Natural Sciences 

About: Natural Sciences is an academic journal. The journal publishes majorly in the area(s): Cold storage & Chemical Dynamics. It has an ISSN identifier of 1505-4667. Over the lifetime, 110 publications have been published receiving 270 citations.

Direct measurement of key exciton properties: Energy, dynamics, and spatial distribution of the wave function

Abstract: 1 Fritz-Haber-Institut derMax-Planck-Gesellschaft, Berlin, Germany 2 Laboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), École polytechnique fédérale de Lausanne, ISIC, Lausanne, Switzerland 3 Institut für Theoretische Physik, Nichtlineare Optik undQuantenelektronik, Technische Universität Berlin, Berlin, Germany 4 Max Planck Institute for the Structure andDynamics ofMatter and Center for Free Electron Laser Science, Hamburg, Germany 5 Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, Fribourg, Switzerland 6 Department of Applied Physics, KTHRoyal Institute of Technology, Stockholm, Sweden 7 SwissFEL, Paul Scherrer Institute, Villigen, Switzerland 8 Department of Physics, Chalmers University of Technology, Gothenburg, Sweden

Chemical dynamics from the gas-phase to surfaces

Abstract: ion of green H by brown H, while the green pathway involves 6 of 42 CHEMICALDYNAMICS FROMTHEGAS-PHASE TO SURFACES: AUERBACH ET AL. F IGURE 5 Quantum interference through a conical intersection: (a) a cut through the HHDPES showing the conical intersection (×) and three transition states (T) that connect three stable arrangements of the atoms. Note the color of the atoms. Direct abstraction visits one transition-state (REDARROW), while the spiral or roaming reaction visits two (GREENARROW). Both paths lead to the same products. (b) The experiment (∙) detects reactive flux arriving in the backward scattering direction producing H2(v = 2, J′ = 3) as the incidence energy is scanned. The oscillations are due to quantum interference between the two topological pathways. The red line shows quantum scattering calculations that neglect the phase-shift of π (geometric phase) introduced by traversal around a conical intersection. The blue curve accounts for the geometric phase.132 Reprinted fromXie Y, ZhaoH,Wang Y, et al. Quantum interference in H plus HD→ H2 +Dbetween direct abstraction and roaming insertion pathways. Science. 2020; 368(6492):767, Copyright 2021, with permission fromAAAS a failed reactive attack of brownHonD followed by a complex internal rearrangement (sometimes called a spiral reaction) allowing abstraction of green H by brown H. Since the identical products, H2 +D, formed via two pathways, interference arises. But beyond this, quantum mechanics requires that when a conical intersection is traversed, the phase of the quantum flux passing on opposite sides of the conical intersection must be shifted by π with respect to one another—Berry’s phase.135 Obviously, this affects the interference.132,140,141 These observations relied on Rydberg-atom tagging, but REMPI-basedmethods like ion imaging and Photoloc142–151 havealsobeencrucial to revealing thedynamics of this system.142–153 The basis of this success and the others that space does not allow us to present is the concept that chemical reactivity involves quantum mechanical motion of nuclei on a Born–Oppenheimer PES. The remarkable agreement between the predictions of the theory and the observations from experiment earns this concept the name the standard model of chemical reactivity.8 Classical roaming reaction The standard model affords the possibility of computing and illustrating the time-dependent motions of individual atoms through a chemically reactive encounter by following, for each atom, either the classical mechanical position or the quantum mechanical expectation value of position. It is even possible to make movies of reactions using calculated trajectories, effectively providing a microscope with time and space resolution far better than will ever be experimentally possible.One of themost inspiring examples of this is the gas-phase roaming reaction, first reported in the unimolecular decomposition ofH2CO. 154 Following up on suspicions that the reaction H2CO→ H2 + CO may proceed by more than one mechanism,155 ion imaging was applied to obtain speed and angular distributions of specific rotation-vibration states of CO(vCO, JCO). Figure 6 shows data revealing that when CO is producedwith low rotational excitation,H2 is producedwith low speed and high vibrational excitation, and vice versa. Using a six-dimensional F IGURE 6 The roaming reaction in formaldehyde. Ion images (right) and CO translational energy distributions (left) for selected rotational states of CO, JCO, formed in formaldehyde photodissociation. (a) JCO = 40; (b) JCO = 28; and (c) JCO = 15. The rings correspond to different quantum states of H2 produced in coincidence with these states of CO. The peaks in the CO translational energy distributions show assignments to specific H2(v) vibrational states—integers in the left panels show v, where experimental results are solid lines and results fromQCT calculations on a full-dimensional PES are the dashed lines.154 In (b) and (c), someH2 rotational assignments are indicated by combs. Reprinted from TownsendD, Lahankar SA, Lee SK, et al. The roaming atom: straying from the reaction path in formaldehyde decomposition. Science. 2004; 306(5699):1158-1161, Copyright 2021, with permission fromAAAS Born–Oppenheimer PES to calculate classical trajectories, theory reproduced the experimental observations—compare blue and black curves. There are two classes of trajectories; one reveals a concerted molecular elimination of H2 achieved by passing over a barrier. This NATURAL SCIENCES 7 of 42 F IGURE 7 Animation of a classical trajectory of the roaming reaction in CH2O decomposition with H (green), C (white), andO (red).154 Note the high vibrational excitation of H2 products seen also experimentally. Usedwith permission of Arthur Suits and Joel Bowman channel leads to low vibrational states of H2 and high rotational states of CO. The second class of reactions is shown in Figure 7. Here, a highly excited formaldehyde molecule breaks one of its C-H bonds, but with insufficient energy for the H atom to escape. It orbits about the HCO fragment until it finds an attack angle toward the other H-atom—H + HCO→ H2 + CO. This is an exoergic early barrier reaction that, just as predicted by the Polanyi rules, leads to highly vibrationally excited H2. This example fulfills the childhood fantasy that drove some of us to become chemists, the wish to be able watch the atoms while they are reacting. Remarkably, this is no fantasy—the classical approximation is highly accurate for many examples in chemistry and we use it often to understand the motion of atoms in reactions. Despite the successes of classical mechanics, it is impossible to avoid the quantum nature of electrons when two (or more) quantized electronic states are involved. Electronically nonadiabatic dynamics The reaction of H+ +H2 → H + 2 +H appears superficially simpler than the H3 reaction—H + 3 has one less electron. However, looks may deceive—this reaction may occur in three ways. Isotopic labeling helps illustrate this. Reacting H+with D2 may involve ion exchange, producing HD +D+, electron transfer, producing H +D 2 , or ion exchange with electron transfer, forming HD+ +D.We need to extend the standard model to consider the quantum motion of protons influenced by an avoided intersection between the two lowest energy electronic states of H 3 . In a reactive encounter, the nonadiabatic coupling—Figure 8(d) —is