The low- and medium-spin level structure of ${}^{77}\mathrm{As}$ nucleus has been investigated using the ${}^{76}\mathrm{Ge}(\alpha ,p2n)$ fusion evaporation reaction and the standard in-beam $\gamma$ ray spectroscopic technique. The deexciting $\gamma$ rays were detected with the Indian National Gamma Array spectrometer comprising Clover and LEPS detectors. The previously known $9/2^{+}$ yrast positive-parity band, based on the $\pi \left(g_{9/2}\right)$ configuration, has been extended to the excitation energy of $E_x\approx 7.5$ MeV with $J^{\pi }=(33/2^{+})$. An extension of the high-lying negative-parity band, based on the $\pi \left[\left(g_{9/2}\right)\right]\otimes \nu \left[\left(g_{9/2}\right)\left(fp\right)\right]$ configuration, has been made up to $E_x\approx 5.1$ MeV, $J^{\pi }=(25/2^{-})$. The yrast positive-parity $\alpha =-1/2$ signature partner band has newly been identified and the occurrence of large signature splitting associated with the partner bands have been observed. A quasi-$\gamma$ vibrational band structure built on the nonyrast $13/2^{+}$ state has also newly been established. The experimental findings have been interpreted on the basis of the predicted results from the total Routhian surface, particle plus triaxial-rotor model, and shell-model calculations. The band crossing phenomena correlated to the $\nu \left(g_{9/2}\right)$ alignment and the perseverance of triaxial shape up to the highest observed excitation regime of the $9/2^{+}$ yrast positive-parity band have been discussed. The evidence for the possible onset of the stapler-like mechanism prevailing in the high-lying negative-parity band structure are presented.

• Received 1 February 2023
• Accepted 9 June 2023

DOI:https://doi.org/10.1103/PhysRevC.107.064320