1,385 research outputs found
Collisional properties of cold spin-polarized nitrogen gas: theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules
We report a combined experimental and theoretical study of collision-induced
dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use
buffer gas cooling to create trapped samples of 14N and 15N atoms with
densities 5+/-2 x 10^{12} cm-3 and measure their magnetic relaxation rates at
milli-Kelvin temperatures. Rigorous quantum scattering calculations based on
accurate ab initio interaction potentials for the 7Sigma_u electronic state of
N2 demonstrate that dipolar relaxation in N + N collisions occurs at a slow
rate of ~10^{-13} cm3/s over a wide range of temperatures (1 mK to 1 K) and
magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are
insensitive to small variations of the interaction potential and to the
magnitude of the spin-exchange interaction, enabling the accurate calibration
of the measured N atom density. We find consistency between the calculated and
experimentally determined rates. Our results suggest that N atoms are promising
candidates for future experiments on sympathetic cooling of molecules.Comment: 48 pages, 17 figures, 3 table
Density Functional Theory of doped superfluid liquid helium and nanodroplets
During the last decade, density function theory (DFT) in its static and
dynamic time dependent forms, has emerged as a powerful tool to describe the
structure and dynamics of doped liquid helium and droplets. In this review, we
summarize the activity carried out in this field within the DFT framework since
the publication of the previous review article on this subject [M. Barranco et
al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive
presentation of the actual implementations of helium DFT is given, which have
not been discussed in the individual articles or are scattered in the existing
literature. This is an Accepted Manuscript of an article published on August 2,
2017 by Taylor & Francis Group in Int. Rev. Phys. Chem. 36, 621 (2017),
available online: http://dx.doi.org/10.1080/0144235X.2017.1351672Comment: 113 pages, 42 figure
Density functional calculations for 4He droplets
A novel density functional, which accounts correctly for the equation of
state, the static response function and the phonon-roton dispersion in bulk
liquid helium, is used to predict static and dynamic properties of helium
droplets. The static density profile is found to exhibit significant
oscillations, which are accompanied by deviations of the evaporation energy
from a liquid drop behaviour in the case of small droplets. The connection
between such oscillations and the structure of the static response function in
the liquid is explicitly discussed. The energy and the wave function of excited
states are then calculated in the framework of time dependent density
functional theory. The new functional, which contains backflow-like effects, is
expected to yield quantitatively correct predictions for the excitation
spectrum also in the roton wave-length range.Comment: 15 pages, REVTEX, 10 figures available upon request or at
http://anubis.science.unitn.it/~dalfovo/papers/papers.htm
Cold collisions of heavy molecules with alkali-metal atoms in a magnetic field: Ab initio analysis and prospects for sympathetic cooling of SrOH by Li(S)
We use accurate ab initio and quantum scattering calculations to explore the
prospects for sympathetic cooling of the heavy molecular radical
SrOH() by ultracold Li atoms in a magnetic trap. A two-dimensional
potential energy surface (PES) for the triplet electronic state of Li-SrOH is
calculated ab initio using the partially spin-restricted coupled cluster method
with single, double and perturbative triple excitations and a large
correlation-consistent basis set. The highly anisotropic PES has a deep global
minimum in the skewed Li-HOSr geometry with cm and saddle
points in collinear configurations. Our quantum scattering calculations predict
low spin relaxation rates in fully spin-polarized Li+SrOH collisions with the
ratios of elastic to inelastic collision rates well in excess of 100 over a
wide range of magnetic fields (1-1000 G) and collision energies
(10~K) suggesting favorable prospects for sympathetic cooling of
SrOH molecules with spin-polarized Li atoms in a magnetic trap. We find that
spin relaxation in Li+SrOH collisions occurs via a direct mechanism mediated by
the magnetic dipole-dipole interaction between the electron spins of Li and
SrOH, and that the indirect (spin-rotation) mechanism is strongly suppressed.
