All-XUV pump-probe transient absorption spectroscopy on the dissociation dynamics of small molecules

In this work, the implementation of an XUV-pump–XUV-probe transient absorp- tion spectroscopy scheme at the free-electron laser facility FLASH in Hamburg is presented. It is applied to the ultrafast dissociation dynamics of the small molecules diiodomethane and oxygen. For the diiodomethane target, the XUV pump pulse enables the resonant site-specific excitation of an iodine 4d electron to an antibonding molecular orbital. The hereby initiated dissociation of the molecule includes a direct, as well as an indirect path through isomeric geometries of the molecular cation. These parallel pathways are traced with the help of simulated absorption spectra and their time constants are extracted via a global fit model. In the case of the oxygen target, dissociation is initiated by the (multiple) ionization of the oxygen molecule. The emergence of absorption resonances of the doubly- charged ionic fragment O2+ is observed in the probe spectrum as a product of the dissociation. The compatibility of the extracted rise times of the fragments with Coulomb explosion is investigated with a simple classical model

Using Noun Phrases for Navigating Biomedical Literature on Pubmed: How Many Updates Are We Losing Track of?

Author-supplied citations are a fraction of the related literature for a paper. The “related citations” on PubMed is typically dozens or hundreds of results long, and does not offer hints why these results are related. Using noun phrases derived from the sentences of the paper, we show it is possible to more transparently navigate to PubMed updates through search terms that can associate a paper with its citations. The algorithm to generate these search terms involved automatically extracting noun phrases from the paper using natural language processing tools, and ranking them by the number of occurrences in the paper compared to the number of occurrences on the web. We define search queries having at least one instance of overlap between the author-supplied citations of the paper and the top 20 search results as citation validated (CV). When the overlapping citations were written by same authors as the paper itself, we define it as CV-S and different authors is defined as CV-D. For a systematic sample of 883 papers on PubMed Central, at least one of the search terms for 86% of the papers is CV-D versus 65% for the top 20 PubMed “related citations.” We hypothesize these quantities computed for the 20 million papers on PubMed to differ within 5% of these percentages. Averaged across all 883 papers, 5 search terms are CV-D, and 10 search terms are CV-S, and 6 unique citations validate these searches. Potentially related literature uncovered by citation-validated searches (either CV-S or CV-D) are on the order of ten per paper – many more if the remaining searches that are not citation-validated are taken into account. The significance and relationship of each search result to the paper can only be vetted and explained by a researcher with knowledge of or interest in that paper

All-XUV pump-probe transient absorption spectroscopy on the dissociation dynamics of small molecules

In this work, the implementation of an XUV-pump–XUV-probe transient absorption spectroscopy scheme at the free-electron laser facility FLASH in Hamburg is presented. It is applied to the ultrafast dissociation dynamics of the small molecules diiodomethane and oxygen. For the diiodomethane target, the XUV pump pulse enables the resonant site-specific excitation of an iodine 4d electron to an antibonding molecular orbital. The hereby initiated dissociation of the molecule includes a direct, as well as an indirect path through isomeric geometries of the molecular cation. These parallel pathways are traced with the help of simulated absorption spectra and their time constants are extracted via a global fit model. In the case of the oxygen target, dissociation is initiated by the (multiple) ionization of the oxygen molecule. The emergence of absorption resonances of the doublycharged ionic fragment O$^{2+}$ is observed in the probe spectrum as a product of the dissociation. The compatibility of the extracted rise times of the fragments withCoulomb explosion is investigated with a simple classical model

All-XUV Pump-Probe Transient Absorption Spectroscopy of the Structural Molecular Dynamics of Di-iodomethane

In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I:4d(5/2)(-sigma*) transition of a gas-phase di-iodomethane (CH2I2) target. This site-specific excitation generates a 4d core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4d resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger.ISSN:2160-330

Electronic Population Reconstruction from Strong-Field-Modified Absorption Spectra with a Convolutional Neural Network

We simulate ultrafast electronic transitions in an atom and corresponding absorption line changes with a numerical, few-level model, similar to previous work. In addition, a convolutional neural network (CNN) is employed for the first time to predict electronic state populations based on the simulated modifications of the absorption lines. We utilize a two-level and four-level system, as well as a variety of laser-pulse peak intensities and detunings, to account for different common scenarios of light–matter interaction. As a first step towards the use of CNNs for experimental absorption data in the future, we apply two different noise levels to the simulated input absorption data

Time-resolved four-wave-mixing spectroscopy for inner-valence transitions

Non-collinear four-wave mixing (FWM) techniques at near-infrared (NIR), visible, and ultraviolet frequencies have been widely used to map vibrational and electronic couplings, typically in complex molecules. However, correlations between spatially localized inner-valence transitions among different sites of a molecule in the extreme ultraviolet (XUV) spectral range have not been observed yet. As an experimental step towards this goal we perform time-resolved FWM spectroscopy with femtosecond NIR and attosecond XUV pulses. The first two pulses (XUV-NIR) coincide in time and act as coherent excitation fields, while the third pulse (NIR) acts as a probe. As a first application we show how coupling dynamics between odd- and even-parity inner-valence excited states of neon can be revealed using a two-dimensional spectral representation. Experimentally obtained results are found to be in good agreement with ab initio time-dependent R-matrix calculations providing the full description of multi-electron interactions, as well as few-level model simulations. Future applications of this method also include site-specific probing of electronic processes in molecules.Comment: 5 pages, 3 figure

CK2 oppositely modulates L-DOPA-induced dyskinesia via striatal projection neurons expressing D1 or D2 receptors

We have previously shown that casein kinase 2 (CK2) negatively regulates dopamine D1 and adenosine A2A receptor signaling in the striatum. Ablation of CK2 in D1 receptor-positive striatal neurons caused enhanced locomotion and exploration at baseline, whereas CK2 ablation in D2 receptor-positive neurons caused increased locomotion after treatment with A2A antagonist, caffeine. Because both, D1 and A2A receptors, play major roles in the cellular responses toL-DOPA in the striatum, these findings prompted us to examine the impact of CK2 ablation on the effects ofL-DOPA treatment in the unilateral 6-OHDA lesioned mouse model of Parkinson’s disease. We report here that knock-out of CK2 in striatonigral neurons reduces the severity ofL-DOPA-induced dyskinesia (LID), a finding that correlates with lowered pERK but unchanged pPKA substrate levels in D1 medium spiny neurons as well as in cholinergic interneurons. In contrast, lack of CK2 in striatopallidal neurons enhances LID and ERK phosphorylation. Coadministration of caffeine with a low dose ofL-DOPA reduces dyskinesia in animals with striatopallidal knock-out to wild-type levels, suggesting a dependence on adenosine receptor activity. We also detect reduced Golf levels in the striatonigral but not in the striatopallidal knock-out in response toL-DOPA treatment. Our work shows, in a rodent model of PD, that treatment-induced dyskinesia and striatal ERK activation are bidirectionally modulated by ablating CK2 in D1- or D2-positive projection neurons, in male and female mice. The results reveal that CK2 regulates signaling events critical to LID in each of the two main populations of striatal neurons