| 1. |
- Gatchell, Michael, 1988-, et al.
(författare)
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Stability of C59 Knockout Fragments from Femtoseconds to Infinity
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have studied the stability of C59 anions as a function of time, from their formation on femtosecond timescales to their stabilization on second timescales and beyond, using a combination of theory and experiments. The C59 fragments were produced in collisions between C60 fullerene anions and neutral helium gas at a velocity of 90 km/s (corresponding to a collision energy of 166 eV in the center-of-mass frame). The fragments were then stored in a cryogenic ion-beam storage ring at the DESIREE facility where they were followed for up to one minute. Classical molecular dynamics simulations were used to determine the reaction cross section and the excitation energy distributions of the products formed in these collisions. We found that about 15 percent of the C59 ions initially stored in the ring are intact after about 100 ms, and that this population then remains intact indefinitely. This means that C60 fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C59, that are fed into interstellar chemical reaction networks.
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| 2. |
- Navarro-Navarrete, José E., 1988-, et al.
(författare)
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High-resolution measurement of the electron affinity of cesium
- 2024
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Ingår i: PHYSICAL REVIEW A. - : American Physical Society. - 2469-9926 .- 2469-9934. ; 109:2
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Tidskriftsartikel (refereegranskat)abstract
- Negative ions are unique quantum systems where electron correlation plays a decisive role in determining their properties. The lack of optically allowed transitions prevents traditional optical spectroscopy and the electron affinity is, therefore, for most elements, the only atomic quantity that can be determined with high accuracy. In this work, we present a high-precision experimental determination of the electron affinity of cesium. A collinear laser-ion beam apparatus was used to investigate the partial photodetachment cross section for the cesium anion, leaving the neutral atom in the 6p 2P3/2 excited state. A resonance ionization scheme was used to obtain final-state selectivity, which enabled the investigation of a sharp onset of the cross section associated with a Wigner s-wave threshold behavior. The electron affinity was determined to be 0.471 598 3(38) eV.
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| 3. |
- Ortiz-Rodriguez, C. A., et al.
(författare)
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Simulations of dynamo action in slowly rotating M dwarfs : Dependence on dimensionless parameters
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 678
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim of this study is to explore the magnetic and flow properties of fully convective M dwarfs as a function of rotation period P-rot and magnetic Reynolds Re-M and Prandlt numbers Pr-M.Methods. We performed three-dimensional simulations of fully convective stars using a star-in-a-box set-up. This set-up allows global dynamo simulations in a sphere embedded in a Cartesian cube. The equations of non-ideal magnetohydrodynamics were solved with the PENCIL CODE. We used the stellar parameters of an M5 dwarf with 0.21 M-circle dot at three rotation rates corresponding to rotation periods (P-rot) of 43, 61, and 90 days, and varied the magnetic Prandtl number in the range from 0.1 to 10.Results. We found systematic differences in the behaviour of the large-scale magnetic field as functions of rotation and Pr-M. For the simulations with P-rot = 43 days and Pr-M <= 2, we found cyclic large-scale magnetic fields. For Pr-M > 2, the cycles vanish and the field shows irregular reversals. In the simulations with P-rot = 61 days for Pr-M <= 2, the cycles are less clear and the reversal are less periodic. In the higher Pr-M cases, the axisymmetric mean field shows irregular variations. For the slowest rotation case with P-rot = 90 days, the field has an important dipolar component for Pr-M <= 5. For the highest Pr-M the large-scale magnetic field is predominantly irregular at mid-latitudes, with quasi-stationary fields near the poles. For the simulations with cycles, the cycle period length slightly increases with increasing Re-M.
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| 4. |
- Bernard, Jérôme, et al.
(författare)
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Efficient radiative cooling of tetracene cations C18H12+ : absolute recurrent fluorescence rates as a function of internal energy
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 25:15, s. 10726-10740
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Tidskriftsartikel (refereegranskat)abstract
- We have measured recurrent fluorescence (RF) cooling rates of internally hot tetracene cations, C18H12+, as functions of their storage times and internal energies in two different electrostatic ion-beam storage rings – the cryogenic ring DESIREE with a circumference of 8.6 meters in Stockholm and the much smaller room temperature ring Mini-Ring in Lyon, which has a circumference of 0.71 meters. The RF rates were measured to be as high as 150 to 1000 s−1 for internal energies in the 7 to 9.4 eV energy range, where we have probed the time evolution of the internal energy distribution with nanosecond laser pulses with a 1 kHz repetition rate. These RF rates are found to be significantly higher than those of previously investigated smaller PAHs such as e.g. anthracene and naphthalene, for which the lowest non-forbidden electronic excited state, the D2 state, is populated with a smaller probability by inverse internal conversion. Furthermore, the D2–D0 transition rate is smaller for these smaller molecules than for tetracene. The complementary features of the two storage rings allow for RF rate measurements in a broader internal energy range than has been possible before. The smaller sampling period of about 6 μs in Mini-Ring is ideal to study the cooling dynamics of the hotter ions that decay fast, whereas DESIREE with a sampling period of about 60 μs is better suited to study the colder ions that decay on longer timescales ranging up to hundreds of milliseconds. The excellent agreement between the two series of measurements in the region where they overlap demonstrates the complementarity of the two electrostatic ion-beam storage rings.
