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In this Letter, we report on the experimental generation of high energy (10 GeV), ultrashort (femtosecond-duration), ultrahigh current (∼ 0.1  MA), petawatt peak power electron beams in a particle accelerator. These extreme beams enable the exploration of a new frontier of high-intensity beam-light and beam-matter interactions broadly relevant across fields ranging from laboratory astrophysics to strong field quantum electrodynamics and ultrafast quantum chemistry. We demonstrate our ability to generate and control the properties of these electron beams by means of a laser-electron beam shaping technique. In conclusion, this experimental demonstration opens the door to on-the-fly customization of extreme beam current profilesmore » for desired experiments and is poised to benefit a broad swath of cross-cutting applications of relativistic electron beams.« less

We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)₅ in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)₄ which are observed following a charge transfer photoexcitation of Fe(CO)₅ as reported in our previous study [Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the ¹A₁ state of Fe(CO)₄. A sub-picosecond time constant of the spin crossover from ¹B₂ tomore » ³B₂ is rationalized by the proposed ¹B₂ → ¹A₁ → ³B₂ mechanism. Ultrafast ligation of the ¹B₂ Fe(CO)₄ state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the ³B₂ Fe(CO)₄ ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via ¹B₂ → ¹A₁ → ¹A' Fe(CO)₄EtOH pathway and the time scale of the ¹A₁ Fe(CO)₄ state ligation is governed by the solute-solvent collision frequency. In conclusion, our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.« less