Laser induced preassociation in the presence of natural predissociation

Laser induced predissociation and its inverse, radiative preassociation are examined in the presence of natural (intrinsic) molecular predissociation. Quantum interferences will occur between the two processes, inducing optical windows in the energy transfer between the radiative channel and the natural predissociation channel. The radiative preassociation can serve as an efficient method of energy transfer to the predissociative channel.     

Effect of orbiting resonance on the elastic differential cross sections

The effects of orbiting resonances on elastic differential cross sections are investigated for the Lennard-Jones (12, 6) model potential. The elastic differential cross section consists of three terms: a resonance term, a potential scattering term and an interference term. The interference term has a significant effect only at small angles of the differential cross section. The importance of the potential scattering term depends proportionally upon the orbiting resonance energy. For the low-lying orbiting resonance states, the resonance term is the dominant term, but at higher energy it appears only in the backward differential cross sections. Generally, by fitting the large angle differential cross sections, the resonance orbital angular momentum can be estimated.     

Chemical effects on oscillatory cross sections in low energy ion scattering

Oscillatory structure in the scattered ion yield for He⁺ scattering from In has been observed to depend upon the chemical environmental of the In. Fourier analysis of the oscillatory structure indicates two major components whose relative amplitudes vary significantly with chemical environment.     

Ab initio calculation of resonance Raman cross sections based on excited state geometry optimization

A method for the calculation of resonance Raman cross sections is presented on the basis of calculation of structural differences between optimized ground and excited state geometries using density functional theory. A vibrational frequency calculation of the molecule is employed to obtain normal coordinate displacements for the modes of vibration. The excited state displacement relative to the ground state can be calculated in the normal coordinate basis by means of a linear transformation from a Cartesian basis to a normal coordinate one. The displacements in normal coordinates are then scaled by root-mean-square displacement of zero point motion to calculate dimensionless displacements for use in the two-time-correlator formalism for the calculation of resonance Raman spectra at an arbitrary temperature. The method is valid for Franck-Condon active modes within the harmonic approximation. The method was validated by calculation of resonance Raman cross sections and absorption spectra for chlorine dioxide, nitrate ion, trans-stilbene, 1,3,5-cycloheptatriene, and the aromatic amino acids. This method permits significant gains in the efficiency of calculating resonance Raman cross sections from first principles and, consequently, permits extension to large systems (>50 atoms).     

Shape resonance and electronic autoionization in vibrationally resolved partial photoionization cross sections of O2

Vibrationally resolved partial photoionization cross sections of O₂ leading to the b⁴Σ^?_g and B²Σ^?1_g(3σ^?1_g) tonic states have been measured at high resolution in a wide photon energy range from 0.5 eV above threshold to 24.5 eV. The σ_u shape resonance is observed around 21.5 eV, in fairly good agreement with “one-electron” theoretical predictions. The series of resonances from 19 to 20 5 eV result from an autoionization process with a strong vibrational selectivity, explained by the similar geometries of the resonance states and the b ionic state. The large resonance widths originate from a strong Rydberg-valence orbital mixing Calculations using multichannel quantum defect theory reproduce the most important features.     

Correlation effects in R-matrix calculations of electron-F2 elastic scattering cross sections

Correlation effects are studied in electron scattering off the fluorine molecule. Fixed-nuclei approximation R-matrix calculations of the elastic collision cross sections are presented for a set of internuclear distances at three levels of correlation. The aim of this work is to study the role of electronic correlation on the properties of the 2Sigmau resonance. The Feshbach-Fano R-matrix method of resonance-background separation is used to study the effect of inclusion of various levels of correlation on the energy and width of the 2Sigmau resonance. Data required for construction of the nonlocal resonance model (construction of a discrete state and its coupling to the continuum) which allows the calculation of inelastic processes such as dissociative electron attachment and vibrational excitation [W. Domcke, Phys. Rep. 208, 97 (1991)] including the correlation are presented.     

Multiple ionization effects on total L-shell X-ray production cross sections by proton impact

The effect of multiple ionization on total L-shell X-ray production cross sections by proton impact, with energies below 1 MeV, on elements with atomic numbers in the range 26–55 was studied. Measurements of those cross sections for several elements were also done to enlarge the experimental database. Several tables for atomic parameters (fluorescence yields and Coster–Kronig transition probabilities) were used. The agreement between theory and experiment was optimized when average fluorescence yields given by Hubbel et al. (J. Phys. Chem. Ref. Data 23(2) (1994) 339) and a multiple ionization model proposed by Lapicki et al. (Phys. Rev. A 34(5) (1986) 5813) were used together. Thus, improvements to theoretical predictions for ionization cross sections should consider first a correct set of atomic parameters.     

