Professor Justin Wark

Abstract:

The x-ray intensities made available by x-ray free electron lasers (FEL) open up new x-ray matter interaction channels not accessible with previous sources. We report here on the resonant generation of Kα emission, that is to say the production of copious Kα radiation by tuning the x-ray FEL pulse to photon energies below that of the K edge of a solid aluminum sample. The sequential absorption of multiple photons in the same atom during the 80 fs pulse, with photons creating L-shell holes and then one resonantly exciting a K-shell electron into one of these holes, opens up a channel for the Kα production, as well as the absorption of further photons. We demonstrate rich spectra of such channels, and investigate the emission produced by tuning the FEL energy to the K-L transitions of those highly charged ions that have transition energies below the K edge of the cold material. The spectra are sensitive to x-ray intensity dependent opacity effects, with ions containing L-shell holes readily reabsorbing the Kα radiation. © 2012 American Physical Society.

Abstract:

Interaction of intense x-ray and solid density Al plasma is studied via K-shell emission spectroscopy. A high fluence, high-intensity x-ray pulse from an x-ray free-electron laser unveils multiphoton ionization pathway and drives hidden resonances. © 2012 OSA.

Abstract:

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the K-α fluorescence, we can directly measure the position of the K edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kröll, which predicts significantly greater depression of the ionization potential. © 2012 American Physical Society.

Abstract:

A common misperception of quantum gravity is that it requires accessing energies up to the Planck scale of 10 19 GeV, which is unattainable from any conceivable particle collider. Thanks to the development of ultra-high intensity optical lasers, very large accelerations can be now the reached at their focal spot, thus mimicking, by virtue of the equivalence principle, a non Minkowski space-time. Here we derive a semiclassical extension of quantum mechanics that applies to different metrics, but under the assumption of weak gravity. We use our results to show that Thomson scattering of photons by uniformly accelerated electrons predicts an observable effect depending upon acceleration and local metric. In the laboratory frame, a broadening of the Thomson scattered x ray light from a fourth generation light source can be used to detect the modification of the metric associated to electrons accelerated in the field of a high power optical laser.