The quasiparticle relaxation dynamics of the iron-based superconductor ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ ($T_c\sim 29$ K) were investigated using ultrafast optical spectroscopy. A pseudogap (${\mathrm{\Delta }}_{\text{PG}}=3.3\pm 0.3$ meV) was observed to open below $T^{*}\approx 60$ K, prior to the emergence of a superconducting gap [$\mathrm{\Delta }\left(0\right)=6.6\pm 0.4$ meV]. At high excitation fluence, a coherent $A_{1g}$ phonon mode at 5.49 THz was identified, exhibiting deviations from anharmonic behavior below $T_c$. The electron-phonon coupling constant for this mode was estimated to be ${\lambda }_{A_{1g}}\approx 0.23$. These results provide insights into the interplay between the electron-phonon interactions, pseudogap, and the superconducting pairing mechanism in ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$.

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• Quasiparticles & collective excitations
• Superconductivity
• Iron-based superconductors
• Unconventional superconductors
• Ultrafast femtosecond pump probe