PAH IR Spectral Database

Duplicating the harsh conditions of cold interstellar space in their laboratories and on their computers, NASA Astrobiology Institute Ames team scientists have created a unique database of polycyclic aromatic hydrocarbon (PAH) spectra, which is primarily used to interpret mysterious infrared (IR) emission detected by ground, air and space-based observatories.

The value of the NASA Ames PAH IR Spectral Database extends far beyond the immediate needs of NASA and the field of astronomy. The PAH spectral database has a large and diverse set of applications. PAHs are a major product of combustion — they remain in the environment and are carcinogenic. Consequently, they are important to scientists, educators, policymakers and consultants working in the fields of medicine, health, chemistry, fuel composition, engine design, environmental assessment, environmental monitoring and protection. The PAH database is a new tool for people working in all these fields.

The database contains over 800 spectra of PAHs in their neutral and electrically charged states, and tools to download PAH spectra ranging in temperature from -470 to 2000 degrees F. PAHs are flat, chicken-wire shaped, nano-sized molecules that are now known to be abundant throughout the universe, but often in exotic forms not readily available on Earth. They are thought to be produced in outflows from carbon-rich stars by processes similar to combustion in oxygen-poor flames that produce PAH-rich soots on Earth.

Mysterious infrared radiation from space was discovered from astronomical observations made in the 1970s and 1980s. While the infrared signature hinted that PAHs might be responsible, laboratory spectra of only a handful of small, individual PAHs were available to test this idea. These were only for neutral PAHs, not for PAHs as they were expected in interstellar space: electrically charged, free, very cold, individual molecules in the gas. The only spectra available were of tiny crystals containing many PAH molecules stuck together and suspended in oils and salt pellets: these PAHs were not isolated, not cold, and not charged. By the mid-1990s, observations from space showed this infrared emission was surprisingly common and widespread across the universe, implying that the unknown carrier was abundant and important. The need to measure PAH spectra under astronomical conditions was critical to make progress.

To provide these spectra, a team of scientists led by Louis Allamandola at NASA Ames Research Center developed a program in the late 1980s to measure PAH spectra under simulated astronomical conditions. According to Allamandola, there are now about 800 spectra in the database: 600 spectra have been theoretically computed and 200 spectra have been measured in the laboratory. The theoretical spectra span the range from 2 to 2000 microns, the experimental spectra cover 2 to 25 microns.

Christiaan Boersma is one of the three astronomers on the team who routinely uses the PAH IR Spectra Database to interpret astronomical observations. Boersma worked on the design and development of the current spectra website and its user tools. “The spectra in the database have given insights into the composition of the PAHs in space that were impossible to obtain any other way,” Boersma said. “In the near future these spectra will be especially valuable for interpreting observations made with NASA’s new airborne observatory, the Stratospheric Observatory for Infrared Astronomy and the European Space Agency’s Herschel Telescope. These telescopes are pioneering the far-infrared radiation region, opening a new window on the sky. Because the database shows that PAHs have many far-infrared transitions, PAHs will add to the pioneering discoveries made by these observatories.”

The easy-to-use website allows users to sort through long tables of numbers and to interact graphically with the data. The database can be interrogated using many criteria, such as PAH charge, composition and spectral signatures. Several tools allow users to do initial data analysis online. For example, spectra can be combined to create a ‘composite’ spectrum that can be directly compared to an unknown. In addition, all data can be downloaded. “Future plans are to expand the database with new sets of data and add useful tools along the way,’‘ Boersma said. Formerly at the University of Groningen in the Netherlands, Boersma is now a NASA postdoctoral fellow at Ames.

Charles Bauschlicher, Jr., also at NASA Ames, is one of the first computational chemists to calculate the infrared spectra of PAHs and their ions. Using quantum chemical calculations he and Alessandra Ricca of the SETI Institute, computed all of the spectra in the computational portion of the database. “When we started this project our hope was to help interpret the experimental spectra, but over time our computational capabilities grew to a point that we were able to study molecules much larger than can be studied in the laboratory,” Bauschlicher said. “In addition, we were soon using theory to study species that are very difficult to create and measure in a laboratory, but could be common in space.”

Ricca added, “I am very excited about the future of this research, because we have only scratched the surface of what theory can contribute to our understanding of PAHs. For example, we are now studying clusters of large PAHs and extremely large individual PAHs containing more than 100 carbon atoms.”

Andrew Mattioda, also at NASA Ames, and Douglas Hudgins now at NASA Headquarters, are both experimental physical chemists who have measured all of the spectra in the experimental portion of the database under astrophysically relevant conditions. “We accomplished this feat by first isolating the molecules in an inert matrix at very low temperatures, nearly -450 degrees F, much like the molecules are isolated in space. We then measured the spectra of both the neutral and ionized PAHs,” Mattioda said.

Mattioda’s and Hudson’s work includes PAHs containing a nitrogen atom in the molecule’s framework, molecules affectionately known as PANHs. “PANHs are very important in biochemistry as well as in the search for life elsewhere in the universe,” Mattioda said. “Our laboratory research has revealed previously unknown spectroscopic and electronic properties of PAHs and PANHs. We are rewriting the organic chemistry textbooks where polycyclic compounds are concerned. Given the biological processes that rely on PAH-type molecules, understanding the distribution of PAHs in the universe could provide insight into the distribution of life in the universe.”

