Terahertz Desorption Emission Spectroscopy

Auriacombe, Olivier (2019). Terahertz Desorption Emission Spectroscopy. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.000103f4

Abstract

Enhancing our understanding of the formation and evolution of the Universe is strongly dependent upon the observation of matter that exists between the stars and that forms the interstellar medium (ISM). Using Radio-telescopes based on heterodyne detection (such as ALMA), nearly 200 molecules have been detected. Whilst molecules, such as H2O and CH3OH, can only form in the solid state, the formation of complex organic molecules is not fully known. Moreover many simple species desorb from ices during the star formation process being useful tracers of these astronomical processes, but only if the thermal and non-thermal desorption mechanisms are fully understood.

In this thesis, I have designed, developed, built and proven a new experimental technique, namely Terahertz Desorption Emission Spectroscopy (THz-DES). The basis of THz-DES is to recreate the chemical conditions found in interstellar environments by emulating the astronomical detection techniques. The emission spectroscopy is based on the integration of a total-power Schottky-barrier based radiometer operating between 320-350 GHz (overlapping with ALMA band 7), coupled to a vacuum chamber where interstellar ice analogues can be grown on a 77 K cold-finger, then thermally desorbed, enabling the gas-ice synergy to identify molecular spectra and desorption energies.

I demonstrate the versatility of the technique observing thermal desorption from pure, layered or mixed ices of nitrous oxide, water and methanol. The results are compared to existing literature values of the desorption energies, and the known spectral features. Finally, there is some evidence that certain energy levels of desorbing species are under-populated relative to the population expected in local thermodynamic equilibrium conditions. This impact of surface constrains on the dynamics and internal energy of desorbing molecules, largely ignored in astronomy, shows interesting promise for the future of the THz-DES technique.

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