Photoabsorption cross sections of sodium clusters: electronic and geometrical effects

We study the origin of the discrepancy between the photoabsorption cross sections of small jellium spheres calculated by the time dependent local density approximation (TDLDA) and experiments for small metallic clusters. We have specifically studied Na ₂₁ ⁺ . We conclude that both non-local exchange-correlation effects beyond the LDA and geometrical effects beyond the jellium approximation should be taken in the same calculation. We also present local and non-local calculations for Na _n ^? (n=19, 91 and 197) within the framework of the jellium model. The large anions show a fragmentation of the plasmon due to its interference with the ionization threshold. This feature is absent in the TDLDA results.     

Partial cross sections and density-of-states effects in the valence-band photoemission from solid nitrogen

Photoelectron energy distribution curves from solid nitrogen were measured for excitation energies to 40 cV using synchrotron radiation. Partial cross sections for emission from 3σ_g, 1π_u and 2σ_u derived valence bands show pronounced mauna 3, 4, 2.9 and 3.0 eV above the vacuum levels, interpreted as due to high density of conduction-band final states. These states are related to π*_g negative-ion shape resonances.     

A sliding mass model to rationalize effects of reagent rotation on reaction cross sections

A sliding mass model for three-center reactions is formulated which proves to be useful to rationalize the dependence of integral reaction cross section (σ_r) on reagent rotation (j) by topographic attributes of the potential energy surface (PES). The model allows a clear definition of frequently used terms like “cone of acceptance” and “disruption of the favoured orientation” by rotation. A computational study of reaction probabilities uncovers intimate relations between steric properties of reagent interaction and the shape of σ_r(j). The oscillatory structure of σ_r(j) predicted by NoorBatcha and Sathyamurthy for the reaction Li + HF → LiF + H is shown to be due to the ill asymptotic behaviour of the used PES (Carter and Murrell's best fit function to the ab initio of Chen and Schaefer).     

Interference effects in the predissociation of H2 and HD

Several absorption lines in the B? ¹Σ⁺_u ← X ¹Σ⁺_g system at 83 nm have been observed with sub-Doppler resolution in H₂ and HD. A detailed analysis of the lineshapes arising from the homogeneously predissociaied B ¹Σ⁺_u states shows that the linewidths are strongly affected by the Franck-Condon factors associated with coupling to the B' ¹Σ⁺_u continuum.     

Chemical effects on K and L shell production cross sections and transfer probabilities in Nb compounds

In this study, the chemical effects on σ_Ki (i = α, β), σ_Lα cross sections, K_β/K_α X-ray intensity ratios and vacancy transfer probabilities from K to L (η _KL) for pure Nb and Nb compounds were investigated. The samples were excited by 59.5 keV γ-rays from ²⁴¹Am and 5.96 keV photon energy from a ⁵⁵Fe annular radioactive sources. K and L X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV. While it was observed that the chemical bonding had an effect on the σ_Kβ, σ_Lα cross sections and K_β/K_α X-ray intensity ratios for compounds, it was almost negligible for σ_Kα cross section because K_α transitions (2P_3/2,1/2→1S_1/2) occurred in inner shells. It is well known that interactions between central element atom and ligands come into existence in valence state, so outer energy levels are sensitive to the chemical environment. The experimental values of σ_Kα cross section and η _KL are in good agreement with theoretically calculated and other experimental values of pure niobium, but the experimental values of the σ_Kβ, σ_Lα cross sections and K_β/K_α X-ray intensity ratios have differences for some compounds because valence electrons have different bond distances and binding energies in different compounds.     

Electron inelastic-scattering cross sections in liquid water

Electron inelastic-scattering cross-section data for use as input in electron track-structure calculations in liquid water are re-examined and improved. The dielectric-response function used in such cross-sections is estimated on the basis of optical data and other experimental and theoretical information. The mean excitation energy for stopping power is obtained to be 81.8 eV, which is close to the recent experimental value, 79.75±0.5 eV, of Bichsel and Hiraoka. Inelastic-scattering cross sections are evaluated within the first Born approximation. Electron-exchange effects and semi-empirical corrections to account for non-Born effects at low energies are also incorporated.     

Rotational energy transfer cross sections in methane (13CD4) from infrared double resonance measurements

Rotational relaxation times have been measured for methane (¹³CD₄) in collisions with itself, He, Ne, Ar, Kr, Xe, and CH₂F₂, using the method of infrared double resonance. Collision efficiencies range from one-half to greater than gas-kinetic, and the measured relaxation times are longer in the vibrational ground state than in the υ₄ = 1 excited state.     

