291 research outputs found
Does electronic coherence enhance anticorrelated pigment vibrations under realistic conditions?
The light-harvesting efficiency of a photoactive molecular complex is largely
determined by the properties of its electronic quantum states. Those, in turn,
are influenced by molecular vibrational states of the nuclear degrees of
freedom. Here, we reexamine two recently formulated concepts that a coherent
vibronic coupling between molecular states would either extend the electronic
coherence lifetime or enhance the amplitude of the anticorrelated vibrational
mode at longer times. For this, we study a vibronically coupled dimer and
calculate the nonlinear two-dimensional (2D) electronic spectra which directly
reveal electronic coherence. The timescale of electronic coherence is initially
extracted by measuring the anti-diagonal bandwidth of the central peak in the
2D spectrum at zero waiting time. Based on the residual analysis, we identify
small-amplitude long-lived oscillations in the cross-peaks, which, however, are
solely due to groundstate vibrational coherence, regardless of having resonant
or off-resonant conditions. Our studies neither show an enhancement of the
electronic quantum coherence nor an enhancement of the anticorrelated
vibrational mode by the vibronic coupling under ambient conditions
Direct laser acceleration of electrons in free-space
Compact laser-driven accelerators are versatile and powerful tools of
unarguable relevance on societal grounds for the diverse purposes of science,
health, security, and technology because they bring enormous practicality to
state-of-the-art achievements of conventional radio-frequency accelerators.
Current benchmarking laser-based technologies rely on a medium to assist the
light-matter interaction, which impose material limitations or strongly
inhomogeneous fields. The advent of few cycle ultra-intense radially polarized
lasers has materialized an extensively studied novel accelerator that adopts
the simplest form of laser acceleration and is unique in requiring no medium to
achieve strong longitudinal energy transfer directly from laser to particle.
Here we present the first observation of direct longitudinal laser acceleration
of non-relativistic electrons that undergo highly-directional multi-GeV/m
accelerating gradients. This demonstration opens a new frontier for direct
laser-driven particle acceleration capable of creating well collimated and
relativistic attosecond electron bunches and x-ray pulses
Nature does not rely on long-lived electronic quantum coherence for photosynthetic energy transfer
During the first steps of photosynthesis, the energy of impinging solar photons is transformed into electronic excitation energy of the light-harvesting biomolecular complexes. The subsequent energy transfer to the reaction center is commonly rationalized in terms of excitons moving on a grid of biomolecular chromophores on typical timescales [Formula: see text]100 fs. Today's understanding of the energy transfer includes the fact that the excitons are delocalized over a few neighboring sites, but the role of quantum coherence is considered as irrelevant for the transfer dynamics because it typically decays within a few tens of femtoseconds. This orthodox picture of incoherent energy transfer between clusters of a few pigments sharing delocalized excitons has been challenged by ultrafast optical spectroscopy experiments with the Fenna-Matthews-Olson protein, in which interference oscillatory signals up to 1.5 ps were reported and interpreted as direct evidence of exceptionally long-lived electronic quantum coherence. Here, we show that the optical 2D photon echo spectra of this complex at ambient temperature in aqueous solution do not provide evidence of any long-lived electronic quantum coherence, but confirm the orthodox view of rapidly decaying electronic quantum coherence on a timescale of 60 fs. Our results can be considered as generic and give no hint that electronic quantum coherence plays any biofunctional role in real photoactive biomolecular complexes. Because in this structurally well-defined protein the distances between bacteriochlorophylls are comparable to those of other light-harvesting complexes, we anticipate that this finding is general and directly applies to even larger photoactive biomolecular complexes
Terahertz-driven linear electron acceleration
The cost, size and availability of electron accelerators is dominated by the
achievable accelerating gradient. Conventional high-brightness radio-frequency
(RF) accelerating structures operate with 30-50 MeV/m gradients. Electron
accelerators driven with optical or infrared sources have demonstrated
accelerating gradients orders of magnitude above that achievable with
conventional RF structures. However, laser-driven wakefield accelerators
require intense femtosecond sources and direct laser-driven accelerators and
suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing
requirements due to the short wavelength of operation. Here, we demonstrate the
first linear acceleration of electrons with keV energy gain using
optically-generated terahertz (THz) pulses. THz-driven accelerating structures
enable high-gradient electron or proton accelerators with simple accelerating
structures, high repetition rates and significant charge per bunch. Increasing
the operational frequency of accelerators into the THz band allows for greatly
