Crystal electric field excitation and vibrational properties of the quantum spin liquid candidate LiYbSe2

${\mathrm{LiYbSe}}_{2}$ was recently discovered and assumed to be a new member of the quantum spin liquid candidates. In the paper, polarization-resolved Raman scattering and temperature-dependent Raman scattering experiments were utilized to study the elementary excitations in ${\mathrm{LiYbSe}}_{2}$. Four phonon modes with ${E}_{g}$, ${T}_{2g}^{1}$, ${T}_{2g}^{2}$, and ${A}_{1g}$ symmetry, respectively, and three crystal electric field (CEF) excitations, including CEF1, CEF2, and CEF3 at 116.2, 200.9, and $232.1\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, respectively, were conclusively identified. With 633 and 532 nm excitation, resonant conditions were satisfied and caused the breakdown of selection rules for ${T}_{2g}$ and also prompted the observation of higher-order modes. Helicity-resolved CEF modes were studied based on circularly polarized light and the conservation law of angular momentum. Moreover, CEF modes showed abnormal redshift with decreasing temperature which can be attributed to the reduction of electronegativity of Se atoms due to the lattice contraction. Contributing to the close energy between ${E}_{g}$ and CEF1, weak CEF-phonon coupling was induced in this system and caused abnormal softening of ${E}_{g}$ at low temperature. The CEF excitations and vibration properties studied here provide information about the electronic elementary excitations, which help to further study or control the frustrated magnetism and novel quantum spin liquid state in ${\mathrm{LiYbSe}}_{2}$.