Microwave spectroscopy in the gas and solid phases: from rotational spectroscopy on organic molecules to the development of inorganic quantum sensors.

This thesis deals with two applications of microwave spectroscopy: rotational spectroscopy and quantum sensing. The first part reports the recording and analysis of rotational spectra under supersonic expansion conditions for two distinct molecular sets. The first set includes 2'-, 3'-, and 4'-aminoacetophenone (2AA, 3AA, and 4AA respectively), while the second set comprises N,N-diethylhydroxylamine (DEHA) and N,N-diethylacetoxylamine (DEAA). The formation of 1:1 weakly bound molecular complexes with water or neon has also been observed, allowing for the spectrum assignment of 2AA:W, 2AA:Ne, and DEHA:W. All these species exhibit complex spectra with hyperfine structures due to the presence of 14N and/or the effects of internal methyl rotation. Several isotopologues at natural abundance have been identified for a total of 39 species analysed. These structures have been interpreted using quantum mechanical models, allowing the determination of nuclear quadrupole coupling constants and methyl rotational barriers. A complementary experimental approach has been used, including Fourier transform microwave spectroscopy, free jet millimeter absorption wave spectroscopy and chirped pulse Fourier transform microwave spectroscopy. Moreover, DEHA has been investigated through photoelectron spectroscopy, which unveils structural rearrangements upon ionization and effects of dissociation in ethylamine and water. The derived spectroscopic constants enable future investigations in various frequency regions and molecular detection applications in analytical and astrochemical fields. Transitioning to condensed matter physics, microwave spectroscopy is employed to coherently manipulate and control the spin state of VB- defects in hexagonal Boron Nitride. Significant extension of coherence times through dynamical decoupling protocols (CPMG, XY8-N, and Spinlock) and the potential use of VB- as an RF field sensor are demonstrated. This enhanced spin control opens up possibilities for nanoscale spin sensing in low-dimensional quantum materials and devices.