Infrared Spectroscopy - An Overview

Except for the very lightest elements (primarily hydrogen and helium), which resulted from the Big Bang, most of the atoms and molecules in the Universe have their origins in the stars. As stars burn and eventually die out, they produce heavier elements which are then ejected into interstellar space as a star blows off its outer layers during its final phases. Some of these elements will then combine to form molecules. Atoms and molecules are the basic building blocks of all matter. Determining which atoms and molecules are present in space, what their distribution and abundance is, and in what environments they exist is critical to our understanding of the Universe, the formation of stars, planets and galaxies, and the possibility of life beyond the Earth.

The infrared part of the spectrum is where the emission and absorption lines of virtually all molecules as well as numerous atoms and ions (electrically charged atoms) lie. Infrared spectroscopy is the primary way to detect these elements in space.

Spectrometers onboard infrared missions like the Kuiper Airborne Observatory (KAO), and the Infrared Space Observatory (ISO), as well as near-infrared spectra from ground based observatories, have led to the discovery of hundreds of atoms and molecules in many different regions of space.

Infrared spectra of the molecule CH4 - one of at least ten new molecules detected by ISO. Credit: ESA/ISO, SWS, H. Feuchtgruber et al.

Since infrared can penetrate heavy dust, which often surrounds objects like newly forming stars and our galactic center, infrared spectroscopy can provide information about environments which are hidden from optical view, such as regions of star formation and the center of our galaxy. As a result of the Big Bang, the Universe is expanding. This causes spectral lines which would normally appear in the ultraviolet and visible part of the spectrum to be doppler shifted into the infrared for very distant objects. This means that infrared spectroscopy is a valuable tool for understanding conditions in the early Universe. An example of this can be seen in the spectrum of the most distant quasar currently known. The doppler shift of spectral lines also allows astronomers to detect and measure the velocity of planets around stars, stars around stars, expanding stellar atmospheres, outflows from star forming regions, rotating rings in galaxies, rotating spiral arms, supernova explosions, and shock waves from colliding galaxies. Infrared light is radiated from any object with a temperature. Even objects which are too cool to be detected optically can be studied in the infrared. For example, the ISO spectra (left) led to the discovery of a new molecule in interstellar space. The molecule, CH4, is one of the most important tracers for the formation of complex carbon-based molecules. It was detected in very cold and thin molecular clouds by its infrared absorption lines. The discovery of this molecule and the measurement of its abundance will lead to a better estimate of the abundance of hydrocarbons in space. To see some of the molecules which have already been discovered in interstellar space click here.

Infrared spectral studies are providing stunning information about the role of interstellar molecules in the formation of stars, planets and possibly even life. For example, infrared spectroscopy has shown that water is abundant in many regions of space and it is likely that the water we have here on Earth originated from stars which died out long before our solar system was formed. Crystaline silicates, the most abundant mineral on Earth, is also found in abundance in interstellar space. Recent infrared spectral data have shown that complex organic molecules can form rapidly (over a few thousand years) in the environments around old stars and are abundant in many regions of space. These elements and molecules will likely find their way into new stars and planets as they form from molecular clouds.

These are just a few examples of discoveries resulting from infrared spectroscopy. New infrared missions such as SOFIA (The Stratospheric Observatory For Infrared Astronomy) and the Spitzer Space Telescope will include infrared spectrometers which will lead to a greater understanding of the chemistry of the Universe.