A novel subpicosecond pulse radiolysis experimental system has been developed in Terawatt Ultrafast High Field
Facility (TUHFF) at Argonne National Laboratory. TUHFF houses a 20 TW Ti:sapphire laser system that generates 2.5
nC sub-picosecond pulses of 4-25 MeV electrons at 10 Hz using laser wakefield acceleration. The system has been
optimized for chemical studies. The subpicosecond electron pulses were used to generate hydrated electrons in pulse
radiolysis of liquid water. Preliminary transient absorption spectroscopy data with picosecond resolution is presented.
A terawatt tabletop laser wakefield acceleration source of relativistic electrons has been developed in our Terawatt Ultrafast High Field Facility (TUHFF). The preliminary results for ultrafast radiolysis of liquid water using this femtosecond electron source are presented. A TUHFF based femtosecond x-ray source is proposed. Thomson scattering of the accelerated electrons off a counterpropagating terawatt laser beam will be used to generate keV x-ray photons. The expected parameters of this x-ray source have been estimated. The short pulse duration, high flux, and good collimation of the resulting x-ray beam would be conducive for ultrafast time-resolved x-ray absorption studies of short-lived transient species in gases, liquids, and solids. It is argued that the solvation dynamics of Br atoms generated in photoinduced electron detachment from aqueous bromide would make a convenient choice for the first pump-probe experiment using this x-ray source.
We describe the application of two-photon processes and fluorescence to 3D optical data storage. We also report the results of two-photon absorption (TPA) cross section measurement of several organic molecules in solution. The data show that the nonlinear transmission method without consideration of other nonlinear effects results in erroneous values for the TPA cross sections. We also find that the cross sections measured by excited state methods, namely two-photon induced fluorescence and a new excited state method, which is based on transient absorption following two-photon excitation, are in good agreement with the accepted values. Therefore one needs to be cognizant of these facts when using the transmission method.
We describe the experimental system used for ultrafast time resolved EXAFS spectroscopy. It consists of a laser driven x- ray diode which generates the ultrashort hard x-ray pulses used in pump/probe experiments and a dispersive spectrometer which makes possible the simultaneous recording of a broad spectrum. The data presented, using this dispersive spectrometer, shows that we can record more than 500 eV simultaneously with a resolution of 8 eV, or 3 eV resolution for 250 eV energy spectrum.
The nonlinear optical properties of four novel azulene-type donor-acceptor compounds have been measured by means of the 2D Z-scan technique. These compounds exhibit considerable nonlinear refraction and one shows strong nonlinear saturated absorption at 527 nm. The transient spectra and kinetics, after excitation of the charge-transfer absorption band, have been studied by means of a picosecond transient absorption spectroscopy, allowing us to measure absorbance changes as small as (Delta) A equals 0.005 and time resolution of 1.5 ps. Several compounds exhibited quite strong reversed saturable absorption in a wide region of the visible spectrum. The reversed saturable absorption induced an increase in the effective absorption of the material at high power, which suggests that, it may be useful as an optical limiting medium.
A two-dimensional Z-scan technique employing a CCD camera was used to study the nonlinear optical parameters of materials. Using the known beam distribution at the lens plane, measured by the CCD camera, and the split step beam propagation method, we simulate the evolution of the beam profile within the sample. This technique may be applied to any arbitrary beam distribution and sample thickness. We applied our two- dimensional Z-scan technique for the investigation of zinc tetraphenyltetrabenzoporphyrin and three azulene-containing donor-acceptor compounds. It was found that some samples exhibit strong nonlinear refraction while others show strong nonlinear absorption. The use of these materials in optical limiting devices is discussed.
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