1,430 research outputs found
Ultrafast Molecular Imaging by Laser Induced Electron Diffraction
We address the feasibility of imaging geometric and orbital structure of a
polyatomic molecule on an attosecond time-scale using the laser induced
electron diffraction (LIED) technique. We present numerical results for the
highest molecular orbitals of the CO2 molecule excited by a near infrared
few-cycle laser pulse. The molecular geometry (bond-lengths) is determined
within 3% of accuracy from a diffraction pattern which also reflects the nodal
properties of the initial molecular orbital. Robustness of the structure
determination is discussed with respect to vibrational and rotational motions
with a complete interpretation of the laser-induced mechanisms
Laser induced electron diffraction: a tool for molecular orbital imaging
We explore the laser-induced ionization dynamics of N2 and CO2 molecules
subjected to a few-cycle, linearly polarized, 800\,nm laser pulse using
effective two-dimensional single active electron time-dependent quantum
simulations. We show that the electron recollision process taking place after
an initial tunnel ionization stage results in quantum interference patterns in
the energy resolved photo-electron signals. If the molecule is initially
aligned perpendicular to the field polarization, the position and relative
heights of the associated fringes can be related to the molecular geometrical
and orbital structure, using a simple inversion algorithm which takes into
account the symmetry of the initial molecular orbital from which the ionized
electron is produced. We show that it is possible to extract inter-atomic
distances in the molecule from an averaged photon-electron signal with an
accuracy of a few percents
Theoretical study of a cold atom beam splitter
A theoretical model is presented for the study of the dynamics of a cold
atomic cloud falling in the gravity field in the presence of two crossing
dipole guides. The cloud is split between the two branches of this laser guide,
and we compare experimental measurements of the splitting efficiency with
semiclassical simulations. We then explore the possibilities of optimization of
this beam splitter. Our numerical study also gives access to detailed
information, such as the atom temperature after the splitting
Conceptually driven and visually rich tasks in texts and teaching practice: the case of infinite series
The study we report here examines parts of what Chevallard calls the institutional dimension of the students’ learning experience of a relatively under-researched, yet crucial, concept in Analysis, the concept of infinite series. In particular, we examine how the concept is introduced to students in texts and in teaching practice. To this purpose, we employ Duval's Theory of Registers of Semiotic Representation towards the analysis of 22 texts used in Canada and UK post-compulsory courses. We also draw on interviews with in-service teachers and university lecturers in order to discuss briefly teaching practice and some of their teaching suggestions. Our analysis of the texts highlights that the presentation of the concept is largely a-historical, with few graphical representations, few opportunities to work across different registers (algebraic, graphical, verbal), few applications or intra-mathematical references to the concept's significance and few conceptually driven tasks that go beyond practising with the application of convergence tests and prepare students for the complex topics in which the concept of series is implicated. Our preliminary analysis of the teacher interviews suggests that pedagogical practice often reflects the tendencies in the texts. Furthermore, the interviews with the university lecturers point at the pedagogical potential of: illustrative examples and evocative visual representations in teaching; and, student engagement with systematic guesswork and writing explanatory accounts of their choices and applications of convergence tests
A hybrid metal/semiconductor electron pump for quantum metrology
Electron pumps capable of delivering a current higher than 100pA with
sufficient accuracy are likely to become the direct mise en pratique of the
possible new quantum definition of the ampere. Furthermore, they are essential
for closing the quantum metrological triangle experiment which tests for
possible corrections to the quantum relations linking e and h, the electron
charge and the Planck constant, to voltage, resistance and current. We present
here single-island hybrid metal/semiconductor transistor pumps which combine
the simplicity and efficiency of Coulomb blockade in metals with the
unsurpassed performances of silicon switches. Robust and simple pumping at
650MHz and 0.5K is demonstrated. The pumped current obtained over a voltage
bias range of 1.4mV corresponds to a relative deviation of 5e-4 from the
calculated value, well within the 1.5e-3 uncertainty of the measurement setup.
Multi-charge pumping can be performed. The simple design fully integrated in an
industrial CMOS process makes it an ideal candidate for national measurement
institutes to realize and share a future quantum ampere
Two-Color Coherent Photodissociation of Nitrogen Oxide in Intense Laser Fields
A simple one-dimensional semi-classical model with a Morse potential is used
to investigate the possibility of two-color infrared multi-photon dissociation
of vibrationally excited nitrogen oxide. The amplitude ratio effects and
adiabatic effects are investigated. Some initial states are found to have
thresholds smaller than expected from single-mode considerations and multiple
thresholds exist for initial states up to 32.
PACS: 42.50.HzComment: 3 pages, old papers, add source files to replace original postscrip
Theoretical analysis of the implementation of a quantum phase gate with neutral atoms on atom chips
We present a detailed, realistic analysis of the implementation of a proposal
for a quantum phase gate based on atomic vibrational states, specializing it to
neutral rubidium atoms on atom chips. We show how to create a double--well
potential with static currents on the atom chips, using for all relevant
parameters values that are achieved with present technology. The potential
barrier between the two wells can be modified by varying the currents in order
to realize a quantum phase gate for qubit states encoded in the atomic external
degree of freedom. The gate performance is analyzed through numerical
simulations; the operation time is ~10 ms with a performance fidelity above
99.9%. For storage of the state between the operations the qubit state can be
transferred efficiently via Raman transitions to two hyperfine states, where
its decoherence is strongly inhibited. In addition we discuss the limits
imposed by the proximity of the surface to the gate fidelity.Comment: 9 pages, 5 color figure
A comparison of the renal effects (ERPF, GFR, and FF) of FK 506 and cyclosporine in patients with liver transplantation
1. The mean ''cost'' in milliliters per minute of ESLD alone, prior to transplantation, was 35% + 23% (1 SD). In GFR it was 15%. 2. The additional burden of CyA + OLT increases the loss in ERPF an additional 18%; in GFR, it increases loss another 10%. Thus, the total loss in CyA-treated patients was 53% and 25%, respectively. 3. The decrease imposed by FK 506 + OLT on ERPF was only 7%, with no decrease in GFR. 4. Therefore, from the renal point of view, FK 506 would appear to be the superior drug. 5. The large error around mean values underlines the desirability of performing these tests on the individual patient rather than on information from groups, since many values fall near the threshold of the azotemic range (ERPF approximately 175 mL/min). 6. As renal mass was compromised, ie, fall in the ERPF, the GFR increased relatively, ie, the renal filtering membrane became more permeable and the FFs gradually increased. 7. The loss of renal function was significantly less in OLT patients on FK 506 than CyA. However, the greatest loss in expected renal function was due to the basic ESLD itself
Coherent Control of Isotope Separation in HD+ Photodissociation by Strong Fields
The photodissociation of the HD+ molecular ion in intense short- pulsed
linearly polarized laser fields is studied using a time- dependent wave-packet
approach where molecular rotation is fully included. We show that applying a
coherent superposition of the fundamental radiation with its second harmonic
can lead to asymmetries in the fragment angular distributions, with significant
differences between the hydrogen and deuterium distributions in the long
wavelength domain where the permanent dipole is most efficient. This effect is
used to induce an appreciable isotope separation.Comment: Physical Review Letters, 1995 (in press). 4 pages in revtex format, 3
uuencoded figures. Full postcript version available at:
http://chemphys.weizmann.ac.il/~charron/prl.ps or
ftp://scipion.ppm.u-psud.fr/coherent.control/prl.p
- …