arXiv subjects

Chris H. Greene

Publications and source records attributed to Chris H. Greene.

At least 127 records · Page 7Linked to original sources

arXiv2011

Laser-induced fluorescence studies of HfF+ produced by autoionization

Autoionization of Rydberg states of HfF, prepared using the optical-optical double resonance (OODR) technique, holds promise to create HfF+ in a particular Zeeman level of a rovibronic state for an electron electric dipole moment (eEDM) search. We characterize a vibronic band of Rydberg HfF at 54 cm-1 above the lowest ionization threshold and directly probe the state of the ions formed from this vibronic band by performing laser-induced fluorescence (LIF) on the ions. The Rydberg HfF molecules show a propensity to decay into only a few ion rotational states of a given parity and are found to preserve their orientation qualitatively upon autoionization. We show empirically that we can create 30% of the total ion yield in a particular |J+,M+> state and present a simplified model describing autoionization from a given Rydberg state that assumes no angular dynamics.

physics.atom-ph↗

arXiv2011

Numerical study of three-body recombination for systems with many bound states

Three-body recombination processes are treated numerically for a system of three identical bosons. The two-body model potentials utilized support many bound states, a major leap in complexity that produces an intricate structure of sharp nonadiabatic avoided crossings in the three-body hyperradial adiabatic potentials at short distances. This model thus displays the usual difficulties of more realistic systems. To overcome the numerical challenges associated with sharp avoided crossings, the slow variable discretization (SVD) approach is adopted in the region of small hyperradii. At larger hyperradii, where the adiabatic potentials and couplings are smooth, the traditional adiabatic method suffices. Despite the high degree of complexity, recombination into deeply bound states behaves regularly due to the dominance of one decay pathway, resulting from the strong coupling between different recombination channels. Moreover, the usual Wigner threshold law must be modified for excited incident recombination channels.

quant-ph↗

arXiv2011

Universal three-body physics for fermionic dipoles

A study of the universal physics for three oriented fermionic dipoles in the hyperspherical adiabatic representation predicts a single long-lived three-dipole state, which exists in only one three-body symmetry, should form near a two-dipole resonance. Our analysis reveals the spatial configuration of the universal state, and the scaling of its binding energy and lifetime with the strength of the dipolar interaction. In addition, three-body recombination of fermionic dipoles is found to be important even at ultracold energies. An additional finding is that an effective long-range repulsion arises between a dipole and a dipolar dimer that is tunable via dipolar interactions.

physics.atom-ph↗

arXiv2011

The Efimov effect for three interacting bosonic dipoles

Three oriented bosonic dipoles are treated using the hyperspherical adiabatic representation, providing numerical evidence that the Efimov effect persists near a two-dipole resonance and in a system where angular momentum is not conserved. Our results further show that the Efimov features in scattering observables become universal, with a known three-body parameter, i.e. the resonance energies depend only on the two-body physics, which also has implications for the universal spectrum of the four-dipole problem. Moreover, the Efimov states should be long-lived, which is favorable for their creation and manipulation in ultracold dipolar gases. Finally, deeply-bound two-dipole states are shown to be relatively stable against collisions with a third dipole, owing to the emergence of a repulsive interaction originating in the angular momentum nonconservation for this system.

physics.atom-ph↗

arXiv2011

The Hyperspherical Four-Fermion Problem

The problem of a few interacting fermions in quantum physics has sparked intense interest, particularly in recent years owing to connections with the behavior of superconductors, fermionic superfluids, and finite nuclei. This review addresses recent developments in the theoretical description of four fermions having finite-range interactions, stressing insights that have emerged from a hyperspherical coordinate perspective. The subject is complicated, so we have included many detailed formulas that will hopefully make these methods accessible to others interested in using them. The universality regime, where the dominant length scale in the problem is the two-body scattering length, is particularly stressed, including its implications for the famous BCS-BEC crossover problem Derivations and relevant formulas are also included for the calculation of challenging few-body processes such as recombination.

