A method to determine the phase-space distribution of a pulsed molecular beam

We demonstrate a method to determine the longitudinal phase-space distribution of a cryogenic buffer gas cooled beam of barium-fluoride molecules based on a two-step laser excitation scheme. Temporal resolution is achieved by a transversely aligned laser beam that drives molecules from the ground state X 2 Σ + to the A 2 Π 1 / 2 state around 860 nm, while the velocity resolution is obtained by a laser beam that is aligned counter-propagating with respect to the molecular beam and that drives the Doppler shifted A 2 Π 1 / 2 to D 2 Σ + transition around 797 nm. Molecules in the D-state are detected background-free by recording the fluorescence from the D − X transition at 413 nm. A temporal resolution of 11 μs and a velocity resolution of 6 m s⁻¹ is obtained. In order to calibrate the absolute velocity, we have determined the Doppler free transition frequencies for the X − A and X − D transitions with an absolute accuracy below 0.3 MHz. The high resolution of the phase-space distributions allows us to observe a variation of the average velocity and velocity spread over the duration of the molecular beam pulse. Our method hence gives valuable insight into the dynamics in the source.