Reconstruction of the time-dependent electronic wave packet arising from molecular autoionization

Autoionizing resonances are paradigmatic examples of two-path wave interferences between direct photoionization, which takes a few attoseconds, and ionization via quasi-bound states, which takes much longer. Time-resolving the evolution of these interferences has been a long-standing goal, achieved recently in the helium atom owing to progress in attosecond technologies. However, already for the hydrogen molecule, similar time imaging has remained beyond reach due to the complex interplay between fast nuclear and electronic motions. We show how vibrationally resolved photoelectron spectra of H2 allow one to reconstruct the associated subfemtosecond autoionization dynamics by using the ultrafast nuclear dynamics as an internal clock, thus forgoing ultrashort pulses. Our procedure should be general for autoionization dynamics in molecules containing light nuclei, which are ubiquitous in chemistry and biologyThis work was supported by European Research Council advanced grant 290853-XCHEM within the seventh framework program of the European Union. We also acknowledge the financial support from MINECO projects FIS2013-42002-R and FIS2016-77889-R, and the European COST (Cooperation in Science and Technology) Action XLIC CM1204, and the computer time from the Centro de Computación Científica de la Universidad Autónoma de Madrid and Marenostrum Supercomputer Center. A.P. acknowledges a Ramón y Cajal contract from the Ministerio de Economía y Competitividad (Spain). F.M. acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, grant SEV-2016-0686) and the “María de Maeztu” Programme for Units of Excellence in R&D (MDM-2014-0377). S.E.C. acknowledges funding from the Helmoltz Recognition Award. The Extreme Light Infrastructure Attosecond Light Pulse Source project (GINOP-2.3.6-15-2015-00001) was financed by the European Union and cofinanced by the European Regional Development Fun

Reconstruction of the time-dependent electronic wave packet arising from molecular autoionization

The time evolution of an electronic molecular wave packet is determined using nuclear motion as an internal clock of the system

Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy

We present a combined Langmuir–Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquid–vapor interfaces in combined Langmuir–Pockels trough and APXPS measurements

Ionic conductivity and the formation of cubic CaH₂ in the LiBH₄-Ca(BH₄)₂ composite

Transient absorption spectroscopy is used to follow the reactive intermediates involved in the first steps in the photochemistry initiated by ultraviolet (266-nm wavelength) excitation of solutions of 1,5-hexadiene, isoprene, and 2,3-dimethylbut-2-ene in carbon tetrachloride or chloroform. Ultraviolet and visible bands centered close to 330 and 500 nm in both solvents are assigned respectively to a charge transfer band of Cl-solvent complexes and the strong absorption band of a higher energy isomeric form of the solvent molecules (iso-CCl3–Cl or iso-CHCl2–Cl). These assignments are supported by calculations of electronic excitation energies. The isomeric forms have significant contributions to their structures from charge-separated resonance forms and offer a reinterpretation of previous assignments of the carriers of the visible bands that were based on pulsed radiolysis experiments. Kinetic analysis demonstrates that the isomeric forms are produced via the Cl–solvent complexes. Addition of the unsaturated hydrocarbons provides a reactive loss channel for the Cl–solvent complexes, and reaction radii and bimolecular rate coefficients are derived from analysis using a Smoluchowski theory model. For reactions of Cl with 1,5-hexadiene, isoprene, and 2,3-dimethylbut-2-ene in CCl4, rate coefficients at 294 K are, respectively, (8.6 ± 0.8) × 109, (9.5 ± 1.6) × 109, and (1.7 ± 0.1) × 1010 M–1 s–1. The larger reaction radius and rate coefficient for 2,3-dimethylbut-2-ene are interpreted as evidence for an H-atom abstraction channel that competes effectively with the channel involving addition of a Cl-atom to a C═C bond. However, the addition mechanism appears to dominate the reactions of 1,5-hexadiene and isoprene. Two-photon excited CCl4 or CHCl3 can also ionize the diene or alkene solute

Effects of Aversive Stimuli on Prospective Memory. An Event-Related fMRI Study

Prospective memory (PM) describes the ability to execute a previously planned action at the appropriate point in time. Although behavioral studies clearly showed that prospective memory performance is affected by the emotional significance attributed to the intended action, no study so far investigated the brain mechanisms subserving the modulatory effect of emotional salience on PM performance. The general aim of the present study was to explore brain regions involved in prospective memory processes when PM cues are associated with emotional stimuli. In particular, based on the hypothesised critical role of the prefrontal cortex in prospective memory in the presence of emotionally salient stimuli, we expected a stronger involvement of aPFC when the retrieval and execution of the intended action is cued by an aversive stimulus. To this aim BOLD responses of PM trials cued by aversive facial expressions were compared to PM trials cued by neutral facial expressions. Whole brain analysis showed that PM task cued by aversive stimuli is differentially associated with activity in the right lateral prefrontal area (BA 10) and in the left caudate nucleus. Moreover a temporal shift between the response of the caudate nucleus that preceded that of aPFC was observed. These findings suggest that the caudate nucleus might provide an early analysis of the affective properties of the stimuli, whereas the anterior lateral prefrontal cortex (BA10) would be involved in a slower and more deliberative analysis to guide goal-directed behaviour