The upper limit to the Li+SrOH reaction rate coefficient calculated for the
singlet PES using adiabatic capture theory is found to decrease from ~cm/s to a limiting value of cm/s with
decreasing temperature from 0.1 K to 1 K
Ultrafast charge transfer and vibronic coupling in a laser-excited hybrid inorganic/organic interface
Hybrid interfaces formed by inorganic semiconductors and organic molecules are intriguing materials for opto-electronics. Interfacial charge transfer is primarily responsible for their peculiar electronic structure and optical response. Hence, it is essential to gain insight into this fundamental process also beyond the static picture. Ab initio methods based on real-time time-dependent density-functional theory coupled to the Ehrenfest molecular dynamics scheme are ideally suited for this problem. We investigate a laser-excited hybrid inorganic/organic interface formed by the electron acceptor molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) physisorbed on a hydrogenated silicon cluster, and we discuss the fundamental mechanisms of charge transfer in the ultrashort time window following the impulsive excitation. The considered interface is p-doped and exhibits charge transfer in the ground state. When it is excited by a resonant laser pulse, the charge transfer across the interface is additionally increased, but contrary to previous observations in all-organic donor/acceptor complexes, it is not further promoted by vibronic coupling. In the considered time window of 100 fs, the molecular vibrations are coupled to the electron dynamics and enhance intramolecular charge transfer. Our results highlight the complexity of the physics involved and demonstrate the ability of the adopted formalism to achieve a comprehensive understanding of ultrafast charge transfer in hybrid materials
Theoretical Studies of Singlet Fission: Searching for Materials and Exploring Mechanisms
In this Review article, a survey is given for theoretical studies in the subject of singlet fission. Singlet fission converts one singlet exciton to two triplet excitons. With the doubled number of excitons and the longer lifetime of the triplets, singlet fission provides an avenue to improve the photoelectric conversion efficiency in organic photovoltaic devices. It has been a subject of intense research in the past decade. Theoretical studies play an essential role in understanding singlet fission. This article presents a Review of theoretical studies in singlet fission since 2006, the year when the research interest in this subject was reignited. Both electronic structure and dynamics studies are covered. Electronic structure studies provide guidelines for designing singlet fission chromophores and insights into the couplings between singleâ and multiâexcitonic states. The latter provides fundamental knowledge for engineering interchromophore conformations to enhance the fission efficiency. Dynamics studies reveal the importance of vibronic couplings in singlet fission
Laser-Induced Fluorescence Detection of the Elusive SiCF Free Radical
The SiCF free radical has been spectroscopically identified for the first time. The radical was produced in an electric discharge jet using CF3Si(CH3)3 or CF3SiH3 vapor in high pressure argon as the precursor. The laser-induced fluorescence spectrum of the Ă 2ÎŁ+ â XÌ 2Î band system in the 610 â 550 nm region was recorded and the 2Î 3/2 spin component of the 0â0 band was studied at high resolution. Rotational analysis gave the B values for the combining states, and by fixing the CF bond lengths at ab initio values we obtained râł(SiâC) = 1.692(1)Ă
and râČ(SiâC) = 1.594(1)Ă
. The bond lengths correspond to a silicon-carbon double bond in the ground state and an unusual SiâC triple bond in the excited state. Single vibronic level emission spectra yielded the ground state bending and stretching energy levels. These were fitted to a Renner-Teller model that included spin-orbit and limited vibrational anharmonicity effects
Nuclear ab initio calculations of He-6 beta-decay for beyond the Standard Model studies
Precision measurements of beta-decay observables offer the possibility to search for deviations from the Standard Model. A possible discovery of such deviations requires accompanying first-principles calculations. Here we compute the nuclear structure corrections for the beta-decay of He-6 which is of central interest in several experimental efforts. We employ the impulse approximation together with wave functions calculated using the ab initio no-core shell model with potentials based on chiral effective field theory. We use these state-of-the-art calculations to give a novel and comprehensive analysis of theoretical uncertainties. We find that nuclear corrections, which we compute within the sensitivity of future experiments, create significant deviation from the naive Gamow-Teller predictions, making their accurate assessment essential in searches for physics beyond the Standard Model. (C) 2022 The Author(s). Published by Elsevier B.V
Hyperfine Rather Than Spin Splittings Dominate the Fine Structure of the \u3cem\u3eB\u3c/em\u3e \u3csup\u3e4\u3c/sup\u3eÎŁ\u3csup\u3e-\u3c/sup\u3eâ\u3cem\u3eX\u3c/em\u3e \u3csup\u3e4\u3c/sup\u3eÎŁ\u3csup\u3e-\u3c/sup\u3e Bands of AIC
Laser-induced fluorescence and wavelength resolved emission spectra of the B 4ÎŁââXâ4ÎŁâ band system of the gas phase cold aluminum carbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high pressure argon. High resolution spectra show that each rotational line of the 0-0 and 1-1 bands of AlC is split into at least three components, with very similar splittings and intensities in both the P- and R-branches. The observed structure was reproduced by assuming bÎČS magnetic hyperfine coupling in the excited state, due to a substantial Fermi contact interaction of the unpaired electron in the aluminum 3s orbital. Rotational analysis has yielded ground and excited state equilibrium bond lengths in good agreement with the literature and our own ab initio values. Small discrepancies in the calculated intensities of the hyperfine lines suggest that the upper state spin-spin constant λâČ is of the order of â0.025â0.030 cmâ1
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