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We investigate experimentally the ultrafast changes in the spectral responseof the Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl ($\kappa$-Cl) uponphotodoping with intense excitation at 1.6 eV and probing with continuum pulsessimultaneously covering both the terahertz and infrared (IR) ranges (from 0 to0.6 eV). A quantitative analysis of the differential reflectivity using amulti-band Lorentzian model provide absolute changes in spectral weights andobjective global time constants for the relaxation vs. temperature. Thetransient conductivity spectra deduced from the analysis suggest that thetransient photoinduced spectral weight is dominated by a progressive closure ofthe Mott gap with increasing excitation density, i.e. due to changes in theinter-Hubbard-band absorption by the remaining singly occupied states. Wecritically examine this scenario compared to that proposed previously, wherebythe low-energy spectral weight is attributed to a Drude-like response ofphotoexcited doublons/holons. We also consider the observed slowing down of therelaxation rate with increasing excitation density, and temperature dependenceof the initial doublon/holon density in terms of the phonon-mediated gaprecombination model.

Ultrafast gap dynamics upon photodoping the Mott-insulating phase of a two-dimensional organic charge-transfer salt