4,711 research outputs found
A simple approach to the correlation of rotovibrational states in four-atomic molecules
The problem of correlation between quantum states of four-atomic molecules in
different geometrical configurations is reviewed in detail. A general, still
simple rule is obtained which allows one to correlate states of a linear
four-atomic molecule with those of any kind of non-linear four-atomic molecule.Comment: 16 pages (+8 figures), Postscript (ready to print!
Counting statistics of tunneling through a single molecule: effect of distortion and displacement of vibrational potential surface
We analyze the effects of a distortion of the nuclear potential of a
molecular quantum dot (QD), as well as a shift of its equilibrium position, on
nonequilibrium-vibration-assisted tunneling through the QD with a single level
() coupled to the vibrational mode. For this purpose, we derive an
explicit analytical expression for the Franck-Condon (FC) factor for a
displaced-distorted oscillator surface of the molecule and establish rate
equations in the joint electron-phonon representation to examine the
current-voltage characteristics and zero-frequency shot noise, and skewness as
well. Our numerical analyses shows that the distortion has two important
effects. The first one is that it breaks the symmetry between the excitation
spectra of the charge states, leading to asymmetric tunneling properties with
respect to and . Secondly, distortion (frequency
change of the oscillator) significantly changes the voltage-activated cascaded
transition mechanism, and consequently gives rise to a different nonequilibrium
vibrational distribution from that of the case without distortion. Taken in
conjunction with strongly modified FC factors due to distortion, this results
in some new transport features: the appearance of strong NDC even for a
single-level QD with symmetric tunnel couplings; a giant Fano factor even for a
molecule with an extremely weak electron-phonon interaction; and enhanced
skewness that can have a large negative value under certain conditions.Comment: 29 pages, 11 figures, published versio
Excited-state Forces within a First-principles Green's Function Formalism
We present a new first-principles formalism for calculating forces for
optically excited electronic states using the interacting Green's function
approach with the GW-Bethe Salpeter Equation method. This advance allows for
efficient computation of gradients of the excited-state Born-Oppenheimer
energy, allowing for the study of relaxation, molecular dynamics, and
photoluminescence of excited states. The approach is tested on photoexcited
carbon dioxide and ammonia molecules, and the calculations accurately describe
the excitation energies and photoinduced structural deformations.Comment: 2 figures and 2 table
Non-extensive study of Rigid and Non-rigid Rotators
The isotropic rigid and non-rigid rotators in the framework of Tsallis
statistics are studied in the high and low temperature limits. The generalized
partition functions, internal energies and heat capacities are calculated. It
has been found that results are in well agreement with the classical
Boltzmann-Gibbs statistics in the limiting Tsallis index. It has also been
observed that nonextensivity parameter q behaves like a scale parameter in the
low temperature regime.Comment: 11 Pages, 3 Figures, Late
Quantum resonance, Anderson localisation and selective manipulations in molecular mixtures by ultrashort laser pulses
We demonstrate that the current laser technology used for field-free
molecular alignment via a cascade of Raman rotational transitions allows for
observing long-discussed non-linear quantum phenomena in the dynamics of the
periodically kicked rotor. This includes the scaling of the absorbed energy
near the conditions of quantum resonance and Anderson-like localisation in the
angular momentum. Based on these findings, we suggest a novel approach to
tunable selective rotational excitation and alignment in a molecular mixture,
using trains of short laser pulses. We demonstrate the efficiency of this
approach by applying it to a mixture of two nitrogen isotopologues (14N2 and
15N2), and show that strong selectivity is possible even at room temperature
Excitation and emission spectra of rubidium in rare-gas thin-films
To understand the optical properties of atoms in solid state matrices, the
absorption, excitation and emission spectra of rubidium doped thin-films of
argon, krypton and xenon were investigated in detail. A two-dimensional
spectral analysis extends earlier reports on the excitation and emission
properties of rubidium in rare-gas hosts. We found that the doped crystals of
krypton and xenon exhibit a simple absorption-emission relation, whereas
rubidium in argon showed more complicated spectral structures. Our sample
preparation employed in the present work yielded different results for the Ar
crystal, but our peak positions were consistent with the prediction based on
the linear extrapolation of Xe and Kr data. We also observed a bleaching
behavior in rubidium excitation spectra, which suggests a population transfer
