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James F. Babb

Publications and source records attributed to James F. Babb.

At least 19 recordsLinked to original sources
arXiv2025

Relativistic hyperpolarizabilities for atomic H, Li, and Be$^+$ systems

The hyperpolarizability of an atom is a property that describes the nonlinear interaction between an atom and an external electric field leading to a higher-order Stark shift. Accurate evaluations of these coefficients for various systems are crucial to improve experimental precision in advanced atom-based clocks. However, there is a dearth of reports on atomic hyperpolarizabilities, particularly regarding relativistic hyperpolarizabilities. Thus, in this paper, we use fourth-order perturbation theory to establish a universal formula for the hyperpolarizability and calculate the relativistic hyperpolarizabilities of low-lying states for the monovalent electronic atomic systems H, Li, and Be$^+$. The highly accurate results given here for the H atom could serve as benchmarks for other theoretical methods.

physics.atom-ph↗
arXiv2024

PDRs4All IX. Sulfur elemental abundance in the Orion Bar

One of the main problems in astrochemistry is determining the amount of sulfur in volatiles and refractories in the interstellar medium. The detection of the main sulfur reservoirs (icy H$_2$S and atomic gas) has been challenging, and estimates are based on the reliability of models to account for the abundances of species containing less than 1% of the total sulfur. The high sensitivity of the James Webb Space Telescope provides an unprecedented opportunity to estimate the sulfur abundance through the observation of the [S I] 25.249 $μ$m line. We used the [S III] 18.7 $μ$m, [S IV] 10.5 $μ$m, and [S l] 25.249 $μ$m lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho- and para- molecular hydrogen were calculated to predict the line intensities. The [S III] 18.7 $μ$m and [S IV] 10.5 $μ$m lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. We estimate a moderate sulfur depletion, by a factor of $\sim$2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 $μ$m line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 $μ$m line is coming from warm ($>$ 40 K) molecular gas located at A$_{\rm V}$ $\sim$ 1$-$5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 $μ$m line is only accounted for if we assume the presence of undepleted sulfur.

astro-ph.GA↗
arXiv2023

Fine-structure transitions of Si and S induced by collisions with atomic hydrogen

Using a quantum-mechanical close-coupling method, we calculate cross sections for fine structure excitation and relaxation of Si and S atoms in collisions with atomic hydrogen. Rate coefficients are calculated over a range of temperatures for astrophysical applications. We determine the temperature-dependent critical densities for the relaxation of Si and S in collisions with H and compare these to the critical densities for collisions with electrons. The present calculation should be useful in modeling environments exhibiting the [S i] 25 μm and [S i] 57 μm far-infrared emission lines or where cooling of S and Si by collisions with H are of interest.

physics.atom-ph↗
arXiv2022

Fine-structure transitions of the carbon isoelectronic sequence C, N$^+$ and O$^{2+}$ induced by collisions with atomic hydrogen

Fine-structure transitions can be involved in various processes including photon absorption, charge transfer and inelastic collision between ions, electrons and neutral atoms. We present fine-structure excitation and relaxation cross sections for the collisions of the first few members of the carbon isoelectronic sequence (C, N$^+$ and O$^{2+}$) with atomic hydrogen calculated using quantum-mechanical methods. For C, the scattering theory and computational approach is verified by comparison with previous calculations. The rate coefficients for the collisional processes are obtained. For N$^+$ and O$^{2+}$, the transitions correspond to the lines [\ion{O}{iii}] 52~{\textmu}m, [\ion{O}{iii}] 88~{\textmu}m, [\ion{N}{ii}] 122~{\textmu}m, and [\ion{N}{ii}] 205~{\textmu}m, observed in the far-infrared in the local Universe and more recently in high-redshift galaxies using radio interferometry. The influence of different potentials on the cross sections and rate coefficients are demonstrated.

physics.atom-ph↗
arXiv2021

Long-range additive and nonadditive potentials in a hybrid system: Ground state atom, excited state atom, and ion

We report a theoretical study on the long-range additive and nonadditive potentials for a three-body hybrid atom-atom-ion system composed of one ground $S$ state Li atom, one excited $P$ state Li atom and one ground $S$ state Li$^+$ ion, Li($2\,^{2}S$)-Li($2\,^{2}P$)-Li$^+(1\,^{1}S$). The interaction coefficients are evaluated with highly accurate wave functions calculated variationally in Hylleraas coordinates. For this hybrid system the three-body nonadditive collective interactions (appearing in second-order) induced by the energy degeneracy and enhanced by the induction effect of the Li$^+$ ion through the internal electric field can be strong and even stronger than the two-body additive interactions at the same order. We find that for particular geometries the two-body additive interactions of the system sum to zero leaving only three-body nonadditive collective interactions making the present system potentially a platform to explore quantum three-body collective effects. We also extract first-principles leading coefficients of the long-range electrostatic, induction, and dispersion energies of Li$^+_2$ electronic states correlating to Li($2\,^{2}P$)-Li$^+(1\,^{1}S$), which until now were not available in the literature. The results should be especially valuable for the exploration of schemes to create trimers with ultracold atoms and ions in optical lattices.

