  Cryogenic buffer gas beams are central to many cold molecule experiments.
Here, we use absorption and fluorescence spectroscopy to directly compare
molecular beams of AlF, CaF, MgF, and YbF molecules, produced by chemical
reaction of laser ablated atoms with fluorine rich reagents. The beam
brightness for AlF is measured as $2\times 10^{12}$ molecules per steradian per
pulse in a single rotational state, comparable to an Al atomic beam produced in
the same setup. The CaF, MgF and YbF beams show an order of magnitude lower
brightness than AlF, and far below the brightness of Ca and Yb beams. The
addition of either NF$_3$ or SF$_6$ to the cell extinguishes the Al atomic
beam, but has a minimal effect on the Ca and Yb beams. NF$_3$ reacts more
efficiently than SF$_6$, as a significantly lower flow rate is required to
maximise the molecule production, which is particularly beneficial for
long-term stability of the AlF beam. We use NO as a proxy for the reactant gas
as it can be optically detected. We demonstrate that a cold, rotationally pure
NO beam can be generated by laser desorption, thereby gaining insight into the
dynamics of the reactant gas inside the buffer gas cell.
