203 research outputs found
Spin Coupling Effect on Geometry-Dependent X-ray Absorption of Diradicals
We theoretically investigate the influence of diradical electron spin
coupling on the time-resolved X-ray absorption spectra of the photochemical
ring opening of furanone. We predict geometry dependent carbon K-edge signals
involving transitions from core orbitals to both singly and unoccupied
molecular orbitals. The most obvious features of the ring opening come from the
carbon atom directly involved in the bond breaking, through its transition to
both the newly formed SOMO and the available LUMO state. In addition to this
primary feature, the singlet spin coupling of four unpaired electrons that
arises in the core-to-LUMO states creates additional geometry dependence in
some spectral features, with both oscillator strengths and relative excitation
energies varying observably as a function of the ring opening. We attribute
this behavior to a spin-occupancy-induced selection rule, which occurs when
singlet spin coupling is enforced in the diradical state. Notably, one of these
geometry-sensitive core-to-LUMO transitions excites core electrons from a
backbone carbon not involved in the bond breaking, providing a novel non-local
X-ray probe of chemical dynamics arising from electron spin coupling.Comment: 52 Pages, 13 Figure
On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium
We consider the dissipation by Fermi acceleration of magnetosonic turbulence
in the Reynolds Layer of the interstellar medium. The scale in the cascade at
which electron acceleration via stochastic Fermi acceleration (STFA) becomes
comparable to further cascade of the turbulence defines the inner scale. For
any magnetic turbulent spectra equal to or shallower than Goldreich-Sridhar
this turns out to be cm, which is much larger than the shortest
length scales observed in radio scintillation measurements. While STFA for such
spectra then contradict models of scintillation which appeal directly to an
extended, continuous turbulent cascade, such a separation of scales is
consistent with the recent work of \citet{Boldyrev2} and \citet{Boldyrev3}
suggesting that interstellar scintillation may result from the passage of radio
waves through the galactic distribution of thin ionized boundary surfaces of
HII regions, rather than density variations from cascading turbulence. The
presence of STFA dissipation also provides a mechanism for the non-ionizing
heat source observed in the Reynolds Layer of the interstellar medium
\citep{Reynolds}. STFA accommodates the proper heating power, and the input
energy is rapidly thermalized within the low density Reynolds layer plasma.Comment: 12 Pages, no figures. Accepted for publication in MNRA
Ultrafast X-ray Spectroscopy of Intersystem Crossing in Hexafluoroacetylacetone: Chromophore Photophysics and Spectral Changes in the Face of Electron Withdrawing Groups
Intersystem crossings between singlet and triplet states represent a crucial
relaxation pathway in photochemical processes. Herein, we probe the intersystem
crossing in hexafluoro-acetylacetone with ultrafast X-ray transient absorption
spectroscopy at the carbon K-edge. We observe the excited state dynamics
following excitation with 266 nm UV light to the (S) state
with element and site-specificity using a broadband soft X-ray pulse produced
by high harmonic generation. These results are compared to X-ray spectra
computed from orbital optimized density functional theory methods. It is found
that the electron withdrawing fluorine atoms decongest the X-ray absorption
spectrum by enhancing separation between features originating from different
carbon atoms. This facilitates the elucidation of structural and electronic
dynamics at the chromophore. The evolution of the core-to-valence resonances at
the carbon K-edge reveals an ultrafast population transfer between the
(S) and (T) states on a ps
timescale, which is similar to the 1.5 ps timescale earlier observed for
acetylacetone [J. Am. Chem. Soc. 139, 16576 (2017)]. It therefore appears that
terminal fluorination has little influence on the intersystem crossing rate of
the acetylacetone chromophore. In addition, the significant role of
hydrogen-bond opened and twisted rotational isomers is elucidated in the
excited state dynamics by comparison of the experimental transient X-ray
spectra with theory
Accurate prediction of core-level spectra of radicals at density functional theory cost via square gradient minimization and recoupling of mixed configurations
State-specific orbital optimized approaches are more accurate at predicting
core-level spectra than traditional linear-response protocols, but their
utility had been restricted on account of the risk of `variational collapse'
down to the ground state. We employ the recently developed square gradient
minimization (SGM, J. Chem. Theory Comput. 16, 1699-1710, 2020) algorithm to
reliably avoid variational collapse and study the effectiveness of orbital
optimized density functional theory (DFT) at predicting second period element
1s core-level spectra of open-shell systems. Several density functionals
(including SCAN, B3LYP and B97X-D3) are found to predict excitation
energies from the core to singly occupied levels to high accuracy ( eV
RMS error), against available experimental data. Higher excited states are
however more challenging by virtue of being intrinsically multiconfigurational.
