| 1. |
- Schönhense, G., et al.
(författare)
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Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens
- 2021
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 92:5
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Tidskriftsartikel (refereegranskat)abstract
- The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e-e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from -20 to -1100 V/mm for E-kin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 mu m above the sample surface for E-kin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at E-kin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm(2) (retarding field -21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm(2), it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at E-kin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments.
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| 2. |
- Seidel, M., et al.
(författare)
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Multi-pass Cell Based Nonlinear Pulse Compression of Yb : YAG Pump-Probe Lasers at FLASH
- 2023
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Ingår i: X-Ray Free-Electron Lasers : Advances in Source Development and Instrumentation VI - Advances in Source Development and Instrumentation VI. - 1996-756X .- 0277-786X. - 9781510662827 ; 12581
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Konferensbidrag (refereegranskat)abstract
- The majority of user experiments at the high repetition-rate free electron laser (FEL) facility FLASH are of pump-probe type, combining the extreme ultraviolet (XUV) or soft x-ray radiation from the FEL with ultrashort pulses generated by optical lasers. In this contribution, we demonstrate the advantages of using high-power Yb:YAG lasers with subsequent nonlinear pulse compression stages based on multi-pass cells (MPC). The approach enables the combination of hundreds of kHz to MHz repetition-rates, hundreds of watts of average powers and excellent intensity stabilities. We present the characteristics of the MPC-based pump-probe laser at the FLASH plane-grating beamlines. Furthermore, we report pulse compression to 8.2 fs pulse duration and the seeding of an optical parametric amplifier generating mid-IR radiation tunable from 1.4 µm to 16 µm.
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| 3. |
- Weber, A., et al.
(författare)
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Ultrafast demagnetization dynamics of thin Fe/W(110) films : Comparison of time- and spin-resolved photoemission with time-resolved magneto-optic experiments
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:13, s. 132412-
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Tidskriftsartikel (refereegranskat)abstract
- We report the results of time- and spin-resolved photoemission (TR-SPES) and time-resolved magneto-optical Kerr effect experiments on iron thin films. In particular, the extracted demagnetization times for both techniques are compared. It is shown that while for the Kerr measurements the demagnetization times are always limited by our time resolution (250 +/- 30 fs), for the TR-SPES measurements this situation occurs only for relative quenching below 30%. Above this value, the measured TR-SPES demagnetization time exceeds 500 fs. Different demagnetization probes can hence track different demagnetization times.
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