The kagome compound ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$ serves as a natural counterpart to the extensively studied ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, offering an ideal platform for directly comparing their emergent physical properties. Here, we employ ultrafast pump-probe spectroscopy to investigate the nonequilibrium carrier dynamics and collective excitations in ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$. The quasiparticle relaxation consists of several decay components, among which the fast subpicosecond decay is governed by the electron-phonon ($e\text{−}\text{ph}$) coupling. The extracted coupling constant of approximately $0.413\pm 0.007$ is smaller than that reported for ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. This weak-coupling strength may be one factor contributing to the absence of a charge-density-wave (CDW) order in ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$, even when its Van Hove singularities are tuned to the Fermi level, suggesting that $e\text{-ph}$ coupling may play an important role in stabilizing the CDW state in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. In addition, a coherent acoustic phonon mode with a frequency of 0.012 THz is identified, corresponding to a sound velocity of $v_s=1460$ m/s. Combined with the absence of any observed coherent optical phonon modes, the anomalous hardening of this mode with increasing temperature further supports weak $e\text{-ph}$ coupling. Together, these results advance the understanding of nonequilibrium carrier and lattice dynamics in kagome materials.

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• Charge density waves
• Kagome metal
• Ultrafast pump-probe spectroscopy