A cryogenic buffer gas beam source of BaF molecules

The search for the electric dipole moment of the electron (eEDM) is a direct test of fundamental theories beyond the Standard Model. The NL-eEDM collaboration aims to perform a competitive measurement of the eEDM with a Stark decelerated and laser cooled beam of barium monofluoride (BaF) molecules. This thesis describes the design, construction, testing and understanding of the cryogenic buffer gas beam source of BaF molecules. The brightness, density and rotational temperature of the beam are measured with rotationally resolved absorption and laser-induced fluorescence detection. The phase-space density – the number of molecules per position and velocity interval – of decelerated molecular beams is proportional to the phase-space density of the initial beam. The success of Stark and Zeeman deceleration techniques depends crucially on the brightness of the initial beam. A method is developed to directly measure the phase-space distribution in the longitudinal direction of the molecular beam, which provides the velocity and position of the BaF molecules uncorrelated from each other. This method is used to determine the influence of various source parameters. Besides the expected dependence on temperature and pressure, the forward velocity of the molecules is strongly correlated with the time at which they exit the cell, revealing the dynamics of the gas inside the cell. The source produces about 120 billion BaF molecules per steradian per pulse in a single rotational level with a forward velocity of 207 m/s.