3 Figure 3: Fluence dependence of the Δ R/R transient ampli- tude and relaxation time in undoped SmAsFeO at two differ- ent temperatures. Figure 5: Normalized Δ R/R transients at selected temper- atures at 3.1-eV pump photon energy as a function of F in superconducting SmFeAsO 0 . 8 F 0 . 2 . Above ∼ 150 µ J/cm 2 the traces start to overlap indicating a linear F -dependence. The arrows indicate the direction of increasing F . ponent has a similar amplitude at both ℏ ω$_{P}$ . At high F the additional negative component becomes undetectable due to a saturation and the transients become virtually identical to those measured at 55 K including the coher- ent phonon response. II. DISCUSSION A. Undoped SmFeAsO Figure 4: Δ R/R transients as a function of temperature at 1.55-eV pump-ph oton energy (a) and 3.1-eV pump-photon en- ergy (b) in superc onducting SmFeAsO 0 . 8 F 0 . 2 . The pump flu- ence was 17 µ J/ cm 2 at 3.1 eV and 15 µ J/cm 2 at 1.55 eV. Below T c the res ponse of the superconducting state is clearly seen. The tempe rature dependence of the magnetization is shown in the inse t. Upon cooling undoped LaFeAsO undergoes a sequence of a structural transition from a tetragonal to orthorhom- bic symmetry, at T s = 156 K, and a magnetic SDW tran- sition at T$_{SDW}$ = 138 K. 34 In SmFeAsO T$_{SDW}$ ∼ 135 K 35 while the structural transition was reported at lower temperature T s = 130 K. 36 So far due to possible differ- ent oxygen deficiencies in the two experiments 35,36 and strong doping dependence of both T$_{SDW}$ and T s it was not possible to reliably distinguish between T$_{SDW}$ and T s in SmFeAsO. Our data show a marked critical slowing down at 125 K (see Fig. 1) in the form of the long-lived relax- ation tail, while the initial picosecond exponential decay time shows a maximum at ∼ 115 K. Since the long-lived relaxation tail affects the quality of the single-exponential picosecond fit (see Fig. 2(a)) we can not reliably iden- tify 115 K as a separate transition temperature. We are tively identical t o those at higher temperatures (see Fig. 5). The only re maining difference is a delay-independent positive vertical shift of the the intermediate T scans with respect to those measured at 250 K and an increased co- herent phonon f requency of 5.1 THz (170 cm $^{1}$). The ℏ ω$_{P}$ = 1 . 55 -eV t ransients are, as at higher temperatures, similar to the ℏ ω$_{P}$ = 3 . 1 -eV transients but weaker. (iii) Below T$_{c}$ an additional negative component ap- pears with a rise time of 0.2-0.6 ps, depending on the ℏ ω$_{P}$ and pump fluen ce, and decay time of ∼ 5 ps. The com-