X-Ray Laser Pumping

X-ray laser pumping, especially optical-field-ionisation pumping... 

Lasers act as sources and sometimes as amplifiers of coherent k—uv radiation. Excitation in lasers is provided by external particle or photon pump sources. The high energy densities requked to create inverted populations often involve plasma formation. Certain plasmas, eg, cadmium, are produced by small electric discharges, which act as laser sources and amplifiers (77). Efforts that were dkected to the improvement of the energy conversion efficiencies at longer wavelengths and the demonstration of an x-ray laser in plasma media were successful (78). 

A second, older type of X-ray laser uses a powerful Nd-YAG pumping laser (e.g., Nova) to excite plasmas (e.g., Ne-like Ti+12 ions), which in turn can emit soft X rays by now even tabletop soft X-ray lasers exist (using the chirped-amplification of a Nd YAG laser fired once every 3 to 4 minutes, and Ni-like Pd1 4 ions). 

Self-contained in a high-temperature high-density plasma is the electron thermal energy per unit voliune NekTe (Ne aud are the electron density and temperature, respectively). This intrinsic energy has been tapped veiy successfully for pumping X-ray laser transitions through inelastic collisions of free electrons with ions. Excitation, recombination, and (innershell) ionization are all possible electron-induced processes for effective pumping. The first two have proven to be most successful to date. 

The first theoretical attempts in the field of time-resolved X-ray diffraction were entirely empirical. More precise theoretical work appeared only in the late 1990s and is due to Wilson et al. [13-16]. However, this theoretical work still remained preliminary. A really satisfactory approach must be statistical. In fact, macroscopic transport coefficients like diffusion constant or chemical rate constant break down at ultrashort time scales. Even the notion of a molecule becomes ambiguous at which interatomic distance can the atoms A and B of a molecule A-B be considered to be free Another element of consideration is that the electric field of the laser pump is strong, and that its interaction with matter is nonlinear. What is needed is thus a statistical theory reminiscent of those from time-resolved optical spectroscopy. A theory of this sort was elaborated by Bratos and co-workers and was published over the last few years [17-19]. 

The experiment was done as follows. A diluted solution of C2H4I2 in CH3OH was pumped by an optical laser, which triggered the elimination of one iodine atom followed by creation of the radicals (C2H4I) and (1). A series of X-ray... 

Time-resolved X-ray diffraction (TRXRD), illustrated in Fig. 3.1, provides a powerful technique to probe directly the structural dynamics of crystals far from the equilibrium. It employs visible pump pulses from a laser, and laser-or accelerator-based X-ray probe pulses [1, 3]. As X-ray diffraction can in principle probe k 0 phonons, TRXRD has the potential to reveal the energy transfer dynamics, for example, from the zone-center to the zone-boundary phonons. 

Fig. 3.1. Left visible pump/X-ray probe scheme for femtosecond TRXRD experiments. Hard X-ray pulses are generated by shining intense femtosecond laser pulses on a metal target (laser plasma X-ray source). Right geometrical structure factor of bismuth as a function of inter-atomic distance for diffraction from (111) and (222) lattice planes. From [1] and [2]...

Many methods of investigation of protein-ligand binding kinetics that are based on linear processes are of a pump-probe type. In this approach an optical pulse, called a pump, starts a photoreaction (such as dissociation of MbCO into Mb and CO), and its progress is probed a time At later. The probe could be, for example, a weak laser pulse, which detects the spectral changes in the heme during the protein-ligand recombination, or an x-ray pulse, which allows determination of the protein structure at a particular instant in time. 

Using time-resolved crystallographic experiments, molecular structure is eventually linked to kinetics in an elegant fashion. The experiments are of the pump-probe type. Preferentially, the reaction is initiated by an intense laser flash impinging on the crystal and the structure is probed a time delay. At, later by the x-ray pulse. Time-dependent data sets need to be measured at increasing time delays to probe the entire reaction. A time series of structure factor amplitudes, IF, , is obtained, where the measured amplitudes correspond to a vectorial sum of structure factors of all intermediate states, with time-dependent fractional occupancies of these states as coefficients in the summation. Difference electron densities are typically obtained from the time series of structure factor amplitudes using the difference Fourier approximation (Henderson and Moffatt 1971). Difference maps are correct representations of the electron density distribution. The linear relation to concentration of states is restored in these maps. To calculate difference maps, a data set is also collected in the dark as a reference. Structure factor amplitudes from the dark data set, IFqI, are subtracted from those of the time-dependent data sets, IF,I, to get difference structure factor amplitudes, AF,. Using phases from the known, precise reference model (i.e., the structure in the absence of the photoreaction, which may be determined from... 

Chen LX, Shaw GB, Novozhilova I, Liu T, Jennings G, Attenkofer K, Meyer GJ, Coppens P (2003) MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations. J Am Chem Soc 125 ... 

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