Reactions of excited atoms and molecules with atoms and molecules

From a He-beam excited by electron impact we eliminated the He(2¹ S) component to better than 0.5% by irradiating light from a He discharge. The quenching process ishv(2¹ P→2¹ S)+He(2¹ S)→He(2¹ P)→He(1¹ S +hv) (2¹ P 1)¹ S. By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He (2¹ S) and He(2³ S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He-beam, 320 and 90 °K. It is found that the cross section ratio of associative — to Penning ionization increases considerably as the temperature is decreased for both, He(2¹ S) and He(2³ S), the effect being much more pronounced for He(2¹ S). The results of this work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.     

Reactions of excited atoms and molecules with atoms and molecules

Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH⁺, RH ₂ ⁺ , and H ₂ ⁺ by collisions of excited rare gas atoms R^* with H₂. With HD as the target molecule the isotope effect for the production of RD⁺ and RH⁺ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.     

Reactions of excited atoms and molecules with atoms and molecules

Penning electron distributions arising from the ionization of Na and K by He (1s 2s ^1,3 S)-metastables in thermal collisions, as well as the absolute cross section for Penning ionization of Na by He (2³ S) and relative cross sections for ionization of Na and K by He(2¹ S) and He(2³ S) are measured. It is shown that under fairly general conditions the well depth ε* of the interaction potential between the metastable and the target particle can be obtained directly from the measured electron distributions. ε*-values are reported for the moleules He(1s 2s ^1,3 S)-Na(² S), K(² S) (^2,2Ω), and for He(1s 2s ^1,3 S)-Hg(¹ S)(^1,3Ω). These latter values are obtained from previously published measurements and are to be considered preliminary. Further, additional evidence is given, that Penning ionization with metastables is an electron exchange process.     

Solving the Kadanoff-Baym equations for inhomogeneous systems: application to atoms and molecules

We implement time propagation of the nonequilibrium Green function for atoms and molecules by solving the Kadanoff-Baym equations within a conserving self-energy approximation. We here demonstrate the usefulness of time propagation for calculating spectral functions and for describing the correlated electron dynamics in a nonperturbative electric field. We also demonstrate the use of time propagation as a method for calculating charge-neutral excitation energies, equivalent to highly advanced solutions of the Bethe-Salpeter equation.     

Density functional calculation of K-shell spectra of small molecules

Both ΔE_KS and time-dependent density functional theory (TD-DFT) methods, with approximations for the singlet–triplet splitting and for the relativistic corrections, were tested for the calculation of K-shell spectra of Ne, HF, H₂O, NH₃, CH₄, and CO. Results from several exchange-correlation functionals as well as diffuse basis sets were compared with available experimental data. Excellent core excitation and core-electron ionization energies for Ne, HF, H₂O, NH₃, CH₄, and CO can be obtained from ΔE with Perdew–Wang 1986 exchange and Perdew–Wang 1991 correlation functionals; and reasonable intensities for singlet excitations, from TD-DFT with exchange-correlation potential known as statistical average of orbital potentials. The dependence of the quality of ΔE on basis set is as expected: excitations to higher Rydberg levels requiring more diffuse functions. However, the oscillator strength seems to be more sensitive to the quality of the basis set. Suggestions are made for extending the procedure to larger systems.     

Electronegativity of fractionally charged atoms in the Density Functional Theory

The electronegativity (identified with the negative of the chemical potential) of atoms and ions has been calculated in several isoelectronic series using the Density Functional Theory. Then, the electronegativities of atoms and ions with fractional nuclear charge have been obtained by interpolation in each isoelectronic series. Similar interpolations have been performed, starting with approximate electronegativities obtained by Mulliken's finite difference approximation. Both sets of results have been compared.     

