Optical pumping of the ground states of sodium can radically alter the shape of the laser-induced fluorescence excitation spectrum, complicating measurements of temperature, pressure, etc., which are based on these spectra. Modeling of the fluorescence using rate equations for the eight hyperfine states of the sodium D manifolds can be used to quantify the contribution to the ground state pumping of transitions among the hyperfine excited states induced by collisions with buffer gas atoms. This model is used here to determine, from the shape of experimental spectra, cross sections for $\Delta F$ transitions of the $P_{3/2}$ state induced by collisions with helium and argon atoms, for a range of values assumed for the $P_{1/2},\hspace{0.5em}\Delta F$ cross sections. The hyperfine cross sections measured using this method, which to our knowledge is novel, are compared with cross sections for transitions involving polarized magnetic substates $m_F$ measured previously using polarization sensitive absorption. Also, fine-structure transition $\left(\Delta J\right)$ cross sections were measured in the pumped vapor, giving agreement with previous measurements made in the absence of pumping.

The decay of individual L subshell vacancies in ${}_{47}\mathrm{Ag}$ has been investigated through selective photoionization by monochromatized synchrotron radiation. By tuning the radiation energy over the L edges, the ionization of particular subshells can be turned off and on. The corresponding subshell ionization cross sections including the subtle structure caused by electron correlations are known from our previous studies. The x-ray fluorescence emitted in the vacancy decay was recorded by a Si (Li) detector. Advanced fitting techniques were employed to extract the intensities of lines originating from different subshells. The extracted line intensities were plotted versus energy of the primary radiation. The relative jumps of the line intensities occurring at the edges directly correspond to the Coster-Kronig yields, whereas the absolute values of the total subshell fluorescence give the fluorescence yields. The following Coster-Kronig yields were obtained: $f_{12}=0.14\mathrm{}\left(3\right),$ $f_{13}=0.58\mathrm{}\left(5\right),$ and $f_{23}=0.18\mathrm{}\left(3\right).$ Adopting the value ${\omega }_3=0.056\mathrm{}\left(7\right)\mathrm{}$ from literature, the following fluorescence yields are obtained: ${\omega }_1=0.013\mathrm{}\left(2\right)\mathrm{}$ and ${\omega }_2=0.067\mathrm{}\left(9\right).$ The results agree reasonably with theoretical calculations. In particular, the rather large value of $f_{13}$ indicates that the $L_1{-L}_3{M}_{4,5}$ Coster-Kronig channel is well allowed.

High-resolution photoabsorption spectra of CO and NO below the O $1s$ ionization threshold are presented. The vibrational fine structure of the O $1\overrightarrow{s}{\pi }^{*}$ and O ${1s}^{-1}$ Rydberg excitations could be resolved for both molecules, allowing a determination of the vibrational energies and intramolecular distances of the core-excitation states in CO and NO from Franck-Condon analyses. Ab initio calculations are performed for the O $1\overrightarrow{s}{\pi }^{*}$ excitation in CO to give an independent confirmation of the spectroscopic parameters derived from the Franck-Condon analysis. The spectral features of the O ${1s}^{-1}$ Rydberg region in CO are reassigned on the basis of the experimental results. The results obtained for the O ${1s}^{-1}3s$ Rydberg state in NO support the idea of a weakening of the molecular bond upon an O ${1s}^{-1}$ ionization process.

Using a Hartree-Fock mesh method and a configuration-interaction approach based on a generalized Gaussian basis set, we investigate the behavior of the exchange and correlation energies of small atoms and molecules, namely, the helium and lithium atoms as well as the hydrogen molecule, in the presence of a magnetic field covering the regime $B=0\mathrm{}–100\hspace{0.5em}\mathrm{a}.\mathrm{u}.\mathrm{}$ In general, the importance of the exchange energy to the binding properties of atoms or molecules increases strongly with increasing field strength. This is due to the spin-flip transitions, and in particular due to the contributions of the tightly bound hydrogenic states which are involved in the corresponding ground states of different symmetries. In contrast to the exchange energy the correlation energy becomes less relevant with increasing field strength. This holds for the individual configurations constituting the ground state and for the crossovers of the global ground state.

