Heat Absorbing Gases

Selections Rules

Even 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 Spectroscopy

Infrared (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.

Wavelength Radiation Transition
10-50 nm X-rays core electron
50-1000 nm UV-visible valence electron
1000-20,000 nm near infrared vibrations
20,000-100,000 nm far infrared rotations
1-100 nm microwaves rotations
100-1000 nm radio waves nuclear spin

Green House Gases

The Earth absorbs solar energy and most of this energy is later released as heat, or IR radiation.

The most abundant gas molecules in the atmosphere, N2 and O2, are not able to absorb this energy and it passes through and out into space.

Carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and certain other gases absorb IR radiation from the Earth's surface and re-emit it in all directions. These gases act like the glass in a greenhouse to increase the temperature of the planet.

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.

Professor Patricia Shapley, University of Illinois, 2011