increased accelerating gradients due to reduced complications with respect to
breakdown and pulsed heating. Electric fields in the GV/m range have been
achieved in the THz frequency band using all optical methods. With recent
advances in the generation of THz pulses via optical rectification of slightly
sub-picosecond pulses, in particular improvements in conversion efficiency and
multi-cycle pulses, increasing accelerating gradients by two orders of
magnitude over conventional linear accelerators (LINACs) has become a
possibility. These ultra-compact THz accelerators with extremely short electron
bunches hold great potential to have a transformative impact for free electron
lasers, future linear particle colliders, ultra-fast electron diffraction,
x-ray science, and medical therapy with x-rays and electron beams
Exploring vibrational ladder climbing in vibronic coupling models: Toward experimental observation of a geometric phase signature of a conical intersection
Conical intersections (CIs) have been widely studied using spectroscopic
techniques. However, CIs have mainly been identified by rapid internal
conversion transitions that take place after the photoexcitation. Such
identifications cannot distinguish various types of intersections as well as to
separate the actual intersection from an avoided crossing. In this paper, we
investigate how ultrafast IR laser pulses can be utilized to stimulate nuclear
dynamics revealing geometric phase features associated with CIs. We consider
two low-dimensional nonadiabatic models to obtain optimal two- and three-pulse
laser sequences for stimulating nuclear dynamics necessary for the CI
identification. Our results provide insights on designing non-linear
spectroscopic schemes for subsequent probes of the nuclear wavepackets by
ultrafast electron diffraction techniques to unambiguously detect CIs in
molecules
Dietary Antioxidants in the Chemoprevention of Prostate Cancer
Prostate cancer is the second most common cancer and the fifth leading cause of cancer death. The incidence of prostate cancer is rising due to increased screening and awareness, and there is epidemiological evidence suggesting an interaction among biological and environmental risk factors in the development and progression of prostate cancer. Vegetables and fruits provide a wide range of antioxidants and phytochemicals that have been demonstrated to have a negative, positive, or no association with prostate cancer risk. Therefore, it is evident that the effect of dietary antioxidants on risk of prostate cancer remains undecided and inconclusive. The main focus of this review was to examine recent and past literature of the chemoprotective properties of five major groups of phytochemicals against prostate cancer development including both in vivo and in vitro findings
Novel applications of Generative Adversarial Networks (GANs) and Convolutional Neural Networks (CNNs) in the analysis of ultrafast electron diffraction (UED) images
We employ generative adversarial networks (GANs) and convolutional neural
networks (CNNs) in the study of ultrafast electron diffraction images. We
propose a machine learning approach that employs a GAN to convert experimental
images into idealized diffraction patterns from which information is extracted
via a CNN trained on synthetic data only. We validate this approach on
ultrafast electron diffraction (UED) data of bismuth samples undergoing
thermalization upon excitation via 800 nm laser pulses. The network was able to
predict transient temperatures with a deviation of less than 6% from
analytically estimated values. Notably, this performance was achieved on a
dataset of 408 images only. We believe employing this network in experimental
settings where high volumes of visual data are collected, such as beam lines,
could provide insights into the structural dynamics of different samples
Unraveling Quantum Coherences Mediating Primary Charge Transfer Processes in Photosystem II Reaction Center
Photosystem II (PSII) reaction center is a unique protein-chromophore complex
that is capable of efficiently separating electronic charges across the
membrane after photoexcitation. In the PSII reaction center, the primary
energy- and charge-transfer (CT) processes occur on comparable ultrafast
timescales, which makes it extremely challenging to understand the fundamental
mechanism responsible for the near-unity quantum efficiency of the transfer.
Here, we elucidate the role of quantum coherences in the ultrafast energy and
CT in the PSII reaction center by performing two-dimensional (2D) electronic
spectroscopy at the cryogenic temperature of 20 K, which captures the distinct
underlying quantum coherences. Specifically, we uncover the electronic and
vibrational coherences along with their lifetimes during the primary ultrafast
processes of energy and CT. We also examine the functional role of the observed
quantum coherences. To gather further insight, we construct a structure-based
excitonic model that provided evidence for coherent energy and CT at low
temperature in the 2D electronic spectra. The principles, uncovered by this
combination of experimental and theoretical analyses, could provide valuable
guidelines for creating artificial photosystems with exploitation of
system-bath coupling and control of coherences to optimize the photon
conversion efficiency to specific functions
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