Ultrafast electron and molecular dynamics induced by short wavelength novel free-electron lasers

Date	Mo, 24.02.2020 - Mo, 24.02.2020
Time	11:00
Speaker	Dr. Tsukasa Takanashi, Molecular spectroscopy laboratory, RIKEN, Wako, Japan
Location	EPFL, CH G1 495
Program	X-ray and extreme ultraviolet (XUV) free-electron lasers (FELs) have broken new ground of atomic and molecular physics. In this talk, two typical studies of ultrafast electron and molecular dynamics induced by X-ray and XUV FEL pulses will be discussed. In the X-ray energy region, the interaction of XFEL pulses with isolated molecules containing heavy atoms, such as iodine has been intensively studied by using Japanese XFEL facility “SACLA” [1-2]. In this study, we performed multiple coincidence ion momentum spectroscopy of diiodomethane (CH2I2) molecules irradiated by 5.5-keV intense single XFEL pulse generated by SACLA [2]. By XFEL irradiation, molecules are multiply ionized via cycles of inner-shell photoionization of the iodine atom and subsequent Auger cascades, then further explodes violently due to Coulomb repulsive forces. Since those decay processes occur competitively, momenta of released ions and those correlations have information about dynamic behavior of molecules. Through comparison of experimental momenta of released ions with simulation, molecular dynamics during Coulomb explosion could be extracted. Italian XUV FEL facility “FERMI” generates fully coherent, intense XUV laser pulses with the precise tunability and narrow bandwidth. We investigated interatomic Coulombic decay (ICD) processes in doubly excited neon (Ne) dimers [3]. ICD is a relaxation process of electronically excited atoms in the environment [4]. These excited atoms can relax non-radiatively by transferring their energy to neighboring species which release the energy by emitting an electron. In 2010, a new ICD process induced by two-photon double excitation in Ne dimer (Ne2) was proposed [5]. In this study, we tried to observe the ICD process in doubly excited Ne2 [3]. The two-photon double excitation of Ne2 and time-resolved measurement of this ICD process were performed by the XUV FEL pump-UV laser probe technique at the Low Density Matter (LDM) end-station of FERMI. We measured yield of Ne2+, product of the ICD process, as a function of FEL photon energy. The ion yield curve clearly showed resonant shape and its peak locates around the photon energy 16.39 eV. Using FEL pulse with resonant photon energy, the Ne2+ yield was measured as a function of the time delay between the XUV and UV pulses. That yield curve showed a clear dip around zero time delay. After a depletion at time delays between -400 and -100 fs, the measured Ne2+ yield increased over a large range of positive delays, with a slope reflecting the lifetime of ICD. By theoretical fitting procedure to experimental data points, the ICD lifetime of 390 (-130/+450) fs was obtained.
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Link	Imperial College London, Prof Misha Ivanov
	OptETH: Laser Seminar