physics.atom-ph↗
arXiv2019

Radiative association of atomic and ionic carbon

We present calculated cross sections and rate coefficients for the formation of the dicarbon cation (C$_2^+$) by the radiative association process in collisions of a C$(^3P)$ atom and a C$^+(^2P^o)$ ion. Molecular structure calculations for a number of low-lying doublet and quartet states of C$_2^+$ are used to obtain the potential energy surfaces and transition dipole moments coupling the states of interest, substantially increasing the available molecular data for C$_2^+$. Using a quantum-mechanical method, we explore a number of allowed transitions and determine those contributing to the radiative association process. The calculations extend the available data for this process down to the temperature of 100 K, where the rate coefficient is found to be about $2\times 10^{-18}$ cm$^3$/s. We provide analytical fits suitable for incorporation into astrochemical reaction databases.

physics.atom-ph↗
arXiv2019

Transition rates for $3s3p^2\;^4P - 3s3p4s\;^4P^o$ transitions in Al I

Fully relativistic calculations have been performed for two multiplets, $3s3p^2\;^4P$ and $3s3p4s\;^4P^o$, in Al I. Wave functions were obtained for all levels of these multiplets using the GRASP programs. Reported are the E1 transitions rates for all transitions between levels of these multiplets. Transition energies and transition rates are compared with observed values and other theory. Our calculated transition rates are smaller by about 10% than observed rates, reducing a large discrepancy between earlier calculations and experiment.

physics.atom-ph↗
arXiv2019

Dicarbon formation in collisions of two carbon atoms

Radiative association cross sections and rates are computed, using a quantum approach, for the formation of C$_2$ molecules (dicarbon) during the collision of two ground state C($^3$P) atoms. We find that transitions originating in the C$\;^1Π_g$, d$\;^3Π_g$, and 1$\;^5Π_u$ states are the main contributors to the process. The results are compared and contrasted with previous results obtained from a semi-classical approximation. New ab initio potential curves and transition dipole moment functions have been obtained for the present work using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pCV5Z basis sets, substantially increasing the available molecular data on dicarbon. Applications of the current computations to various astrophysical environments and laboratory studies are briefly discussed focusing on these rates.

astro-ph.CO↗
arXiv2019

Single photoionization of the Kr-like Rb II ion in the photon energy range 22 - 46.0 eV

Single photoionization cross sections for Kr-like Rb$^+$ ions are reported in the energy (wavelength) range 22 eV (564 Å) to 46 eV (270 Å). Theoretical cross section calculations for this {\it trans}-Fe element are compared with measurements from the ASTRID radiation facility in Aarhus, Denmark and the dual laser plasma (DLP) technique, at respectively 40 meV and 35 meV FWHM energy resolution. In the photon energy region 22 - 32 eV the spectrum is dominated by excitation autoionizing resonance states. Above 32 eV the cross section exhibit classic Fano window resonances features, which are analysed and discussed. Large-scale theoretical photoionization cross-section calculations, performed using a Dirac Coulomb $R$-matrix approximation are bench marked against these high resolution experimental results. Comparison of the theoretical work with the experimental studies allowed the identification of resonance features and their parameters in the spectra in addition to contributions from excited metastable states of the Rb$^+$ ions.

physics.atom-ph↗
arXiv2018

Dipole-dipole interactions between neutrons

In this work we present results of the dipole-dipole interactions between two neutrons, a neutron and a conducting wall, and a neutron between two walls. As input, we use dynamical electromagnetic dipole polarizabilities fitted to chiral EFT results up to the pion production threshold and at the onset of the Delta resonance. Our work can be relevant to the physics of confined ultracold neutrons inside bottles.

nucl-th↗
arXiv2018

Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques

About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between these two extremes. Most of the Universe resides in this intermediate region. We put forward that the next frontier for AMO science is to explore the AMO complexity that describes most of the Cosmos.

physics.atom-ph↗
arXiv2018

Photodissociation of CS from Excited Rovibrational Levels

Accurate photodissociation cross sections have been computed for transitions from the X $^1Σ^+$ ground electronic state of CS to six low-lying excited electronic states. New ab initio potential curves and transition dipole moment functions have been obtained for these computations using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pV6Z basis sets. State-resolved cross sections have been computed for transitions from nearly the full range of rovibrational levels of the X $^1Σ^+$ state and for photon wavelengths ranging from 500 $\textÅ$ to threshold. Destruction of CS via predissociation in highly excited electronic states originating from the rovibrational ground state is found to be unimportant. Photodissociation cross sections are presented for temperatures in the range between 1000 and 10,000 K, where a Boltzmann distribution of initial rovibrational levels is assumed. Applications of the current computations to various astrophysical environments are briefly discussed focusing on photodissociation rates due to the standard interstellar and blackbody radiation fields.

