40 research outputs found
Auger decay in double core ionized molecules
Röntgen Freie Elektronen Laser ermöglichen es Doppel-K-Schalen Löchern in Molekülen in aufeinanderfolgenden mehrfachen Ionisationsschritten in bedeutender Anzahl zu erzeugen. Die Eigenschaften dieser zweifach ionisierten Zustände ist insbesondere relevant für die Strahlungsschäden bei Beugungsexperimenten mit kohärenter Röntgenstrahlung zur Bildgebung einzelner Moleküle. In dieser Arbeit wird der Auger Zerfall doppelt K-Schalen ionisierter Moleküle mittels quantenchemischer ab-initio Methoden untersucht. Zur Beschreibung des emittierten Auger Elektrons im kontinuierlichen Energiespektrum wird dabei die Ein-Zentrums Methode verwendet, in der die elektronische Wellenfunktion auf einem radialen Gitter beschrieben wird unter Verwendung von sphärischen Harmonischen. Wie anhand desWassermoleküls gezeigt wird, ergeben sich durch die Doppel-K-Loch induzierte Protonendynamik in dem Auger Spektrum ausgeprägte Flanken im höherenergetischen Teil jeder Spektralspitze. Die Lebensdauer von Doppel-K-Schalen Löchern in Molekülen ist deutlich verringert im Vergleich zu einfachen K-Löchern durch die K-Loch induzierten Abschirmeffekte der Valenzelektronen. Dieser Mechanismus wird durch ein einfaches Modell erklärt aus dem eine Beziehung zwischen Zerfallsrate und Valenzelektronenpopulation abgeleitet. Mögliche Konsequenzen dieser Ergebnisse für Röntgenbeugungsexperimente sind: Erstens, auch für Röntgenpulse kürzer als 10fs wird das Beugungsbild durch die K-Loch induzierten Umstrukturierungen der Valenzelektronen beeinflußt. Zweitens, die Gesamt-Ionisationsrate ist erhöht aufgrund der schnelleren Neubesetzung der K-Löcher
Simulated XUV Photoelectron Spectra of THz-pumped Liquid Water
Highly intense, sub-picosecond terahertz (THz) pulses can be used to induce
ultrafast temperature jumps (T-jumps) in liquid water. A supercritical state of
gas-like water with liquid density is established, and the accompanying
structural changes are expected to give rise to time-dependent chemical shifts.
We investigate the possibility of using extreme ultraviolet (XUV) photoelectron
spectroscopy as a probe for ultrafast dynamics induced by sub-picosecond THz
pulses of varying intensities and frequencies. To this end, we use ab initio
methods to calculate photoionization cross sections and photoelectron energies
of (H2O) clusters embedded in an aqueous environment represented by
point charges. The cluster geometries are sampled from ab initio molecular
dynamics simulations modeling the THz-water interactions. We find that the
peaks in the valence photoelectron spectrum are shifted by up to 0.4 eV after
the pump pulse, and that they are broadened with respect to unheated water. The
shifts can be connected to structural changes caused by the heating, but due to
saturation effects they are not sensitive enough to serve as a thermometer for
T-jumped water
Efficient electronic structure calculation for molecular ionization dynamics at high x-ray intensity
We present the implementation of an electronic-structure approach dedicated
to ionization dynamics of molecules interacting with x-ray free-electron laser
(XFEL) pulses. In our scheme, molecular orbitals for molecular core-hole states
are represented by linear combination of numerical atomic orbitals that are
solutions of corresponding atomic core-hole states. We demonstrate that our
scheme efficiently calculates all possible multiple-hole configurations of
molecules formed during XFEL pulses. The present method is suitable to
investigate x-ray multiphoton multiple ionization dynamics and accompanying
nuclear dynamics, providing essential information on the chemical dynamics
relevant for high-intensity x-ray imaging.Comment: 28 pages, 6 figure
Looking ahead: Summer offerings for all ages
The advent of ultrashort soft X-ray pulse sources permits the use of established gas-phase spectroscopy methods to investigate ultrafast photochemistry in isolated molecules with element and site specificity. In the present study, we simulate excited-state wavepacket dynamics of a prototypical process, the ultrafast photodissociation of methyl iodide. Using the simulation, we calculate time-dependent excited-state carbon edge photoelectron and Auger electron spectra. We observe distinct signatures in both types of spectra and show their direct connection to C–I bond dissociation and charge rearrangement processes in the molecule. We demonstrate at the CH3I molecule that the observed signatures allow us to map the time-dependent dynamics of ultrafast photoinduced bond breaking with unprecedented detail
Hole dynamics in a photovoltaic donor-acceptor couple revealed by simulated time-resolved X-ray absorption spectroscopy
