Greenhouse Gases

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

Most of the gas molecules in the atmosphere 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.

IR Absorption

The homonuclear diatomic molecules that make up the bulk of the atmosphere (N2 and O2) do not absorb IR radiation because they have no IR active vibrations. What makes a vibrational mode IR-active?

An electromagnetic field interacts with a molecular bond by affecting the dipole moment of that bond. A bond with a dipole moment can be visualized as a positive charge and a negative charge separated by a spring. An electric field will cause this bond to stretch or be compressed. Stretching the bond increases the dipole moment of the bond while compressing the bond decreases the dipole moment of the bond.

A rapidly alternating electric field is one of the components of an electromagnetic wave and will cause a bond with a dipole moment (polar bond) to alternately stretch and compress. When this stretching and compressing occurs at the frequency of the molecule's natural rate of vibration, energy is absorbed.

A bond must have a dipole moment in order to interact with the electromagnetic field of infrared spectroscopy and that dipole moment must be changed by the vibration of the bond in order for the bond to be IR-active.

When infrared radiation is absorbed by molecules, the molecules are promoted into higher energy vibrational states.

The amount of energy required for a molecular vibration is related to bond strength through the force constant.

In IR spectroscopy, we measure absorption peaks by wavenumbers (cm-1). This is the reciprocal of wavelength and is proportional to energy.

Examine the simplest situation. Infrared radiation is absorbed by the stretching vibration of a simple diatomic molecule (CO, HCl, NO, for example). This stretch is the only vibrational mode for a diatomic molecule.

    Stretching Vibrations
  1. There is movement along the bond axis.

  2. Interatomic distances change.

  3. There is NO change in the center of gravity.

Non-linear molecules have more vibrational modes. Water has 3 of these.

    Vibrational Modes
  1. symmetric stretch

  2. asymmetric stretch

  3. scissoring bend

Carbon Dioxide

IR radiation excites some of the possible vibrational modes in carbon dioxide. The vibrational absorption spectrum of CO2 is shown below.

Other Greenhouse Gases

Other gases can absorb even more solar energy than carbon dioxide. Early in the history of our planet, the high methane concentration in the atmosphere kept the Earth's surface warm and water in a liquid form.

Professor Patricia Shapley, University of Illinois, 2010