Complex and Exotic Excitations

In Rydberg atoms, at least one electron is excited to a state with a high principal quantum number. In an ultracold environment, this low-energy electron can scatter off a ground state atom allowing for the formation of a Rydberg molecule consisting of one Rydberg atom and several ground state atoms. Here we investigate those Rydberg molecules created by photoassociation for the spherically symmetric S-states. A step by step increase of the principal quantum number up to n=111 enables us to go beyond the previously observed dimer and trimer states up to a molecule, where four ground state atoms are bound by one Rydberg atom. The increase of bound atoms and the decreasing binding potential per atom with principal quantum number results finally in an overlap of spectral lines. The associated density-dependent line broadening sets a fundamental limit, for example, for the optical thickness per blockade volume in Rydberg quantum optics experiments.

With the use of second-order perturbation theory in the long-range interatomic interaction for the degenerate states of two Rydberg atoms we have obtained a general formula for the dependence of atomic interaction energy on the interatomic distance R in the presence of the Förster resonance. Inside of the ‘Förster sphere’ (R < RF) this dependence transforms to the formula for electric dipole interaction energy ΔEd − d = C3/R3 and for R > RF it transforms to the formula for the van der Waals interaction energy ΔEVdW = −C6/R6. The van der Waals constant C6 is represented as an expansion in terms of irreducible components which define the dependence on the interatomic axis orientation relative to the quantisation axis of projections M of the total angular momentum J. The numerical values of the irreducible components of tensor C6 were calculated for rubidium atoms in the same Rydberg states |nlJM〉 with large quantum numbers n. We present the calculated resonance interaction energy of two rubidium atoms in the states |43D5/2M〉, whose total energy exceeds by only 8 MHz the total energy of one of the atoms in the state |45P3/2M〉 and of the other in the state |41F7/2M〉.

The ground states of small atoms and molecules, namely those of the hydrogen, helium, and lithium atoms and the hydrogen molecular ion and hydrogen molecule, are studied in the presence of an external magnetic field. For the one-electron systems the ground state is of the same symmetry for arbitrary field strengths, and the corresponding binding energy shows a monotonic increase with increasing field strength. More interesting, the two- and three-electron systems show ground-state transitions which involve both transitions in the spin multiplicity and spatial quantum numbers. An outlook on the expected cascade of transitions in many electron atoms and molecules is provided. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 789–795, 1998

Influence of several types of inelastic collision on the electron energy distribution in helium

Electron impact excitation from the ground state of bismuth atoms has been studied. A beam of electrons was scattered from a beam of atoms and the intensity of scattered electrons was measured for scattering angles up to 150° and incident electron energies of 40 and 60 eV. Obtained intensities were used for the calculation of relative differential cross sections (DCS). In addition, we recorded the energy loss-spectra at different incident electron energies and scattering angles. These spectra were analysed in order to identify the energy levels of bismuth atom below and above (autoionisation) the first ionization limit in electron spectroscopy. The presence of bismuth was confirmed in spectra of the chemically peculiar (CP) magnetic Ap 73 Dra and HR 465 and nonmagnetic Hg-Mn HR 7775 and χ Lupi stars. The obtained results for relative DCS and identified autoionised energy levels of bismuth were analysed and compared with previous experimental and theoretical data. The connection between our investigations of bismuth and astrophysical measurements are discussed.

We consider a Dirac one-electron atom placed in a weak, static, uniform\nmagnetic field. We show that, to the first order in the strength $B$ of the\nperturbing field, the only electric multipole moment induced by the field in\nthe ground state of the atom is the quadrupole one. Using the Sturmian\nexpansion of the generalized Dirac-Coulomb Green function [R. Szmytkowski, J.\nPhys. B 30 (1997) 825; erratum 30 (1997) 2747], we derive a closed-form\nexpression for the induced electric quadrupole moment. The result contains the\ngeneralized hypergeometric function $_{3}F_{2}$ of the unit argument. Earlier\ncalculations by other authors, based on the non-relativistic model of the atom,\npredicted in the low-field region the quadratic dependence of the induced\nelectric quadrupole moment on $B$.\n

