92 research outputs found

### A quasiclassical method for calculating the density of states of ultracold collision complexes

We derive a quasiclassical expression for the density of states (DOS) of an
arbitrary, ultracold, $N$-atom collision complex, for a general potential
energy surface (PES). We establish the accuracy of our quasiclassical method by
comparing to exact quantum results for the K$_2$-Rb and NaK-NaK systems, with
isotropic model PESs. Next, we calculate the DOS for an accurate NaK-NaK PES to
be 0.124~$\mu$K$^{-1}$, with an associated Rice-Ramsperger-Kassel-Marcus (RRKM)
sticking time of 6.0~$\mu$s. We extrapolate the DOS and sticking times to all
other polar bialkali-bialkali collision complexes by scaling with atomic
masses, equilibrium bond lengths, dissociation energies, and dispersion
coefficients. The sticking times calculated here are two to three orders of
magnitude shorter than those reported by Mayle et al. [Phys. Rev. A 85, 062712
(2012)]. We estimate dispersion coefficients and collision rates between
molecules and complexes. We find that the sticking-amplified three-body loss
mechanism is not likely the cause of the losses observed in the experiments

### Role of Resonance-Enhanced Multiphoton Excitation in High-Harmonic Generation of N-2: A Time-Dependent Density-Functional-Theory Study

A minimum at similar to 39 eV is observed in the high-harmonic-generation spectra of N-2 for several laser intensities and frequencies. This minimum appears to be invariant for different molecular orientations. We reproduce this minimum for a set of laser parameters and orientations in time-dependent density-functional-theory calculations, which also render orientation-dependent maxima at 23-26 eV. Photon energies of these maxima overlap with ionization potentials of excited states observed in photoelectron spectra. Time profile analysis shows that these maxima are caused by resonance-enhanced multiphoton excitation. We propose a four-step mechanism, in which an additional excitation step is added to the well-accepted three-step model. Excitation to a linear combination of Rydberg states c(4)\u27 (1)Sigma(+)(u) and c(3) (1)Pi(u) gives rise to an orientation-invariant minimum analogous to the Cooper minimum in argon. When the molecular axis is parallel to the polarization direction of the field, a radial node goes through the atomic centers, and hence the Cooper-like minimum coincides with the minimum predicted by a modified two-center interference model that considers the de-excitation of the ion and symmetry of the Rydberg orbital. DOI: 10.1103/PhysRevA.87.01343

### Photo-induced two-body loss of ultracold molecules

The lifetime of nonreactive ultracold bialkali gases was conjectured to be
limited by sticky collisions amplifying three-body loss. We show that the
sticking times were previously overestimated and do not support this
hypothesis. We find that electronic excitation of NaK+NaK collision complexes
by the trapping laser leads to the experimentally observed two-body loss. We
calculate the excitation rate with a quasiclassical, statistical model
employing ab initio potentials and transition dipole moments. Using longer
laser wavelengths or repulsive box potentials may suppress the losses

### Cold Magnetically trapped \u3csup\u3e2\u3c/sup\u3eD\u3csub\u3eg\u3c/sub\u3e scandium atoms. I. Interaction potential

We present a first principles description of the interaction of two ground-state scandium atoms. Scandium has a 2Dg ground state. Thirty molecular states correlate to the lowest dissociation limit of the dimer. In the short range, potential energy curves are calculated using second-order n-electron valence state perturbation theory. The first-order long-range interaction is calculated at the complete active space self-consistent field level. We determine the second-order long-range dispersion interaction from atomic dynamic polarizabilities at imaginary frequencies. These polarizabilities are calculated using time-dependent density functional theory. We merge the short-range approach with the long-range model to obtain a physical description of the 30 potential energy curves correlating to the 2Dg + 2Dg limit. Diabatic potentials are presented that can be used in quantum scattering calculations, in order to study Zeeman relaxation of ultracold scandium atoms

### Electronic Anisotropy Between Open Shell Atoms in First and Second Order Perturbation Theory

The interaction between two atoms in states with nonzero electronic orbital angular momenta is anisotropic and can be represented by a spherical tensor expansion. The authors derive expressions for the first order (electrostatic) and second order (dispersion and induction) anisotropic interaction coefficients in terms of the multipole moments and dynamic polarizabilities of the atoms and show that a complete description of the second order interaction requires odd rank or out-of-phase polarizabilities. The authors relate the tensorial expansion coefficients to the adiabatic Born-Oppenheimer potentials of the molecule and show that there are linear, and in some cases nonlinear, constraints on the van der Waals coefficients of these potentials