Els Peeters, formerly a postdoctoral fellow at NASA Ames, and now a professor of astronomy at the University of Western Ontario, pioneered the application of the spectral database to astronomical spectra, and guided the direction of its expansion from the astronomer’s perspective. Analyzing observational data obtained with the Infrared Space Observatory and the Spitzer Space Telescope, Peeters identified several classes of astronomical PAH spectra that are related to different types of astronomical objects. “Just as with handwriting and language analysis, by comparing the spectral signature from each of these different astronomical objects with the different PAH signatures in the database, we were able to pinpoint the types of PAHs in these objects.” Peeters said. “When you realize that this works not only in our galaxy, but across the universe, it’s pretty amazing.”

According to Peeters, scientists are now able to use each ‘letter’ in the spectral signature to study specifics about the PAHs in space — their electrical charge, size and shape, etc. In addition, the NASA scientists who created the PAH IR Spectral Database have found small but real mismatches within the database, indicating that something was missing. Bringing together all the information on cosmic emission has revealed the types and amounts of different PAHs present in space and how they evolve from their birth site in red giant stars, to the interstellar medium between the stars, and ultimately into star-forming regions and proto-planetary disks.

“Thanks to this synthesis of lab data with astronomical observations, just as a weatherwoman uses satellite pictures to forecast the weather, these emission bands are being developed into a diagnostic tool to probe the local environmental conditions in our galaxy, out to nearby galaxies and all across the distant universe,” said Peeters.

Jan Cami, a former postdoctoral fellow at NASA Ames and now a professor of physics and astronomy at the University of Western Ontario, laid the conceptual foundation for the database and the website. Cami developed algorithms to match astronomical observations with spectra from the database. “Having the capability to almost perfectly reproduce astronomical observations with Earth-based laboratory experiments and theoretical calculations is very cool and rewarding,” Cami said. “That alone makes the NASA Ames PAH database unique — it is the only database in the world with enough PAH information to be relevant to astrophysical environments. Not only that, the database reveals what kind of PAHs like to hang out together and in what corner of the universe they do so.”

Creation of the PAH IR Spectral Database has led to some unexpected scientific results: 1) a significant fraction of PAHs in space are negatively charged, which was thought unlikely until now; 2) emission at certain wavelengths originates from smaller molecules, while larger molecules dominate other wavelength ranges.

Douglas Hudgins joined the team in the 1990s and pioneered the experimental techniques that are now routinely used in many laboratories to measure PAH spectra under extraterrestrial conditions. Now a program scientist at NASA Headquarters, Hudgins is thrilled to see the Ames database coming to fruition. “The reason that NASA’s Astrophysics Division supports laboratory research is because the resultant data are essential for analyzing and interpreting the observations of NASA’s space observatories. Doing the experiments and calculations are only part of the job. It is just as important to get those data into the hands of scientists in a convenient and useful format and this new database will do exactly that,” Hudgins said.

According to Allamandola, the NASA scientists who created the PAH Spectral Database began their work motivated to test the PAH hypothesis and provide the spectra needed to exploit the PAH model and develop it into a new “probe” of the wide variety and vast number of astronomical objects that show the PAH emission spectrum. “This field has exploded far beyond my wildest dreams and, in my opinion, this spectral database is what broke the spell that astrochemistry was limited to simple species and marginal for astrophysics,” Allamandola said.

“Thanks to the incredible sensitivity of the Spitzer Space Telescope, the PAH signature is seen across the universe, removing any doubt of the importance of these species. There is even evidence for PAH emission from very distant galaxies at red shifts of three, indicating these complex organic molecules were produced only a few billion years after the Big Bang. This means that enough carbon was available to drive a rich organic chemistry far earlier in the history of the universe than people thought only a few years ago,” stated Allamandola. “When you consider that the discovery of simple, garden-variety molecules like ammonia, formaldehyde and carbon monoxide in space made headlines in the 1960s and 1970s, this is incredible. Until then, space was thought to be chemically barren. If this isn’t enough, Messier 82 (the prototype nearby starburst galaxy about 12 million light years away) and Spitzer have shown there are even PAHs in the space between galaxies. Beyond a doubt, PAHs are an important part of modern astrophysics.”

Pioneering observations made by the European Infrared Space Observatory and the unprecedented sensitivity of NASA’s Spitzer Space Telescope have seen the PAH infrared signature from many astronomical objects within our galaxy and from most other galaxies across the universe. This database and accompanying user tools will see immediate use by astronomers throughout the world as they probe PAH emission to the edge of the universe with increasingly sensitive telescopes.

For additional information:

The PAH Spectral Database and tools are available at http://www.astrochem.org/pahdb

The Astrophysical Journal Supplement Series published, “The NASA Ames Polycyclic Aromatic Hydrocarbon Infrared Spectroscopic Database: The Computed Spectra,” which describes the website and details about the computational spectra in the database.

This research is supported by the Space Science and Astrobiology Division at NASA Ames Research Center and the Science Mission Directorate at NASA Headquarters, Washington, D.C.