66 research outputs found
A kinetic Monte Carlo study of desorption of H2 from graphite (0001)
The formation of H2 in the interstellar medium proceeds on the surfaces of
silicate or carbonaceous particles. To get a deeper insight of its formation on
the latter substrate, this letter focuses on H2 desorption from graphite (0001)
in Temperature-Programmed-Desorption Monte-Carlo simulations. The results are
compared to experimental results which show two main peaks and an intermediate
shoulder for high initial coverage. The simulation program includes barriers
obtained by ab-initio methods and is further optimised to match two independent
experimental observations. The simulations reproduce the two experimental
observed desorption peaks. Additionally, a possible origin of the intermediate
peak is given.Comment: 9 pages, 5 figures, Chem. Phys. Lett. in pres
Adsorption and STM imaging of polycyclic aromatic hydrocarbons on graphene
International audienceThe structural characterization of polycyclic aromatic hydrocarbon molecules adsorbed on graphene is of fundamental importance in view of the use of graphene or graphene nanoribbons for electronic applications. Before reaching this point, one has to determine the structure of the adsorbed molecules. Here, we study the case of benzene, coronene, and hexabenzocoronene on a graphene layer. First, the adsorption properties of single molecules are calculated using first-principles calculations at the level of density functional theory. We benefit from a recent scheme, particularly adapted for weakly adsorbed molecules, allowing us to precisely calculate the van der Waals contribution. Then, scanning tunneling microscopy (STM) is used to produce images of self-assembled molecules comparing different theoretical approaches to experimental observations. Finally, we consider the imaging of isolated molecules, and we show how the STM tip influences the molecule position by soft mechanical interaction during the scanning process
Mobility of D atoms on porous amorphous water ice surfaces under interstellar conditions
Aims. The mobility of H atoms on the surface of interstellar dust grains at
low temperature is still a matter of debate. In dense clouds, the hydrogenation
of adsorbed species (i.e., CO), as well as the subsequent deuteration of the
accreted molecules depend on the mobility of H atoms on water ice.
Astrochemical models widely assume that H atoms are mobile on the surface of
dust grains even if controversy still exists. We present here direct
experimental evidence of the mobility of H atoms on porous water ice surfaces
at 10 K. Methods. In a UHV chamber, O2 is deposited on a porous amorphous water
ice substrate. Then D atoms are deposited onto the surface held at 10 K.
Temperature-Programmed Desorption (TPD) is used and desorptions of O2 and D2
are simultaneously monitored. Results. We find that the amount of O2 that
desorb during the TPD diminishes if we increase the deposition time of D atoms.
O2 is thus destroyed by D atoms even though these molecules have previously
diffused inside the pores of thick water ice. Our results can be easily
interpreted if D is mobile at 10 K on the water ice surface. A simple rate
equation model fits our experimental data and best fit curves were obtained for
a D atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can take
place efficiently on interstellar dust grains. These experimental results are
in line with most calculations and validate the hypothesis used in several
models.Comment: 4 pages (Submitted to A&A
Spatially-resolved potential measurement with ion crystals
We present a method to measure potentials over an extended region using
one-dimensional ion crystals in a radio frequency (RF) ion trap. The
equilibrium spacings of the ions within the crystal allow the determination of
the external forces acting at each point. From this the overall potential, and
also potentials due to specific trap features, are calculated. The method can
be used to probe potentials near proximal objects in real time, and can be
generalized to higher dimensions.Comment: 7 pages (double spaced), 3 figure
Ab initio simulations of the kinetic properties of the hydrogen monomer on graphene
The understanding of the kinetic properties of hydrogen (isotopes) adatoms on
graphene is important in many fields. The kinetic properties of
hydrogen-isotope (H, D and T) monomers were simulated using a composite method
consisting of density functional theory, density functional perturbation theory
and harmonic transition state theory. The kinetic changes of the magnetic
property and the aromatic bond of the hydrogenated graphene during the
desorption and diffusion of the hydrogen monomer was discussed. The vibrational
zero-point energy corrections in the activation energies were found to be
significant, ranging from 0.072 to 0.205 eV. The results obtained from
quantum-mechanically modified harmonic transition state theory were compared
with the ones obtained from classical-limit harmonic transition state theory
over a wide temperature range. The phonon spectra of hydrogenated graphene were
used to closely explain the (reversed) isotope effects in the prefactor,
activation energy and jump frequency of the hydrogen monomer. The kinetic
properties of the hydrogen-isotope monomers were simulated under conditions of
annealing for 10 minutes and of heating at a constant rate (1.0 K/s). The
isotope effect was observed; that is, a hydrogen monomer of lower mass is
desorbed and diffuses more easily (with lower activation energies). The results
presented herein are very similar to other reported experimental observations.
This study of the kinetic properties of the hydrogen monomer and many other
involved implicit mechanisms provides a better understanding of the interaction
between hydrogen and graphene.Comment: Accepted by J. Phys. Chem.
Molecular Dynamics Simulation of Sympathetic Crystallization of Molecular Ions
It is shown that the translational degrees of freedom of a large variety of
molecules, from light diatomic to heavy organic ones, can be cooled
sympathetically and brought to rest (crystallized) in a linear Paul trap. The
method relies on endowing the molecules with an appropriate positive charge,
storage in a linear radiofrequency trap, and sympathetic cooling. Two
well--known atomic coolant species, and
, are sufficient for cooling the molecular mass range
from 2 to 20,000 amu. The large molecular charge required for simultaneous
trapping of heavy molecules and of the coolant ions can easily be produced
using electrospray ionization. Crystallized molecular ions offer vast
opportunities for novel studies.Comment: Accepted for publication in Phys. Rev.
All-Optical Broadband Excitation of the Motional State of Trapped Ions
We have developed a novel all-optical broadband scheme for exciting,
amplifying and measuring the secular motion of ions in a radio frequency trap.
Oscillation induced by optical excitation has been coherently amplified to
precisely control and measure the ion's secular motion. Requiring only laser
line-of-sight, we have shown that the ion's oscillation amplitude can be
precisely controlled. Our excitation scheme can generate coherent motion which
is robust against variations in the secular frequency. Therefore, our scheme is
ideal to excite the desired level of oscillatory motion under conditions where
the secular frequency is evolving in time. Measuring the oscillation amplitude
through Doppler velocimetry, we have characterized the experimental parameters
and compared them with a molecular dynamics simulation which provides a
complete description of the system.Comment: 8 pages, 10 figure
Quantum Computing with Trapped Ion Hyperfine Qubits
We discuss the basic aspects of quantum information processing with trapped ions, including the principles of ion trapping, preparation and detection of hyperfine qubits, single-qubit operations and multi-qubit entanglement protocols. Recent experimental advances and future research directions are outlined.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45527/1/11128_2004_Article_489417.pd
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