Orientational effects on atomic photoelectric cross sections and their relevance to photoelectron spectra from single crystals

We have examined the dependence of the photoelectric cross section from individual atomic orbitals and from groups of orbitals adapted to crystal symmetries in order to ascertain the relevance of this effect to the photoelectron spectra arising from single crystals.     

Measurements of rotational relaxation cross sections for H2CO

Cross sections for the 1₁₁, 1₁₀, 2₁₂, and 2₁₁ rotational states and for total beam attenuation of H₂CO were measured using a beam maser spectrometer. Significant inelastic scattering in the forward direction was obtained for He and CF₃H. Significant inelastic contributions were not observed using H₂ as a scattering gas. Observed cross sections for the 1₁₁ state are larger than those for the 1₁₀ state. The relations of the present cross sections to various models for obtaining a low excitation temperature for the 1₁₁-1₁₀ doublet of formaldehyde in interstellar space is discussed.     

Molecular beam maser measurements of relaxation cross sections in NH3

Direct measurements of scattering cross sections for NH₃ in well-defined quantum states are made using a molecular beam maser spectrometer. Cross sections are compared for a pure inversion state with those for a coherent superposition state for scattering gases He, Ar, N₂, N₂O, NH₃, CH₃H and CH₃CN. The cross sections are significantly larger for scattering of the pure inversion state by NH₃, CF₃H and CH₃CN than for scattering of the superposition state. The scattering is the same for both states on non-polar gases. These cross sections are related to relaxation parameters describing transitions between rotation and inversion states.     

Two-photon absorption cross sections: an investigation of solvent effects. Theoretical studies on formaldehyde and water

The effects of a solvent on the two-photon absorption of microsolvated formaldehyde and liquid water have been studied using hybrid coupled-cluster/molecular mechanics (CC/MM) response theory. Both water and formaldehyde were considered solvated in water, where the solvent water molecules were described within the framework of molecular mechanics. Prior to the CC/MM calculations, molecular dynamics simulations were performed on the water/formaldehyde and water/water aggregates and many configurations were generated. By carrying out CC/MM response calculations on the individual configurations, it was possible to obtain statistically averaged results for both the excitation energies and two-photon absorption cross sections. For liquid water, the comparison between one- and two-photon absorption spectra is in good agreement with the experimental data available in the literature. In particular, the lowest energy transition occurring in the one-photon absorption spectrum of water only occurs with a relatively small strength in the two-photon absorption spectrum. This result is important for the interpretation of two-photon absorption data as these results show that in the absence of selection rules that determine which transitions are forbidden, the spectral profile of the two-photon absorption spectrum can be significantly different from the spectral profile of the one-photon absorption spectrum.     

Measurements of differential cross sections in the backscattering region

A review of experimental techniques which enable detection of electrons scattered in the backward direction, with an emphasis on recent developments in the magnetic angle-changing technique is presented. Results of measurements of differential cross sections for elastic electron scattering in the range of large scattering angles in rare gases and small molecules are discussed and compared with selected theoretical investigations.     

Anomalous neutron Compton scattering cross sections in ammonium hexachlorometallates

The authors have performed neutron Compton scattering measurements on ammonium hexachloropalladate (NH(4))(2)PdCl(6) and ammonium hexachlorotellurate (NH(4))(2)TeCl(6). Both substances belong to the family of ammonium metallates. The aim of the experiment was to investigate the possible role of electronic environment of a proton on the anomaly of the neutron scattering intensity. The quantity of interest that was subject to experimental test was the reduction factor of the neutron scattering intensities. In both samples, the reduction factor was found to be smaller than unity, thus indicating the anomalous neutron Compton scattering from protons. Interestingly, the anomaly decreases with decreasing scattering angle and disappears at the lowest scattering angle (longest scattering time). The dependence of the amount of the anomaly on the scattering angle (scattering time) is the same in both substances (within experimental error). Also, the measured widths of proton momentum distributions are equal in both metallates. This is consistent with the fact that the attosecond proton dynamics of ammonium cations is fairly well decoupled from the dynamics of the sublattice of the octahedral anions PdCl(6) (2-) and TeCl(6) (2-), respectively. The hypothesis is put forward that proton-electron decoherence processes are responsible for the considered effect. Decoherence processes may have to do rather with the direct electronic environment of ammonium protons and not with the electronic structure of the metal-chlorine bond.