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| 5. |
- Bull, James N., et al.
(författare)
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Autoionization from the plasmon resonance in isolated 1-cyanonaphthalene
- 2023
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Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 158:24
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Tidskriftsartikel (refereegranskat)abstract
- Polycyclic aromatic hydrocarbons have widely been conjectured to be ubiquitous in space, as supported by the recent discovery of two isomers of cyanonaphthalene, indene, and 2-cyanoindene in the Taurus molecular cloud-1 using radioastronomy. Here, the photoionization dynamics of 1-cyanonaphthalene (1-CNN) are investigated using synchrotron radiation over the hν = 9.0–19.5 eV range, revealing that prompt autoionization from the plasmon resonance dominates the photophysics for hν = 11.5–16.0 eV. Minimal photo-induced dissociation, whether originating from an excited state impulsive bond rupture or through internal conversion followed by a statistical bond cleavage process, occurs over the microsecond timescale (as limited by the experimental setup). The direct photoionization cross section and photoelectron angular distributions are simulated using an ezDyson model combining Dyson orbitals with Coulomb wave photoejection. When considering these data in conjunction with recent radiative cooling measurements on 1-CNN+, which showed that cations formed with up to 5 eV of internal energy efficiently stabilize through recurrent fluorescence, we conclude that the organic backbone of 1-CNN is resilient to photodestruction by VUV and soft XUV radiation. These dynamics may prove to be a common feature for the survival of small polycyclic aromatic hydrocarbons in space, provided that the cations have a suitable electronic structure to support recurrent fluorescence.
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| 6. |
- Lee, Jason W. L., et al.
(författare)
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Cooling dynamics of energized naphthalene and azulene radical cations
- 2023
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Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 158:17
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Tidskriftsartikel (refereegranskat)abstract
- Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np+) and azulene (Az(+)) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording molecular cloud in a box conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np+ reversible arrow Az(+) quasi-equilibrium. Consequently, dissociation by C2H2-elimination proceeds predominantly through common Az(+) decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np+ D-0 <- D-2 transition, efficiently quenches dissociation for vibrational energies up to approximate to 1 eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation.
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| 7. |
- Navarro Navarrete, José E., 1988-, et al.
(författare)
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Experimental radiative cooling rates of a polycyclic aromatic hydrocarbon cation
- 2023
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Ingår i: Faraday discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 245, s. 352-367
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Tidskriftsartikel (refereegranskat)abstract
- Several small Polycyclic Aromatic Hydrocarbons (PAHs) have been identified recently in the Taurus Molecular Cloud (TMC-1) using radio telescope observations. Reproducing the observed abundances of these molecules has been a challenge for astrochemical models. Rapid radiative cooling of PAHs by Recurrent Fluorescence (RF), the emission of optical photons from thermally populated electronically excited states, has been shown to efficiently stabilize small PAHs following ionization, augmenting their resilience in astronomical environments and helping to rationalize their observed high abundances. Here, we use a novel method to experimentally determine the radiative cooling rate of the cation of 1-cyanonaphthalene (C10H7CN, 1-CNN), the neutral species of which has been identified in TMC-1. Laser-induced dissociation rates and kinetic energy release distributions of 1-CNN cations isolated in a cryogenic electrostatic ion-beam storage ring are analysed to track the time evolution of the vibrational energy distribution of the initially hot ion ensemble as it cools. The measured cooling rate is in good agreement with the previously calculated RF rate coefficient. Improved measurements and models of the RF mechanism are needed to interpret astronomical observations and refine predictions of the stabilities of interstellar PAHs.
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| 8. |
- Stockett, Mark H., 1984-, et al.
(författare)
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Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds
- 2023
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence - radiative relaxation via thermally populated electronic excited states - efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.
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