| 1. |
- Callegari, Carlo, et al.
(författare)
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Application of matched-filter concepts to unbiased selection of data in pump-probe experiments with free electron lasers
- 2017
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Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 7:6
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Tidskriftsartikel (refereegranskat)abstract
- Pump-probe experiments are commonly used at Free Electron Lasers (FEL) to elucidate the femtosecond dynamics of atoms, molecules, clusters, liquids and solids. Maximizing the signal-to-noise ratio of the measurements is often a primary need of the experiment, and the aggregation of repeated, rapid, scans of the pump-probe delay is preferable to a single long-lasting scan. The limited availability of beamtime makes it impractical to repeat measurements indiscriminately, and the large, rapid flow of single-shot data that need to be processed and aggregated into a dataset, makes it difficult to assess the quality of a measurement in real time. In post-analysis it is then necessary to devise unbiased criteria to select or reject datasets, and to assign the weight with which they enter the analysis. One such case was the measurement of the lifetime of Intermolecular Coulombic Decay in the weakly-bound neon dimer. We report on the method we used to accomplish this goal for the pump-probe delay scans that constitute the core of the measurement; namely we report on the use of simple auto- and cross-correlation techniques based on the general concept of "matched filter". We are able to unambiguously assess the signal-to-noise ratio (SNR) of each scan, which then becomes the weight with which a scan enters the average of multiple scans. We also observe a clear gap in the values of SNR, and we discard all the scans below a SNR of 0.45. We are able to generate an average delay scan profile, suitable for further analysis: in our previous work we used it for comparison with theory. Here we argue that the method is sufficiently simple and devoid of human action to be applicable not only in post-analysis, but also for the real-time assessment of the quality of a dataset.
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| 2. |
- Alagia, Michele, et al.
(författare)
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Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene : Experiment and theory
- 2005
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Ingår i: Journal of chemical physics Online. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 122:12, s. 124305-
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Tidskriftsartikel (refereegranskat)abstract
- The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to * virtual orbitals, and broader structures at higher energy, involving * virtual orbitals. The sharp absorption structures to the * empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of * symmetry, from the six chemically shifted C 1s core orbitals. ©2005 American Institute of Physics
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| 3. |
- Andersson, Åke, et al.
(författare)
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Single-photon hot-electron ionization of C70
- 2023
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Ingår i: Physical Review A. - 2469-9926 .- 2469-9934. ; 107:1
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Tidskriftsartikel (refereegranskat)abstract
- Gas phase C70 molecules have been ionized with single photons of energies between 16 and 70 eV, and the electron spectra measured with velocity map imaging in coincidence with the ions. The doubly ionized and unfragmented species was present at photon energies of 22 eV and up, and triply charged ions were present from 55 eV. The low-kinetic-energy parts of the spectra are explained with thermal emission of transient hot electrons. We propose a generally applicable mechanism, named resonance ionization shadowing, for the creation of hot electrons by absorption of above-threshold energy photons.
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| 4. |
- Falcinelli, Stefano, et al.
(författare)
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The Fragmentation Dynamics of Simple Organic Molecules of Astrochemical Interest Interacting with VUV Photons
- 2019
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Ingår i: ACS Earth and Space Chemistry. - : American Chemical Society (ACS). - 2472-3452. ; 3:9, s. 1862-1872
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Tidskriftsartikel (refereegranskat)abstract
- An experimental investigation on the fragmentation dynamics following the double photoionization of simple organic molecules of astrochemical interest, propylene oxide and N-methylformamide molecules, induced by VUV photons has been reported. Experiments used linearly polarized light in the 18–37 eV (propylene oxide) and 26–45 eV (N-methylformamide) photon energy range at the ELETTRA Synchrotron Facility of Trieste (Italy), coupling ion imaging and electron–ion–ion coincidence techniques with time-of-flight mass spectrometry. In the case of propylene oxide, six different two-body fragmentation processes have been recorded with the formation of CH2+/C2H4O+, CH3+/C2H3O+, O+/C3H6+, OH+/C3H5+, C2H3+/CH3O+, C2H4+/CH2O+ ion pairs. On the other hand, the double photoionization of N-methylformamide occurs producing two main fragmentation reactions, forming CH3+ + CH2NO+ and H+ + C2H4NO+. The relative cross sections and the threshold energies for all fragmentation channels are recorded as a function of the photon energy. Furthermore, in the case of the double photoionization of propylene oxide, the measure of the kinetic energy released distribution for the CH3+/C2H3O+ final ions with their angular distributions allowed the identification of a bimodal behavior indicating the possible formation of two different stable isomers of C2H3O+: acetyl and oxiranyl cations. The obtained results are important to clarify the physical chemistry of the elementary processes induced by the interaction of ionizing radiations with simple organic molecules of astrochemical interest: propylene oxide and N-methylformamide.
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| 5. |
- Gorczyca, Thomas, et al.
(författare)
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Radiative and Relativistic Effects in the Decay of Highly Excited States in Helium
- 2000
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 85:6, s. 1202-1205
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Tidskriftsartikel (refereegranskat)abstract
- A recent experimental study [J.-E. Rubensson et al., Phys. Rev. Lett. 83, 947 (1999)] measured a significant fluorescence yield of the He( 2lnl′) photoexcited resonances, showing major qualitative differences from nonrelativistic predictions. We present a further theoretical study of these states, and perform R-matrix multichannel quantum defect theory calculations to extract fluorescence and ionization cross sections. These theoretical results are in excellent agreement with newer, higher-resolution measurements. Radiative and spin-orbit effects are quantified and shown to play an important role in the overall characterization of highly excited states.
