Abstract; Kurzzusammenfassung; Contents; List of Figures; List of Tables; Chapter 1 Molecular Spectroscopy: a Tool to Unveil the Universe; Chapter 2 Theoretical Considerations; Chapter 3 Experimental Setup; Chapter 4 Accurate Rest Frequencies for CO+, ideal tracer for Photon-Dominated Regions; Chapter 5 First Interstellar Detection of c-C3D2; Chapter 6 Tentative Detection of l-C3HD; Appendix A MAPLE mini model; Bibliography; Acknowledgements; Erklärung; Weitere Publikationen; Lebenslauf