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Capturing ring opening in photoexcited enolic acetylacetone upon hydrogen bond dissociation by ultrafast electron diffraction
No full textPhotoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular hydrogen bond. Following excitation into the \ce{S2} () state at 266~nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization and bond elongation all within the first 700~fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the \ce{S1} potential energy surface, formed after very rapid transfer from \ce{S2} to \ce{S1}, allowing AcAc to reach the conical intersection to intersystem crossing. Such timescales of nuclear motion are contrasted with the timescales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (100~fs) and intersystem crossing ( 1.5~ps)
Real-Time Monitoring of Thermally Induced Twisting−Untwisting of Noncubic Domains in Au Microcrystallites using X‑ray Diffraction Microscopy
No full textAu bipyramids hosting body-centered orthorhombic and tetragonal lattices (bc(o,t)) exhibit extraordinary stability at ambient conditions and even under high-temperature/high-pressure conditions. The phases undergo conversion to a conventional face-centered cubic (fcc) lattice only during annealing at 700 °C due to the unlocking of the geometrically induced stresses. The spatial distribution of the phases in the crystallite volume has revealed fcc capped bc(o,t) lattices with two halves of the bipyramid twisted by ∼6° along the length with approximately ± 5% strain. Understanding the spatial distribution and dynamics of these phases at high temperatures can provide detailed information on their thermal stability. Herein, using nanoprobe scanning X-ray diffraction microscopy (SXDM), in situ annealing of the bc(o,t) Au bipyramid (∼1.5 μm long and 300 nm wide) has been performed at different temperatures (up to 800 °C). The study reveals untwisting of the domains assisted by the supplied high temperature, while the existing lattices undergo variation in parameters with negligible changes in proportion. The study reveals and picturizes the dynamic change in diffracting volumes across a wide temperature range. Notably, despite annealing, ∼83% of the bc(o,t) content is still retained (with different lattice parameters), proposing the annealing route to produce unusual metastable lattices of gold
Ionization-induced proton and energy transfer in liquid water
No full textWe report computational simulation results addressing the ionization response of liquid water upon valence ionization. The simulations cover ionizations in the whole valence-orbital range of liquid water, i.e., vacancies in 1b, 3a, 1b, and 2a orbitals. It is found that ionization in any of these valence orbitals leads to rapid proton-transfer dynamics. The timescale on which the proton transfer occurs depends on which type of orbital is ionized. For ionization in the 2a orbitals, the proton transfer takes place in about 22 fs, competing with the intermolecular Coulombic decay mechanism that takes place on a similar timescale. This result is discussed in the context of earlier experimental results (Richter et al., Nat. Commun. 9, 4988) regarding the intermolecular Coulombic decay in water. For ionization in the outer-valence orbitals (1b, 3a, 1b), we see rapid internal conversion via non-adiabatic transitions to the electronic ground state. The proton transfer occurs 46, 70, and 91 fs after the initial ionization from a 1b, 3a, and 1b orbital, respectively. The initial valence ionization induces strong vibrational excitations in the surrounding water molecules, leading to a considerable increase in the local effective temperature. The created heat diffuses into the liquid environment on a timescale of several hundred femtoseconds. We compare the results using two different embedding schemes, subtractive and electrostatic embedding, and find overall very similar dynamics
Pd single atoms on g-CN photocatalysts: minimum loading for maximum activity
No full textNoble metal single atoms (SAs) on semiconductors are increasingly explored as co-catalysts to enhance the efficiency of photocatalytic hydrogen production. In this study, we introduce a “spontaneous deposition” approach to anchor Pd SAs onto graphitic carbon nitride (g-CN) using a highly dilute tetraaminepalladium(II) chloride precursor. Maximized photocatalytic activity and significantly reduced charge transfer resistance can be achieved with a remarkably low Pd loading of 0.05 wt% using this approach. The resulting Pd SA-modified g-CN demonstrates a remarkable hydrogen production efficiency of 0.24 mmol h mg Pd, which is >50 times larger than that of Pd nanoparticles deposited on g-CNvia conventional photodeposition. This significant enhancement in catalytic performance is attributed to improved electron transfer facilitated by the optimal coordination of Pd SAs within the g-CN structure
Collinear and TMD distributions with dynamical soft-gluon resolution scale