physics.atom-ph↗

arXiv2010

Resonant structure of low-energy H3+ dissociative recombination

New high-resolution dissociative recombination rate coefficients of rotationally cool and hot H3+ in the vibrational ground state have been measured with a 22-pole trap setup and a Penning ion source, respectively, at the ion storage ring TSR. The experimental results are compared with theoretical calculations to explore the dependence of the rate coefficient on ion temperature and to study the contributions of different symmetries to probe the rich predicted resonance spectrum. The break-up energy was investigated by fragment imaging to derive internal temperatures of the stored parent ions under differing experimental conditions. A systematic experimental assessment of heating effects is performed which, together with a survey of other recent storage-ring data, suggests that the present rotationally cool rate-coefficient measurement was performed at 380^{+50}_{-130} K and that this is the lowest rotational temperature so far realized in storage-ring rate-coefficient measurements on H3+. This partially supports the theoretical suggestion that higher temperatures than assumed in earlier experiments are the main cause for the large gap between the experimental and theoretical rate coefficients. For the rotationally hot rate-coefficient measurement a temperature of below 3250K is derived. From these higher-temperature results it is found that increasing the rotational ion temperature in the calculations cannot fully close the gap between the theoretical and experimental rate coefficients.

physics.chem-ph↗

arXiv2010

Collisional Aspects of Bosonic and Fermionic Dipoles in Quasi-Two-Dimensional Confining Geometries

Fundamental aspects of ultracold collisions between identical bosonic or fermionic dipoles are studied under quasi-two-dimensional (Q2D) confinement. In the strongly dipolar regime, bosonic and fermion species are found to share important collisional properties as a result of the confining geometry, which suppresses the inelastic rates irrespective of the quantum statistics obeyed. A potential negative is that the confinement causes dipole-dipole resonances to be extremely narrow, which could make it difficult to explore Q2D dipolar gases with tunable interactions. Such properties are shown to be universal, and a simple WKB model reproduces most of our numerical results. In order to shed light on the many-body behavior of dipolar gases in Q2D we have analyzed the scattering amplitude and developed an energy-analytic form of the pseudopotentials for dipoles. For specific values of the dipolar interaction, the pseudopotential coefficient can be tuned to arbitrarily large values, indicating the possibility of realizing Q2D dipolar gases with tunable interactions.

physics.atom-ph↗

arXiv2010

Quantum-defect analysis of $np$ and $nd$ $\rm{H_3}$ Rydberg energy levels

In this work, Rydberg energy levels of the triatomic hydrogen molecule ($\rm{H_3}$) are studied with multichannel quantum-defect theory. We extract the body-frame p-wave quantum defects from highly accurate \emph{ab initio} electronic potential surfaces and calculate the quantum defects of higher momentum states by using the long-range multipole potential model. To obtain laboratory-frame quantum defects, we perform a rovibrational-frame transformation with accurate rovibrational states of $\rm{H_3^+}$. Finally, we use the laboratory-frame quantum defects to calculate Rydberg energy levels for the fundamental neutral triatomic molecule $\rm{H_3}$.

quant-ph↗

arXiv2010

Green's Functions and the Adiabatic Hyperspherical Method

We address the few-body problem using the adiabatic hyperspherical representation. A general form for the hyperangular Green's function in $d$-dimensions is derived. The resulting Lippmann-Schwinger equation is solved for the case of three-particles with s-wave zero-range interactions. Identical particle symmetry is incorporated in a general and intuitive way. Complete semi-analytic expressions for the nonadiabatic channel couplings are derived. Finally, a model to describe the atom-loss due to three-body recombination for a three-component fermi-gas of $^{6}$Li atoms is presented.

physics.atom-ph↗

arXiv2010

Calculation of rate constants for vibrational and rotational excitation of the H3+ ion by electron impact

We present theoretical thermally-averaged rate constants for vibrational and rotational (de-)excitation of the H3+ ion by electron impact. The constants are calculated using the multi-channel quantum-defect approach. The calculation includes processes that involve a change Delta J<=2 in the rotational angular momentum J of H3+. The rate constants are calculated for states with J<=5 for rotational transitions of the H3+ ground vibrational level. The thermal rates for transitions among the lowest eight vibrational levels are also presented, averaged over the rotational structure of the vibrational levels. The conditions for producing non-thermal rotational and vibrational distributions of H3+ in astrophysical environments are discussed.

physics.atom-ph↗

arXiv2009

Theory of dissociative recombination of a linear triatomic ion with permanent electric dipole moment: Study of HCO$^{+}$

We present a theoretical description of dissociative recombination of triatomic molecular ions having large permanent dipole moments. The study has been partly motivated by a discrepancy between experimental and theoretical cross-sections for dissociative recombination of the HCO$^+$ ion. The HCO$^{+}$ ion has a considerable permanent dipole moment ($D\approx$4 debye), which has not been taken explicitly into account in previous theoretical studies. In the present study, we include explicitly the effect of the permanent electric dipole on the dynamics of the incident electron using the generalized quantum defect theory, and we present the resulting cross section obtained. This is the first application of generalized quantum defect theory to the dissociative recombination of molecular ions.