Theoretical and experimental methodologies that can characterize electronic and nuclear dynamics, and the coupling between the two, are needed to understand photoinduced charge transfer in molecular building blocks used in organic photovoltaics. Ongoing developments in ultrafast pump-probe techniques such as time-resolved X-ray absorption spectroscopy, using an X-ray free electron laser in combination with an ultraviolet femtosecond laser, present desirable probes of coupled electronic and nuclear dynamics. In this work, we investigate the charge transfer dynamics of a donor-acceptor pair, which is widely used as a building block in low bandgap block copolymers for organic photovoltaics. We simulate the dynamics of the benzothiadiazole-thiophene molecule upon photoionization with a vacuum ultraviolet (VUV) pulse and study the potential of probing the subsequent charge dynamics using time-resolved X-ray absorption spectroscopy. The photoinduced dynamics are calculated using on-the-fly nonadiabatic molecular dynamics simulations based on Tully's Fewest Switches Surface Hopping approach. We calculate the X-ray absorption spectrum as a function of time after ionization at the Hartree-Fock level. The changes in the time-resolved X-ray absorption spectrum at the sulfur K-edge reveal the ultrafast charge carrier dynamics in the molecule occurring on a femtosecond time scale. These theoretical findings anticipate that ultrafast time-resolved X-ray absorption spectroscopy using an X-ray probe in combination with a VUV pump offers a new approach to investigate the detailed dynamics of organic photovoltaic materials
Strategies for solving the excited-state self-consistent-field problem for highly excited and multiply ionized states
The dynamics of molecules exposed to intense x-ray radiation involve a large number of multiply ionized and highly excited electronic configurations. To model these dynamics a reliable and efficient electronic structure model is imperative. Employing the Hartree-Fock-Slater electronic structure model in combination with the maximum overlap method, we quantify the associated convergence failures when calculating electronic states of carbon monoxide with multiple vacancies in the core and valence levels. We characterize these cases and describe strategies to overcome the convergence problems. The described techniques not only eliminate all convergence issues for CO but also result in a significant reduction of convergence failures for simulations of the x-ray-induced multiple ionization dynamics of the phenol molecule
Core hole screening and decay rates of double core ionized first row hydrides
Because of the high intensity, X-ray free electron lasers allow one to create and probe double core
ionized states in molecules. The decay of these multiple core ionized states crucially determines the
evolution of radiation damage in single molecule diffractive imaging experiments. Here we have
studied the Auger decay in hydrides of first row elements after single and double core ionization by
quantum mechanical ab initio calculations. In our approach the continuum wave function of the emitted
Auger electron is expanded into spherical harmonics on a radial grid. The obtained decay rates of
double K-shell vacancies were found to be systematically larger than those for the respective single
K-shell vacancies, markedly exceeding the expected factor of two. This enhancement is attributed to
the screening effects induced by the core hole. We propose a simple model, which is able to predict
core hole decay rates in molecules with low Z elements based on the electron density in the vicinity
of the core hole.peerReviewe
Capturing electronic decoherence in quantum-classical dynamics using the ring-polymer-surface-hopping–density-matrix approach
Simulations of coupled electronic and nuclear dynamics in molecules can be quite challenging due to the involved interplay of the many degrees of freedom. Because a full quantum treatment of both electrons and nuclei is computationally very demanding, it is generally restricted to model systems or rather small molecules and short timescales. Mixed quantum-classical dynamics methods such as Tully's fewest switches surface hopping (FSSH) can be used to overcome this limitation. However, FSSH is known to poorly describe electronic coherences and decoherence phenomena. Here, we present an approach that combines FSSH with ring-polymer molecular dynamics (RPMD) in a specific way that aims to alleviate the coherence problem. Termed the ring-polymer-surface-hopping–density-matrix approach, this method uses an electronic density-matrix formulation to calculate surface hopping rates. This incorporates decoherence effects into FSSH in a natural way by taking into account the spatial spreading of the ring polymer that mimics the width of a nuclear wave packet in each RPMD trajectory. By applying our method to Tully's one-dimensional model system, we demonstrate that this method captures a crucial decoherence mechanism that is missing in FSSH. Furthermore, our method turns out to be superior at describing electronic coherences compared with earlier attempts at combining RPMD and FSSH