We thus present a CI inspired route to self-consistently recouple single
determinant mixed configurations obtained from DFT, in order to obtain
approximate doublet states. This recoupling scheme is used to predict the C
K-edge spectra of the allyl radical, the O K-edge spectra of CO and the N
K-edge of NO to high accuracy relative to experiment, indicating
substantial promise in using this approach for computation of core-level
spectra for doublet species (vs more traditional time dependent DFT, EOM-CCSD
or using unrecoupled mixed configurations). We also present general guidelines
for computing core-excited states from orbital optimized DFT.Comment: Added more dat
Femtosecond Symmetry Breaking and Coherent Relaxation of Methane Cations at the Carbon K-Edge
Understanding the relaxation pathways of photoexcited molecules is essential
to gain atomistic level insight into photochemistry. Herein, we perform a
time-resolved study of ultrafast molecular symmetry breaking via geometric
relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient
absorption spectroscopy with soft X-rays at the carbon K-edge reveals that the
distortion occurs within femtoseconds after few-femtosecond
strong-field ionization of methane. The distortion activates coherent
oscillations in the scissoring vibrational mode of the symmetry broken cation,
which are detected in the X-ray signal. These oscillations are damped within
femtoseconds, as vibrational coherence is lost with the energy
redistributing into lower-frequency vibrational modes. This study completely
reconstructs the molecular relaxation dynamics of this prototypical example and
opens new avenues for exploring complex systems
Laboratory Plasma Dynamos, Astrophysical Dynamos, and Magnetic Helicity Evolution
The term ``dynamo'' means different things to the laboratory fusion plasma
and astrophysical plasma communities. To alleviate the resulting confusion and
to facilitate interdisciplinary progress, we pinpoint conceptual differences
and similarities between laboratory plasma dynamos and astrophysical dynamos.
We can divide dynamos into three types: 1. magnetically dominated helical
dynamos which sustain a large scale magnetic field against resistive decay and
drive the magnetic geometry toward the lowest energy state, 2. flow-driven
helical dynamos which amplify or sustain large scale magnetic fields in an
otherwise turbulent flow, and 3. flow-driven nonhelical dynamos which amplify
fields on scales at or below the driving turbulence. We discuss how all three
types occur in astrophysics whereas plasma confinement device dynamos are of
the first type. Type 3 dynamos requires no magnetic or kinetic helicity of any
kind. Focusing on type 1 and 2 dynamos, we show how different limits of a
unified set of equations for magnetic helicity evolution reveal both types. We
explicitly describe a steady-state example of a type 1 dynamo, and three
examples of type 2 dynamos: (i) closed volume and time dependent; (ii)
steady-state with open boundaries; (iii) time dependent with open boundaries.Comment: accepted by MNRA
Jahn-Teller Distortion and Dissociation of CCl by Transient X-ray Spectroscopy Simultaneously at the Carbon K- and Chlorine L-Edge
X-ray Transient Absorption Spectroscopy (XTAS) and theoretical calculations
are used to study CCl prepared by 800 nm strong-field ionization. XTAS
simultaneously probes atoms at the carbon K-edge (280-300 eV) and chlorine
L-edge (195-220 eV). Comparison of experiment to X-ray spectra computed by
orbital-optimized density functional theory (OO-DFT) indicates that after
ionization, CCl undergoes symmetry breaking driven by Jahn-Teller
distortion away from the initial tetrahedral structure (T) in 62 fs.
The resultant symmetry-broken covalently bonded form subsequently separates to
a noncovalently bound complex between CCl and Cl over 9010 fs, which
is again predicted by theory. Finally, after more than 800 fs, L-edge signals
for atomic Cl are observed, indicating dissociation to free CCl and Cl.
The results for Jahn-Teller distortion to the symmetry-broken form of CCl
and formation of the Cl -- CCl complex characterize previously unobserved
new species along the route to dissociation
Recommended from our members
Reduced local mutation density in regulatory DNA of cancer genomes is linked to DNA repair
Carcinogenesis and neoplastic progression are mediated by the accumulation of somatic mutations. Here we report that the local density of somatic mutations in cancer genomes is highly reduced specifically in accessible regulatory DNA defined by DNase I hypersensitive sites. This reduction is independent of any known factors influencing somatic mutation density and is observed in diverse cancer types, suggesting a general mechanism. By analyzing individual cancer genomes1, we show that the reduced local mutation density within regulatory DNA is linked to intact global genome repair machinery, with nearly complete abrogation of the hypomutation phenomenon in individual cancers that possess mutations in multiple nucleotide excision repair components. Together, our results connect chromatin structure, gene regulation and cancer-associated somatic mutation
The Human Mitochondrial Transcriptome
SummaryThe human mitochondrial genome comprises a distinct genetic system transcribed as precursor polycistronic transcripts that are subsequently cleaved to generate individual mRNAs, tRNAs, and rRNAs. Here, we provide a comprehensive analysis of the human mitochondrial transcriptome across multiple cell lines and tissues. Using directional deep sequencing and parallel analysis of RNA ends, we demonstrate wide variation in mitochondrial transcript abundance and precisely resolve transcript processing and maturation events. We identify previously undescribed transcripts, including small RNAs, and observe the enrichment of several nuclear RNAs in mitochondria. Using high-throughput in vivo DNaseI footprinting, we establish the global profile of DNA-binding protein occupancy across the mitochondrial genome at single-nucleotide resolution, revealing regulatory features at mitochondrial transcription initiation sites and functional insights into disease-associated variants. This integrated analysis of the mitochondrial transcriptome reveals unexpected complexity in the regulation, expression, and processing of mitochondrial RNA and provides a resource for future studies of mitochondrial function (accessed at http://mitochondria.matticklab.com)
- …