arXiv subjects

Jesús Pérez-Ríos

Publications and source records attributed to Jesús Pérez-Ríos.

72 records · Page 4Linked to original sources

arXiv2016

Ultracold chemical reactions of a single Rydberg atom in a dense gas

Within a dense environment ($ρ\approx 10^{14}\,$atoms/cm$^3$) at ultracold temperatures ($T < 1\,μ\text{K}$), a single atom excited to a Rydberg state acts as a reaction center for surrounding neutral atoms. At these temperatures almost all neutral atoms within the Rydberg orbit are bound to the Rydberg core and interact with the Rydberg atom. We have studied the reaction rate and products for $nS$ $^{87}$Rb Rydberg states and we mainly observe a state change of the Rydberg electron to a high orbital angular momentum $l$, with the released energy being converted into kinetic energy of the Rydberg atom. Unexpectedly, the measurements show a threshold behavior at $n\approx 100$ for the inelastic collision time leading to increased lifetimes of the Rydberg state independent of the densities investigated. Even at very high densities ($ρ\approx4.8\times 10^{14}\,\text{cm}^{-3}$), the lifetime of a Rydberg atom exceeds $10\,μ\text{s}$ at $n > 140$ compared to $1\,μ\text{s}$ at $n=90$. In addition, a second observed reaction mechanism, namely Rb$_2^+$ molecule formation, was studied. Both reaction products are equally probable for $n=40$ but the fraction of Rb$_2^+$ created drops to below 10$\,$% for $n\ge90$.

physics.atom-ph↗

arXiv2016

Short-range Photoassociation of LiRb

We have observed short-range photoassociation of LiRb to the two lowest vibrational states of the $d\,^3Π$ potential. These $d\,^3Π$ molecules then spontaneously decay to vibrational levels of the $a^3\,Σ^+$ state with generation rates of $\sim10^3$ molecules per second. This is the first observation of many of these $a\,^3Σ^+$ levels. We observe an alternation of the peak heights in the rotational photoassociation spectrum that suggests a $p$-wave shape resonance in the scattering state. Franck-Condon overlap calculations predict that photoassociation to higher vibrational levels of the $d\,^3Π$, in particular the sixth vibrational level, should populate the lowest vibrational level of the $a\,^3Σ^+$ state with a rate as high as $10^4$ molecules per second. These results encourage further work to explain our observed LiRb collisional physics using PECs. This work also motivates an experimental search for short-range photoassociation to other bound molecules, such as the $c\,^3Σ^+$ or $b\,^3Π$, as prospects for preparing ground-state molecules.

physics.atom-ph↗

arXiv2016

Controlling Rydberg atom excitations in dense background gases

We discuss the density shift and broadening of Rydberg spectra measured in cold, dense atom clouds in the context of Rydberg atom spectroscopy done at room temperature, dating back to the experiments of Amaldi and Segrè in 1934. We discuss the theory first developed in 1934 by Fermi to model the mean-field density shift and subsequent developments of the theoretical understanding since then. In particular, we present a model whereby the density shift is calculated using a microscopic model in which the configurations of the perturber atoms within the Rydberg orbit are considered. We present spectroscopic measurements of a Rydberg atom, taken in a Bose-Einstein condensate (BEC) and thermal clouds with densities varying from $5\times10^{14}\textrm{cm}^{-3}$ to $9\times10^{12}\textrm{cm}^{-3}$. The density shift measured via the spectrum's center of gravity is compared with the mean-field energy shift expected for the effective atom cloud density determined via a time of flight image. Lastly, we present calculations and data demonstrating the ability of localizing the Rydberg excitation via the density shift within a particular density shell for high principal quantum numbers.

physics.atom-ph↗

arXiv2016

Mapping trilobite state signatures in atomic hydrogen

A few-body approach relying on static line broadening theory is developed to treat the spectroscopy of a single Rydberg excitation to a trilobite-like state immersed in a high density ultracold medium. The present theoretical framework implements the compact treatment of polyatomic Rydberg molecules by Eiles et al. [arXiv:1601.06881], allowing for an accurate treatment of a large number of perturbers within the Rydberg orbit. This system exhibits two unique spectral signatures: its lineshape depends on the Rydberg quantum number $n$ but, strikingly, is independent of the density of the medium, and it is characterized by sharply peaked features reflecting the oscillatory structure of the potential energy landscape.