No full textSoft-gluon resolution scales characterize parton branching Monte Carlo implementations of the evolution equations for parton distribution functions in Quantum Chromodynamics (QCD). We examine scenarios with dynamical, i.e., branching-scale dependent, resolution scale, and discuss physical implications for both collinear and transverse-momentum dependent (TMD) distributions. We perform the first determination of parton distributions with dynamical resolution scale, at next-to-leading order (NLO) in perturbation theory, from fits to precision deep-inelastic scattering measurements from HERA. We present an application of TMD distributions with dynamical resolution scale to Drell-Yan lepton-pair transverse momentum spectra at the LHC, and comment on the extraction of non-perturbative intrinsic-kT parameters from Drell-Yan data at small transverse momenta
Interpolating families of integrable AdS backgrounds
No full textWe construct families of integrable deformations that interpolate between and either or . They preserve half of the supersymmetry of the original background, namely one copy of the algebra. From this it follows a similar integrable interpolation between and , which also preserves half of the supersymmetry, namely a copy of the algebra. In all cases, the interpolating backgrounds are constructed by using TsT transformations, which makes it easy to implement them in the integrability formalism in the full quantum theory. To illustrate this point, we discuss the lightcone gauge fixing of the models and compute their pp-wave Hamiltonian
Absence of CP Violation in the Strong Interaction: Vacuum thwarts Axion
No full textQCD admits a contribution to the action, the term, which potentially gives rise to nontrivial phases and violates CP. This is essentially a question of how the vacuum reacts to the term. In this talk I will address the problem using new developments on the lattice. The overall solution is contrasted with the axion `solution'
Theory of neutrino slow flavor evolution. Part II. Space-time evolution of linear instabilities
Get PDFSlow flavor evolution (defined as driven by neutrino masses and not necessarily ``slow'') is receiving fresh attention in the context of compact astrophysical environments. In Part~I of this series, we have studied the slow-mode dispersion relation following our recently developed analogy to plasma waves. The concept of resonance between flavor waves in the linear regime and propagating neutrinos is the defining feature of this approach. It is best motivated for weak instabilities, which probably is the most relevant regime in self-consistent astrophysical environments because these will try to eliminate the cause of instability. We here go beyond the dispersion relation alone (which by definition applies to infinite media) and consider the group velocities of unstable modes that determines whether the instability relaxes within the region where it first appears (absolute), or away from it (convective). We show that all weak instabilities are convective so that their further evolution is not local. Therefore, studying their consequences numerically in small boxes from given initial conditions may not always be appropriate
Fast flavor pendulum: Instability condition
No full textEven in the absence of neutrino masses, a neutrino gas can exhibit a homogeneous flavor instability that leads to a periodic motion known as the fast flavor pendulum. A well-known necessary condition is a crossing of the angular flavor lepton distribution. In an earlier work, some of us showed that homogeneous flavor instabilities also obey a Nyquist criterion, inspired by plasma physics. This condition, while more restrictive than the angular crossing, is only sufficient if the unstable branch of the dispersion relation is bounded by critical points that both lie under the light cone (points with subluminal phase velocity). While the lepton-number angle distribution, assumed to be axially symmetric, easily allows one to determine the real-valued branch of the dispersion relation and to recognize if instead superluminal critical points exist, this graphical method does not translate into a simple instability condition. We discuss the homogeneous mode in the more general context of the dispersion relation for modes with arbitrary wave number and stress that it plays no special role on this continuum, except for its regular but fragile long-term behavior, owed to its many symmetries
NuSTAR Bounds on Radiatively Decaying Particles from M82
No full textAxions and other putative feebly interacting particles (FIPs) with a mass of tens to several hundreds of keVs can be produced in stellar cores with a Lorentz boost factor . Thus, starburst galaxies such as M82 are efficient factories of slow axions. Their decay would produce a large flux of X-ray photons, peaking around keV and spread around the galaxy by an angle that can be relatively large. We use observations of the Nuclear Spectroscopic Telescope Array (NuSTAR) mission to show that the absence of these features can constrain keV axion masses into uncharted regions for axion-photon coupling of . Our argument can be applied to other heavy FIPs and astrophysical sources that are hot enough to produce them, yet cold enough to avoid large boost factors which slow down the decay