| 1. |
- Asmussen, J. D., et al.
(författare)
-
Unravelling the full relaxation dynamics of superexcited helium nanodroplets
- 2021
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 23:28, s. 15138-15149
-
Tidskriftsartikel (refereegranskat)abstract
- The relaxation dynamics of superexcited superfluid He nanodroplets is thoroughly investigated by means of extreme-ultraviolet (XUV) femtosecond electron and ion spectroscopy complemented by time-dependent density functional theory (TDDFT). Three main paths leading to the emission of electrons and ions are identified: droplet autoionization, pump-probe photoionization, and autoionization induced by re-excitation of droplets relaxing into levels below the droplet ionization threshold. The most abundant product ions are He-2(+), generated by droplet autoionization and by photoionization of droplet-bound excited He atoms. He+ appear with some pump-probe delay as a result of the ejection He atoms in their lowest excited states from the droplets. The state-resolved time-dependent photoelectron spectra reveal that intermediate excited states of the droplets are populated in the course of the relaxation, terminating in the lowest-lying metastable singlet and triplet He atomic states. The slightly faster relaxation of the triplet state compared to the singlet state is in agreement with the simulation showing faster formation of a bubble around a He atom in the triplet state.
|
|
| 2. |
- Michiels, R., et al.
(författare)
-
Enhancement of above Threshold Ionization in Resonantly Excited Helium Nanodroplets
- 2021
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 127
-
Tidskriftsartikel (refereegranskat)abstract
- Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.
|
|
| 3. |
- Mirian, N. S., et al.
(författare)
-
Generation and measurement of intense few-femtosecond superradiant extreme-ultraviolet free-electron laser pulses
- 2021
-
Ingår i: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 15, s. 523-529
-
Tidskriftsartikel (refereegranskat)abstract
- Free-electron lasers producing ultrashort pulses with high peak power promise to extend ultrafast non-linear spectroscopic techniques into the extreme-ultraviolet-X-ray regime. Key aspects are the synchronization between pump and probe, and the control of the pulse properties (duration, intensity and coherence). Externally seeded free-electron lasers produce coherent pulses that can be synchronized with femtosecond accuracy. An important goal is to shorten the pulse duration, but the simple approach of shortening the seed is not sufficient because of the finite-gain bandwidth of the conversion process. An alternative is the amplification of a soliton in a multistage, superradiant cascade: here, we demonstrate the generation of few-femtosecond extreme-ultraviolet pulses, whose duration we measure by autocorrelation. We achieve pulses four times shorter, and with a higher peak power, than in the standard high-gain harmonic generation mode and we prove that the pulse duration matches the Fourier transform limit of the spectral intensity distribution. By amplifying a soliton in a multistage cascade, few-femtosecond extreme-ultraviolet free-electron laser pulses are achieved.
|
|
| 4. |
- 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.
|
|
| 5. |
- Maroju, P. K., et al.
(författare)
-
Analysis of two-color photoelectron spectroscopy for attosecond metrology at seeded free-electron lasers
- 2021
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:4
-
Tidskriftsartikel (refereegranskat)abstract
- The generation of attosecond pulse trains at free-electron lasers opens new opportunities in ultrafast science, as it gives access, for the first time, to reproducible, programmable, extreme ultraviolet (XUV) waveforms with high intensity. In this work, we present a detailed analysis of the theoretical model underlying the temporal characterization of the attosecond pulse trains recently generated at the free-electron laser FERMI. In particular, the validity of the approximations used for the correlated analysis of the photoelectron spectra generated in the two-color photoionization experiments are thoroughly discussed. The ranges of validity of the assumptions, in connection with the main experimental parameters, are derived.
|
|