physics.atom-ph↗
arXiv2017

Feasibility of the optical fiber clock

We explore the feasibility of a compact high-precision Hg atomic clock based on a hollow core optical fiber. We evaluate the sensitivity of the $^1S_0$-$^3P_0$ clock transition in Hg and other divalent atoms to the fiber inner core surface at non-zero temperatures. The Casimir-Polder interaction induced $^1S_0$-$^3P_0$ transition frequency shift is calculated for the atom inside the hollow capillary as a function of atomic position, capillary material, and geometric parameters. For $^{199}\mathrm{Hg}$ atoms on the axis of a silica capillary with inner radius $\geq 15 \,μ\mathrm{m}$ and optimally chosen thickness $d\sim 1 \,μ\mathrm{m}$, the atom-surface interaction induced $^1S_0$-$^3P_0$ clock transition frequency shift can be kept on the level $δν/ν_{\mathrm{Hg}} \sim10^{-19}$. We also estimate the atom loss and heating due to the collisions with the buffer gas, lattice intensity noise induced heating, spontaneous photon scattering, and residual birefringence induced frequency shifts.

physics.atom-ph↗
arXiv2017

Dipole-dipole dispersion interactions between neutrons

We investigate the long-range interactions between two neutrons utilizing recent data on the neutron static and dynamic electric and magnetic dipole polarizabilities. The resulting long-range potentials are used to make quantitative comparisons between the collisions of a neutron with a neutron and a neutron with a proton. We also assess the importance of the first pion production threshold and first excited state of the nucleon, the $Δ$-resonance ($J^π$ = + 3/2, I = 3/2). We found both dynamical effects to be quite relevant for distances r between ~ 50 fm up to ~$10^3$ fm in the nn system, the neutron-wall system and in the wall-neutron-wall system, reaching the expected asymptotic limit beyond that. Relevance of our findings to the confinement of ultra cold neutrons inside bottles is discussed.

nucl-th↗
arXiv2017

Radiative association of C(${}^3P$) and H${}^+$: Triplet states

The radiative association of C(${}^3P$) and H${}^+$ is investigated by calculating cross sections for photon emission into bound ro-vibrational states of CH${}^+$ from the vibrational continua of initial triplet d$\,{}^3Π$ or b$\,{}^3Σ^-$ states. Potential energy curves and transition dipole moments are calculated using multi-reference configuration interaction (MRCI) methods with AV6Z basis sets. The cross sections are evaluated using quantum-mechanical methods and rate coefficients are calculated. The rate coefficients are about 100 times larger than those for radiative association of C${}^+({}^2{P^o})$ and H from the A$\,{}^1Π$ state. We also confirm that the formation of CH${}^+$ by radiative association of C${}^+({}^2{P^o})$ and H via the triplet c$\,{}^3Σ^+$ state is a minor process.

physics.atom-ph↗
arXiv2017

Radiative charge transfer in collisions of C with He+

Radiative charge exchange collisions between a carbon atom C(${}^3$P) and a helium ion He+, both in their ground state, are investigated theoretically. Detailed quantum chemistry calculations are carried out to obtain potential energy curves and transition dipole matrix elements for doublet and quartet molecular states of the HeC+ cation. Radiative charge transfer cross sections and rate coefficients are calculated and are found at thermal and lower energies to be large compared to those for direct charge transfer. The present results might be applicable to modelling the complex interplay of [C II] (or C+), C, and CO at the boundaries of interstellar photon dominated regions (PDRs) and in xray dominated regions (XDRs), where the abundance of He+ affects the abundance of CO.

physics.atom-ph↗
arXiv2016

Calculations of long-range three-body interactions for Li($2\,^2S$)-Li($2\,^2S$)-Li($2\,^2P$)

General formulas for calculating the several leading long-range interactions among three identical atoms where two atoms are in identical $S$ states and the other atom is in a $P$ state are obtained using perturbation theory for the energies up to second order. The first order (dipolar) interactions depend on the geometrical configurations of the three atoms. In second order, additive and nonadditive dispersion interactions are obtained. The nonadditive interactions depend on the geometrical configurations in marked contrast to the case where all three atoms are in identical $S$ states, for which the nonadditive (also known as triple-dipole or as Axilrod-Muto-Teller) dispersion interactions appear at the third order. The formalism is demonstrated by the calculation of the coefficients for the Li($2\,^2S$)-Li($2\,^2S$)-Li($2\,^{2}P$) system using variationally-generated atomic lithium wave functions in Hylleraas coordinates. The present dipolar coefficients and additive and nonadditive dispersion coefficients may be useful in constructing precise potential energy surfaces for this three lithium atom system.

physics.atom-ph↗