173 research outputs found
Proposed imaging of the ultrafast electronic motion in samples using x-ray phase-contrast
Tracing the motion of electrons has enormous relevance to understanding
ubiquitous phenomena in ultrafast science, such as the dynamical evolution of
the electron density during complex chemical and biological processes.
Scattering of ultrashort x-ray pulses from an electronic wavepacket would
appear to be the most obvious approach to image the electronic motion in
real-time and real-space with the notion that such scattering patterns, in the
far-field regime, encode the instantaneous electron density of the wavepacket.
However, recent results by Dixit {\em et al.} [Proc. Natl. Acad. Sci. U.S.A.,
{\bf 109}, 11636 (2012)] have put this notion into question and shown that the
scattering in the far-field regime probes spatio-temporal density-density
correlations. Here, we propose a possible way to image the instantaneous
electron density of the wavepacket via ultrafast x-ray {\em phase contrast
imaging}. Moreover, we show that inelastic scattering processes, which plague
ultrafast scattering in the far-field regime, do not contribute in ultrafast
x-ray phase contrast imaging as a consequence of an interference effect. We
illustrate our general findings by means of a wavepacket that lies in the time
and energy range of the dynamics of valence electrons in complex molecular and
biological systems. This present work offers a potential to image not only
instantaneous snapshots of non-stationary electron dynamics, but also the
Laplacian of these snapshots which provide information about the complex
bonding and topology of the charge distributions in the systems.Comment: 12 pages, 2 figures, article accepted in Physical Review Letters
(2013
Tailoring Photocurrent in Weyl Semimetals via Intense Laser Irradiation
Generating and tailoring photocurrent in topological materials has immense
importance in fundamental studies and the technological front. Present work
introduces a universal method to generate ultrafast photocurrent in {\it both}
inversion-symmetric and inversion-broken Weyl semimetals with degenerate Weyl
nodes at the Fermi level. Our approach harnesses the asymmetric electronic
population in the conduction band induced by an intense {\it single-color}
circularly polarized laser pulse. It has been found that the induced
photocurrent can be tailored by manipulating helicity and ellipticity of the
employed laser. Moreover, our approach generates photocurrent in realistic
situations when the Weyl nodes are positioned at different energies and have
finite tilt along a certain direction. Present work adds a new dimension on
practical applications of Weyl semimetals for optoelectronics and
photonics-based quantum technologies.Comment: 13 pages, 5 figure
Theoretical spectroscopic studies of the atomic transitions and lifetimes of low-lying states in Ti IV
The astrophysically important electric quadrupole (E2) and magnetic dipole
(M1) transitions for the low-lying states of triply ionized titanium (Ti IV)
are calculated very accurately using a state-of-art all-order many-body theory
called Coupled Cluster (CC) theory in the relativistic frame-work. Different
many-body correlations of the CC theory has been estimated by studying the core
and valence electron excitations to the unoccupied states. The calculated
excitation energies of different states are in very good agreement with the
measurements. Also we compare our calculated electric dipole (E1) transition
amplitudes of few transitions with recent many-body calculations by different
groups. We have also carried out the calculations for the lifetimes of the
low-lying states of Ti IV. A long lifetime is found for the first excited
3d state, which suggested that Ti IV may be one of the useful
candidates for many important studies. Most of the results reported here are
not available in the literature, to the best of our knowledge.Comment: 15 pages submitted to J. Phys.
X-ray imaging of chemically active valence electrons during a pericyclic reaction
Time-resolved imaging of chemically active valence electron densities is a
long sought goal, as these electrons dictate the course of chemical reactions.
However, x-ray scattering is always dominated by the core and inert valence
electrons, making time-resolved x-ray imaging of chemically active valence
electron densities extremely challenging. To image such electron densities, we
demonstrate an effective and robust method, which emphasizes the information
encoded in weakly scattered photons. The degenerate Cope rearrangement of
semibullvalene, a pericyclic reaction, is used as an example to visually
illustrate our approach. Our work also provides experimental access to the
long-standing problem of synchronous versus asynchronous bond formation and
breaking during pericyclic reactions.Comment: 19 pages, 3 figures, comments are most welcom
- …