Plasmonic atoms and plasmonic molecules

The proposed paradigm of plasmonic atoms and plasmonic molecules allows one to describe and predict the strongly localized plasmonic oscillations in the clusters of nanoparticles and some other nanostructures in uniform way. Strongly localized plasmonic molecules near the contacting surfaces might become the fundamental elements (by analogy with Lego bricks) for the construction of fully integrated opto-electronic nanodevices of any complexity and scale of integration. PACS 78.67.-n; 73.20.Mf; 32.50.+d     

Role of metastable atoms and molecules of helium in excitation transfer to hydrogen atoms

The afterglow of a discharge in helium with a small admixture of hydrogen is studied spectroscopically (p=40 Torr, [e]≤10¹¹ cm^?3). The time-resolved measurements of intensities of the first four lines of the Balmer series are performed. The concentrations of metastable helium atoms and molecules are evaluated from the relative intensity of the absorption lines. The ratios of excitation transfer rates from atoms He(2 ³ S ₁) k ₁(n) and molecules of helium He₂(a 2sσ ³Σ _u ⁺ ) k ₂(n) to atomic hydrogen H*(n) are measured to be k ₁(n=3)/k ₂(n=3)=0.04±0.02 and k ₁(n=4)/k ₂(n=4)=0.01±0.02. The ratios of excitation rate constants k ₂(n) corresponding to different states H(n) are measured to be k ₁(n=4)/k ₂(n=3)=0.023±0.01; k ₁(n=5)/k ₂(n=3)≤0.013; and k ₁(n=6)/k ₂(n=3)≤0.007.     

Calculation of optical properties within the Local Density Approximation to Density Functional Theory: application to palladium

We calculate the optical functions of Pd using the ab initio, all-electron Full Potential Linear Muffin Tin Orbital method within the framework of the Density Functional Theory in the Local Density approximation. We test, in the case of Pd, the convergence of the dielectric function and energy loss function in different energy ranges vs. the completeness of the basis and give a quantitative estimate of the accuracy. The present approach opens the possibility of extending the energy range where the optical functions can be calculated with good accuracy without increasing the computational effort. Received 4 September 2001     

Highly spin-polarized atoms and molecules from rotationally state-selected molecules

It is shown that large nuclear polarizations in isolated molecules may be created via the hyperfine interaction following excitation to selected rotational states |JM>. Explicit time-dependent nuclear polarization expressions for pulsed rotational excitation are presented for the case of one nuclear spin I, and for the case of two nuclear spins I1 and I2 in the hierarchical coupling limit. Photodissociation of the polarized molecules allows the production of polarized atoms, on short time scales if pulsed, at densities close to the parent molecule density.     

Femtosecond dynamics and coherent control in atoms, molecules and clusters: Theory and experiments

The understanding and control of the dynamics in atoms, molecules and clusters has been a tremendously growing field in the past decade. This has been acknowledged with the 1999 Nobel prize in chemistry awarded to Ahmed Zewail. The present issue collects some of the newest theoretical and experimental results in the field of ultrafast dynamics and coherent control in the gas phase. The papers are grouped into three categories. The first section contains work on the “Coherent Control with Femtosecond Laser Pulses”. Topics like the general theory of quantum control, the control of electron transfer processes, transformation of chiral molecules and coherent population transfer are treated here. The “Femtosecond Dynamics” taking place in molecules and clusters is the topic of the second section. New insight into the nature of atomic motion within molecules and relaxation processes is provided. The last section collects most recent work on the interaction of ultrashort laser pulses with matter. In particular, high harmonic generation, multi-photon ionization and interference effects, as well as the possible orientation of molecules in external fields are discussed. We think that the present compilation demonstrates that the field of ultrashort pulse spectroscopy is of still growing importance and exciting new phenomena have been revealed in the past and will be discovered in the future.     

Isolated atoms,molecules and clusters

The basic constituents of matter, namely, atoms molecules and small clusters can be isolated in a solid rare-gas-matrix (RGM) and be characterized by Mössbauer Spectroscopy (MS). Experiments can be conducted both by the absorption method using a co-evaporation method of Mössbauer isotopes or by emission spectroscopy where source nuclei are isolated in the RGM. Experimental aspects of both techniques are outlined. Examples of applications of RGMI-MS are described, namely,⁵⁷Fe clustering with noble-metals (Cu, Ag, Au); unusual electronic states of Fe following⁵⁷Co e.c. decay; species formed by the bonding of Fe⁰, in its ground state, to small molecules are compared with those formed by Fe in its various excited states; and finally, the characterization of atomic I⁰ and Te⁰ and Te^?1 embedded in a solid argon matrix.     

