In two-dimensional ultrathin materials, lower dielectric screening enables stronger exciton binding, which plays a crucial role for large excitonic radiative lifetimes. In this work, we have investigated a polar monolayer of $\mathrm{Hf}{}_3{\mathrm{ZrS}}_8$ from the perspective of many-body perturbation theory and the Bethe-Salpeter equation (BSE) to determine the corresponding excited state properties, particularly the excitonic radiative lifetime. We have envisaged the impact of external biaxial compressive and tensile strain on the excitonic lifetime, optical absorption, and excitonic wave function in the monolayer, based on the $GW$-BSE formalism, while including relativistic spin-orbit coupling effect. The polar monolayer possesses strong binding energy of the first bright exciton within the quasiparticle band-gap range, while the excitonic radiative lifetime is determined in picoseconds. The optical band gap increases and decreases with tensile and compressive strain, respectively. We have obtained an excitonic radiative lifetime approximately three times larger than the pristine one under biaxial compressive strain. Our finding reveals that the $\mathrm{Hf}{}_3{\mathrm{ZrS}}_8$ monolayer under the influence of biaxial strain could be useful for miniaturized optoelectronic and photonic devices.

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• Electronic structure
• Electronic structure of atoms & molecules
• Excitons
• Spin-orbit coupling