Photodissociation Dynamics of 2-Iodotoluene Investigated by Femtosecond Time-Resolved Mass Spectrometry

The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond time-resolved mass spectrometry. The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 38±50 fs was measured from the rising time of the co-fragments of toluene radical (C₇H₇) and iodine atom (I), which is attributed to the averaged time needed for the C-I bond breaking for the simultaneously excited nσ^* and ππ^* states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated nσ^* and ππ^* states. And a rise time of 40±50 fs is extracted from the fitting of time-dependent I⁺ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.     

Ground-state energies of atomic chains in superstrong magnetic fields

The ground-state energies of hydrogenic and helium chains in superstrong magnetic fields (B ≈? 10⁷?10⁹ Tesla) have been calculated within the single-particle scheme of a heuristic density functional method. Applying the local density approximation (LDA ), the simple Dirac exchange functional was used neglecting correlation effects. The equations were solved iteratively using a basis set of Landau functions in the transverse directions and directly integrating along the longitudinal direction. We obtained binding lengths and, by comparing the binding energies of the chains with the ground-state energies of single atoms, we found condensation energies. © 1995 John Wiley & Sons, Inc.     

Electronic energy transfer between metastable argon atoms and ground-state oxygen atoms

The transfer of electronic energy between metastable argon and ground-state oxygen atoms has been studied in a discharge-flow apparatus. The excitation energy of the argon metastables is transferred to the 3p ³P state of atomic oxygen with a cross section of 3 A². The energy transfer is discussed in terms of an ionic-intermediate, curve-crossing mechanism for which the calculated cross section is 13 A².     

On the third virial coefficient for the alkali metal vapours

Theoretical calculations of the third virial coefficient for the pure components of all alkali metal vapours have been carried out using recently reported potential energy surfaces for the doublet and quartet states in which the interaction of three ²S ground-state atoms may evolve. The discrepancy between the theoretical and available experimental estimates for those coefficients is pointed out.     

Erratum

Peculiar transitions in the optical spectra of Mn atoms isolated in argon matrices confirm the existence of Mn₂ molecule. The temperature variation of the absorption and MCD intensity in the range 13-27 K demonstrate that the Mn₂ molecule is antiferromagnetic and allows a precise determination of its ground-state exchange energy: J = ?10.3 ± 0.6 cm^?1.     

Ultraviolet spectra and structure of the isomeric dihydroxy- and diamino-substituted benzenes

The semi-empirical all-valence-electron SCF-LCAO-MO-CI method in its Del Bene and Jaffé formalism has been applied to the study of the electronic absorption spectra and structure of isomeric dihydroxy- and diamino-substituted benzenes. The calculated π-π^* singlet excitation energies and corresponding oscillator strengths have been compared with earlier π-electron results and discussed in the light of experimental findings. Ionization potentials, electron affinities, ground-state atomic orbital populations and net electronic charges of atoms, have also been presented.     

Atomic calculations through a modified Thomas-Fermi-Dirac-Weizsäcker theory

By using an approximate analytical trial density and the consideration of a Thomas-Fermi-Dirac Weizsäcker energy density functional, total ground-state energies, its different components and the expectation values 〈r ⁿ 〉 withn=?1, 1 and 2 are calculated for several atoms in the Periodic Table. The results are compared with Hartree-Fock values.     

Reaction of oxygen plasma active species with polyethylene

The composition of the polyethylene surface upon treatment in an oxygen plasma and its afterglow was studied by attenuated total reflectance IR spectroscopy and X-ray photoelectron spectroscopy. The oxidation of the surface at the lowest destruction rates was attained upon simultaneous action of excited O₂(a¹Δ_g) and ground-state oxygen molecules. However, O(³ P) atoms are involved in both the formation of oxygen-containing groups and their destruction accompanied by polymer degradation.     

Temporally resolved laser diagnostic measurements of atomic hydrogen concentrations in RF plasma discharges

Ground-state atomic hydrogen produced in radio-frequency plasma discharges (20 KHz-5 MHz) has been detectedin situ using two-photon absorption laser-induced fluorescence (TALIF). Atomic ground-state concentration measurements have demonstrated excellent spatial resolution in the interelectrode gap of a planar discharge configuration with 10 nsec temporal resolution at all phases of the RF driving voltage waveform. Concentrations were measured in gas mixtures of helium and hydrogen down to 2×10¹³ H atoms/cm³.     

Cluster beams from passivated nanocrystals

Stable, intense beams of large metal clusters (from 10² to 10³ Au or Ag atoms) can be produced by laser desorption of molecular films of passivated nanocrystals that have undergone fractionation by size and a separate structural characterization. The mass onset of the desorbed species corresponds directly to the dimensions of the nanocrystal core, indicating that only the surfactant shell is lost during desorption. Photofragmentation, photoionization, and photodetachment from beams of large metal clusters, generated in this way, have all been observed.     

Radiative lifetimes and depopulation cross-sections of excited rubidium2 S and2 D states

The radiative lifetimes of excited rubidiumn ² S (n=8?13) andn ² D (n=6?11) states have been measured using the time resolved laser induced fluorescence method. The cross-sections for depopulation of the excited states in collisions with the ground-state Rb atoms have been determined. The influence of the blackbody radiation on the measured values is discussed.     

