Dissociation dynamics of fluorinated ethene cations:\ud from time bombs on a molecular level to double-regime dissociators\ud

The dissociative photoionization mechanism of internal energy selected C$_2$H$_3$F$^+$, 1,1-C$_2$H$_2$F$_2^+$, C$_2$HF$_3^+$ and C$_2$F$_4^+$ cations have been studied in the 13−20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C–C bond post H or F-atom migration, and cleavage of the C=C bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E$_0$, of the daughter ions have been determined. Both C$_2$H$_3$F$^+$ and 1,1-C$_2$H$_2$F$_2^+$ are veritable time bombs with respect to dissociation via HF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C$_2$HF$_3$ and C$_2$F$_4$ involves an atom migration across the C=C bond, giving CF–CHF$_2^+$ and CF–CF$_3^+$, respectively, which then dissociate to form CHF$_2^+$ and CF$_3^+$. The nature of the F-loss pathway has been found to be bimodal for C$_2$H$_3$F and 1,1-C$_2$H$_2$F$_2$, switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C$_2$F$_4$ is found to be comprised of two regimes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E$_0$ of CF$_3^+$ and CF$^+$ formation from C$_2$F$_4$ together with the now well established ∆$_f$Hº of C$_2$F$_4$ yield self-consistent enthalpies of formation for the CF$_3$, CF, CF$_3^+$, and CF$^+$ species

Shining new light on the multifaceted dissociative photoionisation dynamics of CCl₄

Statisticality restored: high internal energy CCl4+ dissociates mostly according to statistical theory, and an intersystem crossing path precludes fluorescence.</p

Context-dependent detection of fungal parasites in four ant species

The reaction between the allyl radical (C3H5˙) and acetylene (C2H2) in a heated microtubular reactor has been studied at the VUV beamline of the Swiss Light Source. The reaction products are sampled from the reactor and identified by their photoion mass-selected threshold photoelectron spectra (ms-TPES) by means of imaging photoelectron photoion coincidence spectroscopy. Cyclopentadiene is identified as the sole reaction product by comparison of the measured photoelectron spectrum with that of cyclopentadiene. With the help of quantum-chemical computations of the C5H7 potential energy surface, the C2H2 + C3H5˙ association reaction is confirmed to be the rate determining step, after which H-elimination to form C5H6 is prompt in the absence of re-thermalization at low pressures. The formation of cyclopentadiene as the sole product from the allyl + acetylene reaction offers a direct path to the formation of cyclic hydrocarbons under combustion relevant conditions. Subsequent reactions of cyclopentadiene may lead to the formation of the smallest polycyclic aromatic molecule, naphthalene

A Halomethane thermochemical network from iPEPICO experiments and quantum chemical calculations

Internal energy selected halomethane cations CH3Cl+, CH2Cl2+, CHCl3+, CH3F+, CH2F2+, CHClF2+ and CBrClF2+ were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH3F+, CH2F2+ and CH3Cl+, chlorine atom loss for CH2Cl2+, CHCl3+ and CHClF2+, and bromine atom loss for CBrClF2+. Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl3+, CHClF2+ and CBrClF2+ do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV and 11.23 ± 0.03 eV for CHCl3, CHClF2 and CBrClF2, respectively. The updated 0 K enthalpies of formation, ∆fHo0K(g) for the ions CH2F+, CHF2+, CHCl2+, CCl3+, CCl2F+ and CClF2+ have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3 and 552.2 ± 3.4 kJ mol–1, respectively. The ∆fHo0K(g) values for the neutrals CCl4, CBrClF2, CClF3, CCl2F2 and CCl3F and have been determined to be –94.0 ± 3.2, –446.6 ± 2.7, –702.1 ± 3.5, –487.8 ± 3.4 and –285.2 ± 3.2 kJ mol–1, respectively

Nitrogen matters: the difference between PANH and PAH formation

A solid understanding of and a reliable model for the formation pathways of polycyclic aromatic (nitrogen containing) hydrocarbons (PA(N)Hs) is needed to account for their abundance in the interstellar medium and in Titan's atmosphere as well as to mitigate the emission of these carcinogens in our terrestrial environment. We have investigated the phenyl + acrylonitrile reaction mechanism between 600 and 1200 K in a hot microreactor. Radical intermediates (C9H8N˙), formed by addition, and closed-shell C9H7N products, formed by subsequent hydrogen elimination, are isomer-selectively identified using photoion mass-selected threshold photoelectron spectroscopy in conjunction with Franck–Condon simulations. Although quinoline is the most stable product, the calculated potential energy surface and a kinetic model confirm that the reaction is kinetically controlled and yields four open-chain isomers instead. The absence of quinoline is in stark contrast with the isoelectronic phenyl + vinylacetylene reaction that produces naphthalene. Ab initio calculations suggest that this change is brought about by the stability of the nitrile group, which inhibits ring formation. Therefore, it is unlikely that nitrile precursors form nitrogen-containing rings, which calls for alternative pathways by which nitrogen atoms can be incorporated in aromatic systems to explain their presence in the ISM and Titan's atmosphereLaboratory astrophysics and astrochemistr

End-systole and end-diastole detection in short axis cine MRI using a fully convolutional neural network with dilated convolutions

[EN] The correct assessment and characterization of heart anatomy and functionality is usually done through inspection of magnetic resonance image cine sequences. In the clinical setting it is especially important to determine the state of the left ventricle. This requires the measurement of its volume in the end-diastolic and end-systolic frames within the sequence trough segmentation methods. However, the first step required for this analysis before any segmentation is the detection of the end-systolic and end-diastolic frames within the image acquisition. In this work we present a fully convolutional neural network that makes use of dilated convolutions to encode and process the temporal information of the sequences in contrast to the more widespread use of recurrent networks that are usually employed for problems involving temporal information. We trained the network in two different settings employing different loss functions to train the network: the classical weighted cross-entropy, and the weighted Dice loss. We had access to a database comprising a total of 397 cases. Out of this dataset we used 98 cases as test set to validate our network performance. The final classification on the test set yielded a mean frame distance of 0 for the end-diastolic frame (i.e.: the selected frame was the correct one in all images of the test set) and 1.242 (relative frame distance of 0.036) for the end-systolic frame employing the optimum setting, which involved training the neural network with the Dice loss. Our neural network is capable of classifying each frame and enables the detection of the end-systolic and end-diastolic frames in short axis cine MRI sequences with high accuracy.Funding sources This work was partially supported by the Conselleria d'Innovació, Universitats, Ciència i Societat Digital, Generalitat Valenciana (grants AEST/2020/029 and AEST/2021/050) .Pérez-Pelegrí, M.; Monmeneu, JV.; López-Lereu, MP.; Maceira, AM.; Bodi, V.; Moratal, D. (2022). End-systole and end-diastole detection in short axis cine MRI using a fully convolutional neural network with dilated convolutions. Computerized Medical Imaging and Graphics. 99:1-8. https://doi.org/10.1016/j.compmedimag.2022.102085189

Probing different spin states in xylyl radicals and ions

Resonant one-color two-photon ionization spectroscopy and mass-selected threshold photoelectron spectroscopy were applied to study the electronic doublet states of the three xylyl (methyl-benzyl) radicals above 3.9 eV as well as the singlet and triplet states of the cations up to 10.5 eV.</p