62 research outputs found
Sustainable early-stage lasing in a low-emittance electron storage ring
In this Letter, we report on the concept and analysis of a low-emittance
electron storage ring, in which the electron beams undergo an early-stage
self-amplified spontaneous emission lasing process on a turn-by-turn basis. The
lasing process for each pass through a long undulator in the ring is terminated
when the radiated power is still negligible compared to the total synchrotron
loss of each circulation, and the electron beams can be maintained in an
equilibrium state that supports sustainable lasing. A self-consistent model is
derived for evaluation of the properties of the electron beams, and a design
with numerical modeling is presented that demonstrates the feasibility of
generating short-wavelength radiation at the kW power level
Diffractive imaging of dissociation and ground state dynamics in a complex molecule
We have investigated the structural dynamics in photoexcited
1,2-diiodotetrafluoroethane molecules (C2F4I2) in the gas phase experimentally
using ultrafast electron diffraction and theoretically using FOMO-CASCI excited
state dynamics simulations. The molecules are excited by an ultra-violet
femtosecond laser pulse to a state characterized by a transition from the
iodine 5p orbital to a mixed 5p|| hole and CF2 antibonding orbital, which
results in the cleavage of one of the carbon-iodine bonds. We have observed,
with sub-Angstrom resolution, the motion of the nuclear wavepacket of the
dissociating iodine atom followed by coherent vibrations in the electronic
ground state of the C2F4I radical. The radical reaches a stable classical
(non-bridged) structure in less than 200 fs.Comment: 13 pages, 11 figure
Ultrafast manipulation of mirror domain walls in a charge density wave
Domain walls (DWs) are singularities in an ordered medium that often host
exotic phenomena such as charge ordering, insulator-metal transition, or
superconductivity. The ability to locally write and erase DWs is highly
desirable, as it allows one to design material functionality by patterning DWs
in specific configurations. We demonstrate such capability at room temperature
in a charge density wave (CDW), a macroscopic condensate of electrons and
phonons, in ultrathin 1T-TaS. A single femtosecond light pulse is shown to
locally inject or remove mirror DWs in the CDW condensate, with probabilities
tunable by pulse energy and temperature. Using time-resolved electron
diffraction, we are able to simultaneously track anti-synchronized CDW
amplitude oscillations from both the lattice and the condensate, where
photo-injected DWs lead to a red-shifted frequency. Our demonstration of
reversible DW manipulation may pave new ways for engineering correlated
material systems with light
Concurrent probing of electron-lattice dephasing induced by photoexcitation in 1T-TaSeTe using ultrafast electron diffraction
It has been technically challenging to concurrently probe the electrons and
the lattices in materials during non-equilibrium processes, allowing their
correlations to be determined. Here, in a single set of ultrafast electron
diffraction patterns taken on the charge-density-wave (CDW) material 1T-TaSeTe,
we discover a temporal shift in the diffraction intensity measurements as a
function of scattering angle. With the help of dynamic models and theoretical
calculations, we show that the ultrafast electrons probe both the
valence-electron and lattice dynamic processes, resulting in the temporal shift
measurements. Our results demonstrate unambiguously that the CDW is not merely
a result of the periodic lattice deformation ever-present in 1T-TaSeTe but has
significant electronic origin. This method demonstrates a novel approach for
studying many quantum effects that arise from electron-lattice dephasing in
molecules and crystals for next-generation devices.Comment: 13 pages and 4 figures in main tex
Investigating dissociation pathways of nitrobenzene via mega-electron-volt ultrafast electron diffraction
As the simplest nitroaromatic compound, nitrobenzene is an interesting model
system to explore the rich photochemistry of nitroaromatic compounds. Previous
measurements of nitrobenzene's photochemical dynamics have probed structural
and electronic properties, which, at times, paint a convoluted and sometimes
contradictory description of the photochemical landscape. A sub-picosecond
structural probe can complement previous electronic measurements and aid in
determining the photochemical dynamics with less ambiguity. We investigate the
ultrafast dynamics of nitrobenzene triggered by photoexcitation at 267 nm
employing megaelectronvolt ultrafast electron diffraction with femtosecond time
resolution. We measure the first 5 ps of dynamics and, by comparing our
measured results to simulation, we unambiguously distinguish the lowest singlet
and triplet electronic states. We observe ground state recovery within 160 +/-
60 fs through internal conversions and without signal corresponding to
photofragmentation. Our lack of dissociation signal within the first 5 ps
indicates that previously observed photofragmenation reactions take place in
the vibrationally "hot" ground state on timescales considerably beyond 5 ps.Comment: 5 pages, 3 figures, and 1 tabl
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