A University of Delaware-led research team reports an advance inthe June 1 issue of Science that may help astrophysicists moreaccurately analyze the vast molecular clouds of gas and dust wherestars are born. Krzysztof Szalewicz, professor of physics and astronomy at UD, wasthe principal investigator on the National Science Foundationfunded research project, which solved equations of quantummechanics to more precisely describe the interactions betweenmolecules of hydrogen and carbon monoxide, the two most abundantgases in space. Such calculations are important to spectroscopy, the science thatidentifies atoms or molecules by the color of light they absorb oremit. Sir Isaac Newton discovered that sunlight shining through aprism would separate into a rainbow of colors. Today, spectroscopyis essential to fields ranging from medical diagnostics to airportsecurity. |
In astrophysics, spectrometers attached to telescopes orbiting inspace measure light across the visible, infrared, and microwavespectrum to detect and quantify the abundance of chemical elementsand molecules, as well as their temperatures and densities, inplaces such as the vast Orion Nebula, a celestial maternity wardcrowded with newborn stars, some 1,500 light years away. Whereas carbon monoxide - the second-most abundant molecule inspace - is easily detected by spectrometers, such is not the casefor hydrogen. Despite ranking as the most abundant molecule inspace, hydrogen emits and absorbs very little light in the spectralranges that can be observed. Thus, researchers must deduceinformation about molecular hydrogen from its weak interactionswith carbon monoxide in the interstellar medium (the stuff betweenthe stars).
"The hydrogen spectra get lost on the way, but carbon monoxide islike a lighthouse - its spectra are observed more often than thoseof any other molecule," Szalewicz says. "You can indirectly tellwhat the density of hydrogen is from the carbon monoxide spectra." Szalewicz and co-authors Piotr Jankowski, a former UD postdoctoralresearcher who is now on the chemistry faculty at NicolausCopernicus University in Torun, Poland, and A. Robert W. McKellar,from the National Research Council in Ottawa, Canada, wanted torevisit the spectra of the hydrogen and carbon monoxide complex.The first time such a calculation was done was 14 years ago bySzalewicz and Jankowski, parallel to an accurate measurement byMcKellar.
In their computational model, the scientists needed to determinefirst how electrons move around nuclei. To this end, they includedsimultaneous excitations of up to four electrons at a time. Theenergy levels produced by the rotations and vibrations of thenuclei then were computed and used to build a theoretical spectrumthat could be compared with the measured one. The team's calculations, accomplished with the high-powered koloscomputing cluster at UD, have resulted in theoretical spectra 100times more accurate than those published 14 years ago. Thetheoretical and experimental spectra are now in near-perfectagreement, which allowed the team to "assign" the spectrum, thatis, to determine how each spectral feature is related to theunderlying motion of the nuclei, Szalewicz says.
The combined theoretical and experimental knowledge about thismolecular complex now can be used to analyze recent results fromsatellite observatories to search for its direct spectral signal.Even more importantly, this knowledge can be used to get betterinformation about the hydrogen molecule in space from indirectobservations, Szalewicz notes. "Spectroscopy provides the most precise information about matterthat is available," he says. "I am pleased that our computationshave untangled such a complex problem." Szalewicz's expertise is in numerically solving the equations forthe motions of electrons resulting in molecules attracting orrepelling each other and then using these interactions to look atdifferent properties of clusters and condensed phases of matter. His research has unveiled hidden properties of water and found amissing state in the beryllium dimer, both results previouslyreported in Science, and his findings about helium may lead to moreaccurate standards for measuring temperature and pressure. Szalewicz was elected to the International Academy of QuantumMolecular Science in 2010 and is a fellow of the American PhysicalSociety.
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