ИСТИНА

Проект имеет две основные цели, которые будут достигаться в совместных экспериментальных и теоретических исследованиях: выяснение механизмов кратной ионизации атомов под действием излучения рентгеновских лазеров на свободных электронах; разработка метода реконструкции и синтеза аттосекундных импульсов, генерируемых лазерами на свободных электронах.

With the advent of free-electron lasers (FELs) operating in the extreme ultraviolet and x-ray range as well as the progress of lasers used to generate high harmonics (HHG), researchers obtained sources of ultrashort (femto- and attosecond) pulses with short wavelengths. Their use has led to the birth of new branches of the physics of interaction of electromagnetic radiation with matter: nonlinear processes in the x-ray domain and attosecond physics. The planned study covers both of these branches. The production of ions with a charge of several dozen units from atoms being irradiated by FEL pulses was discovered a few years ago. The mechanism of this phenomenon is known only in the most general terms. It involves a combination of photoionization of the inner electron shells and the Auger decay of the vacancies which both are competing with each other on the femtosecond time scale. Experimental methods for observing this phenomenon are still mainly limited to mass spectroscopy, which measures the charge distribution of the final ions. The applied theoretical methods are adapted to describe this type of experiments. To reveal the mechanisms of the process, a selective approach is needed, which makes it possible to identify individual electronic transitions. As a tool for this approach, the project proposes electron spectroscopy, which includes coincidences with ions of a certain charge state and is accompanied by evincive theoretical calculations. Experiments will be conducted at the FEL FERMI (Trieste, Italy) and the European XFEL (Schenefeld, Germany) for multiple ionization of krypton, in a wide photon energy range from 90 to 2000 eV. These energies cover the ionization thresholds of the L, M, N shells, which will make it possible to consider the results as a showcase for a detailed study of the phenomenon of generating multiply charged ions. The duration of an electromagnetic pulse is one of the key parameters of these sources. It largely determines the range of nonstationary phenomena available for studies with the sources. Record values of several dozen attoseconds here belong to the HHG sources. Ordinary FEL pulses have a duration not shorter than a few femtoseconds. On the other hand, the FEL intensity is several orders of magnitude higher than the intensity of the HHG sources, and with a much wider photon energy range. Therefore, reducing the duration of FEL pulses to an attosecond scale is an attractive task, leading to far-going implications. One of the ways to solve the task is the synthesis of coherent femtosecond FEL pulses of different frequencies with an adjustable relative phase. The project proposes to carry out such a synthesis at the FEL FERMI (Trieste, Italy). However, reliable methods for the reconstruction of attosecond FEL pulses do not yet exist. We plan to justify and practically apply the new method of reconstruction, and generate attosecond pulses on FEL FERMI, including pulses with adjustable polarization. Joint research will be conducted by the groups from M.V. Lomonosov Moscow State University, the Pacific State University