Search

The electric-field-dependent $g$ factor and the electron electric dipole moment (eEDM)-induced Stark splittings for the lowest rotational levels of $^{207,208}$PbF are calculated. Observed and calculated Zeeman shifts for $^{207}$PbF are found to be in very good agreement. It is shown that the $^{207}$PbF hyperfine sublevels provide a promising system for the eEDM search and related experiments., 6 pages, 5 figures

We report calculations on the q(Yb) electric field gradient (EFG) for the X2Σ+ and A2Π1/2 electronic states of the ytterbium monofluoride (YbF) molecule at the molecular mean-field Dirac-Coulomb-Gaunt as well as scalar-relativistic coupled-cluster levels of theory using large uncontracted basis sets. Vibrational contributions are included in the final results. Our estimated nuclear quadrupole coupling constants of -3386(78) MHz and -2083(153) MHz for the X2Σ+ and A2Π1/2 states of 173YbF are in stark contrast to the only available experimental results (-2050(170) MHz and -1090(160) MHz) respectively, where the only similarity is the difference between the two values. Perturbative triple contributions in the coupled cluster treatment are significant and point towards the necessity to go to higher order in the coupled-cluster treatment in future calculations. We also present density functional calculations which show rather large variations for the Yb EFG with different functionals used; the best result was obtained using the CAM-B3LYP∗ functional.

Absolute single- and double-charge-exchange cross sections for the astrophysically prominent ${\mathrm{O}}^{6+}$ ion with the atomic and molecular targets He and ${\mathrm{H}}_{2}$ are reported. These collisions give rise to x-ray emissions in the interplanetary medium, planetary atmospheres, and comets as they approach the sun. Measurements have been carried out using the Caltech Jet Propulsion Laboratory electron cyclotron resonance ion source with ${\mathrm{O}}^{6+}$ at energies of 1.17 and 2.33 keV/u characteristic of the slow and fast components of the solar wind. Absolute charge-exchange (CE) data are derived from knowledge of the target gas pressure, target path length, incident ion current, and charge-exchanged ion currents. These data are compared with results obtained using the $n$-electron classical trajectory Monte Carlo method. The radiative and Auger evolution of ion populations following one- and two-electron transfers is calculated with the time-dependent collisional-radiative code nomad using atomic data from the flexible atomic code. Calculated CE emission spectra for $100\phantom{\rule{4pt}{0ex}}\AA{}l\ensuremath{\lambda}l1400\phantom{\rule{4pt}{0ex}}\AA{}$ are reported as well and compared with experimental sublevel spectra and cross sections.

We report measured absolute single and double charge-exchange cross sections (CSs) for Feq+ ions with He at the ion impact energy of 7.0 ? q keV for q = 5?13. The range of ion velocities is 350 km s?1 for q = 5, to 560 km s?1 for q = 13, comparable to typical solar wind velocities. A retarding-field method is used for charge-state analysis of the ion beam after traversing the stationary target gas cell. The CSs are determined from absolute measurements of the ion currents and He target gas pressure.

We report measured absolute single and double charge-exchange cross sections for 3He2+ on CO at ion impact energies between 1 and 14 keV (velocities between 254 and 949 km s-1). A retarding-field method is used for charge-state analysis of the ion beam after traversing the target gas cell. Cross sections are determined from absolute measurements of current ratios and the CO target gas pressure. For testing the experimental technique, proton-electron capture cross sections are measured at energies between 2.5 and 7 keV. The measured He2+ and H+ cross sections are compared to available data.

Rotational spectra of 6LiNaF2 and 7LiNaF2 were measured by microwave absorption and Fourier‐transform spectroscopy at different low rotational temperatures obtained by an adiabatic expansion in a beam and by collisional cooling in a cold absorption cell. Precise rotational constants, quartic centrifugal constants, quadrupole coupling constants, and the electric dipole moment were determined from least‐squares fits of the experimental transition frequencies. Bond lengths and angles and the dipole moment are found to be in good agreement with recent ab initio calculations. As in homogeneous dimers, monomer to dimer bond expansions calculated from an ionic model are too large. The electric dipole moment from the ionic model is in excellent agreement with the ab initio result when experimental bond distances and angles are used and a 100% ionic character of the dimer bonds is assumed.

