Highest resolution Fourier transform infrared spectroscopy of aromatic and chiral molecules and of molecules of astrophysical interest

Interferometric high resolution Fourier transform infrared (FTIR) spectroscopy is one of the most powerful tools in spectroscopy. At the highest resolution limitations arise from weakness of the light source and the resulting noise at long interferometer optical paths. The use of new bright light sources like synchrotron radiation overcomes the resolution and noise limitations of thermal sources and opens the route for more highly resolving interferometers by extending the maximum optical path difference to new dimensions. We have set up two new worldwide unique high resolution FTIR experiments. The first experiment consists of a nine chamber Bruker interferometer, the Zurich Bruker spectrometer prototype ZP2001, which is capable of measuring infrared spectra using a resolution of better than 33 MHz. This spectrometer was set up in our laboratory in Zurich and is equipped with a collisional cooling cell. This cell makes it possible to record the highly resolved IR spectra of complicated and heavy molecules at temperatures as low as 10 K. This setup was used to record the FTIR spectra of methane and its isotopomers under similar temperature conditions as in Titan’s atmosphere at 80 K and at lower temperatures down to 15 K. It was also used to measure the spectrum of the dichloro fluro hydrocarbon CHCl2F (HCFC-21) at temperatures of 160 K important in the context of the atmospheric greenhouse effect and the ozone depletion. The second FTIR experiment consists of an eleven chamber Bruker interferometer which uses as a light source the synchrotron light of the Swiss Light Source (SLS). We have interfaced this prototype spectrometer, the ETHSLS Bruker IFS 125 HR prototype 2009, with the currently worldwide highest available maximum optical path difference of dMOPD = 11.7 m, including transfer optics, to the infrared port available at the SLS located at the Paul-Scherrer-Institute (PSI). This setup makes it possible to record IR spectra with a resolution of better than 20 MHz and with a 5 to 20 times better signal-to-noise ratio compared to spectra recorded using thermal sources. The THz region below 10 THz becomes now easily accessible using this FTIR setup which is of evident importance in the context of high resolution atmospheric and interstellar molecular spectroscopy. Both setups have been used to bring solutions to several fundamental scientific questions closer. The first is in regard to the analysis of important atmospheric trace gases like methane and fluoro chloro hydrocarbons important in the context of global warming and of understanding of extraterrestrial atmospheres. We have helped improve the spectral analysis of these molecules using new cold FTIR spectra. The second question is in regard to the experimental detection of molecular parity violation. Based on the high resolution spectral analysis of the FTIR spectra of the chiral molecule CDBrClF, rovibrationally analysed in the ν5, ν4, ν3 and 2ν4 regions, and CHBrIF, analysed in the ν5, ν4 and 2ν4 regions, one can now propose that the CF-stretching band (ν4) and its overtone region (2ν4) of the chiral isotopomers CHBrIF and CHBrIF are very promising candidates to identify line shifts due to parity violation using ultra high resolution CO2 laser or frequency comb spectroscopy. The measurements of the rotationally resolved FTIR spectra of CHBrIF and CDBrClF spectra were only possible thanks to the new synchrotron light source and the use of collisional cooling. The rovibrational analysis of such complicated spectra with rotational constants as small as 0.026 cm−1 was only possible thanks to newly developed spectroscopic tools like pattern recognition methods. A further question is in regard to the identification of the interstellar Unidentified Infrared Bands (UIBs) detected in different interstellar objects. Using synchrotron radiation it was possible to rotationally resolve and analyse the FTIR spectrum of naphthalene in the 12.8 μm region as a prototype spectrum for a Polycyclic Aromatic Hydrocarbon (PAH) spectrum. PAHs are considered to be the carrier of the UIBs. Based on the analysis and the highly resolved infrared spectra a new strategy was developed which should easily identify the UIBs below 12 μm using the Q branches of the out-of-plane modes of PAHs. Coincidences between naphthalene, biphenyl bands and UIBs at 12.8, 14.2 and 13.6 μm have been identified. In this context general questions dealing with the recording and analysing of spectra of large aromatic compounds must be answered. Therefore, we have measured the highly rotationally resolved infrared spectra of pyrimidine, pyridine, indole, fluorobenzene, chlorobenzene and azulene in the range 600 to 1300 cm−1 and partly up to 3600 cm−1 and fully analysed these spectra for the first time. The heterocyclic aromatic compounds pyridine, pyrimidine and indole are also of evident interstellar interest because they are building blocks of biomolecules. Based on our analysis we propose a search for these compounds in the 700 cm−1 region. Detection of these molecules using rotational spectroscopy in the submm wave range was so far unsuccessful. We have measured and analysed the first rotationally resolved infrared spectrum of azulene finishing a story which began more than 60 years ago with the analyses of azulene‘s infrared spectrum by Günthard and Heilbronner. Another important question pertains to mode selective dynamics of tunneling processes in polyatomic molecules. As an example we present an analysis of the torsional splitting of the phenol spectra from the THz to mid-IR region (200 to 1500 cm−1) and discuss the change of the splitting upon excitation of the torsional , the out-of-plane and in-plane modes. Rotationally resolved aniline FTIR spectra within the THz and infrared regions have been measured for the first time. Their analysis leads to the determination of the inversion splitting of aniline in the ground state and a rovibrational analysis of the torsional subbands. The highly resolved FTIR spectra of these