| 1. |
- Ciavardini, Alessandra, et al.
(författare)
-
Ultra-Fast-VUV Photoemission Study of UV Excited 2-Nitrophenol
- 2019
-
Ingår i: Journal of Physical Chemistry A. - : AMER CHEMICAL SOC. - 1089-5639 .- 1520-5215. ; 123:7, s. 1295-1302
-
Tidskriftsartikel (refereegranskat)abstract
- The initial deactivation pathways of gaseous 2-nitrophenol excited at 268 nm were investigated by time-resolved photoelectron spectroscopy (TRPES) with femtosecond-VUV light, produced by a monochromatized high harmonic generation source. TRPES allowed us to obtain new, valuable experimental information about the ultrafast excited-state dynamics of 2-nitrophenol in the gas phase. In accord with recent ab initio on-the-fly nonadiabatic molecular dynamic simulations, our results validate the occurrence of an ultrafast intersystem crossing leading to an intermediate state that decays on a subpicosecond time scale with a branched mechanisms. Two decay pathways are experimentally observed. One probably involves proton transfer, leading to the most stable triplet aci-form of 2-nitrophenol; the second pathway may involve OH rotation. We propose that following intersystem crossing, an ultrafast fragmentation channel leading to OH or HONO loss could also be operative.
|
|
| 2. |
- Khan, Shaukat, et al.
(författare)
-
Evolution of density-modulated electron beams in drift sections
- 2024
-
Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 27:4
-
Tidskriftsartikel (refereegranskat)abstract
- Initiating the amplification process in a short-wavelength free-electron laser (FEL) by external seed laser pulses results in radiation with a high degree of longitudinal and transverse coherence. The basic layout in seeded harmonic generation involves a periodic electron energy modulation by laser-electron interaction in a short undulator (the "modulator"), which is converted into a density modulation in a dispersive section immediately followed by a long FEL undulator (the "radiator") tuned to a harmonic of the seed laser wavelength. With the advent of more complex seeding schemes, density-modulated beams may need to be transported in drift sections before entering the radiator. Long FEL undulators may also contain several drift spaces to accommodate focusing elements and diagnostics. Therefore, it is of general interest to study the evolution of density-modulated electron beams in drift sections under the influence of repulsive Coulomb forces. At FERMI, a seeded FEL user facility in Trieste, Italy, systematic studies of the impact of varying drift length on coherent harmonic emission were undertaken. In order to make the underlying physics transparent, the emphasis of this paper is on reproducing the experimental findings with analytical estimates and a simple one-dimensional numerical model. Furthermore, the Coulomb forces in a drift section may be employed to enhance the laser-induced energy modulation and yield an improved density modulation before entering the FEL radiator.
|
|
| 3. |
- Fushitani, Mizuho, et al.
(författare)
-
Time-resolved photoelectron imaging of complex resonances in molecular nitrogen
- 2021
-
Ingår i: The Journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 154:14
-
Tidskriftsartikel (refereegranskat)abstract
- We have used the FERMI free-electron laser to perform time-resolved photoelectron imaging experiments on a complex group of resonances near 15.38 eV in the absorption spectrum of molecular nitrogen, N2, under jet-cooled conditions. The new data complement and extend the earlier work of Fushitani et al. [Opt. Express 27, 19702–19711 (2019)], who recorded time-resolved photoelectron spectra for this same group of resonances. Time-dependent oscillations are observed in both the photoelectron yields and the photoelectron angular distributions, providing insight into the interactions among the resonant intermediate states. In addition, for most states, we observe an exponential decay of the photoelectron yield that depends on the ionic final state. This observation can be rationalized by the different lifetimes for the intermediate states contributing to a particular ionization channel. Although there are nine resonances within the group, we show that by detecting individual photoelectron final states and their angular dependence, we can identify and differentiate quantum pathways within this complex system.
|
|
| 4. |
- Fushitani, Mizuho, et al.
