Femtosecond X-ray Absorption Spectroscopy at a Hard X-ray Free Electron Laser: Application to Spin Crossover Dynamics

Lemke HT; Bressler C; Chen LX; Fritz DM; Gaffney KJ; Galler A; Gawelda W; Haldrup K; Hartsock RW; Ihee H; Kim J; Kim KH; Lee JH; Nielsen MM; Stickrath AB; Zhang WK; Zhu DL; Cammarata M

Journal of Physical Chemistry A, Vol.117, No.4, 735-740, 2013

Lemke HT, Bressler C, Chen LX, Fritz DM, Gaffney KJ, Galler A, Gawelda W, Haldrup K, Hartsock RW, Ihee H, Kim J, Kim KH, Lee JH, Nielsen MM, Stickrath AB, Zhang WK, Zhu DL, Cammarata M

X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (similar to 10(12) photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2'-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is similar to 160 fs.