Leticia González, Philipp Marquetand
This perspective on laser control provides a quick overview over different schemes for coherent control of chemical reactions and photophysical processes. It originally appeared in Bunsen Magazin 1, 13 - 23 (2012).
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Recently developed circularly polarized X-ray light sources can probe ultrafast chiral electronic and nuclear dynamics through spatially localized resonant core transitions. We present simulations of time-resolved circular dichroism (TRCD) signals given by the difference of left and right circularly polarized X-ray probe transmission following an excitation by a circularly polarized optical pump with variable time delay. Application is made to formamide which is achiral in the ground state and assumes two chiral geometries upon optical excitation to the first valence excited state. Probes reso…
We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find…
Phenomena involving electron transfer are ubiquitous in nature, photosynthesis and enzymes or protein activity being prominent examples. Their deep understanding thus represents a mandatory scientific goal. Moreover, controlling the separation of photogenerated charges is a crucial prerequisite in many applicative contexts, including quantum electronics, photo-electrochemical water splitting, photocatalytic dye degradation, and energy conversion. In particular, photoinduced charge separation is the pivotal step driving the storage of sun light into electrical or chemical energy. If properly ma…
The ultrafast nonadiabatic internal conversion in azomethane is explored by the on-the-fly trajectory surface-hopping simulations of photoinduced dynamics and femtosecond transient absorption (TA) pump-probe (PP) spectra at three electronic-structure theory levels, OM2/MRCI, SA-CASSCF, and XMS-CASPT2. All these dynamics simulations predict ultrafast internal conversion. On the one hand, the OM2/MRCI and SA-CASSCF methods yield similar excited-state dynamics, while the XMS-CASPT2 method predicts a much slower population decay. On the other hand, the TA PP signals simulated at the SA-CASSCF and …
Ultrafast Charge Migration Dynamics in Enol Keto Tautomerization Monitored with a Local Soft-X-Ray Probe
本研究通过模拟证明飞秒软X射线光谱结合全量子动力学处理，能揭示光致烯醇式-酮式互变异构中质子耦合电子转移的超快电荷迁移机制。
We investigate photoinduced proton-coupled electron transfer (PI-PCET) reaction through a recently devel- oped quasi-diabatic (QD) quantum dynamics propagation scheme. This scheme enables interfacing accurate diabatic-based quantum dynamics approaches with adiabatic electronic structure calculations for on-the-fly simulations. Here, we use the QD scheme to directly propagate PI-PCET quantum dynamics with the di- abatic Partial Linearized Density Matrix (PLDM) path-integral approach with the instantaneous adiabatic electron-proton vibronic states. Our numerical results demonstrate the importanc…
In situ and real-time ultrafast spectroscopy of photoinduced reactions in perovskite nanomaterials
本研究开发了异步干涉瞬态吸收（AI-TA）方法，利用同步飞秒激光器实现了对钙钛矿纳米材料中光致反应的原位实时观察，揭示了光取代与光致合并过程中的载流子动力学变化。
Organic-inorganic perovskites have shown great promise towards their application in optoelectronics. The success of this class of material is dictated by the complex interplay between various underlying microscopic phenomena. The structural dynamics of organic cations and the inorganic sublattice after photoexcitation is hypothesized to have a direct effect on the material properties, thereby affecting the overall device performance. Here, we use two-dimensional (2D) electronic spectroscopy to reveal impulsively excited vibrational modes of methylammonium (MA) lead iodide perovskite, which dri…
Ab initio simulation of the ultrafast circular dichroism spectrum of provitamin D ring-opening
提出基于含时密度泛函理论轨迹表面跳跃的超快TRCD光谱模拟方法，揭示维生素D原光致开环过程中的激发态弛豫与旋转异构体异构化机制，为光致手性变化亚皮秒动力学提供灵敏分析工具。
Chiral Porphyrin Monolayers on Ferromagnetic Thin Films: Ultrafast Spectroscopy of Hybrid Interfaces
研究铁磁多层结构对含血红素染料的分子动态的影响，发现表面吸附缩短激发态寿命，但未观察到长寿命电荷转移产物。
The Photochemistry of Rydberg Excited Cyclobutanone: Photoinduced Processes and Ground State Dynamics
本研究结合量子与激发态轨迹跳跃分子动力学模拟，揭示了环丁酮在3s←n激发至Rydberg态后的超快弛豫动力学，预测了电子衍射信号，并阐明了其非辐射弛豫至基态后产生C2和C3碎裂产物的光化学通道。
In-situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to investigate many physical science phenomena, ranging from phase transitions, chemical reaction and crystal growth to grain boundary dynamics. A major limitation of in-situ XRD and TEM is a compromise that has to be made between spatial and temporal resolution. Here, we report the development of in-situ X-ray nanodiffraction to measure atomic-resolution diffraction patterns from single grains with up to 5 millisecond temporal resolution, and make the first real-time observation of grain rotation and lat…
Wave packet propagation succeeding electron transfer (ET) from alizarin dye molecules into the nanocrystalline TiO2 semiconductor has been studied by ultrafast transient absorption spectroscopy. Due to the ultrafast time scale of the ET reaction of about 6 fs the system shows substantial differences to molecular ET systems. We show that the ET process is not mediated by molecular vibrations and therefore classical ET theories lose their applicability. Here the ET reaction itself prepares a vibrational wave packet and not the electromagnetic excitation by the laser pulse. Furthermore, the gener…
Ultrafast X-Ray Scattering: New Views of Chemical Reaction Dynamics
本文介绍超快X射线散射的理论与实验要点，指出其可通过时间分辨分子结构为化学反应动力学研究提供新观测维度，并展望了未来技术发展方向。
The photodissociation dynamics and ultrafast electron diffraction image of cyclobutanone from the surface hopping dynamics simulation
本文基于轨迹表面跳跃方法预测了环丁酮的气相电子衍射（GUED）谱，揭示了其光诱导的超快非绝热动力学中C2和C3通道的主导作用，为监测飞秒分子结构演化提供了独特视角。
