2 Figure 2: Δ R/R transients at representative temperatures in undoped SmFeAsO with single-exponential decay fits (a). The relaxation time at two pump photon energies b) and am- plitude (c) as functions of temperature at F = 15 µ J/cm $^{2}$. The red solid line in (b) is fit of equation (1) to τ ud above 230K. The blue dashed line in (b) is equation (2) with λ = 0 . 2 and Θ$_{D}$ = 175 K. Black thin solid line in (a) represents the fit of equation (28) from $^{16}$discussed in detail in text. Thin lines in (b) represent the fits of equation (6) from 16 with different magnitudes of the gap. Figure 1: Δ R/R transients a s a function of temperature at 1.55 eV pump-ph oton energy an d 18 µ J/cm 2 (a) and 3.1 eV pump-photon ene rgy and 15 µ J /cm 2 (b) in undoped Sm- FeAsO. In (a) th e coherent p h o non, shown expanded in the insert, is artificial ly smeared beyond 4 ps delay due to a de- creased time resol ution of scans. the ℏ ω$_{P}$ one can clearly resolve three temperature regions with different characteristic behaviors. (i) At the high temperatures a negative transient is observed with initial 0.25-ps decay followed by a slower response consisting from a weak peak at 12 ps (see Fig. 5 (a)) at ℏ ω$_{P}$ = 3 . 1 eV. The transients linearly scale with increasing fluence except in the region of the initial 0.25- ps decay where a weak F -dependence is observed at low F . At ℏ ω$_{P}$ = 3 . 1 eV the transients have the same shape and similar amplitude as in undoped SmFeAsO without the coherent phonon. At ℏ ω$_{P}$ = 1 . 55 eV the negative hight- T transients are much weaker than at ℏ ω$_{P}$ = 3 . 1 eV so only the initial 0.25-ps decay is resolved from the noise (see Fig. 6). In addition a coherent phonon is observed with a softer frequency than in undoped SmFeAsO of 4.6 THz (153 cm − $^{1}$) having similar amplitude at both ℏ ω$_{P}$ . (ii) At the intermediate temperatures above T c and low F the transients are positive on the sub-ps timescale cross zero around 4 ps with slow dynamics similar to the high-temperature one. At high F the positive part of the transients vanishes and the transients become qualita- The temperat ure dependence of the Δ R/R -transients in the near opti mally doped sample is shown in Fig. 4. Contrary to the undoped case the transients show com- plex time and te mperature dependencies. Independent of B. Su perconducting SmFeAsO 0 . 8 F 0 . 2 eV, which is absent at ℏ ω$_{P}$ = 3 . 1 eV, consistent with Ra- man data $^{33}$. Apart of the coherent phonon oscillation the transients consist from a negative-amplitude single- exponential relaxation with a temperature independent rise time of ∼ 180 fs (see Fig. 2 (a)). Around ∼ 125 K an additional long lived response appears with decay time beyond our measurement delay range. The amplitude of the transients, A$_{0}$ , linearly increases with decreasing tem- perature down to ∼ 170 K (see Fig. 2 (c)) with the relax- ation time, τ$_{ud}$ , of 220 fs time being virtually constant above 200K. Below 200 K τ$_{ud}$ starts to increase while the amplitude starts to depart from the linear dependence only below ∼ 170 K rapidly increasing below ∼ 140 K, and achieving a maximum at 115 K, upon entering the SDW state. With further decrease of the temperature the amplitude slightly drops at first and then remains constant below 50 K. Simultaneously with the maximum of the amplitude τ$_{ud}$ shows a remarkable divergent-like peak at ∼ 120 K and then drops to a temperature inde- pendent value of 0.8 ps below 50 K. The rise and decay times are virtually independent of the fluence, F , at all temperatures (see Fig. 3) while the amplitude increases linearly with F at 295K and shows a weak saturation above F = 25 µ J/cm 2 at 5K.