4 where Θ$_{D}$ is the Debye temperature. From the fit of equation (1) to the relaxation time above 230 K (see Fig. 2) we obtain λ 〈 ω $^{2}$〉 = 135 ± 10 meV $^{$^{2}$. If we estimate}$〈 ω $^{2}$〉 ≈ 25 2 meV 2 from inelastic neutron data 40 we obtain λ ≈ 0 . 2 indicating a rather weak electron phonon coupling, which can not explain high T$_{c}$ in the doped compound within a single band BCS model. However, owing to the multiband nature of iron-pnictides it is possible, that due to optical selection rules some bands with possible higher couplings are not detected by Δ R/R transients. To check the consistency of the resulting value of λ we plot in Fig. 2 (b) also the low- T result (2) indicating validity of the high- T approximation (1) above 230K. Below T$_{SDW}$ a gap opens at the Fermi surface introduc- ing a bottleneck in the relaxation. The relaxation across a temperature dependent gap was analyzed by Kabanov et al. . 16 We use equation (6) from Kabanov et al. $^{16}$, which describes the photo-excited change in quasiparticle den- sity in the presence of a temperature dependent gap, to fit the amplitude below T$_{SDW}$ = 125 K. Using a single SDW gap energy with the BCS temperature dependence and 2Δ$_{SDW}$ /k$_{B}$T$_{SDW}$ ≃ 5 results in a rather good fit to the amplitude temperature dependence (see Fig. 2 (c)). Equation (28) for the relaxation time from Kabanov et al. 16 with the same Δ$_{SDW}$ ( T ) describes well also the tem- perature dependence of the relaxation time (see Fig. 2 (c)). However, equation (28) from Kabanov et al. 16 also predicts a fast decrease of the relaxation time with F , which is not observed in our data. The reason for this might originate in the fact that the SDW state is not fully gaped and the energy relaxation is not limited by the an- harmonic energy transfer from the high frequency to the low frequency phonons as assumed in the derivation. 16 therefore unable to differentiate between the structural and spin transitions so we will only refer to a single tran- sition temperature T$_{SDW}$ ≈ T$_{s}$ ≈ 125 K in the rest of the paper. From our data T$_{SDW}$ is lower than reported in literature $^{35,37}$. The apparent lower T$_{SDW}$ can originate in an elevated temperature of the excited volume with respect to the cryostat temperature due to the laser heat- ing. The ∼ 10 -K shift of T$_{SDW}$ is larger than the shift of T$_{c}$ of a few (2-3) K observed in the superconducting crystals under similar excitation conditions. Due to a variable thermal coupling to the sapphire substrate for such small crystals a small decrease of T$_{SDW}$ originating from the oxygen deficiency can not be distinguished from the laser heating in our samples. Above T$_{SDW}$ undoped pnictides are bad metals with re- sistivities in the m Ω cm range 7,38 and plasma frequency in an ∼ 1 eV range $^{6,7}$. The Δ R/R transients in this tem- perature range are therefore attributed to the relaxation of electrons in the states near ϵ$_{F}$ and can be analyzed by means of the recent theoretical results 30 on electron relaxation in metals. The F -independent relaxation time warrants use of the low excitation expansion $^{30}$, where in the high temperature limit the relaxation time is propor- tional to the temperature, 39 B. Decomposition of the Δ R/R transients in superconducting SmFeAsO 0 . 8 F 0 . 2 into components. Figure 7: Decomposition of Δ R/R transients into different components in superconducting SmFeAsO 0 . 8 F 0 . 2 . Figure 6: Δ R/R transients at different pump-photon energies and selected temperatures in superconducting SmFeAsO 0 . 8 F 0 . 2 . For comparison the 295 K transient from undoped SmFeAsO is also shown. While the transients in the undoped sample show a simple single-exponential relaxation, which is sensitive to the phase transition to the orthorhombic SDW state, the transients in the doped sample show a clear muticompo- Here λ 〈 ω $^{2}$〉 is the second moment of the Eliashberg function $^{30}$, λ the electron-phonon coupling constant and k$_{B}$ the Boltzman constant. In the low temperature limit the relaxation time is predicted to diverge at low T : 30 τ = 2 πk$_{B}$T 3 ℏ λ 〈 ω $^{2}$〉 . (1) τ = 2 ℏ Θ$_{D}$ π $^{3}$λk$_{B}$ T $_{2}$, (2)