In a dc-magnetron discharge with argon feed gas, densities of copper atoms in the ground state Cu(2S1/2) and metastable state Cu*(2D5/2) were measured by the resonance absorption technique, using a commercial hollow cathode lamp as light source. The operating conditions were 0.3–14 µbar argon pressure and 10–200 W magnetron discharge power. The deposition rate of copper in a substrate positioned at 18 cm from the target was also measured with a quartz microbalance. The gas temperature, in the range 300–380 K, was deduced from the emission spectral profile of N2(C 3Πu − B 3Πg) 0–0 band at 337 nm when trace of nitrogen was added to the argon feed gas. The isotope-shifts and hyperfine structures of electronic states of Cu have been taken into account to deduce the emission and absorption line profiles, and hence for the determination of atoms' densities from the measured absorption rates. To prevent error in the evaluation of Cu density, attributed to the line profile distortion by auto-absorption inside the lamp, the lamp current was limited to 5 mA. Density of Cu(2S1/2) atoms and deposition rate both increased with the enhanced magnetron discharge power. But at fixed power, the copper density augmented with argon pressure whereas the deposition rate followed the opposite trend. Whatever the gas pressure, the density of Cu*(2D5/2) metastable atoms remained below the detection limit of 1 × 1010 cm−3 for magnetron discharge powers below 50 W and hence increased much more rapidly than the density of Cu(2S1/2) atoms, over passing this later at some discharge power, whose value decreases with increasing argon pressure. This behaviour is believed to result from the enhancement of plasma density with increasing discharge power and argon pressure, which would increase the excitation rate of copper into metastable states. At fixed pressure, the deposition rate followed the same trend as the total density of copper atoms in the ground and metastable states. Two important conclusions of this work are (i) copper atoms sputtered from the target under ion bombardment are almost all in the ground state Cu(2S1/2) and hence in the plasma volume they can be excited into the metastable states; (ii) all atoms in the long-lived ground and metastable states contribute to the deposition of copper layer on the substrate.

Excitation, ionization, and electron capture cross sections for collisions of Li 3+ with ground state and excited hydrogen atoms

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were observed making use of state dependent quenching effects caused by small admixtures of ${\mathrm{H}}_{2}$ molecules. By selectively shortening the lifetimes of states with higher principal quantum number $n$ as compared to those of lower $n$, this method for the first time provides access to all initially populated metastable states of $\overline{p}{\mathrm{He}}^{+}$ atoms. This was demonstrated by observing the transitions $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(38,l)\ensuremath{\rightarrow}(39,l+1)$, $l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}35,36,37$ and $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(37,l)\ensuremath{\rightarrow}(38,l+1)$, $l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}34,35,36$.

An extended Heitler-London-Pauling-Slater calculation is given for methane. In addition to the usual quadrivalent configuration of the carbon atom, divalent and other configurations with which it may ``resonate'' are considered, in order to ascertain whether or not s2p2 carbon plays a more important part than usually is supposed. The mathematical formulation, an example of the use of the vector model, leads to a seventh degree secular equation, which is solved for empirical values of the integrals. The influence of resonance upon the energy is found to be small, but not negligible, and increases the stability of the molecule about 1.2 volts over the value for the usual Pauling-Slater configuration. The state of the carbon atom when it forms four electron-pair bonds is obtained explicitly in terms of the atomic states, and is found to comprise 5S, 3D and 1D of sp3, together with 3P and 1D of s2p2 and of p4. This, the ``valence state'' of Van Vleck, is 7 volts above the ground state of the carbon atom; resonance lowers it about 2.5 volts. The resonance causes a greater modification in the wave function and in the valence state than it does in the energy. In the final section the relative energies of CH, CH2, CH3 and CH4 are examined under the approximation of electron pairing, and are shown to have the regularly graded values of 4.0, 8.0, 12.1 and 17.0 volts when the experimental values for CH and CH4 are used to determine the unknown integrals. There is no indication that CH2 occupies a favored position, and the valence of four for carbon remains understandable in the light of the theory.