### Time-Domain Measurement of Spontaneous Vibrational Decay of Magnetically Trapped NH

The v = 1 -> 0 radiative lifetime of NH (X triplet-Sigma-, v=1,N=0) is
determined to be tau_rad,exp. = 37.0 +/- 0.5 stat +2.0 / -0.8 sys miliseconds,
corresponding to a transition dipole moment of |mu_10| = 0.0540 + 0.0009 /
-0.0018 Debye. To achieve the long observation times necessary for direct
time-domain measurement, vibrationally excited NH (X triplet-Sigma-, v=1,N=0)
radicals are magnetically trapped using helium buffer-gas loading. Simultaneous
trapping and lifetime measurement of both the NH(v=1, N=0) and NH(v=0,N=0)
populations allows for accurate extraction of tau_rad,exp. Background helium
atoms are present during our measurement of tau_rad,exp., and the rate constant
for helium atom induced collisional quenching of NH(v=1,N=0) was determined to
be k_q < 3.9 * 10^-15 cm^3/s. This bound on k_q yields the quoted systematic
uncertainty on tau_rad,exp. Using an ab initio dipole moment function and an
RKR potential, we also determine a theoretical value of 36.99 ms for this
lifetime, in agreement with our experimental value. Our results provide an
independent determination of tau_rad,10, test molecular theory, and furthermore
demonstrate the efficacy of buffer-gas loading and trapping in determining
metastable radiative and collisional lifetimes.Comment: 10 pages + 3 figures (11 pages total) v2 has minor corrections and
explanations accepted for publication in PR

### State-to-state rovibrational transition rates for CO2 in the bend mode in collisions with He atoms

Modeling environments that are not in local thermal equilibrium, such as
protoplanetary disks or planetary atmospheres, with molecular spectroscopic
data from space telescopes requires knowledge of the rate coefficients of
rovibrationally inelastic molecular collisions. Here, we present such rate
coefficients in a temperature range from 10 to 500 K for collisions of CO$_2$
with He atoms in which CO$_2$ is (de)excited in the bend mode. They are
obtained from numerically exact coupled-channel (CC) calculations as well as
from calculations with the less demanding coupled-states approximation (CSA)
and the vibrational close-coupling rotational infinite-order sudden (VCC-IOS)
method. All of the calculations are based on a newly calculated accurate ab
initio four-dimensional CO$_2$-He potential surface including the CO$_2$ bend
($\nu_2$) mode. We find that the rovibrationally inelastic collision cross
sections and rate coefficients from the CSA and VCC-IOS calculations agree to
within 50% with the CC results at the rotational state-to-state level, except
for the smaller ones and in the low energy resonance region, and to within 20%
for the overall vibrational quenching rates except for temperatures below 50 K
where resonances provide a substantial contribution. Our CC quenching rates
agree with the most recent experimental data within the error bars. We also
compared our results with data from Clary et al. calculated in the 1980's with
the CSA and VCC-IOS methods and a simple atom-atom model potential based on ab
initio Hartree-Fock calculations and found that their cross sections agree
fairly well with ours for collision energies above 500 cm$^{-1}$, but that the
inclusion of long range attractive dispersion interactions is crucial to obtain
reliable cross sections at lower energies and rate coefficients at lower
temperatures.Comment: The article has been accepted to the Journal of Chemical Physic

### Controlling the nature of a charged impurity in a bath of Feshbach dimers

We theoretically study the dynamics of a trapped ion that is immersed in an
ultracold gas of weakly bound atomic dimers created by a Feshbach resonance.
Using quasi-classical simulations, we find a crossover from dimer dissociation
to molecular ion formation depending on the binding energy of the dimers. The
location of the crossover strongly depends on the collision energy and the
time-dependent fields of the Paul trap. Deeply bound dimers lead to fast
molecular ion formation, with rates approaching the Langevin collision rate
$\Gamma'_\text{L}\approx4.8\times10^{-9}\,$cm$^3$s$^{-1}$. The kinetic energies
of the created molecular ions have a median below $1\,$mK, such that they will
stay confined in the ion trap. We conclude that interacting ions and Feshbach
molecules may provide a novel approach towards the creation of ultracold
molecular ions with applications in precision spectroscopy and quantum
chemistry.Comment: 9 pages and 12 figures including appendice

### Characterization of methanol as a magnetic field tracer in star-forming regions

Magnetic fields play an important role during star formation. Direct magnetic
field strength observations have proven specifically challenging in the
extremely dynamic protostellar phase. Because of their occurrence in the
densest parts of star forming regions, masers, through polarization
observations, are the main source of magnetic field strength and morphology
measurements around protostars. Of all maser species, methanol is one of the
strongest and most abundant tracers of gas around high-mass protostellar disks
and in outflows. However, as experimental determination of the magnetic
characteristics of methanol has remained largely unsuccessful, a robust
magnetic field strength analysis of these regions could hitherto not be
performed. Here we report a quantitative theoretical model of the magnetic
properties of methanol, including the complicated hyperfine structure that
results from its internal rotation. We show that the large range in values of
the Land\'{e} g-factors of the hyperfine components of each maser line lead to
conclusions which differ substantially from the current interpretation based on
a single effective g-factor. These conclusions are more consistent with other
observations and confirm the presence of dynamically important magnetic fields
around protostars. Additionally, our calculations show that (non-linear) Zeeman
effects must be taken into account to further enhance the accuracy of
cosmological electron-to-proton mass ratio determinations using methanol.Comment: 23 pages, 3 figures, excluding Supplementary information. Author
manuscript version before editorial/copyediting by Nature Astronomy. Journal
version available via http://rdcu.be/FPeB . Supplementary material available
via
https://static-content.springer.com/esm/art%3A10.1038%2Fs41550-017-0341-8/MediaObjects/41550_2017_341_MOESM1_ESM.pd

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