New Physics with Cold Molecules : Precise Microwave Spectroscopy of CH and the Development of a Microwave Trap

Cold polar molecules provide unique opportunities to test fundamental physics and chemistry. Their permanent electric dipole moments and rich internal structure arising from their vibrational and rotational motion, makes them sensitive probes for new physics. These features also make them ideal for studying ultracold chemistry, for simulating the behaviour of strongly interacting many-body quantum systems, and for quantum information science. This thesis describes a number of advances in cold molecule physics. The optimum method for producing an intense, pulsed, supersonic beam of cold CH molecules is investigated, resulting in a beam with 3.5x10^9 ground state CH molecules per steradian per shot. The beam has a translational temperature of 400 mK and a velocity that is tuneable between 400 and 1800 m/s. The lowest-lying Λ-doublet transitions of ground state CH, at 3.3 GHz and 0.7 GHz, are exceptionally sensitive to variations in the fine-structure constant α and the electron-to-proton mass ratio μ. Many modern theories predict that these constants may depend on time, position, or local matter density. Using a novel spectroscopic method, the frequencies of these microwave transitions are measured with accuracy down to 3 Hz. By comparing to radio-astronomical observations, the hypothesis that fundamental constants may differ between the high and low density environments of the Earth and the interstellar medium of the Milky Way is tested. These measurements find no variation and set upper limits of |Δα/α| < 2.1x10^-7 and |Δμ/μ| < 4.3x10^-7. The frequency of the lowest millimetre-wave transition of CH, near 533 GHz, is also measured with an accuracy of 0.6 kHz. The development of a novel type of trap for ground state polar molecules is presented. Trapping polar molecules is a necessary condition to cool them to ultracold temperatures of 1 μK and below. The trap uses a high intensity microwave field in a Fabry-Pérot resonator. Experimental and theoretical investigations are presented that explore the modes of the cavity, how to obtain the highest possible quality factor, and how to optimally couple the microwave power into the cavity. Finally, a cold supersonic beam of BH molecules is developed. This molecule appears to be particularly well-suited to direct laser cooling due to its favourable rotational structure and Franck-Condon factors. The laser cooling concept is described, and a first spectroscopic investigation of the relevant molecular structure is presented.Open Acces