quant-ph↗

arXiv2009

A general theoretical description of N-body recombination

A formula for the cross section and event rate constant describing recombination of N particles are derived in terms of general S-matrix elements. Our result immediately yields the generalized Wigner threshold scaling for the recombination of N bosons. A semi-analytic formula encapsulates the overall scaling with energy and scattering length, as well as resonant modifications by the presence of borromean N-body states near the threshold collision energy in the entrance channel. We then apply our model to the case of four-boson recombination into an Efimov trimer and a free atom.

physics.atom-ph↗

arXiv2009

Vibrational interference of Raman and high-harmonic generation pathways

Experiments have shown that the internal vibrational state of a molecule can affect the intensity of high harmonic light generated from that molecule. This paper presents a model which explains this modulation in terms of interference between different vibrational states occurring during the high harmonic process. In addition, a semiclassical model of the continuum electron propagation is developed which connects with rigorous treatments of the electron-ion scattering.

physics.atom-ph↗

arXiv2009

One- and two-photon ionization cross sections of the laser excited 6s6p^1P_1 state of barium

Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned 6s6p^1P_1 state of barium in the energy range between the 5d_{3/2} and 5d_{5/2} states of Ba^+. We have also measured these photionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel R-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The non-resonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one and two-photon cross sections, and confirm other aspects of experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce the experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one-photon and two-photon ionization processes.

physics.atom-ph↗

arXiv2009

Theoretical study of the quenching of NH (singlet Delta) molecules via collisions with Rb atoms: preliminary results

We examine the quenching reaction Rb (doublet S) + NH (singlet Delta) goes to Rb (doublet P one-half) + NH (ground triplet Sigma minus). This reaction may be utilized to produce ground state NH molecules for studies of ultracold physics or for other purposes, and is interesting in that it involves initial and final states that are nearly degenerate. This near degeneracy is expected to lead to a large reaction rate. We examine this system using ab initio quantum chemistry calculations and scattering calculations, which include spin-orbit effects, and find that the reaction rate is large and, in fact, approaches the quantum mechanical unitarity limit. We discuss the prospects for an experimental examination of this system.

physics.chem-ph↗

arXiv2009

Correlated Gaussian Hyperspherical Method for Few-Body Systems

We develop an innovative numerical technique to describe few-body systems. Correlated Gaussian basis functions are used to expand the channel functions in the hyperspherical representation. The method is proven to be robust and efficient compared to other numerical techniques. The method is applied to few-body systems with short range interactions, including several examples for three- and four-body systems. Specifically, for the two-component, four-fermion system, we extract the coefficients that characterize its behavior at unitarity.

physics.atom-ph↗

arXiv2009

Universal Four-Boson States in Ultracold Molecular Gases: Resonant Effects in Dimer-Dimer Collisions

We study the manifestations of universal four-body physics in ultracold dimer-dimer collisions. We show that resonant features associated with three-body Efimov physics and dimer-dimer scattering lengths are universally related. The emergence of universal four-boson states allows for the tunability of the dimer-dimer interaction, thus enabling the future study of ultracold molecular gases with both attractive and repulsive interactions. Moreover, our study of the interconversion between dimers and Efimov trimers shows that $B_{2}+B_{2}\to B_{3}+B$ rearrangement reactions can provide an efficient trimer formation mechanism. Our analysis of the temperature dependence of this reaction provides an interpretation of the available experimental data and sheds light on the possible experimental realization of rearrangement processes in ultracold gases.

physics.atom-ph↗

arXiv2008

Dimer-dimer collisions at finite energies in two-component Fermi gases

We introduce a major theoretical generalization of existing techniques for handling the three-body problem that accurately describes the interactions among four fermionic atoms. Application to a two-component Fermi gas accurately determines dimer-dimer scattering parameters at finite energies and can give deeper insight into the corresponding many-body phenomena. To account for finite temperature effects, we calculate the energy-dependent complex dimer-dimer scattering length, which includes contributions from elastic and inelastic collisions. Our results indicate that strong finite-energy effects and dimer dissociation are crucial for understanding the physics in the strongly interacting regime for typical experimental conditions. While our results for dimer-dimer relaxation are consistent with experiment, they confirm only partially a previously published theoretical result.

physics.atom-ph↗