Rhenium disulfide (${\mathrm{ReS}}_2$), a prototypical 2D semiconductor with an anisotropic ${1\mathrm{T}}^{\prime }$ structure due to the pronounced Peierls distortion, has demonstrated great potential for polarization-dependent optoelectronics and photonics. Here, we report an ultrafast phase transition occurring within 1 ps, accompanied by a one-dimensional Peierls-distortion relaxation in ${1\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$ revealed with combined 4D electron microscopy and time-dependent density functional theory calculations. Upon femtosecond laser pulse excitation, the Re-Re dimerization morphology rapidly transforms from a diamond cluster to zigzag chains and persists for several nanoseconds. This ultrafast Peierls-distortion relaxation is further verified by transient changes in optical anisotropy via polarization-dependent transient absorption spectroscopy. Time-dependent density functional theory calculations attribute this novel phase transition to strong correlation between Peierls distortion and impulsive photoexcited carrier doping, predicting a transient band-gap collapse. This Letter opens up an exciting avenue for ultrafast control of Peierls-distortion-induced anisotropy and the metal-insulator transition in ${1\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$.

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• Electron-phonon coupling
• Lattice dynamics
• Peierls transition
• Density functional theory
• Electron probe micro-analysis
• High-resolution electron microscopy
• Photoexcitation
• Transmission electron microscopy
• Ultrafast femtosecond pump probe