Generation of ultrashort x-ray pulses and their applications in dynamics studies.

摘要

Ultrashort x-ray pulses hold great potential in structural dynamics studies because x-ray wavelength is on the atomic scale. This dissertation summarizes our effort in this field. Chapter 1 reviews various methods for generating ultrashort x-ray pulses, as well as their applications in dynamics studies, mainly as time-resolved x-ray spectroscopy techniques.; Chapter 2 describes the development of ultrafast x-ray sources in our lab. A newly built source used a terawatt laser system as the driving force, and W as the x-ray target material. At the optimum conditions, it was able to emit a total yield of 6 x 1010 photon/(4pi Sr s) over W Lalpha two lines.; Chapter 3 presents the x-ray absorption spectra of six Ni and Zn species measured with the above ultrafast x-ray source in a new way which was termed as Ultrafast Selected Energy X-ray Absorption Spectroscopy (USEXAS). This method utilized the intense characteristic x-rays from the x-ray target material to probe the materials that have absorption edges close to these line positions. The measured spectra were in general agreement with the simulated spectra and those measured with synchrotron x-ray radiation, which demonstrated the feasibility of this method.; Chapter 4 addresses the idea to determine the x-ray emitting species in our x-ray source with the high-resolution USEXAS measurements, as absorption edges can provide absolute energy calibration. It was found that tungsten atoms, rather than tungsten ions, were responsible for ultrafast hard x-ray generation, which provided the first experimental evidence to the mechanism for hard x-ray generation via high-energy electrons interacting with cold atoms in the solid target.; Chapter 5 discusses the studies on plume dynamics of laser ablation over metal films in helium at ambient pressure with time-resolved x-ray imaging method. The speed of the x-ray absorbing plume was measured as 100 m/sec. Attempts were also made to identify whether nanoparticles dominated the composition of the plumes with a dual pulse evaporation scheme.; Chapter 6 shows the investigation on the effect of our ultrafast x-ray source on DNA radiation damage. It was found that energetic electrons were mainly responsible for the elevated damages.