Abstract
X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ∼1 Ångstrom, and HHG provides unprecedented time resolution (∼50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ∼280 eV (44 Ångstroms) and the bond length in methane of ∼1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science.
Highlights
Linda YoungArgonne National Laboratory and University of Chicago The roadmap starts with topics generally familiar to the AMO community: femtochemistry viewed with an x-ray probe, multidimensional spectroscopy extended to the x-ray regime, and winds toward the less commonly encountered areas of high-intensity x-ray phenomena and attosecond science
The second category describes multidimensional x-ray spectroscopies enabled by X-ray free-electron lasers (XFELs): a theoretical perspective where analogy to NMR, infrared and optical realizations is used to highlight the potential of x-rays to monitor the phase and dynamics of non-equilibrium valence wavepackets (Mukamel, section 3), a discussion of routes from an atomic x-ray laser to control of stimulated Raman processes with XFELs (Rohringer, section 3), and an account of the realization of coherent control and four-wave mixing in the XUV regime using only the fully coherent, seeded XFEL, FERMI (Prince and Masciovecchio, section 3)
The XUV/soft x-ray four-wave mixing (FWM) technique is a powerful tool for investigating many systems, which include: (i) spin, charge and structural dynamics of macrocycles, dyes, and metal-based molecular complexes, which are important in solar energy applications; (ii) in condensed matter, charge carrier and spin dynamics of metal oxides; (iii) field-driven phase transitions
Summary
Linda YoungArgonne National Laboratory and University of Chicago The roadmap starts with topics generally familiar to the AMO community: femtochemistry viewed with an x-ray probe, multidimensional spectroscopy extended to the x-ray regime, and winds toward the less commonly encountered areas of high-intensity x-ray phenomena and attosecond science. We start with a general perspective on table-top-scale ultrafast coherent x-ray science that leads toward a future that can be ‘smaller, cheaper and (ultra)faster’ (Kapteyn and Murnane, section 5), followed by a description of a route to highaverage-power soft x-ray ultrashort pulses via mid-infrared drive lasers (Ibrahim and Légaré, section 5), a discussion of attosecond and femtosecond XUV science (Vrakking, section 5), quantitative studies of photoionization time delays in atoms (Isinger, Kroon, Gisselbrecht and L’Huillier, section 5), evolution of attosecond spectroscopies from the XUV to the x-ray regime and from isolated molecules to the liquid phase (Wörner, section 5), and a description of soft x-ray transient absorption and the first multidimensional spectroscopies in the attosecond domain (Leone, section 5). In figure 1, the performance of accelerator-based XFEL and HHG sources are shown Both are labeled with photons/pulse/1% bandwidth for individual pulses. The incredible progress of the past few years and the logical paths for source improvement augur a very exciting future for ultrafast x-ray science
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