radiation to probe the absorption of energy by molecules between electronic energy levels. IR spectroscopy probes the absorption of energy by molecules between vibrational energy levels. The principle behind all forms of absorption spectroscopy is the same.
Electromagnetic radiation interacts with atoms or molecules within a sample. When radiation is absorbed, there is a transition from a low energy state to a higher energy state.
Some of this radiation is absorbed and the remainder passes through. A detector measures the radiation that passes through the sample. In a split-beam spectrometer, half of the radiation goes through the sample and the other half goes directly to the detector. The detector compares the intensity of the two signals.
Water vapor is a significant greenhouse gas but its concentration in the air is regulated by the equilibrium between H2O(g) in the atmosphere and H2O(l) in surface bodies of water. The increases in other greenhouse gases is a direct result of human activity. Below is a table that shows the most important of these.