  The odd isotopologues of ytterbium monohydroxide, $^{171,173}$YbOH, have been
identified as promising molecules in which to measure parity (P) and time
reversal (T) violating physics. Here we characterize the
$\tilde{A}^{2}\Pi_{1/2}(0,0,0)-\tilde{X}^2\Sigma^+(0,0,0)$ band near 577 nm for
these odd isotopologues. Both laser-induced fluorescence (LIF) excitation
spectra of a supersonic molecular beam sample and absorption spectra of a
cryogenic buffer-gas cooled sample were recorded. Additionally, a novel
spectroscopic technique based on laser-enhanced chemical reactions is
demonstrated and utilized in the absorption measurements. This technique is
especially powerful for disentangling congested spectra. An effective
Hamiltonian model is used to extract the fine and hyperfine parameters for the
$\tilde{A}^{2}\Pi_{1/2}(0,0,0)$ and $\tilde{X}^2\Sigma^+(0,0,0)$ states. A
comparison of the determined $\tilde{X}^2\Sigma^+(0,0,0)$ hyperfine parameters
with recently predicted values (M. Denis, et al., J. Chem. Phys. $\bf{152}$,
084303 (2020), K. Gaul and R. Berger, Phys. Rev. A $\bf{101}$, 012508 (2020),
J. Liu et al., J. Chem. Phys. $\bf{154}$, 064110 (2021)) is made. The measured
hyperfine parameters provide experimental confirmation of the computational
methods used to compute the P,T-violating coupling constants $W_d$ and $W_M$,
which correlate P,T-violating physics to P,T-violating energy shifts in the
molecule. The dependence of the fine and hyperfine parameters of the
$\tilde{A}^{2}\Pi_{1/2}(0,0,0)$ and $\tilde{X}^2\Sigma^+(0,0,0)$ states for all
isotopologues of YbOH are discussed and a comparison to isoelectronic YbF is
made.