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| 6. |
- Hansen, Klavs, 1958, et al.
(författare)
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Single Photon Thermal Ionization of C60
- 2017
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 118
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Tidskriftsartikel (refereegranskat)abstract
- We report on experiments which show that C60 can ionize in an indirect, quasithermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding 104 K. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons. The astrophysical consequences are briefly discussed.
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| 7. |
- Li, Cui, et al.
(författare)
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Experimental and theoretical XPS and NEXAFS studies of N-methylacetamide and N-methyltrifluoroacetamide
- 2016
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 18:3, s. 2210-2218
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Tidskriftsartikel (refereegranskat)abstract
- Experimental Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectra of N-methyltrifluoroacetamide (FNMA), which is a peptide model system, measured at the C, N, O and F K-edges are reported. The features in the spectra have been assigned by Static-Exchange (STEX) calculations. Using the same method, we have also assigned previously measured NEXAFS spectra of another peptide model system, N-methylacetamide (NMA). To facilitate the NEXAFS feature assignments, X-ray Photoelectron Spectroscopy (XPS) measurements for NMA and FNMA have been carried out with the aim of obtaining the 1s electron ionization potentials, which are compared with the values predicted by our Hartree-Fock (Delta HF) and Multi Configuration Self Consistent Field (Delta MCSCF) calculations. We also demonstrate an approach to compensate for screening effects that are neglected in the STEX method. Ion yield measurements of FNMA associated with the excitation of several C, N, O, and F K-shell pre-edge resonances have revealed site-specific fragmentation in some cases which we interpret with the aid of our theoretical calculations.
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| 8. |
- Salén, Peter, et al.
(författare)
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Electronic state influence on selective bond breaking of core-excited nitrosyl chloride (ClNO)
- 2022
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 157:12
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Tidskriftsartikel (refereegranskat)abstract
- The potential for selective bond breaking of a small molecule was investigated with electron spectroscopy and electron-ion coincidence experiments on ClNO. The electron spectra were measured upon direct valence photoionization and resonant core excitation at the N 1s- and O 1s-edges, followed by the emission of resonant-Auger (RA) electrons. The RA spectra were analyzed with particular emphasis on the assignment of the participator and spectator states. The states are of special relevance for investigating how distinct electronic configurations influence selective bond breaking. The electron-ion coincidence measurements provided branching fractions of the produced ion fragments as a function of electron binding energy. They explicitly demonstrate how the final electronic states created after photoionization and RA decay influence fragmentation. In particular, we observed a significantly different branching fraction for spectator states compared with participator states. In addition, it was also observed that the bonds broken for the spectator states correlate with the antibonding nature of the spectator-electron orbital.
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| 9. |
- Salén, Peter, et al.
(författare)
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Investigating core-excited states of nitrosyl chloride (ClNO) and their break-up dynamics following Auger decay
- 2018
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 149:16
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Tidskriftsartikel (refereegranskat)abstract
- The fragmentation of ClNO upon resonant core-electron excitation to the LUMO and LUMO+1 orbitals at the N and O K-edges is investigated. The produced fragment ions were detected in coincidence with a position sensitive ion time-of-flight detector which enables deduction of the angular distribution of the ions. This facilitates a comparison between the two resonances and the two Kedges with respect to fragmentation time, transition dipole moment orientation, fragment yield of single-ion and ion-pair channels, and fragmentation mechanisms. We observe significant correlations between the core-excited site and the location of the bonds that are broken, as well as the dissociation time. Moreover, we observe preferential cleavage of specific bonds upon excitation to the LUMO and LUMO+1 states which can be attributed to their orbital character.
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| 10. |
- Salén, Peter, et al.
(författare)
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Resonant Auger electron-ion-coincidence spectroscopy of N-methyltrifluoroacetamide : Site-specific fragmentation studies
- 2020
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : AMER PHYSICAL SOC. - 2469-9926 .- 2469-9934. ; 102:3
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical and experimental study of the electronic structure of N-methyltrifluoroacetamide (FNMA) and examine the resonant Auger (RA) decay upon site- and state-specific excitation at the C 1s, N 1s, and O is edges. Based on the calculated energies for cationic states of 1 h and 2h-lp configurations, we assign the peaks of recorded VUV photoelectron and RA spectra. Specifically, we identify the spectral features associated with participator and spectator RA decay. To aid in a deeper understanding of previous observations of limited site-specific bond breaking in FNMA, the influence of the site-specific excitation and the character of the Auger decay on the fragmentation is investigated using Auger electron ion coincidence experiments. Moreover, to investigate the potentially different capacity for selective bond cleavage of participator and spectator Auger states, we apply this technique to the spectator Auger state with the main electronic configuration of (31a')(-1) (32e '')(-1) (34a '')(1) which is well resolved in the experimental spectrum. Finally, the influence of the molecular excess energy and the electronic character of the RA final states on fragmentation is addressed. Based on our findings, we argue that the excess energy the FNMA molecule gains upon RA decay is the dominant factor determining the fragmentation pattern.
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