We report the discovery of predissociation channels in H{sub 3} at energies within a few meV of the lowest ionization threshold. The predissociation affects selected {ital p}- and {ital d}-Rydberg states with high principal quantum numbers. For the {ital d} states, the predissociation process is induced by very weak electric fields, of order less than 1 V/cm. The predissociations are due to interaction with vibrationally excited states of low principal quantum number that exhibit significant coupling to the repulsive ground-state surface of H{sub 3}, 2{ital p}{sup 2}{ital E}{prime}. We discuss the possible significance of these predissociation channels for decay paths in dissociative recombination of H{sub 3}{sup +} with low-energy electrons.

Precision calculations of the ground and low-lying excited electronic states of the Rf atom (Z = 104) were performed. The relativistic effects were taken into account with the Dirac-Coulomb-Breit Hamiltonian. The finite nucleus size was taken into account in the Fermi model (A = 267). The correlation effects were described by the combined scheme that we suggested previously, which proved to be efficient: The relativistic coupled clusters method with one- and two-particle cluster amplitudes in the Fock space was used for describing the correlations of 44 outer electrons of the Rf atoms in a fairly large atomic basis set, and for the four valence electrons occupying the 7s, 7p, and 6d states, the contributions from three- and four-particle cluster amplitudes were taken into account using the complete configuration interaction method, but in a relatively small basis set. The relativistic configuration and term for the ground state of the Rf atom were refined, and the energies of its excitation to low-lying states were calculated. The accuracy of the calculation by the generalized relativistic effective core potential method was evaluated by comparing the results with those obtained similarly using the Dirac-Coulomb-Breit Hamiltonian.

Ground and low-lying electronic states of Cr atoms in the gas phase were generated from photolysis of <TEX>$Cr(CO)_6$</TEX> vapor in He or Ar using an unfocussed weak UV laser pulse and their reactions with <TEX>$O_2$</TEX> and <TEX>$N_2O$</TEX> were studied. When 0.5-1.0 Torr of <TEX>$Cr(CO)_6$</TEX> /<TEX>$O_2$</TEX> /He or Ar mixtures were photolyzed using 295-300 nm laser pulses, broadband chemiluminescence peaked at ~420 and ~500 nm, respectively, was observed in addition to the atomic emissions from <TEX>$z^7P^{\circ}$</TEX>, <TEX>$z^5P^{\circ}$</TEX>, and <TEX>$y^7P^{\circ}$</TEX> states of Cr atoms. When <TEX>$N_2O$</TEX> was used instead of <TEX>$O_2$</TEX>, no chemiluminescence was observed. The chemiluminescence intensities as well as the LIF intensities for those three low-lying electronic states (<TEX>$a^7S_3,\;a^5S_2\;and\;a^5D_J$</TEX>) showed second-order dependence on the photolysis laser power. Also, the chemiluminescence intensities were first-order in <TEX>$O_2$</TEX> pressure, but the presence of excess Ar showed a strong inhibition effect on them. Based on the experimental results, the chemiluminecent species in this work is attributed to <TEX>$CrO_2^*$</TEX> generated from hot ground state Cr atoms with <TEX>$O_2$</TEX>. The apparent radiative lifetimes of the chemiluminescent species and collisional quenching rate constants by <TEX>$O_2$</TEX> and Ar also were investigated.

DFT study on the reaction of neutral Ti and Ni atoms with CS 2

The X-ray signature of L-shell charge exchange in sulfur was studied in the laboratory. A comparison of the charge exchange (CX) spectra with those obtained under electron-impact excitation showed marked differences. In the CX spectra, an enhancement was observed in the transitions from levels with high principal quantum numbers, n = 4, 5, 6 → n = 2 in comparison with the n = 3 → n = 2 transitions that dominate the direct excitation spectra. An even greater enhancement was recorded in the transitions from the levels of electron capture to the ground states: n = 7, 8, 9 → n = 2. The spectra mainly consist of emission from S XIV, but lower charge states such as S XIII, S XII, and S XI also contribute. The results have been compared with observations made by the Chandra and XMM-Newton X-ray Observatories of Jupiter's polar regions. The enhancement we noticed in transitions from the high-n levels is not seen in the Chandra spectra.