Long-range interaction between 1s and 2s or 2p hydrogen atoms

Long-range interaction energy between two hydrogen atoms has been computed in the second order of the perturbation theory. All states of the system arising when one of the atoms is in the 1s and the other in the 2s or 2p state have been considered. The energy represented by a series expansion in inverse powers of the internuclear distance, R, has been computed up to the terms in R^?8. The results are believed to give reliable interaction energies for R > 15 a.u. Accurate interaction energy for two ground-state hydrogen atoms has also been obtained up to the terms in R^?10. Results for the B′ ¹∑ state are employed to discuss the experimental ground-state dissociation energy of H₂, D₂, and HD. For H₂ all values of the dissociation energy obtained from various experimental absorption limits, by using the computed potential energy curve to separate off the effect of rotation, are shown to be satisfactorily consistent. The resulting total energy of H₂ is, however, higher than the most accurate theoretical value.     

Validity of <Emphasis Type="Italic">B</Emphasis>-splines as a universal basis set for atomic Hartree–Fock–Roothaan calculations

The validity of B-splines as a universal basis set for atomic Hartree–Fock–Roothaan calculations is studied. In order to accomplish our aim, the ground-state energies of neutral atoms He–Xe, cations Li ⁺–Xe ⁺, and anions H ^-–I ^- with the nuclear charge Z=54 are calculated by the Hartree–Fock–Roothaan method with the B-spline sets. All radial functions of the atoms and singly charged ions are expanded by common B-spline sets regardless of atomic systems and symmetries of atomic orbitals. The energies obtained by the best B-spline set are in excellent agreement with ten-digit numerical Hartree–Fock results.     

Molecular beam kinetics: The excitation function of the reaction Xe(3P0,2) + Br2 → XeBr* + Br

Fast beams of metastable xenon atoms, Xe(³P_0,2), have been generated by combining the high speed rotor technique with electron bombardment excitation. In a crossed-beam apparatus, the excitation function σ(E_t) of the reaction Xe(³P_0,2) + Br₂ → XeBr* + Br has been measured. The results show a close similarity to the behaviour of the alkali metal atoms, with which the metastable rate gas atoms have been previously compared.     

Inhomogeneous electron liquid in the free-space building block Li2C2 plus its dimer and trimer

Quantum-chemical calculations are first reported on the building block Li₂C₂. Two low-lying isomers are predicted, and hence the equilibrium ground-state geometry by the coupled cluster method (CCSD). The ground-state is non-linear, contrasting with the other planar isomer. The dimer (Li₂C₂)₂ is then examined by the Hartree–Fock plus second-order Møller–Plesset (HF+MP2) method. The ground-state geometry is predicted to have C₄ on a linear chain. As to the trimer, the C₆ atoms are predicted at the same ab initio level to lie on a hexagon. But the Li atoms prefer each to bond to two C atoms, rather than reproduce the geometry of benzene. Finally, some contact is made with a recent density functional theory study of solid lithium carbide under pressure.     

Quantum Monte Carlo for 3d transition-metal atoms

The domain Green's function Monte Carlo method has been used to calculate the ground-state energy of the atoms Sc through Zn. The fixed node approximation with single-configuration explicitly correlated wave functions is used. A comparison with variational Monte Carlo energies is carried out. The quality of the ground-state energies reported here is similar to that achieved for few-electron atoms using similar techniques.     

Production of a spin-polarized,metastable He(23 S) beam for studies in atomic and surface physics

This beam was developed as a target for a crossed-beam electron-atom scattering experiment on the interaction of a polarized spin-1/2 electron with a polarized spin-1 atom. In the future this beam will be used in “Spin-Polarized Metastable Atom Deexcitation Spectroscopy” (SPMDS) for studying ferromagnetic surfaces without and with adsorbate layers. We use a discharge source for producing a beam of metastable helium atoms, a permanent sextupole magnet with a central stop at its exit for selecting He(2³ S) atoms in the Zeeman substatem _s =+1, a zero-field spin flipper for reversing the atomic beam polarization with respect to a magnetic guiding field, and a Stern-Gerlach magnet for analyzing the atomic polarization. At a distance of 90 cm beyond the exit of the sextupole, in the “interaction region” of an experiment, the polarized beam has a circular cross section of about 6 mm FWHM and a particle density of 1 · 10⁷ atoms/cm³. The reversible spin polarization was determined asP=0.90±0.02. A possible contamination of the beam with metastable singlet atoms is included within this value; the ground-state He atoms are not considered to be part of the polarized beam. An observed contamination with long-lived Rydberg atoms can easily be destroyed by applying a high electric field.     

Analysis of the 350–400 nm oscillatory continuum from I2 (D 1Σ+u)

The 350–400 nm oscillatory continuum, observed in emission when I₂ is excited to the D¹Σ⁺_u state (λ = 193 nm, u′ ≈ 134), has been analysed. The lower potential state, which correlates with two ground-state 1 atoms, is found to be purely repulsive between 2.75 and 3.8 Å, the range defined by the observed fluorescence. The band is partially overlapped by another system and there are two possible positions for the band origin, leading to very similar lower states.     

Fine structure excitation transfer between the potassium 42P states induced by collisions with caesium atoms

Applying diode-laser resonant fluorescence method, the cross sections for the excitation energy transfer of the collisional process K*(4² P _1/2)+Cs(6² S _1/2)?K*(4² P _3/2)+Cs(6² S _1/2) have been measured. The values we have obtained are σ(1/2→3/2)=77 Ų and σ(3/2→1/2)=48 Ų. These results complete the sequence of data for the fine-structure mixing of the first-resonance states of alkali atoms colliding with the ground-state caesium atoms.     

Predicted rate constants for the exothermic reactions of ground state oxygen atoms and CH radicals

A potential function has been derived for the ground-state surface of HCO which reproduces the spectroscopic properties of the equilibrium molecule and the results of ab-initio calculations at other stationary points on the surface. The potential has been used for a classical trajectory study of the vibrational excitation of CO on collision with fast H atoms and for a study of the reaction of ground-state oxygen atoms and CH radicals.