physics.atom-ph↗

arXiv2016

Energy scaling of cold atom-atom-ion three-body recombination

We study three-body recombination of Ba$^+$ + Rb + Rb in the mK regime where a single $^{138}$Ba$^{+}$ ion in a Paul trap is immersed into a cloud of ultracold $^{87}$Rb atoms. We measure the energy dependence of the three-body rate coefficient $k_3$ and compare the results to the theoretical prediction, $k_3 \propto E_{\textrm{col}}^{-3/4}$ where $E_{\textrm{col}}$ is the collision energy. We find agreement if we assume that the non-thermal ion energy distribution is determined by at least two different micro-motion induced energy scales. Furthermore, using classical trajectory calculations we predict how the median binding energy of the formed molecules scales with the collision energy. Our studies give new insights into the kinetics of an ion immersed into an ultracold atom cloud and yield important prospects for atom-ion experiments targeting the s-wave regime.

physics.atom-ph↗

arXiv2016

Observation of pendular butterfly Rydberg molecules

Obtaining full control over the internal and external quantum states of molecules is the central goal of ultracold chemistry and allows for the study of coherent molecular dynamics, collisions and tests of fundamental laws of physics. When the molecules additionally have a permanent electric dipole moment, the study of dipolar quantum gases and spin-systems with long-range interactions as well as applications in quantum information processing are possible. Rydberg molecules constitute a class of exotic molecules, which are bound by the interaction between the Rydberg electron and the ground state atom. They exhibit extreme bond lengths of hundreds of Bohr radii and giant permanent dipole moments in the kilo-Debye range. A special type with exceptional properties are the so-called butterfly molecules, whose electron density resembles the shape of a butterfly. Here, we report on the photoassociation of butterfly Rydberg molecules and their orientation in a weak electric field. Starting from a Bose-Einstein condensate of rubidium atoms, we fully control the external degrees of freedom and spectroscopically resolve the rotational structure and the emerging pendular states in an external electric field. This not only allows us to extract the bond length, the dipole moment and the angular momentum of the molecule but also to deterministically create molecules with a tunable bond length and orientation. We anticipate direct applications in quantum chemistry, many-body quantum physics and quantum information processing.

physics.atom-ph↗

arXiv2015

Probing a scattering resonance in Rydberg molecules with a Bose-Einstein condensate

We present spectroscopy of a single Rydberg atom excited within a Bose-Einstein condensate. We not only observe the density shift as discovered by Amaldi and Segre in 1934, but a line shape which changes with the principal quantum number n. The line broadening depends precisely on the interaction potential energy curves of the Rydberg electron with the neutral atom perturbers. In particular, we show the relevance of the triplet p-wave shape resonance in the Rydberg electron-Rb(5S) scattering, which significantly modifies the interaction potential. With a peak density of 5.5x10^14 cm^-3, and therefore an inter-particle spacing of 1300 a0 within a Bose-Einstein condensate, the potential energy curves can be probed at these Rydberg ion - neutral atom separations. We present a simple microscopic model for the spectroscopic line shape by treating the atoms overlapped with the Rydberg orbit as zero-velocity, uncorrelated, point-like particles, with binding energies associated with their ion-neutral separation, and good agreement is found.

physics.atom-ph↗

arXiv2015

Effective Single Photon Decay Mode of Positronium Decay via Electroweak Interactions

We consider the decay of positronium to a neutrino-antineutrino accompanied by a single photon. Since the neutrino pair go undetected, this appears as a single photon decay of positronium. These decay channel are mediated through the exchange of the massive $W$ and $Z$ vector bosons of the electroweak interaction. After summing over the various neutrino channels, the standard model calculation yields the rate for such a single photon decay process of $Γ_{Ps \rightarrow γ}$ = 1.72 $\times 10^{-19}$ s$^{-1}$.

physics.atom-ph↗

arXiv2015

Universal threshold law for ion-neutral-neutral three-body recombination

A very recently method for classical trajectory calculations for three-body collision [J. Pérez-Ríos, S. Ragole, J. Wang and C. H. Greene, J. Chem. Phys. {\bf 140}, 044307 (2014)] has been applied to describe ion-neutral-neutral ternary processes for low energy collisions: 0.1 mK - 10 mK. As a result, a threshold law for the three-body recombination cross section is obtained and corroborated both, experimentally and numerically. The derived threshold law predicts the formation of weakly bound dimers, with binding energies comparable to the collision energy of the collisional partners. In this low energy range, this analysis predicts that molecular ions should dominate over molecular neutrals as the most products formed.

