Why do certain compounds, such as carbon dioxide, absorb and emit infrared energy?
A. Molecules can absorb and emit three kinds of energy: energy from the excitation of electrons, energy from rotational motion, and energy from vibrational motion. The first kind of energy is also exhibited by atoms, but the second and third are restricted to molecules. A molecule can rotate about its center of gravity (there are three mutually perpendicular axes through the center of gravity). Vibrational energy is gained and lost as the bonds between atoms, which may be thought of as springs, expand and contract and bend. The three kinds of energy are associated with different portions of the spectrum: electronic energy is typically in the visible and ultraviolet portions of the spectrum (for example, wavelength of 1 micrometer, vibrational energy in the near infrared and infrared (for example, wavelength of 3 micrometers), and rotational energy in the far infrared to microwave (for example, wavelength of 100 micrometers). The specific wavelength of absorption and emission depends on
