  We study the ground state properties, the electronic excitations and lattice
dynamics in spin-liquid candidate TbInO$_3$. By employing polarization resolved
Raman spectroscopy we define the inter- and intra-multiplet excitations, and
establish the low-energy crystal-field (CF) level scheme. In particular, we
demonstrate that the ground state of the Tb$^{3+}$ ions is a non-Kramers
doublet, and relate the enhanced linewidth of the CF modes to the magnetic
fluctuations near the spin-liquid ground state. We identify all 38 allowed
Raman-active phonon modes at low temperature. Moreover, we observe hybrid
vibronic excitations involving coupled CF and low-lying phonon modes,
suggesting strong spin-lattice dynamics. We develop a model for vibronic states
and obtain the parameters of the bare responses and coupling strength. We
further demonstrate that the obtained CF level scheme is consistent with
specific heat data.
