$\alpha -\mathrm{RuC}{\mathrm{l}}_3$ is a Kitaev material suggested to be a proximate quantum spin liquid in a certain temperature and magnetic field range. Nonequilibrium measurements of transient dynamics have been proposed to detect fractionalized particles that emerge in the spin liquid and to possibly drive the system into novel photoinduced magnetic states that cannot be accessed by conventional equilibrium probes. Here we study ultrafast spin dynamics of photoinduced excitations in $\alpha -\mathrm{RuC}{\mathrm{l}}_3$ using pump-probe transient grating spectroscopy. In the real part of the complex transient reflection coefficient $\mathrm{\Delta }r/r$, we observe evidence of long-range antiferromagnetic correlations near the Néel temperature. Most intriguingly, above the Néel temperature in the Kitaev paramagnetic phase, we reveal a photoexcitation component sensitive to the in-plane magnetic field in the imaginary part of $\mathrm{\Delta }r/r$. This component exhibits two distinct lifetimes of about tens of picoseconds. This photoexcitation component may be connected to novel photoexcited states in the Kitaev quantum spin liquid, and its lifetimes likely reflect the dynamics of unconventional spin excitations in the Kitaev model.

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• Quantum spin liquid
• Spin dynamics
• Ultrafast magnetic effects
• Ultrafast phenomena
• Ultrafast pump-probe spectroscopy