Microwave - Infrared Double Resonance Spectrometer

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Double resonance spectroscopy (DRS) shall advance our high-resolution spectroscopy experiments in many substantial ways. In the proposed configuration we want to combine rotational spectroscopy in the microwave/THz frequency range and ro-vibrational spectroscopy in the infrared wavelength regime. The general idea is to address molecules in the gas phase by one radiation source in order to create a very sensitive and molecule specific signal. The second radiation source is used to create a difference signal which only arises when the two photons address a common molecular state. One part of the DRS instrument is a chirped pulse Fourier Transform Spectrometer (CPFTS) with a minimum frequency range of 2.5 – 18 GHz. This setup is able to address the lowest rotational states of most molecules over a very wide bandwidth in one chirp excitation pulse. The electronics to create a broad band chirp signal and to record the free induction decay (FID) signal is available in our laboratory but high-power amplifiers and an intense molecular beam to create the necessary sensitivity are missing and shall be purchased. For many experiments the broad band signal of the CPFTS serves as a very sensitive detector signal. In the second part radiation from a narrow line width but widely tunable Quantum Cascade laser addresses a ro-vibrational level associated with one molecule, one specific isomer or conformer and/or a specific vibrational state of this species. Therefore, it becomes possible to decipher complex spectra of molecular mixtures, experiments where different isomers, conformers and vibrational states are present at the same time. Thus DRS will be instrumental in analysing spectra of complex organic molecules and finally finding those species in astrophysical observations. In our highly sophisticated action spectroscopy experiments in ion traps the microwave/THz and infrared radiation sources change their role. The IR QCL shall be used to create a practically background free action spectroscopy signal, e.g. by promoting a chemical reaction through vibrational excitation. The microwave/THz photon is used to change the rotational population which results in recording the rotational spectra of molecular ions which is extremely hard or practically impossible to acchieve otherwise. Such a DRS instrument is not available commercially in one piece but building it from pieces becomes now possible and will put our research in a unique position.