  We excite YbF molecules to low-lying vibrational levels of the $A^2\Pi_{1/2}$
state, and of the nearby perturber state sometimes called [18.6]0.5. By
dispersing the fluorescence, we measure branching ratios for the radiative
decay to vibrational levels of the $X^2\Sigma^+$ state. These ratios help to
determine the optimum laser cooling scheme for the molecule. Our results
establish the practicality of a scheme that was previously suggested [Tarbutt
et al., New J. Phys. 15 (2013) 053034] and show how the scheme can be modified
to provide a stronger radiative cooling force.
