Christian Y. Cote

ULTRAFAST X-RAY ABSORPTION PROBING OF A CHEMICAL REACTION

Ultrafast x‐ray techniques can, in principle, allow us to more directly watch the time evolution of matter, with atomic spatial resolution and with time resolution on the scale of atomic motions such as the making and breaking of chemical bonds, in order to more directly observe the fundamental molecular dynamics underlying the concept of ‘‘mechanism’’ in inorganic, organic, and biochemical reactions. As a step toward this goal, we have observed a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near‐edge x‐ray absorption spectroscopy with time resolutions of 1.5–3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A).

Ultrafast x‐ray sources*

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

Picosecond dynamics of a hot solid-density plasma

We present an experimental study of the dynamics of a hot near-solid-density plasma generated by the interaction of a very-high-contrast laser pulse (main-pulse-to-prepulse intensity ratio, ∼1010) with solids at intensities of as high as 1018 W/cm2. Time-resolved spectroscopy with a temporal resolution of 1.6 ps is used to follow line broadening, ionization, and recombination time histories. It is shown that thermal line emission can still be very short even at these high laser intensities and that the Kα radiation is surprisingly long.

Time-resolved kiloelectron-volt spectroscopy of ultrashort plasmas.

We describe a technique for spectrally and temporally resolving the kilo-electron-volt emission fromultrashort plasmas produced from solid targets with a tabletop terawatt 400-fs laser. The firsttime-resolved Al spectra (near 8 Å) obtained with a 2-ps time resolution are presented. The results clearly demonstrate that the resonance emission width decreases as the plasma density increases. The ultrafast K(α) emission component is also measured in our experimental conditions.

Development of a subpicosecond large-dynamic-range x-ray streak camera

A novel subpicosecond x-ray streak camera (called PX1) was developed by the INRS group for subpicosecond time resolved spectroscopy in x rays and X-UV range. Using the PX1 camera, we have measured keV x-ray pulses with a 950 fs FWHM and a 850 fs rise time. The camera has also been coupled to ultrafast photoconductive switches and tested in jitter-free mode as a signal averaging detector. This instrument allows to analyze ultrafast changes in short wavelength signals with an unlimited dynamic range.

Ultrafast 2.5-keV x-ray absorption probing of a chemical reaction with 3-ps time resolution

We perform pump-probe measurements in which intense ultrashort optical pulses are the pump pulses that initiate a chemical reaction and ultrafast x-ray pulses are the probe pulses that monitor the response of the system. We present experimental results on the observation of a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast x-ray absorption spectroscopy with 3 ps time resolution near the sulfur K edge at a photon energy of 2.48 keV (4.98 A). High contrast light pulses of 400 fs duration (500 mJ energy and 0.53 micrometers wavelength) from the INRS terawatt laser were focused on high atomic number targets at an intensity of 5 X 1017 W/cm2 in order to generate an x-ray continuum around the sulfur K edge. The SF6 molecule exhibits intense near shape resonances at the sulfur K and L edges, due to the multiple scattering and interference of the emitted photoelectrons by the fluorine atoms that symmetrically surround the central sulfur atom. The shape resonance of the molecule is clearly resolved in the absence of any pump pulse, and the variation of the x-ray absorption spectrum was measured as a function of the delay between the optical pump and x-ray probe pulses. As expected from theory, the reaction process is faster than can be resolved with the 3 picosecond duration x-ray pulses used in this initial experiment. This fast response can, in principle, be used to measure the duration of ultrashort x-ray pulses.

Ultrafast x-ray emission from ultrashort plasmas

We present recent results of our effort to develop an efficient, user-friendly, table-top ultrafast X-ray source. The factors affecting the duration and the intensity of the X-ray emission in the keV range are studied. Time-dependent calculation of the atomic physics coupled to a Fokker- Planck code is used for a quantitative analysis of the experimental results.

Ultrafast x-ray absorbtion and diffraction

Our goal is to watch the evolution of matter on the atomic length scale and on the time scale on which elementary chemical reactions take place. We present initial experiments made in collaboration between UCSD and the INRS laboratory in Canada, on time-resolved ultrafast, 3 ps temporal resolution, near-edge x-ray absorption of gas phase SF6 at 2.4 keV (4.89 A). We can see both the initial presence of the F atoms around the S and their absence after photodissociation produced by pumping with an intense optical pulse. Simulations of ultrafast EXAFS and diffraction experiments are presented. We are constructing an ultrahigh intensity laser to generate ultrafast x-ray pulses from laser-produced plasmas. This laser is especially designed to achieve high average power, short pulse duration and high intensity to produce very high temperature solid density plasmas and ultrahot electrons for ultrafast hard x-ray production at high x-ray photon flux, which should enable us to perform a variety of ultrafast x-ray absorption and diffraction experiments. Finally, we discuss several means to measure the duration of subpicosecond x-ray pulses.

Laser-triggered ultrafast streak camera for the measurement of ultrashort events on the femtosecond time scale

We have developed a new diagnostic that allows accurate time- and space-resolved measurements of low intensity femtosecond light pulses, from the infrared to the hard x- rays. Based on an ultrafast streak camera, this revolutionary tool allows the accumulation of streak traces at rates up to a state-of-the-art 1 kHz which is of great interest to understand the dynamics of ultrafast phenomena in physics and chemistry. Axis Photonique Inc. has commercialized a unique subpicosecond streak camera which was especially developed at INRS-U. of Quebec to record ultrafast events in the x-rays. A laser-triggered sweep unit developed by Medox Electro-Optics Inc. and using high- voltage photoconductive switches designed by Alliage was coupled to the AXIS-PX camera. We present here of the characterization tests performed on a 10 Hz high-power laser at the Max Planck Institut fur Quantenoptik.

X-ray conversion efficiency in short-pulse laser plasmas

We present a comparative study of the x-ray emission produced by laser plasma sources in short ((tau) L equals 0.6 ps) and long ((tau) L equals 0.6 ns) pulse regimes for copper and tantalum targets. The experiments at (tau) L equals 0.6 ps show that the x-ray conversion efficiency (eta) is still increasing with laser intensity between 2 X 1015 W/cm2 and 5 X 1016 W/cm2 for both sub-keV (0.1 - 0.75 keV) and keV (0.75 - 2 keV) ranges. In addition, we found that the optimum values (eta) at (tau) L equals 0.6 ps are at least 2 - 4 times lower than those at (tau) L equals 0.6 ns.