physics.chem-ph↗

arXiv2015

Theory of long-range ultracold atom-molecule photoassociation

The creation of ultracold molecules is currently limited to diatomic species. In this letter we present a theoretical description of the photoassociation of ultracold atoms and molecules to create ultracold excited triatomic molecules, thus being a novel example of light-assisted ultracold chemical reaction. The calculation of the photoassociation rate of ultracold Cs atoms with ultracold Cs$_2$ molecules in their rovibrational ground state is reported, based on the solution of the quantum dynamics involving the atom-molecule long-range interactions, and assuming a model potential for the short-range physics. The rate for the formation of excited Cs$_3$ molecules is predicted to be comparable with currently observed atom-atom photoassociation rates. We formulate an experimental proposal to observe this process relying on the available techniques of optical lattices and standard photoassociation spectroscopy.

physics.atom-ph↗

arXiv2015

Formation of ultracold $^{7}$Li$^{85}$Rb molecules in the lowest triplet electronic state by photoassociation and their detection by ionization spectroscopy

We report the formation of ultracold $^{7}$Li$^{85}$Rb molecules in the $a^{3}Σ^{+}$ electronic state by photoassociation (PA) and their detection via resonantly enhanced multiphoton ionization (REMPI). With our dual-species Li and Rb magneto-optical trap (MOT) apparatus, we detect PA resonances with binding energies up to ~62 cm$^{-1}$ below the $^{7}$Li 2s $^{2}S_{1/2}$ $+$ $^{85}$Rb 5p $^{2}P_{1/2}$ asymptote. In addition, we use REMPI spectroscopy to probe the $a^{3}Σ^{+}$ state and excited electronic $3^{3} Π$ and $4^{3} Σ^{+}$ states, and identify $a^{3}Σ^{+} (v" = 7 - 13)$, $3^{3} Π(v'_Π = 0 - 10)$ and $4^{3} Σ^{+} (v'_Σ = 0 - 5)$ vibrational levels. Our line assignments agree well with ab initio calculations. These preliminary spectroscopic studies on previously unobserved electronic states are crucial to discovering transition pathways for transferring ultracold LiRb molecules created via PA to deeply bound rovibrational levels of the electronic ground state.

physics.atom-ph↗

arXiv2014

Two-Photon Total Annihilation of Molecular Positronium

The rate for complete two-photon annihilation of molecular positronium Ps$_{2}$ is reported. This decay channel involves a four-body collision among the fermions forming Ps$_{2}$, and two photons of 1.022 MeV, each, as the final state. The quantum electrodynamics result for the rate of this process is found to be $Γ_{Ps_{2} \rightarrow γγ}$ = 9.0 $\times 10^{-12}$ s$^{-1}$. This decay channel completes the most comprehensive decay chart for Ps$_{2}$ up to date.

physics.atom-ph↗

arXiv2014

Quantum Defect Theory description of weakly bound levels and Feshbach resonances in LiRb

The multichannel quantum defect theory (MQDT) in combination with the frame transformation (FT) approach is applied to model the Fano-Feshbach resonances measured for $^{7}$Li$^{87}$Rb and $^{6}$Li$^{87}$Rb [Marzok {\it et al.} Phys. Rev. A {\bf 79} 012717 (2009)]. The MQDT results show a level of accuracy comparable to that of previous models based on direct, fully numerical solutions of the the coupled channel Schrödinger equations (CC). Here, energy levels deduced from 2-photon photoassociation spectra for $^{7}$Li$^{85}$Rb are assigned by applying the MQDT approach, obtaining the bound state energies for the coupled channel problem. Our results confirm that MQDT yields a compact description of photoassociation observables as well as the Fano-Feshbach resonance positions and widths.

physics.atom-ph↗

arXiv2013

Comparison of classical and quantal calculations of helium three-body recombination

A general method to study classical scattering in $n$-dimension is developed. Through classical trajectory calculations, the three-body recombination is computed as a function of the collision energy for helium atoms, as an example. Quantum calculations are also performed for the $J^Π$ = $0^{+}$ symmetry of the three-body recombination rate in order to compare with the classical results, yielding good agreement for $E\gtrsim$ 1 K. The classical threshold law is derived and numerically confirmed for the Newtonian three-body recombination rate. Finally, a relationship is found between the quantum and classical three-body hard hypersphere elastic cross sections which is analogous to the well-known shadow scattering in two-body collisions.

