Double-Resonance Spectroscopy of Small Molecules Using an Optical Frequency Comb
Satellite- and ground observations of hot-Jupiter exoplanets provide information about conditions at these remote objects. To decipher, we need accurate theoretical models of the observed high-temperature spectra, verified by high-precision laboratory measurement. Double-resonance spectroscopy with a frequency comb probe to measure and analyze transitions to highly-excited energy levels of methane, ammonia and acetylene, provide high-precision data improving accuracy of theoretical models.
The project is supported by the Swedish Research Council Consolidator Grant.
The first exoplanet orbiting a star similar to our Sun was observed in 1995 and since then astronomers have found 4000 more. Many of them are the so-called hot-Jupiters - they are as large as Jupiter but orbit so close to their stars that their temperature reaches up to 700 ° C. Although life as we know it probably cannot exist on these exoplanets, studying them provides unique information about our universe.
All information we have about astronomical objects comes from satellite and terrestrial observations. The observed spectra carry information about the composition, conditions and photo-chemistry in the exoplanet's atmosphere, as well as the planet formation. To extract this information, accurate theoretical models of high-temperature spectra are needed. The accuracy of these models, in turn, needs to be verified by data obtained by high-precision laboratory measurements. Such data is missing for many molecular species.
So far, water, methane, carbon dioxide, carbon monoxide and titanium oxide have been detected on exoplanets, but many more species are expected there, such as ammonia or acetylene. Although these molecules are relatively simple, their energy level structure is complex. At high temperatures, their spectra become rich in lines and are difficult to analyze. Our understanding of their energy level structure is limited by the lack of high-precision experimental data for transitions to highly-excited energy levels. This prevents accurate modeling of high-temperature spectra observed from exoplanets and other hot astrophysical objects.
Within our project, we measure and identify transitions to highly-excited energy levels of methane, ammonia and acetylene and provide experimental data that enables verification of the theoretical calculations used to model high-temperature spectra. To do this, we use a technique called double-resonance spectroscopy, which uses a high power pump laser to increase the population of a single chosen energy level and a weaker probe laser to measure the spectrum from this selectively populated level to higher energy levels. To detect a large number of transitions, we use an optical frequency comb as a probe. An optical frequency comb is a type of laser whose spectrum consists of hundreds of thousands of equidistant lines. This provides a unique combination of broad spectral coverage and high resolution – a measurement with a frequency comb is equivalent to a measurement with hundreds of thousands of synchronized lasers.
The data provided by our project will allow improving the accuracy of the theoretical calculations of high-temperature spectra used to interpret the spectral information from hot-Jupiter exoplanets and other astronomical objects. This is particularly relevant for the missions planned by NASA (James Webb Space Telescope) and ESA (ARIEL) that will investigate the temperature structure and chemical composition of the exoplanetary atmospheres using high-resolution infrared spectroscopy.