| 1. |
- Arnold, Cord L., et al.
(författare)
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Spatiotemporal coupling of attosecond pulses
- 2019
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Ingår i: 2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019. - 9781728104690 ; Part F140-CLEO_Europe 2019
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Konferensbidrag (refereegranskat)abstract
- Attosecond pulses in the extreme ultraviolet (XUV) spectral range are today routinely generated via high-order harmonic generation (HHG), when intense ultrashort laser pulses are focused into a gaseous generation medium. The effect is most easily understood in a semi-classical picture [1]. An electron can tunnel ionize from the distorted atomic potential, pick up kinetic energy in the laser field, potentially return to its parent ion and recombine. The excess energy is emitted as XUV photon. The process repeats for every half-cycle of the driving field, resulting in a train of attosecond pulses and in the frequency domain in the well-known, odd-order comb of harmonics. Two main families of electron trajectories leading to the same photon energy can be distinguished into 'short' and 'long', according to their time of travel in the continuum. Due to the complicated nature of the HHG process, attosecond pulses usually cannot be separated into their temporal and spatial profiles, but instead have strong chromatic aberration and are spatio-temporally coupled [2-4].
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| 2. |
- Busto, David, et al.
(författare)
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Fano's Propensity Rule in Angle-Resolved Attosecond Pump-Probe Photoionization
- 2019
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 123:13
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Tidskriftsartikel (refereegranskat)abstract
- In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. We generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular dependence of the photoionization time delays and the delay dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40 eV range.
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| 3. |
- Dacasa, Hugo, et al.
(författare)
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Single-shot extreme-ultraviolet wavefront measurements of high-order harmonics
- 2019
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Ingår i: Optics Express. - 1094-4087. ; 27:3, s. 2656-2670
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Tidskriftsartikel (refereegranskat)abstract
- We perform wavefront measurements of high-order harmonics using an extreme-ultraviolet (XUV) Hartmann sensor and study how their spatial properties vary with different generation parameters, such as pressure in the nonlinear medium, fundamental pulse energy and duration as well as beam size. In some conditions, excellent wavefront quality (up to 휆/11) was obtained. The high throughput of the intense XUV beamline at the Lund Laser Centre allows us to perform single-shot measurements of both the full harmonic beam generated in argon and individual harmonics selected by multilayer mirrors. We theoretically analyze the relationship between the spatial properties of the fundamental and those of the generated high-order harmonics, thus gaining insight into the fundamental mechanisms involved in high-order harmonic generation (HHG).
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| 4. |
- Oostenrijk, Bart, et al.
(författare)
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Fission of charged nano-hydrated ammonia clusters - microscopic insights into the nucleation processes
- 2019
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 21:46, s. 25749-25762
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Tidskriftsartikel (refereegranskat)abstract
- While largely studied on the macroscopic scale, the dynamics leading to nucleation and fission processes in atmospheric aerosols are still poorly understood at the molecular level. Here, we present a joint experimental-theoretical study of a model system consisting of hydrogen-bonded ammonia and water molecules. Experimentally, the clusters were produced via adiabatic co-expansion. Double ionization ionic products were prepared using synchrotron radiation and analyzed with coincidence mass- and 3D momentum spectroscopy. Calculations were carried out using ab initio molecular dynamics to understand the fragmentation within the first similar to 500 fs. Further exploration of the potential energy surfaces was performed at a DFT level of theory to gain information on the energetics of the processes. Water was identified as an efficient nano-droplet stabilizer, and is found to have a significant effect even at low water content. On the molecular level, the stabilizing role of water can be related to an increase in the dissociation energy between ammonia molecules and the water enriched environment at the cluster surface. Furthermore, our results support the role of ammonium as a charge carrier in the solution, preferentially bound to surrounding ammonia molecules, which can influence the atmospheric nucleation process.
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| 5. |
- Wikmark, Hampus, et al.
(författare)
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Spatiotemporal coupling of attosecond pulses
- 2019
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 116:11, s. 4779-4787
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Tidskriftsartikel (refereegranskat)abstract
- The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme UV range and bandwidths exceeding tens of electronvolts. They are often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely used approximation consisting of writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argument uses a simple analytical model based on Gaussian optics, numerical propagation calculations, and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring highquality focusing while retaining the broadband/ultrashort characteristics of the radiation.
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