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Chirped-pulse spectroscopy of methyl cyanide

There are different ways to obtain the fingerprint-like rotational spectra of astrophysical interesting molecules. A rather new technique is the chirped-pulse Fourier transform spectroscopy, which was developed by the group of Prof. Brooks Pate (University of Virginia, USA) in 2006. The technique consists of a short and powerful chirped-pulse covering a  broad frequency range to excite and polarise molecules in the gas phase.  These molecules then emit in free induction decay, which gives us their spectrum. In comparison to conventional absorption spectroscopy, it has the main advantage of being broadband. The technique was first implemented in the microwave range. Nowadays it is even possible to bring it into the millimeter-wave range. The used state-of-the-art spectrometer is working in the range between 75 and 110 GHz (also known as W band). As a result, there is an overlap with the frequency range of the Atacama Large Millimeter Array (ALMA) where many complex molecules are discovered in space. In addition, a supersonic jet expansion is used to cool down the molecules.

As preparation:

  • Brown et al., A broadband Fourier transform microwave spectrometer based on chirped pulse excitation, Review of Scientific Instruments 79, 053103 (2008); DOI

  • Read about „Heterodyne“ (a radio signal processing technique)

  • Hermanns & Wehres et al., Performance of a chirped-pulse Fourier transform millimeter wave spectrometer in the range of 75-110 GHz, Review of Scientific Instruments 94, 034705 (2023); DOI

  • Instruction

  • Datasheet methyl cyanide

Extended literature:

  • Peter F. Bernath, Spectra of atoms and molecules, Oxford University Press, 2005.

  • Giacinto Scoles, Atomic and Molecular Beam Methods, Chapter 2: Free Jets, Oxford University Press, 1988.

  •  W. Gordy and R. L. Cook, Microwave Molecular Spectra, John Wiley and Sons, 1984.


  • Dr. Mariyam Fatima (

  • Bettina Heyne (