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We employed femtosecond time- and angle-resolved photoelectron spectroscopyto analyze the response of the electronic structure of the 122 Fe-pnictideparent compounds Ba/EuFe_2As_2 and optimally doped BaFe_{1.85}Co_{0.15}As_2near the \Gamma point to femtosecond optical excitation. We identify pronouncedchanges of the electron population within several 100 meV above and below theFermi level, which we explain as combination of (i) coherent latticevibrations, (ii) a hot electron and hole distribution, and (iii) transientmodifications of the chemical potential. The response of the three differentmaterials is very similar. In the Fourier transformation of the time-dependentphotoemission intensity we identify three modes at 5.6, 3.3, and 2.6 THz. Whilethe highest frequency mode is safely assigned to the A_{1g} mode, the other twomodes require a discussion in comparison to literature. The time-dependentevolution of the hot electron distribution follows a simplified description ofa transient three temperature model which considers two heat baths of latticevibrations, which are more weakly and strongly coupled to transiently excitedelectron population. Still the energy transfer from electrons to the stronglycoupled phonons results in a rather weak, momentum-averaged electron-phononcoupling quantified by values for \lambda<\omega^2> between 30 and 70 meV^2.The chemical potential is found to present a transient modulation induced bythe coherent phonons. This change in the chemical potential is particularlystrong in a two band system like in the 122 Fe-pnictide compounds investigatedhere due to the pronounced variation of the electrons density of states closeto the equilibrium chemical potential.

journal of physics. condensed matter

Coherent excitations and electron phonon coupling in Ba/EuFe_2As_2  compounds investigated by femtosecond time- and angle-resolved photoemission  spectroscopy