In semiempirical ionic models a number of adjustable parameters have to be fitted to experimental data of either monomer molecules or crystals. This leads to strong correlations between these constants and prevents a unique test and a clear physical interpretation of the fit parameters. Moreover, it is not clear whether these constants remain unchanged when the model is applied to dimers or larger clusters. It is shown that these correlations can be substantially reduced when reliable information about dimers is available from experiments or ab initio calculations. Starting with Dunham coefficients of the monomer potential determined from microwave measurements, we have calculated the monomer to dimer bond expansion and the bond angle without any additional adjustable parameter. Assuming that the overlap repulsion between nearest neighbors remains unchanged, the bond expansion is mainly determined by the simple Coulomb repulsion between equally charged ions and depends only very little on the effective ion polarizabilities. Deviation of the bond angle from 90° sensitively tests the difference of effective polarizabilities of the two ions. A comparison with previously available data and new ab initio MP2 results presented here for the heavy‐atom containing dimers shows that bond angles can be modeled reasonably well with Seitz–Ruffa corrected Pauling polarizabilities while calculated bond expansions are much too long. This shows that changes of the overlap repulsion term must be considered for reliable predictions of the structure of dimers and larger clusters.

Reported are total, absolute charge-exchange cross sections for collisions of ${}^{3}$He${}^{2+}$ ions with He and H${}_{2}$. Measurements are reported at fixed energies between 0.33 and 4.67 keV/amu. Both the present results and earlier results of others are analyzed in terms of available experimental small-angle differential cross sections as a function of collision energy, and hence the geometry of the exit aperture of the gas-collision cells used by the various experimental groups. In addition, the effective length of gas-collision cells is studied using fluid dynamic and molecular flow simulations to address the density patterns near the cell entrance and exit apertures. When small acceptance-angle corrections were applied, the results of present and previous measurements for the single electron capture in these systems were brought into good accord in the relevant energy ranges. Taken in their entirety, the present data for ${}^{3}$He${}^{2+}$ with He and H${}_{2}$ lend themselves to new theoretical calculations of the multichannel charge-exchange cross sections.

Electron-excitation cross sections are reported for the 3s 2S yields 3p 2P(h, k) resonance transition in Mg(+) at energies from threshold (4.43 eV) to approximately 9 times threshold (40.0 eV). The electron-energy-loss merged-beams technique used in these measurements is described in detail. In addition, the method of separating contributions of the elastically scattered (Coulomb) and the inelastically scattered electrons in the present Mg(+) case and previously reported Zn(+) results is described. Comparisons in the experimental energy range are made for Mg(+) with the two five-state close-coupling theoretical calculations carried out herein, and with other published close-coupling, distorted-wave, and semiempirical calculations. The present Mg(+) cross sections and Zn(+) cross sections from earlier measurements are tabulated.

The importance of measuring electron-ion excitation cross sections in singly and multiply charged positive ions is outlined, and recent results for Mg II and O II ions are given using the JPL's electron energy-loss merged-beams apparatus. Theoretical comparisons are given with two five-state close-coupling calculations. The energy variation of the collision strength is fitted with a semiempirical analytic function which includes approximations to polarization, resonance, and exchange contributions. In O II, first spectra anywhere of electron excitation of the optically allowed transitions are presented. In addition, excitations of two low lying, optically forbidden transitions are detected for the first time.