Photoionization of Xenon Clusters

High resolution photoelectron spectra of xenon clusters of about 100 atoms, measured with photon energies of 20 to 60 eV are reported. Peaks due to the clusters are observed at lower binding energies than the corresponding atomic lines and exhibit different behaviors. The peak associated with the Xe 5p1/2 state at higher binding energy is relatively unchanged with photon energy whereas the Xe 5p3/2 peak changes its shape with increasing photon energy. The change is attributed to differences in the escape depth of the electrons at different kinetic energies and the bulk and surface components of the cluster

Academic Performance, Age, Gender, and Ethnicity in Online Courses Delivered by Two-Year Colleges

This study investigated the effects the demographic variables age, gender, and ethnicity and their interactions had on academic performance in online courses delivered by public two-year colleges in Kentucky. The study controlled for previous academic performance measured by cumulative grade point average (GPA). The study used a random sample (N = 320) of all students who had enrolled in at least one online course delivered by the institutions of the Kentucky Community and Technical College System in the spring 2008 semester. A linear hierarchical multiple regression acting as ANCOVA served as the main analysis, with the order entry as follows: cumulative GPA; independent variables (age, gender, ethnicity); interaction vectors; and product vectors. Final course grade served as the dependent variable. The results of the analysis indicated that only cumulative GPA was a significant predictor, explaining approximately 40% of the variance of the final grade. Although differences in final grades were present among the variables age and ethnicity, these differences disappeared when controlling for cumulative GPA. Significance of the results and recommendations for future research are discussed

Direct observation of Young's double-slit interferences in vibrationally resolved photoionization of diatomic molecules.

Vibrationally resolved valence-shell photoionization spectra of H(2), N(2) and CO have been measured in the photon energy range 20-300 eV using third-generation synchrotron radiation. Young's double-slit interferences lead to oscillations in the corresponding vibrational ratios, showing that the molecules behave as two-center electron-wave emitters and that the associated interferences leave their trace in the angle-integrated photoionization cross section. In contrast to previous work, the oscillations are directly observable in the experiment, thereby removing any possible ambiguity related to the introduction of external parameters or fitting functions. A straightforward extension of an original idea proposed by Cohen and Fano [Cohen HD, Fano U (1966) Phys Rev 150:30] confirms this interpretation and shows that it is also valid for diatomic heteronuclear molecules. Results of accurate theoretical calculations are in excellent agreement with the experimental findings

Towards efficient time-resolved X-ray absorption studies of electron dynamics at photocatalytic interfaces.

We present a picosecond time-resolved X-ray absorption spectroscopy (tr-XAS) setup designed for synchrotron-based studies of interfacial photochemical dynamics. The apparatus combines a high power, variable repetition rate picosecond laser system with a time-resolved X-ray fluorescence yield detection technique. Time-tagging of the detected fluorescence signals enables the parallel acquisition of X-ray absorption spectra at a variety of pump-probe delays employing the well-defined time structure of the X-ray pulse trains. The viability of the setup is demonstrated by resolving dynamic changes in the fine structure near the O1s X-ray absorption edge of cuprous oxide (Cu2O) after photo-excitation with a 355 nm laser pulse. Two distinct responses are detected. A pronounced, quasi-static, reversible change of the Cu2O O1s X-ray absorption spectrum by up to ∼30% compared to its static line shape corresponds to a redshift of the absorption edge by ∼1 eV. This value is small compared to the 2.2 eV band gap of Cu2O but in agreement with previously published results. The lifetime of this effect exceeds the laser pulse-to-pulse period of 8 μs, resulting in a quasi-static spectral change that persists as long as the sample is exposed to the laser light, and completely vanishes once the laser is blocked. Additionally, a short-lived response corresponding to a laser-induced shift of the main absorption line by ∼2 eV to lower energies appears within <200 ps and decays with a characteristic timescale of 43 ± 5 ns. Both the picosecond rise and nanosecond decay of this X-ray response are simultaneously captured by making use of a time-tagging approach - highlighting the prospects of the experimental setup for efficient probing of the electronic and structural dynamics in photocatalytic systems on multiple timescales