Excitation energies and oscillator strengths of neutral calcium (Ca I) are calculated through third order using a variant of the multireference many-body perturbation theory, known as the effective valence-shell Hamiltonian ${(H}^v)$ method. Transition energies, oscillator strengths, and binding energies of various states are computed and compared with experimental and other theoretical data. These quantities are in favorable agreement with the experiment and other correlated calculations. The basis and valence space dependence of the $H^v$ scheme are addressed with some illustrative examples.

The existence of three-body resonances in the molecular ion $\mathrm{pp}\pi$ has been shown by means of a variational calculation in Jacobi coordinates. The relativistic and strong-force corrections to the Coulombic binding energies and the decay widths of the resonances have been calculated. We consider two mechanisms that supply the $p\pi$ atom with large kinetic energies before the moment of pion absorption by the proton, with significant impact on the interpretation of experiments measuring the mass difference $m_{{\pi }^{-}}-m_{{\pi }^0}$ from pion absorption in pionic atoms. These mechanisms are (a) the formation and subsequent Coulomb decay of the metastable $\mathrm{pp}\pi$ states and (b) pion absorption from the molecular $\mathrm{pp}\pi$ state, giving, respectively, a discrete or a continuous distribution of kinetic energies of the resulting $p\pi$ atoms.

The calculational scheme for an approximate ground-state energy of a many-electron system, recently proposed by Goedecker and Umrigar (GU) [Phys. Rev. Lett. 81, 866 (1998)], is analyzed in terms of the reduced-density-matrix approach. Its underlying object, the GU density-matrix functional, is generalized, possibly leading to improved energies. Properties of the solutions of the ground-state problem obtained from the original and the generalized GU functionals are established.

The problem of calculating Franck-Condon overlap integrals in molecular transitions between vibrational states in different electronic configurations is addressed. An exact and easily applicable analytical expression is obtained when the vibrational states can be approximated by eigenstates of Morse potentials with different strengths and locations but identical ranges. An approximate analytical expression is obtained for the general case. The method is applied to the stretching S-S mode corresponding to $C^1{v}_2\leftarrow X^1{v}_1$ transitions in the ${\mathrm{S}}_2\mathrm{O}$ molecule.

Autoionization widths of atomic states with an outer electron in a large angular momentum l are analyzed using Coulomb Green’s function. The analytical representation and the Sturmian expansion of this function are both used. The method is applied here to low doubly excited states of alkaline-earth-metal atoms, among which ${3d}_j\mathrm{ng}$ in Ca, ${4d}_j\mathrm{ng}$ in Sr, and ${5d}_j\mathrm{nl}$ in Ba. For the ${5d}_{5/2}\mathrm{nh}\hspace{0.5em}J=5\mathrm{}$ levels of barium, the predicted width accurately agrees with previous measurements. Concerning the ${5d}_j\mathrm{ng}$ levels of barium, the method provides figures in generally good agreement with experimental or theoretical data if available. It demonstrates that a series of previously reported widths arise only from experimental broadening; new values are reported for such states.

A systematic measurement of the lifetimes and tensor polarizabilities of the lowest-lying odd-parity levels of Sm I was performed. The lifetimes were measured by detecting time-resolved fluorescence following pulsed-laser excitation of atoms in an atomic beam; polarizabilities were measured employing the method of Stark-induced quantum beats. An analysis of the data is undertaken to find the best even-parity candidate states for an atomic electric dipole moment (EDM) measurement. For the most favorable candidate state ${(}^7{G}_1),$ the electron EDM enhancement factor is evaluated to be in the range $|R|\approx 100–3800.$ Critical analysis of the present data along with earlier results in Sm shows the necessity of a term reassignment for several odd-parity states. This term reassignment is also used in an estimate of the parity-nonconserving ${}^7{\overrightarrow{F_0}}^7{G}_1$ amplitude.

