Development of a tabletop ultrashort hard X-ray source and the structure of solvated transition metal carbonyls.

摘要

With the rapid advance of laser technology, tabletop ultrashort hard x-ray sources have been developed and applied to the study of time-resolved physical dynamics. In order to extend such experiments into chemical research, a tabletop ultrashort hard x-ray source is developed. The radiation is expected to have a pulse width on the 100-fs timescale. It is based on a newly designed Ti:Sapphire laser system that consists of a high-intensity seed-pulse injection section, which includes a Kerr-lens mode-locked cavity-dumped oscillator and a two-pass preamplifier and a pulse cleaner, and a CPA section that contains a pulse expander, two stages of multi-pass amplifiers, and a pulse compressor. This laser delivers 8.5 W average light power onto the target. The laser pulse is centered at 800 nm wavelength and has a 40 fs pulse width at 2-kHz repetition rate. The light is focused to 3-μm diameter on a copper wire target. This results in a laser focus intensity of about 1018 W/cm 2. The generated x-ray spectrum contains continuum radiation (Bremsstrahlung) suitable for ultrafast x-ray absorption spectroscopy and characteristic line radiations. The x-ray flux near the iron K-edge is 6 × 107 photons/(s 4π eV). The total x-ray flux within 1 keV spectral range is 3 × 109 photons/(s 4π keV), while the flux at the copper K_α line is 1.5 × 10 10 photons/(s 4π).; The chemical application of this ultrashort x-ray source will be the observation of time-resolved chemical dynamics of solvated transition metal carbonyls. The overall objective is to observe, by ultrafast x-ray absorption measurements, photo-induced ligand dissociation and association processes occurring in solvated complexes such as iron pentacarbonyl. The equilibrium structure of iron pentacarbonyl is essential for ultrafast chemical dynamics studies. Therefore, it is explored first by FTIR spectroscopy and density-functional theory calculations. The results show that most Fe(CO)₅ molecules deform to C_4v-symmetry in solution instead of keeping their gas-phase D_3h-symmetry. The photo-induced ligand dissociation and association processes for Fe(CO)₅ are expected to occur within a few hundred femtoseconds for the dissociation of a single CO ligand. No triplet states are predicted to be involved in the ultrafast process.