Astronomical observations of cold regions in the universe show a rich inventory of ices. Part of these ices may end up on planets like our own, but in that journey they will be exposed to... Show moreAstronomical observations of cold regions in the universe show a rich inventory of ices. Part of these ices may end up on planets like our own, but in that journey they will be exposed to considerable amounts of radiation. As water is the main component of these ices, the optical and photochemical properties of water ice largely determine how the radiation affects the molecules embedded in the ice. In this thesis, water ice is investigated as a host for photochemical reactions. A new laboratory setup is constructed, and two types of molecules are studied: glycine, an amino acid, and triphenylene, a polycyclic aromatic hydrocarbon. The photochemistry upon exposure to ultraviolet radiation is studied using UVvis and infrared spectroscopy. In addition, the optical properties of water ice are constrained in the UV-vis range, resulting in high-resolution optical constants, relevant for all ice-rich environments - from far away in space to our atmosphere. Water itself does not absorb light in this range, but as most organic molecules do, and are destroyed by radiation in this range, this is of high importance for molecules within the ice. Show less
This thesis presents the anharmonic infrared spectra of a series of polycyclic aromatic hydrocarbons. Comparisons and characterizations are made based on high-resolution low-temperature gas-phase... Show moreThis thesis presents the anharmonic infrared spectra of a series of polycyclic aromatic hydrocarbons. Comparisons and characterizations are made based on high-resolution low-temperature gas-phase experimental spectra. Show less
We report on an experimental and theoretical investigation of the importance of anharmonicity in the 3-μm CH stretching region of polycyclic aromatic hydrocarbon (PAH) molecules. We present mass... Show moreWe report on an experimental and theoretical investigation of the importance of anharmonicity in the 3-μm CH stretching region of polycyclic aromatic hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra of the gas-phase cold (∼4 K) linear PAH molecules naphthalene, anthracene, and tetracene. The measured IR spectra show a surprisingly high number of strong vibrational bands. For naphthalene, the observed bands are well separated and limited by the rotational contour, revealing the band symmetries. Comparisons are made to the harmonic and anharmonic approaches of the widely used Gaussian software. We also present calculated spectra of these acenes using the computational program SPECTRO, providing anharmonic predictions with a Fermi-resonance treatment that utilizes intensity redistribution. We demonstrate that the anharmonicity of the investigated acenes is strong, dominated by Fermi resonances between the fundamental and double combination modes, with triple combination bands as possible candidates to resolve remaining discrepancies. The anharmonic spectra as calculated with SPECTRO lead to predictions of the main bands that fall within 0.5% of the experimental frequencies. The implications for the aromatic infrared bands, specifically the 3-μm band, are discussed. Show less
Mid-infrared (mid-IR) astronomical observations show that molecules freeze out on interstellar grains to form interstellar ices. These ices play an important role in the chemical evolution of... Show moreMid-infrared (mid-IR) astronomical observations show that molecules freeze out on interstellar grains to form interstellar ices. These ices play an important role in the chemical evolution of molecules in space. Understanding the physical interactions and chemical reactions that take place in these ices gives us important clues to how the ices are formed and which role they play in the formation of complex molecules. The physical interactions in H2O:CO binary ices and in CH3OH and NH3 containing ices are studied both in the laboratory and in space by means of mid-IR spectroscopy, resulting in the unambiguous detection of NH3 in a low-mass protostar. Additionally, mid-IR and near-UV/VIS spectroscopic measurements on the ultraviolet induced photochemistry of large molecules, Polycyclic Aromatic Hydrocarbons (PAHs), trapped in ices are described. These molecules are known to be abundantly present in space, but their influence on the chemical evolution of interstellar ices has never been studied in detail. The experiments indicate that the processes are important in the formation of complex and charged molecules and that new astrochemical models should take the role of PAHs in ice into account. Show less