Charge exchange (CE) plays a fundamental role in the collisions of solar- and stellar-wind ions with lunar and planetary exospheres, comets, and circumstellar clouds. Reported herein are absolute cross sections for single, double, triple, and quadruple CE of Fe^(q+) (q = 5-13) ions with H_2O at a collision energy of 7q keV. One measured value of the pentuple CE is also given for Fe^(9+) ions. An electron cyclotron resonance ion source is used to provide currents of the highly charged Fe ions. Absolute data are derived from knowledge of the target gas pressure, target path length, and incident and charge-exchanged ion currents. Experimental cross sections are compared with new results of the n-electron classical trajectory Monte Carlo approximation. The radiative and non-radiative cascades following electron transfers are approximated using scaled hydrogenic transition probabilities and scaled Auger rates. Also given are estimates of cross sections for single capture, and multiple capture followed by autoionization, as derived from the extended overbarrier model. These estimates are based on new theoretical calculations of the vertical ionization potentials of H_2O up to H_2O^(10+).

Absolute cascade-free excitation cross-sections in an ion have been measured for the resonance 2S to 2P transition in Zn(+) using electron-energy-loss and merged electron-ion beams methods. Measurements were carried out at electron energies of below threshold to 6 times threshold. Comparisons are made with 2-, 5-, and 15-state close-coupling and distorted-wave theories. There is good agreement between experiment and the 15-state close-coupling cross-sections over the energy range of the calculations.

Reported herein are measured absolute single, double, and triple charge exchange (CE) cross sections for the highly charged ions (HCIs) ${\mathrm{C}}^{q+}$ $(q=5,6)$, ${\mathrm{O}}^{q+}$ $(q=6,7,8)$, and ${\mathrm{Ne}}^{q+}$ $(q=7,8)$ colliding with the molecular species ${\mathrm{H}}_{2}\mathrm{O}$, CO, and $\mathrm{C}{\mathrm{O}}_{2}$. Present data can be applied to interpreting observations of x-ray emissions from comets as they interact with the solar wind. As such, the ion impact energies of $7.0q\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ $(1.62--3.06\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu})$ are representative of the fast solar wind, and data at $1.5q\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ for ${\mathrm{O}}^{6+}$ $(0.56\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu})$ on CO and $\mathrm{C}{\mathrm{O}}_{2}$ and $3.5q\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ for ${\mathrm{O}}^{5+}$ $(1.09\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu})$ on CO provide checks of the energy dependence of the cross sections at intermediate and typical slow solar wind velocities. The HCIs are generated within a $14\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ electron cyclotron resonance ion source. Absolute CE measurements are made using a retarding potential energy analyzer, with measurement of the target gas cell pressure and incident and final ion currents. Trends in the cross sections are discussed in light of the classical overbarrier model (OBM), extended OBM, and with recent results of the classical trajectory Monte Carlo theory.

The wavelength range and high resolution of the space instruments Chandra, Newton, SOHO, Suzaku, Herschel, Spitzer, and the upcoming ASTRO-H and James Webb Space Telescope have increased the need for laboratory collision-physics measurements to interpret astrophysical phenomena. A review will be given of charge exchange of highly-charged ions with neutral comet and planet atmospheres; and the formation of complex molecules in stellar regions. These space observations are linked to laboratory measurements of absolute charge-exchange cross sections; and molecular formation of species such as CO2, CH3OH, and CH3CH2OH involving fast H- and O-atom collisions with abundant interstellar molecules adsorbed on dust-grain analogues.

Using the electron energy‐loss method in a merged electron‐ion beams geometry absolute, cascade‐free excitation cross sections have been measured for the resonance 2S→2P transition in Zn+. Measurements were carried out at electron energies of below threshold (threshold at 6.011 eV) to 40 eV. Results are in very good agreement with close‐coupling calculations and, away from threshold, lie below absolute line‐emission cross sections which include effects of cascade into the 2P state from higher levels.

Within the independent atom scattering model, we derive an approximate formula for the rotationally and vibrationally averaged three-atom terms in the series expansion of the electron scattering cross section. This formula uses the atomic scattering factors as well as the atomic scattered wavefunctions as input, and rotational averaging is performed numerically. We compare our results to previous theoretical multiple scattering approaches for the molecules F(3), F(4), and SF(6) and to experimental data for TeF(6). Our results are consistent with those of previous calculations and inclusion of the three-atom term produces a dramatically better least squares fit of the TeF(6) data. The algorithm presented here is sufficiently fast and simple to be incorporated easily into existing electron diffraction codes.