physics.chem-ph↗

arXiv2011

Effect of the anisotropy on the glory structure of molecule-molecule scattering cross sections

Total (elastic + rotationally inelastic) integral cross sections are computed for O$_2(^3Σ_g^-)$-O$_2(^3Σ_g^-)$ using a recent ab initio potential energy surface. The sampled velocity range allows us a thorough comparison of the glory interference pattern observed in molecular beam experiments. The computed cross sections are about 10% smaller than the measured ones, however, a remarkable agreement in the velocity positions of the glory extrema is achieved. By comparing with models where the anisotropy of the interaction is reduced or removed, it is found that the glory pattern is very sensitive to the anisotropy, especially the positions of the glory extrema.

physics.chem-ph↗

arXiv2011

The Molecular Oxygen Tetramer: Intermolecular Interactions and Implications for the $ε$ Solid Phase

Recent data have determined that the structure of the high pressure $ε$ phase of solid oxygen consists of clusters composed of four O$_2$ molecules. This finding has opened the question about the nature of the intermolecular interactions within the molecular oxygen tetramer. We use multiconfigurational ab initio calculations to obtain an adequate characterization of the ground singlet state of (O$_2$)$_4$ which is compatible with the non magnetic character of the $ε$ phase. In contrast to previous suggestions implying chemical bonding, we show that (O$_2$)$_4$ is a van der Waals like cluster where exchange interactions preferentially stabilize the singlet state. However, as the cluster shrinks, there is an extra stabilization due to many-body interactions that yields a significant softening of the repulsive wall. We show that this short range behavior is a key issue for the understanding of the structure of $ε$-oxygen.

physics.atm-clus↗

arXiv2011

Ultracold $O_2$+$O_2$ collisions in a magnetic field: on the role of the potential energy surface

The collision dynamics of $^{17}O_2(^3Σ_g^-) +^{17}O_2(^3Σ_g^-)$ in the presence of a magnetic field is studied within the close-coupling formalism in the range between 10 nK and 50 mK. A recent global {\em ab initio} potential energy surface (PES) is employed and its effect on the dynamics is analyzed and compared with previous calculations where an experimentally derived PES was used [New J. Phys {\bf 11}, 055021 (2009)]. In contrast to the results using the older PES, magnetic field dependence of the low-field-seeking state in the ultracold regime is characterized by quite a large background scattering length, $a_{bg}$, and, in addition, cross sections exhibit broad and pronounced Feshbach resonances. The marked resonance structure is somewhat surprising considering the influence of inelastic scattering, but it can be explained by resorting to the analytical van der Waals theory, where the short range amplitude of the entrance channel wave function is enhanced by the large $a_{bg}$. This strong sensitivity to the short range of the {\em ab initio} PES persists up to relatively high energies (10 mK). After this study and despite quantitative predictions are very difficult, it can be concluded that the ratio between elastic and spin relaxation scattering is generally small, except for magnetic fields which are either low or close to an asymmetric Fano-type resonance. Some general trends found here, such as a large density of quasibound states and a propensity towards large scattering lengths, could be also characteristic of other anisotropic molecule-molecule systems.

cond-mat.quant-gas↗

arXiv2010

External field control of collective spin excitations in an optical lattice of $^2Σ$ molecules

We show that an ensemble of $^2Σ$ molecules in the rotationally ground state trapped on an optical lattice exhibits collective spin excitations that can be controlled by applying superimposed electric and magnetic fields. In particular, we show that the lowest energy excitation of the molecular ensemble at certain combinations of electric and magnetic fields leads to the formation of a magnetic Frenkel exciton. The exciton bandwidth can be tuned by varying the electric or magnetic fields. We show that the exciton states can be localized by creating vacancies in the optical lattice. The localization patterns of the magnetic exciton states are sensitive to the number and distribution of vacancies, which can be exploited for engineering many-body entangled spin states. We consider the dynamics of magnetic exciton wavepackets and show that the spin excitation transfer between molecules in an optical lattice can be accelerated or slowed down by tuning an external magnetic or electric field.

physics.chem-ph↗