Electron scattering by atoms and molecules

Recent advances in the scattering of electrons by free atoms and molecules are considered with an emphasis on the interface between experiment and theory. A survey is made of the theory and methods of measurement and their trend towards the perfect scattering experiment. For atomic targets a detailed coverage is given of the various theoretical models and the results compared with experimental data. For elastic and excitation collisions, the low and high energy regions are reasonably well understood. In the medium energy region, extending from a few times the ionization threshold down to about the ionization threshold there are still difficulties arising from the infinite number of open channels. For ionizing collisions, the three body Coulomb interaction still presents unanswered theoretical problems but has inspired some elegant experimental studies. For molecular targets the additional complexities arising from the nuclear motion, the lack of a centre of symmetry and the evaluation of multicentre integrals are considered. A brief discussion of some current problems and possible future developments is given.     

Optomechanical control of atoms and molecules

We briefly review some of our recent and ongoing work on nanoscale optomechanics, an emerging area at the confluence of atomic, condensed matter and gravitational wave physics. A central tenet of optomechanics is the laser cooling of a moving mirror, typically an end mirror of a Fabry-Perot resonator, to a point near its quantum-mechanical ground state of vibration. Following a general introduction we discuss how the motion of such a macroscopic quantum oscillator can be squeezed, and then show how the placement of a ferroelectric tip on the oscillator allows the coherent manipulation and control of the center-of-mass motion of ultracold polar molecules.     

A density functional theory and quantum theory of atoms in molecules study on hydrogen bonding interaction between paracetamol and water molecules

To consider the hydrogen bonding interactions between paracetamol and water molecules, probable complexes of paracetamol from three active sites (carbonyl oxygen atom, hydroxyl oxygen atom, and nitrogen atom) with H₂O molecule were formed. The optimized geometries and total energies have been obtained at the B3PW91/6-31+G(d, p) level of theory. Comparison of hydrogen bond lengths and the energies of complexes showed hydrogen bond that form between the oxygen atom of the carbonyl group and hydrogen atom is stronger than others. Moreover, an increase in the number of hydrogen bonds increases stability of paracetamol-water complexes. At the end, the QTAIM was applied to explain the nature of the hydrogen bonds and confirm the more stability by complexation.     

Ultracold metastable helium: from atoms to exotic molecules

We have written this article in the honor of our friend Vladilen Lethokov, who has produced so many brilliant ideas and results in the field of atomic and molecular physics and with whom we have had frequent unlightening discussions. We hope that the work described here will contribute to illustrate the richness of the field.     

Nonlinear adiabatic passage from fermion atoms to boson molecules

We study the dynamics of an adiabatic sweep through a Feshbach resonance in a quantum gas of fermionic atoms. Analysis of the dynamical equations, supported by mean-field and many-body numerical results, shows that the dependence of the remaining atomic fraction Gamma on the sweep rate alpha varies from exponential Landau-Zener behavior for a single pair of particles to a power-law dependence for large particle number N. The power law is linear, Gamma is proportional to alpha, when the initial molecular fraction is smaller than the 1/N quantum fluctuations, and Gamma is proportional to alpha(1/3) when it is larger. Experimental data agree well with a linear dependence, but do not conclusively rule out the Landau-Zener model.     

Vibrational excitation of molecules by resonant electron scattering: Theory and application to benzene

A formalism to describe the vibrational excitation of molecules via resonances is presented. For the limiting cases of short and long lifetime of the negative ion the excitation probability can be calculated using a golden rule expression and a simple Hamiltonian. The intensity distribution of the electron energy loss spectrum is expressed in terms of electron-vibration coupling constants which depend on the lifetime of the negative ion. Spectra for resonant scattering on benzene-d₆ are calculated and good agreement with experiment is obtained.