It has come to our attention that some of the ab initio results presented are incorrect due to errors in the Cs and C1 basis sets, and a small error in the binding energy of Rb2F2. The corrected results are presented below for the species that were affected, modifying the results in Table III of the original paper see Table I . Only those values which are different from the results of the original Table III are included. Note that some of these results are used for comparison with

Charge exchange (also referred to also charge transfer or electron capture) has been studied by the atomic collisions community for many years. Apart from the fundamental interest in the subject, much of the research has been driven by the needs of the fusion community where it plays an important role in radiative heat loss and diagnostics in tokamak plasmas. It is therefore surprising that it has generally not been applied as extensively in astrophysical plasmas. Atomic data for astrophysics includes comprehensive data for electron process but little for charge exchange, due to the fact that the data does not exist or is not considered important. However, it has been shown in the last few years that charge exchange is the dominant mechanism for x-ray production for a number of newly discovered x-ray sources in the solar system. These results have demonstrated the need to understand this mechanism for the particular species of interest. Recent experimental work in our laboratory, complimented by other groups, has shown that the relative intensities of these x-ray and extreme ultra-violet lines can be highly dependent on the nature of the neutral target and the collision velocity.

Configuration-interaction calculations of the differential high-energy electron-scattering cross sections of C${\mathrm{O}}_{2}$ have been performed in the first Born approximation leading to encouraging results for the total as well as the elastic and inelastic scattering, separately. A study of the potential-energy differences accessible from the cross-section curves by numerical integration points to a serious problem connected with the experimentally limited range of integration.

Differential electron scattering cross sections have been measured for dipole-forbidden and resonance transitions in Mg II, Zn II and Cd II in the angular range theta = 4-17 deg at 50 eV. These provide the first recorded angular distributions for an optically forbidden transition. It is found that while the cross section for excitation of the 4s (2)S-3d(9)4s(2) (2)D transition in Zn II is small, those for the 3s (2)S-3d (2)D, 4s (2)S (unresolved lines) in Mg II, and the 5s (2)S-4d(9)5s(2) D in Cd II are comparable in magnitude with the cross sections for resonance excitation. In addition, for Cd II it is found that the allowed and forbidden transitions have very similar angular distributions, and it is proposed that excitation to the 2D state may be dominated by a virtual 'double-dipole' transition via the 2P state. Also, the total excitation cross section of the resonance 2P state in Cd II is a factor of four higher than that predicted by the Gaunt factor approximation, suggesting that the accepted value for the oscillator strength may be too low.

The ab initio Hartree–Fock calculation of the elastic electron scattering cross section of sulphur hexafluoride is reported. The integrals for the Born scattering amplitudes have been evaluated by a new simple technique, based on Gaussian quadrature. Spherical averaging was performed numerically; different techniques for spherical averaging are discussed. An estimated re structure was used and vibrational effects have been neglected. In order to assess the effect of the basis set on the intensities, calculations have been performed for the nitrogen molecule with six different basis sets. Using these results as a guide, a triple‐zeta basis set, augmented with polarization functions and diffuse functions, was selected for SF6. The results confirm Bartell’s hypothesis that the observed residuals from the independent atom model intensities are due to electronic bonding effects. It is suggested that, for very accurate electron diffraction work, the effect of electron redistribution has to be taken into account...

A study of monomers and dimers of the four alkali chlorides NaCl, KCl, RbCl, and CsCl in the vapor phase using the counting method of high energy electron diffraction is reported. Nozzle temperatures from 850-960 K were required to achieve the necessary vapor pressures of approximately 0.01 torr. Using harmonic calculations for the monomer and dimer 1 values, a consistent set of structures for all four molecules was obained. The corrected monomer distances reproduce the microwave values very well. The experiment yields information on the amount of dimer present in the vapor, and these results are compared with thermodynamic values.