6,243 research outputs found
Isolating quantum coherence with pathway-selective coherent multi-dimensional spectroscopy
Coherent coupling between spatially separated systems has long been explored
as a necessary requirement for quantum information and cryptography. Recent
discoveries suggest such phenomena appear in a much wider range of processes,
including light-harvesting in photosynthesis. These discoveries have been
facilitated by developments in coherent multi-dimensional spectroscopy (CMDS)
that allow interactions between different electronic states to be identified in
crowded spectra. For complex systems, however, spectral broadening and multiple
overlapping peaks limit the ability to separate, identify and properly analyse
all contributions. Here we demonstrate how pathway-selective CMDS can overcome
these limitations to reveal, isolate and allow detailed analysis of weak
coherent coupling between spatially separated excitons localised to different
semiconductor quantum wells. Selective excitation of the coherence pathways, by
spectrally shaping the laser pulses, provides access to previously hidden
details and enables quantitative analysis that can facilitate precise and
detailed understanding of interactions in this and other complex systems
Time resolved structural dynamics of butadiyne-linked porphyrin dimers
In this work the timescales and mechanisms associated with the structural dynamics of butadiyne-linked porphyrin dimers are investigated through time resolved narrowband pump / broadband probe transient absorption spectroscopy. Our results confirm previous findings that the broadening is partly due to a distribution of structures with different (dihedral) angular conformations. Comparison of measurements with excitations on the red and blue sides of the Q-band unravel the ground and excited state conformational re-equilibration timescales. Further comparison to a planarized dimer, through addition of a ligand, provide conclusive evidence for the twisting motion performed by the porphyrin dimer in solution
A Consistent Dark Matter Interpretation For CoGeNT and DAMA/LIBRA
In this paper, we study the recent excess of low energy events observed by
the CoGeNT collaboration and the annual modulation reported by the DAMA/LIBRA
collaboration, and discuss whether these signals could both be the result of
the same elastically scattering dark matter particle. We find that, without
channeling but when taking into account uncertainties in the relevant quenching
factors, a dark matter candidate with a mass of approximately ~7.0 GeV and a
cross section with nucleons of sigma_{DM-N} ~2x10^-4 pb (2x10^-40 cm^2) could
account for both of these observations. We also comment on the events recently
observed in the oxygen band of the CRESST experiment and point out that these
could potentially be explained by such a particle. Lastly, we compare the
region of parameter space favored by DAMA/LIBRA and CoGeNT to the constraints
from XENON 10, XENON 100, and CDMS (Si) and find that these experiments cannot
at this time rule out a dark matter interpretation of these signals.Comment: 8 pages, 6 figure
The Effect of Isotropic Pressure on the Electronic Structure and Superatomic Orbitals of Molecular [Ag44(SPhCOOH)30]4-, [Ag44(SPhF2)30]4- & [Ag25(SPhMe2)18]- Nanoclusters
Mapping the Excited‐State Potential Energy Surface of a Photomolecular Motor
A detailed understanding of the operation and efficiency of unidirectional photomolecular rotary motors is essential for their effective exploitation in molecular nanomachines. Unidirectional motion relies on light‐driven conversion from a stable (1 a) to a metastable (1 b) conformation, which then relaxes through a thermally driven helix inversion in the ground state. The excited‐state surface has thus far only been experimentally characterised for 1 a. Here we probe the metastable, 1 b, excited state, utilising ultrafast transient absorption and femtosecond stimulated Raman spectroscopy. These reveal that the “dark” excited‐state intermediate between 1 a and 1 b has a different lifetime and structure depending on the initial ground‐state conformation excited. This suggests that the reaction coordinate connecting 1 a to 1 b differs to that for the reverse photochemical process. The result is contrasted with earlier calculations
Ultrafast Structure and Dynamics in the Thermally Activated Delayed Fluorescence of a Carbene-Metal-Amide
Thermally activated delayed fluorescence has enormous potential for the development of efficient light emitting diodes. A recently discovered class of molecules (the carbene – metal – amides, CMAs) are exceptionally promising as they combine the small singlet - triplet energy gap required for thermal activation with a large transition moment for emission. Calculations suggest that excited state structural dynamics modulate the critical coupling between singlet and triplet states, but do not agree on the nature of those dynamics. Here we report ultrafast time resolved transient absorption and Raman studies of CMA photodynamics. The measurements reveal complex structural evolution following intersystem crossing on the tens to hundreds of picoseconds timescale, and a change in the low frequency vibrational spectrum between singlet and triplet states. The latter is assigned to a change in frequency or amplitude associated with a Raman active mode localized on the metal centre
Femtosecond pulses at 50-W average power from an Yb:YAG planar waveguide amplifier seeded by an Yb:KYW oscillator
Control of polymorphism in coronene by the application of magnetic fields
Coronene, a polyaromatic hydrocarbon, has been crystallized for the first
time in a different polymorph using a crystal growth method that utilizes
magnetic fields to access a unit cell configuration that was hitherto unknown.
Crystals grown in magnetic field of 1 T are larger, have a different appearance
to those grown in zero field and retain their structure in ambient conditions.
We identify the new form, beta-coronene, as the most stable at low
temperatures. As a result of the new supramolecular configuration we report
significantly altered electronic, optical and mechanical properties.Comment: 32 pages, 17 figure
An introduction to communicating science
It is becoming increasingly recognised that students in Higher Education must acquire the skills necessary for professional and personal development, as well as for academic progress. The media have recently focused on the issue of declining public interest in the sciences and the lack of accurate reporting of science. We have developed a new programme, which endeavours to address both issues involving a three day intensive course covering writing, TV and radio. In addition to the targeted activities of learning the skills of science communication, the programme encourages partnerships, and exploits the resources and expertise available from various institutions. The undertaking of this type of programme is not limited to the acquisition of time slots in a studio such as Bush House. Most university campuses are now home to their own recording studios and even have television facilities. However, the programme requires only a video camera and audio recording equipment. The success of this science communication module and oftwo others run by MOAC and CBC (Team Development and Decision-making and Leadership) has encouraged us to develop a complete postgraduate certificate in transferable skills. We anticipate the certificate will be a valuable vehicle for consolidating and enhancing the training discussed in this article
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