Title: Hyperfine Structure of the $B^3Π_1$ State and Predictions of Optical Cycling Behavior in the $X\rightarrow B$ transition of TlF,
Abstract: The rotational and hyperfine spectrum of the $X^1\Sigma^+ \rightarrow
B^3\Pi_1$ transition in TlF molecules was measured using laser-induced
fluorescence from both a thermal and a cryogenic molecular beam. Rotational and
hyperfine constants for the $B$ state are obtained. The large magnetic
hyperfine interaction of the Tl nuclear spin leads to significant mixing of the
lowest $B$ state rotational levels. Updated, more precise measurements of the
$B\rightarrow X$ vibrational branching fractions are also presented. The
combined rovibrational branching fractions allow for the prediction of the
number of photons that can be scattered in a given TlF optical cycling scheme.