Taewoo Lee

High-average-power 2-kHz laser for generation of ultrashort x-ray pulses.

We describe a Ti:sapphire-based laser-x-ray system specifically designed for generation of ultrafast x-ray pulses in the tenths-of-nanometers spectral range at a 2-kHz repetition rate. To obtain high-contrast laser pulses we divide the laser system into a section for generation of microjoule, high-contrast pulses with pulse cleaning and a subsequent section for chirped-pulse amplification and pulse compression. This laser section operates in conjunction with an x-ray-generation section based on a moving copper wire in a He atmosphere. The high reliability of the entire system permits maintenance-free production of x-ray pulses over tens of hours. Average x-ray fluxes of 10(13) photons/(s 4pi sr 1 keV) at 3 keV and 10(9) photons/(s 4pi sr) above 5 keV of photon energy are produced.

Ultrafast tabletop laser-pump–x-ray probe measurement of solvated Fe(CN)64−

We report on the first ultrafast laser-pump-picosecond x-ray probe measurements of solvated transition metal complexes carried out with a tabletop ultrafast laser-driven plasma x-ray source. The x-ray absorption fine structure (XAFS) spectra of Fe(CN)(6) (4-) solvated in water have been measured before and tens of picoseconds after photoexcitation with ultrashort UV laser pulses. The XAFS spectra after photoexcitation exhibits a K-edge shift indicating the increase of the iron-ligand distances. Reference spectra of Fe(CN)(6) (4-) and Fe(CN)(6) (3-) measured at a synchrotron source yield structural data that show static solvation-induced bond length changes of the metal complexes.

Generation of ultrashort hard-x-ray pulses with tabletop laser systems at a 2-kHz repetition rate

We describe the generation of ultrafast x-ray pulses at a 2-kHz repetition rate with a new tabletop laser and x-ray generation system. The range of the emitted x-radiation wavelength is approximately a single angstrom, from 0.3 to 0.1 nm. Specific emphasis is placed on the generation of broad continuum radiation for x-ray absorption spectroscopy. High-contrast laser pulses are desirable for generation of ultrashort pulses, which we achieve by dividing the laser system into a section for generation of high-contrast microjoule pulses followed by pulse cleaning and a section for chirped pulse amplification and pulse compression. Using this system, we generated x radiation from solid-copper and liquid-mercury targets in a helium atmosphere. From copper targets an average x-ray flux of 1013 photons/(s 4π sr keV) at 3 keV and 109 photons/(s 4π sr) above 5 keV photon energy was produced. X radiation from the mercury target did not exhibit emission lines within the spectral range 3–13 keV.

Ultrafast laser-driven x-ray spectrometer for x-ray absorption spectroscopy of transition metal complexes.

We report what are to our knowledge the first measurements of an x-ray absorption near-edge structure (XANES) spectrum of solvated iron pentacarbonyl by use of an ultrafast laser-driven plasma x-ray source. This source is operating at a 2-kHz repetition rate. Hard x radiation that might falsify the XANES measurements is suppressed by an x-ray optical setup that consists of a fiber-optic lens and a silicon single crystal.

Ultrafast x-ray absorption spectroscopy: observing the equilibrium structure and structural dynamics of solvated molecules

Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate solid-density plasmas that emit x-ray pulses with sub-picosecond temporal structure and significant spatial coherence. Such ultrafast laser-driven plasma x-ray sources based on solid and liquid targets are currently under construction in our laboratory. Performance details at several kilohertz laser pulse repetition rates are discussed. As an application of the temporal structure of laser-generated x-ray pulses, ultrafast x-ray absorption fine structure (UXAFS), currently under development, is discussed. It allows, in principle, to measure the structural dynamics of atoms during a chemical process in solution. An overview over UXAFS is presented and properties of our ultrafast x-ray absorption spectrometer are discussed. First calculations of time dependent UXAFS-spectra for ironpentacarbonyl are presented. Ultrafast molecular dynamics depend on the structure of the solvated molecule at the moment of photo-excitation. This structure depends on the solutes interaction with the solvent. Furthermore, the solutes vibrational modes and structure are correlated, solvent dependent, and can be measured by mid-infrared and x-ray absorption spectroscopy. Such measured spectra are presented and correlated with semi-empirical quantum calculations in order to elucidate the solvation environment of transition metal coordination complexes in various solvents.

Ultrafast laboratory-based x-ray sources and their applications in chemical research

Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate high-density plasmas that emit x-ray pulses with sub-picosecond temporal structure, significant spatial coherence, and high brightness at kilohertz repetition rates. Such laser-driven plasma x-ray sources based on solid and liquid metal targets have been developed in our laboratory. Essential performance features are discussed along with a feasibility evaluation for future routine application in chemical research. Laser-driven x-ray sources are usable for ultrafast x-ray diffraction and ultrafast x-ray absorption spectroscopy. X-ray absorption near-edge spectra of solvated transition metal complexes are presented. Ultrafast molecular dynamics depends on the structure of the solvated molecule before photo-excitation. This solvation structure, in turn, depends on the solutes interaction with the solvent molecules. Furthermore, the solutes vibrational modes and its structure are correlated, solvent dependent, and can be measured by mid-infrared and x-ray absorption spectroscopy. Such measured spectra are presented and correlated with quantum calculations in order to elucidate the solvation environment of various transition metal coordination complexes.

Ultrafast tabletop x-ray sources and their application to XAFS measurements of transition metal coordination complexes

We report on the first measurements of a XANES spectrum of solvated Fe(CO)5 using an ultrafast laser driven plasma x-ray source operating at 2 kilohertz repetition rate. The measured spectrum is compared to theoretical XANES spectra based on DFT structure calculations of the solvated complex. The used x-radiation is generated by irradiating a solid metal target with ultrafast high-intensity laser pulses. The subsequently generated high-density plasma emits x-ray pulses with sub-picosecond temporal resolution and an x-ray spectrum extends to energies far higher than the desired spectral range. Since these spectral components potentially falsify the XANES measurements, they were suppressed by the x-ray optical setup.

Ultrafast laser-pump x-ray probe measurements of solvated transition metal complexes

Ultrafast laser pump–x-ray absorption fine structure (XAFS) probe spectra of [Fe(CN) 6 ] 4 t- solvated in water are measured using table-top ultrafast laser driven plasma x-ray sources. Close to the x-ray absorption edge, in the region of x-ray absorption near edge structure, large electron scattering cross-sections cause intense modulations and favor multiple photoelectron scattering events with several atoms. This permits the measurements of metal–ligand distances and bond angles. The modulation depth for highly symmetric molecules can reach 100% of the absorption edge jump. At higher x-ray energies, corresponding to the region of extended x-ray absorption fine structure (EXAFS), spectra are determined by single scattering events between the x-ray absorbing atom and surrounding atoms. Thus, the EXAFS spectra are a measure of the first-order atomic pair correlation function, which can be approximated by Fourier-transforming the EXAFS spectrum.

Ultrafast XAFS of transition metal complexes

Ultrafast laser pump-XAFS probe spectra of ironhexacyanide solvated in water have been measured using a laser-driven plasma x-ray source. A new x-ray source driven by a 15-W, 5-kHz laser system has been developed.