from one to another spectral feature due to hole-burning. The observed optical
response implies that rubidium in rare-gas thin-films is detectable with
extremely high sensitivity, possibly down to a single atom level, in low
concentration samples.Comment: 7 pages, 5 figure
Anisotropy induced Feshbach resonances in a quantum dipolar gas of magnetic atoms
We explore the anisotropic nature of Feshbach resonances in the collision
between ultracold magnetic submerged-shell dysprosium atoms, which can only
occur due to couplings to rotating bound states. This is in contrast to
well-studied alkali-metal atom collisions, where most Feshbach resonances are
hyperfine induced and due to rotation-less bound states. Our novel
first-principle coupled-channel calculation of the collisions between
open-4f-shell spin-polarized bosonic dysprosium reveals a striking correlation
between the anisotropy due to magnetic dipole-dipole and electrostatic
interactions and the Feshbach spectrum as a function of an external magnetic
field. Over a 20 mT magnetic field range we predict about a dozen Feshbach
resonances and show that the resonance locations are exquisitely sensitive to
the dysprosium isotope.Comment: 5 pages, 4 figure
Non-adiabatic Effects in the Dissociation of Oxygen Molecules at the Al(111) Surface
The measured low initial sticking probability of oxygen molecules at the
Al(111) surface that had puzzled the field for many years was recently
explained in a non-adiabatic picture invoking spin-selection rules [J. Behler
et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to
conserve the initial spin-triplet character of the free O2 molecule during the
molecule's approach to the surface. A new locally-constrained
density-functional theory approach gave access to the corresponding
potential-energy surface (PES) seen by such an impinging spin-triplet molecule
and indicated barriers to dissociation which reduce the sticking probability.
Here, we further substantiate this non-adiabatic picture by providing a
detailed account of the employed approach. Building on the previous work, we
focus in particular on inaccuracies in present-day exchange-correlation
functionals. Our analysis shows that small quantitative differences in the
spin-triplet constrained PES obtained with different gradient-corrected
functionals have a noticeable effect on the lowest kinetic energy part of the
resulting sticking curve.Comment: 17 pages including 11 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Reactions of C({\it a}) with selected saturated alkanes: A temperature dependence study
We present a temperature dependence study on the gas phase reactions of the
C({\it a}) radical with a selected series of saturated alkanes
(CH, CH, n-CH, i-CH, and n-CH) by
means of pulsed laser photolysis/laser-induced fluorescence technique. The
bimolecular rate constants for these reactions were obtained between 298 and
673 K. A pronounced negative temperature effect was observed for n-CH,
i-CH, and n-CH and interpreted in terms of steric hindrance
of the more reactive secondary or tertiary C-H bonds by less reactive CH
groups. Detailed analysis of our experimental results reveals quantitatively
the temperature dependence of reactivities for the primary, secondary, and
tertiary C-H bonds in these saturated alkanes and further lends support to a
mechanism of hydrogen abstraction.Comment: 26 pages, 8 figures, 1 table, 30 references; accepted to JC
C_2 in Peculiar DQ White Dwarfs
White dwarfs (WDs) with carbon absorption features in their optical spectra
are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects
(T_eff<6000 K) which show molecular absorption bands that have centroid
wavelengths ~100-300 Angstroms shortward of the bandheads of the C_2 Swan
bands. These "peculiar DQ bands" have been attributed to a hydrocarbon such as
C_2H. We point out that C_2H does not show strong absorption bands with
wavelengths matching those of the peculiar DQ bands and neither does any other
simple molecule or ion likely to be present in a cool WD atmosphere. The most
straightforward explanation for the peculiar DQ bands is that they are
pressure-shifted Swan bands of C_2. While current models of WD atmospheres
suggest that, in general, peculiar DQ WDs do not have higher photospheric
pressures than normal DQ WDs do, that finding requires confirmation by improved
models of WD atmospheres and of the behavior of C_2 at high pressures and
temperatures. If it is eventually shown that the peculiar DQ bands cannot be
explained as pressure-shifted Swan bands, the only explanation remaining would
seem to be that they arise from highly rotationally excited C_2 (J_peak>45). In
either case, the absorption band profiles can in principle be used to constrain
the pressure and the rotational temperature of C_2 in the line-forming regions
of normal and peculiar DQ WD atmospheres, which will be useful for comparison
with models. Finally, we note that progress in understanding magnetic DQ WDs
may require models which simultaneously consider magnetic fields, high
pressures and rotational excitation of C_2.Comment: ApJ in press. 8 pages emulateapj style, 1 figur
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