Borrowing heavily from the literature, we develop a transport relaxation equation for the off-diagonal elements of the density matrix. It is applicable to a large range of topics in laser theory, nonlinear optics and spectral line shapes, etc. Applying this single equation to the case of infrared electric dipole transitions permits us to recover, quickly and simply, all of the well-known spectral line shapes found in the literature. It also offers further insight into the process of line mixing, particularly in the weak mixing limit. An outline of a new treatment of Dicke narrowing and other speed-dependent contributions to spectral profiles is also given in terms of the transport relaxation equation. The treatment also provides a numerical method for including the effects of statistical correlation between the evolution of the internal and translational degrees of freedom.

In the preceeding paper it was shown that the general problem of spectral line shapes in multiline, IR spectra may be determined by solving a transport relaxation equation for the off-diagonal elements of the density matrix. This is a semiclassical equation at the Wang-Chang–Uhlenbeck level, i.e., it treats the quantized internal states as nondegenerate. Here we apply the master equation to the case of Dicke narrowing, and by discretizing the velocity distribution show that Dicke narrowing of a single line may be treated in exactly the same manner as line mixing. Both effects lead to a narrowing of a spectral distribution. We indicate how the numerical technique can be extended and used to calculate profiles in the general case of spectra with speed-dependent broadening, shifting, and line mixing.

We have observed the Stark-induced ${6s}^2{}^1{S}_0\to 5d6s{}^3{D}_1$ transition in Yb, and measured properties relevant to the study of atomic parity nonconservation in this transition. An atomic beam of Yb is excited by 408-nm light in the presence of an external dc electric field, resulting in cascade fluorescence at 556 nm as the excitation decays through the $6s6p{}^3{P}_1$ state. The density of the atomic beam and the fluorescence detection efficiency are calibrated by directly exciting the $6s6p{}^3{P}_1$ state with 556-nm light. Using an estimate of the ${}^3{D}_1{-}^3{P}_1$ branching fraction, we obtain a measurement of the vector transition polarizability, $|\beta |=2.18\mathrm{}\left(33\right)\mathrm{}\times {10}^{-8}\hspace{0.5em}{\mathrm{ea}}_0/(\mathrm{V}/\mathrm{cm}).\mathrm{}$ In addition, we have observed the ${6s}^2{}^1{S}_0\to 5d6s{}^3{D}_2$ transition at 404 nm. The electric quadrupole transition amplitude and tensor transition polarizability have been measured. We have also measured the dc Stark shifts, hyperfine structure, and isotope shifts for both transitions.

At relativistic energies the cross section for the atomic photoelectric effect drops off as does the cross section for liberating any bound electron through Compton scattering. However, when the photon energy exceeds twice the rest mass of the electron, ionization may proceed via electron-positron pair creation. The cross section for this channel saturates at several times the threshold energy, and hence the most probable way of photoionizing an atom at high energy is to make the vacuum spark. We present estimates for the cross section of this “vacuum-assisted photoionization” which proceeds either via direct pair creation on the bound electron or via pair creation on the atomic nucleus binding the inner-shell electron followed by an electron-electron or positron-electron encounter.

Absolute differential cross sections (DCS’s) are reported for charge-transfer scattering of 0.5-, 0.85-, 1.5-, 2.8-, and 5.0-keV ${\mathrm{O}}^{+}$ ions by ${\mathrm{H}}_2$ molecules at angles between 0.04° and 5.4° in the laboratory frame. Cross sections for ${\mathrm{O}}^{+}{(}^{4}S)$ ground state and ${\mathrm{O}}^{+}{(}^2{D,}^{2}P)$ metastable ions are presented. The ground-state differential cross section is much less forward peaked than is the metastable differential cross section. Over most of the range of impact energies studied, the ${\mathrm{O}}^{+}{(}^{4}S)$ total cross section is found to be approximately six times smaller than that for ${\mathrm{O}}^{+}{(}^2{D,}^{2}P).\mathrm{}$ Based on the DCS data reported here, a possible explanation for the large discrepancies in the published ground-state total cross sections is given. The present data confirm the existence of a minimum in the ground-state total cross section at about 2 keV, which has been observed by some other workers.

