47 research outputs found
Long-range Rydberg molecules, Rydberg macrodimers and Rydberg aggregates in an ultracold Cs gas
We present an overview of our recent investigations of long-range
interactions in an ultracold Cs Rydberg gas. These interactions are studied by
high-resolution photoassociation spectroscopy, using excitation close to
one-photon transitions into p Rydberg states with pulsed and
continuous-wave ultraviolet laser radiation, and lead to the formation of
long-range Cs molecules. We observe two types of molecular resonances. The
first type originates from the correlated excitation of two atoms into
Rydberg-atom-pair states interacting at long range via multipole-multipole
interactions. The second type results from the interaction of one atom excited
to a Rydberg state with one atom in the electronic ground state. Which type of
resonances is observed in the experiments depends on the laser intensity and
frequency and on the pulse sequences used to prepare the Rydberg states. We
obtain insights into both types of molecular resonances by modelling the
interaction potentials, using a multipole expansion of the long-range
interaction for the first type of resonances and a Fermi-contact
pseudo-potential for the second type of resonances. We analyse the relation of
these long-range molecular resonances to molecular Rydberg states and ion-pair
states, and discuss their decay channels into atomic and molecular ions. In
experiments carried out with a two-colour two-photon excitation scheme, we
observe a large enhancement of Rydberg-excitation probability, which we
interpret as a saturable autocatalytic antiblockade phenomenon.Comment: 28 pages, 11 figures, submitted to EPJ S
Pulsed excitation of Rydberg-atom-pair states in an ultracold Cs gas
Pulsed laser excitation of a dense ultracold Cs vapor has been used to study
the pairwise interactions between Cs atoms excited to p Rydberg
states of principal quantum numbers in the range . Molecular
resonances were observed that correspond to excitation of Rydberg-atom-pair
states correlated not only to the pp dissociation
asymptotes, but also to ss, sf,
and ff dissociation asymptotes. These
pair resonances are interpreted as arising from dipole-dipole, and higher-order
long-range-interaction terms between the Rydberg atoms on the basis of i) their
spectral positions, ii) their response to static and pulsed electric fields,
and iii) millimeter-wave spectra between pair states correlated to different
pair-dissociation asymptotes. The Rydberg-atom--pair states were found to
spontaneously decay by Penning ionization and the dynamics of the ionization
process were investigated during the first 10 s following initial
photoexcitation. To interpret the experimental observations, a potential model
was derived that is based on the numerical determination of the eigenvalues and
eigenfunctions of the long-range interaction Hamiltonian. With this potential
model, which does not include adjustable parameters, all experimental
observations could be accounted for, and the results demonstrate that
long-range-interaction models provide a global and accurate description of
interactions in ultracold Rydberg gases and that they correctly account for,
and enable the analysis of, phenomena as diverse as the formation of Rydberg
macrodimers, Penning ionization in dense Rydberg gases, and Rydberg-excitation
blockade effects.Comment: 17 pages, 12 figure
Experimental characterization of singlet scattering channels in long-range Rydberg molecules
We observe the formation of long-range Cs Rydberg molecules consisting of
a Rydberg and a ground-state atom by photoassociation spectroscopy in an
ultracold Cs gas near 6s(=3,4)np resonances
(n=26-34). The spectra reveal two types of molecular states recently predicted
by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518
(2014)]: states bound purely by triplet s-wave scattering with binding energies
ranging from 400 MHz at n=26 to 80 MHz at n=34, and states bound by mixed
singlet-triplet s-wave scattering with smaller and F-dependent binding
energies. The experimental observations are accounted for by an effective
Hamiltonian including s-wave scattering pseudopotentials, the hyperfine
interaction of the ground-state atom, and the spin-orbit interaction of the
Rydberg atom. The analysis enabled the characterization of the role of singlet
scattering in the formation of long-range Rydberg molecules and the
determination of an effective singlet s-wave scattering length for low-energy
electron-Cs collisions.Comment: v2 with corrections and modifications - to appear in Phys. Rev. Let
Calculations of static dipole polarizabilities of alkali dimers. Prospects for alignment of ultracold molecules
The rapid development of experimental techniques to produce ultracold alkali
molecules opens the ways to manipulate them and to control their dynamics using
external electric fields. A prerequisite quantity for such studies is the
knowledge of their static dipole polarizabilities. In this paper, we computed
the variations with internuclear distance and with vibrational index of the
static dipole polarizability components of all homonuclear alkali dimers
including Fr, and of all heteronuclear alkali dimers involving Li to Cs, in
their electronic ground state and in their lowest triplet state. We use the
same quantum chemistry approach than in our work on dipole moments (M. Aymar
and O. Dulieu, J. Chem. Phys. 122, 204302 (2005)), based on pseudopotentials
for atomic core representation, Gaussian basis sets, and effective potentials
for core polarization. Polarizabilities are extracted from electronic energies
using the finite-field method. For the heaviest species Rb, Cs and
Fr and for all heteronuclear alkali dimers, such results are presented for
the first time. The accuracy of our results on atomic and molecular static
dipole polarizabilities is discussed by comparing our values with the few
available experimental data and elaborate calculations. We found that for all
alkali pairs, the parallel and perpendicular components of the ground state
polarizabilities at the equilibrium distance scale as , which
can be related to a simple electrostatic model of an ellipsoidal charge
distribution. Prospects for possible alignment and orientation effects with
these molecules in forthcoming experiments are discussed.Comment: Accepted for publication in J Chem Phy
The role of Coulomb anti-blockade in the photoassociation of long-range Rydberg molecules
We present a new mechanism contributing to the detection of photoassociated
long-range Rydberg molecules via pulsed-field ionization: ionic products,
created by the decay of a long-range Rydberg molecule, modify the excitation
spectrum of surrounding ground-state atoms and facilitate the excitation of
further atoms into Rydberg states by the photoassociation light. Such an
ion-mediated excitation mechanism has been previously called "Coulomb
anti-blockade". Pulsed-field ionisation typically doesn't discriminate between
the ionization of a long-range Rydberg molecule and an isolated Rydberg atom,
and thus the number of atomic ions detected by this mechanism is not
proportional to the number of long-range Rydberg molecules present in the probe
volume. By combining high-resolution UV and RF spectroscopy of a dense,
ultracold gas of cesium atoms, theoretical modeling of the molecular level
structures of long-range Rydberg molecules bound below nP_3/2 Rydberg states of
cesium, and a rate model of the photoassociation and decay processes, we
unambiguously identify the signatures of this detection mechanism in the
photoassociation of long-range Rydberg molecules bound below atomic asymptotes
with negative Stark shifts.Comment: 8 pages, 7 figure
Observation of dipole-quadrupole interaction in an ultracold gas of Rydberg atoms
We observe the direct excitation of pairs of Cs atoms from the ground state
to molecular states correlating asymptotically to asymptotes. The
molecular resonances are interpreted as originating from the dipole-quadrupole
interaction between the pair states and close-by asymptotes
(). This interpretation is supported by Stark spectroscopy of
the pair states and a detailed modeling of the interaction potentials. The
dipole-quadrupole interaction mixes electronic states of opposite parity and
thus requires a coupling between electronic and nuclear motion to conserve the
total parity of the system. This non-Born-Oppenheimer coupling is facilitated
by the near-degeneracy of even and odd partial waves in the atom-atom
scattering which have opposite parity.Comment: 5 pages, 3 figure