The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Here, we present direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and sub-Ångström length scale by megaelectronvolt ultrafast electron diffraction. We follow the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by direct measurement of time-dependent changes in the distribution of interatomic distances…
The ultrafast hole dynamics triggered by the photoexcitation of molecular targets is a highly correlated process even for those systems, like organic molecules, having a weakly correlated ground state. We here provide a unifying framework and a numerically efficient matrix formulation of state-of-the-art non-equilibrium Green's function (NEGF) methods like second-Born as well as $GW$ and $T$-matrix without and {\em with} exchange diagrams. Numerical simulations are presented for a paradigmatic, exactly solvable molecular system and the shortcomings of the established NEGF methods are highlight…
We develop a detailed theoretical model of photo-induced proton-coupled electron transfer (PPCET) processes, which are at the basis of solar energy harvesting in biological systems and photovoltaic materials. Our model enables to analyze the dynamics and the efficiency of a PPCET reaction under the influence of a thermal environment by disentangling the contribution of the fundamental electron transfer (ET) and proton transfer (PT) steps. In order to study quantum dynamics of the PPCET process under an interaction with non-Markovian environment we employ the hierarchical equations of motion (H…
We study a simple model for photoinduced electron transfer reactions for the case of many donor-acceptor pairs that are collectively and homogeneously coupled to a photon mode of a cavity. We describe both coherent and dissipative collective effects resulting from this coupling within the framework of a quantum optics Lindblad master equation. We introduce a method to derive an effective rate equation for electron transfer, by adiabatically eliminating donor and acceptor states and the cavity mode. The resulting rate equation is valid both for weak and strong coupling to the cavity mode, and d…
Unraveling the ultrafast dynamics of thermal-energy chemical reactions
本文探讨如何结合超短中红外激光脉冲、气相受控分子与前沿成像技术，实时启动、成像并解析热能化学反应的超快基元步骤，从而加深对一般化学反应动力学的理解。
We present a general formalism of ultrafast diffractive voltammetry approach as a contact-free tool to investigate the ultrafast surface charge dynamics in nanostructured interfaces. As case studies, the photoinduced surface charging processes in oxidized silicon surface and the hot electron dynamics in nanoparticle-decorated interface are examined based on the diffractive voltammetry framework. We identify that the charge redistribution processes appear on the surface, sub-surface, and vacuum levels when driven by intense femtosecond laser pulses. To elucidate the voltammetry contribution fro…
Ultrafast science depends on different implementations of the well-known pump-probe method. Here we provide a formal description of ultrafast disruptive probing, a method in which the probe pulse disrupts a transient species that may be a metastable ion or a transient state of matter. Disruptive probing has the advantage of allowing simultaneous tracking of the yield of tens of different processes. Our presentation includes a numerical model and experimental data on multiple products resulting from the strong-field ionization of two different molecules, partially-deuterated methanol and norbor…
Black phosphorus (BP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations take place concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and BP's puckered structure, with the alignment of the electronic band structure, i…
Exploring the impact of vibrational cavity coupling strength on ultrafast CN + $c$-C$_6$H$_{12}$ reaction dynamics
本研究通过瞬态吸收技术探究不同振动腔耦合强度下CN与环己烷-三氯甲烷体系的超快反应动力学，发现反应速率不受腔耦合条件及强度影响，表明耦合强度不足并非相关氢提取反应腔效应微弱的决定因素。
Regions of energy localization referred to as hotspots are known to govern shock initiation and the run-to-detonation in energetic materials. Mounting computational evidence points to accelerated chemistry in hotspots from large intramolecular strains induced via the interactions between the shockwave and microstructure. However, definite evidence mapping intramolecular strain to accelerated or altered chemical reactions has so far been elusive. From a large-scale reactive molecular dynamics simulation of the energetic material TATB, we map molecular temperature and intramolecular strain energ…
We applied angular X-ray Cross-Correlation analysis (XCCA) to scattering images from a femtosecond resolution LCLS X-ray free-electron laser (XFEL) pump-probe experiment with solvated PtPOP ([Pt$_2$(P$_2$O$_5$H$_2$)$_4$]$^{4-}$) metal complex molecules. The molecules were pumped with linear polarized laser pulses creating an excited state population with a preferred orientational (alignment) direction. Two time scales of $1.9\pm1.5$ ps and $46\pm10$ ps were revealed by model-independent XCCA, associated with an internal structural changes and rotational dephasing, respectively. Our studies ill…
Light absorption of myoglobin triggers diatomic ligand photolysis and spin crossover transition of iron(II) that initiate protein conformational change. The photolysis and spin crossover reactions happen concurrently on a femtosecond timescale. The microscopic origin of these reactions remains controversial. Here, we apply quantum wavepacket dynamics to elucidate the ultrafast photochemical mechanism for a heme--carbon monoxide (heme--CO) complex. We observe coherent oscillations of the Fe-CO bond distance with a period of 42 fs and an amplitude of $\sim$1 Å. These nuclear motions induce prono…
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