An implicit fast Fourier transform algorithm is implemented to solve the time-dependent Schrödinger equation with application to charge-exchange collisions. Cross sections are calculated for ${\mathrm{He}}^{2+}$ on H and compared with experiment and other theoretical results. A disagreement between previously published theoretical results is resolved.

Using a multiparameter multicoincidence spectrometer, we have measured the coplanar $\mathrm{}(e,3e)\mathrm{}$ angular distributions following the double ionization of helium at an incident energy of $\approx 5.6\hspace{0.5em}\mathrm{keV}$ and under a small projectile’s scattering angle of $0.45°$. The two ejected electrons have been detected with equal energies $E_b{=E}_c=10\mathrm{}\hspace{0.5em}\mathrm{eV}$. The absolute value of the cross section is determined with an accuracy of $25\%$. The origin of dips and peaks in the spectra is exposed by analyzing the corresponding theoretical calculations. These calculations have been done using a four-body final-state wave function for the three electrons moving in the field of ${\mathrm{He}}^{2+}$. The dipolar limit is investigated and the manifestation of the deviation from this limit are pointed out. General features and possible trends for other targets are proposed.

A method is suggested to find positions and widths of resonances with the use of an arbitrary real square integrable basis. The procedure is based on the fact that the parameters of a resonance are related to the continuum level density. The method is a generalization of the box quantized $L^2$-stabilization technique of Mandelshtam, Ravuri, and Taylor [Phys. Rev. Lett. 70, 1932 (1993)]. For illustration, the method is applied to the s-wave potential scattering case.

Multi-ionization and fragmentation of ${\mathrm{C}}_{60}$ fullerenes induced by collisions with ${\mathrm{Ar}}^{8+}$ ions have been studied in correlation with the energy gain and the number of electrons captured and stabilized by the projectile ion. The method allows us to separate electron capture reactions from transfer ionization processes and to determine the number (r) of active electrons. When one electron is stabilized on the projectile, the target ion ${\mathrm{C}}_{60}^{r+}$ is left intact and the energy gain increases with the charge r, which ranges up to $r=4.\mathrm{}$ The corresponding mean energy gain values for production of ${\mathrm{C}}_{60}^{+}$ through ${\mathrm{C}}_{60}^{4+}$ are used together with three different models for the electronic response of ionized ${\mathrm{C}}_{60}$ in order to deduce semiempirical electron transfer distances for the first four electrons. A model with localized and mobile charges on the surface of the molecule gives a slightly better agreement with earlier measured recoil ion production cross sections than the metal sphere model or an assumption with localized charges kept fixed closest to the projectile during the collisions. The mean energy gain depends on the number of stabilized electrons s. It increases between $s=1\mathrm{}$ and 2, then it stays constant, and finally decreases between $s=5\mathrm{}$ and 6. The energy distribution for $s=6\mathrm{}$ extends to the energy-loss side, which is attributed to close collisions causing a strong electronic excitation of ${\mathrm{C}}_{60}.$

Ab initio potential-energy curves and coupling matrix elements of the molecular states involved in the collision of the ${\mathrm{B}}^{3+}{(}^{1}S)$ multicharged ion on molecular hydrogen have been determined by means of configuration-interaction methods. The total and partial electron-capture cross sections have been evaluated in the frame of a semiclassical approach in the 10–170-keV laboratory energy range. Anisotropy and vibrational dependence have been examined, at both the molecular calculation and collision dynamics levels.