(författare)
-
Wave packet dynamics and control in excited states of molecular nitrogen
- 2024
-
Ingår i: The Journal of chemical physics. - 0021-9606. ; 160:10
-
Tidskriftsartikel (refereegranskat)abstract
- Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4-9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.
|
|
| 5. |
- Hennecke, Martin, et al.
(författare)
-
Ultrafast opto-magnetic effects in the extreme ultraviolet spectral range
- 2024
-
Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 7:1
-
Tidskriftsartikel (refereegranskat)abstract
- Coherent light-matter interactions mediated by opto-magnetic phenomena like the inverse Faraday effect (IFE) are expected to provide a non-thermal pathway for ultrafast manipulation of magnetism on timescales as short as the excitation pulse itself. As the IFE scales with the spin-orbit coupling strength of the involved electronic states, photo-exciting the strongly spin-orbit coupled core-level electrons in magnetic materials appears as an appealing method to transiently generate large opto-magnetic moments. Here, we investigate this scenario in a ferrimagnetic GdFeCo alloy by using intense and circularly polarized pulses of extreme ultraviolet radiation. Our results reveal ultrafast and strong helicity-dependent magnetic effects which are in line with the characteristic fingerprints of an IFE, corroborated by ab initio opto-magnetic IFE theory and atomistic spin dynamics simulations.
|
|
| 6. |
- Kumar Maroju, Praveen, et al.
(författare)
-
Attosecond pulse shaping using a seeded free-electron laser
- 2020
-
Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 578, s. 386-391
-
Tidskriftsartikel (refereegranskat)abstract
- Attosecond pulses are central to the investigation of valence- and core-electron dynamics on their natural timescales. The reproducible generation and characterization of attosecond waveforms has been demonstrated so far only through the process of high-order harmonic generation. Several methods for shaping attosecond waveforms have been proposed, including the use of metallic filters, multilayer mirrors and manipulation of the driving field. However, none of these approaches allows the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free-electron lasers, by contrast, deliver femtosecond, extreme-ultraviolet and X-ray pulses with energies ranging from tens of microjoules to a few millijoules. Recent experiments have shown that they can generate subfemtosecond spikes, but with temporal characteristics that change shot-to-shot. Here we report reproducible generation of high-energy (microjoule level) attosecond waveforms using a seeded free-electron laser. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with an approach for its temporal reconstruction. The results presented here open the way to performing attosecond time-resolved experiments with free-electron lasers.
|
|
| 7. |
- Maroju, Praveen K., et al.
(författare)
-
A Novel Attosecond Timing Tool for Free-Electron Laser Experiment
- 2020
-
Ingår i: High Intensity Lasers and High Field Phenomena 2020. - 9781943580736
-
Konferensbidrag (refereegranskat)abstract
- We demonstrate a novel timing tool for Free-Electron Lasers to determine the delay between an attosecond pulse train and infrared pulse with sub-optical-cycle resolu-. tion.
|
|
| 8. |
- Maroju, Praveen Kumar, et al.
(författare)
-
Complex attosecond waveform synthesis at fel fermi
- 2021
-
Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 11:21
-
Tidskriftsartikel (refereegranskat)abstract
- Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields.
|
|
| 9. |
- Pop, Mihai, et al.
(författare)
-
Single-shot transverse coherence in seeded and unseeded free-electron lasers: A comparison
- 2022
-
Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 25:040701
-
Tidskriftsartikel (refereegranskat)abstract
- The advent of x-ray free-electron lasers (FELs) drastically enhanced the capabilities of several analytical techniques, for which the degree of transverse (spatial) coherence of the source is essential. FELs can be operated in self-amplified spontaneous emission (SASE) or seeded configurations, which rely on a qualitatively different initialization of the amplification process leading to light emission. The degree of transverse coherence of SASE and seeded FELs has been characterized in the past, both experimentally and theoretically. However, a direct experimental comparison between the two regimes in similar operating conditions is missing, as well as an accurate study of the sensitivity of transverse coherence to key working parameters. In this paper, we carry out such a comparison, focusing in particular on the evolution of coherence during the light amplification process.
|
|
