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Mathematisch-Naturwissenschaftliche FakultätAstrophysik I.Physik

Chirped-Pulse Spectroscopy

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Investigators

  • Mariyam Fatima
  • Bettina Heyne
  • Prachi Misra
  • Lise von Rötel
  • Timo Gaßen
  • Natalie Sommer

Description

Molecules are essential to our understanding of the universe. Since they are widely distributed in space their characteristic rotational spectra can be detected with telescopes on earth. Hence, it is necessary to study the molecules in the laboratory. Besides classical absorption spectroscopy, there is also Chirped-Pulse Fourier Transform Spectroscopy (CP-FTS). CP-FTS is a rather new technique of molecular spectroscopy which consists of several steps as you can see in the upper figure. First, a linearly  increasing frequency signal - the chirped-pulse, is broadcasted by an antenna in the chamber to excite and polarize our molecules, which are rotationally cooled down by a supersonic jet. After that the molecules fall back in their original state and emit a free induction decay (FID). The FID is detected by a second antenna. By using Fourier transformation we receive the rotational spectrum of the molecule. This technique allows the following: i) recording of broad band spectra in the centimeter and millimeter wavelength ranges and ii) fast excitation and data acquisition which results in the possibility to record time dependent signals.

Instruments

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There are two CP-FT spectrometers in our laboratories operating at different frequency ranges. The instruments are designed to achieve high stability and sensitivity, making it possible to measure isotopic species of molecules in natural abundance.

One spectrometer operates between 12 and 26.5 GHz which is interesting for rotational spectra of complex molecules. These molecules are relevant for astrophysics and the frequency range  matches the Green Bank Telescope. In addition, many organic molecules present or possibly present in space have their maximum transition intensity in this region at cold temperatures. In a recent modification, we are using state-of-the-art RF modulation and detection technology to directly generate and receive signals in this frequency range. Thus, abandoning the up- and down-mixing processes of our previous chirped-pulse microwave spectrometer setup (Hermanns et al. 2019). We are using supersonic jets to cool down our neutral molecules in a carrier gas. The jet can also be used to study Van der Waals complexes.

The other chirped pulse spectrometer is operational in the millimeter-wave range between 75 and 110 GHz coincident with the Atacama Large Millimeter/Submillimeter Array (ALMA) Band 3. Besides the possibility to detect isotopologs in natural abundance, we can also observe fragments of molecules and study their recombination processes. This can be done by a high voltage DC discharge in combination with a supersonic jet. With this DC discharge attachment it is also possible to create ions and radicals.

 

Selected Publications

Acknowledgments

This project has been supported via Collaborative Research Centre 956, sub-project B4. B. H. acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG; project ID WE 5874/1-1).