Experimental and theoretical studies of the valence-shell dipole excitation spectrum of molecular fluorine

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X¹Σ_g⁺→¹Σ_u⁺ bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π_g→npπ_u Rydberg and 3σ_g→3σ_u intravalence excitations. A depleted X→V_σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F₂ below the 3σ_g^?1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X¹Σ_g⁺→¹Π_u excitations the origins of which are attributed to configuration mixing between 1π_g→npσ_u and 1π_u→nsσ_g Rydberg series.     

Inner- and valence-shell excitation of SF4 studied by photoabsorption and electron energy loss spectroscopy

The S 1s photoabsorption spectrum of SF₄ recorded with synchrotron radiation is reported along with the electron energy loss spectrum of SF₄ in the regions of S 2p, S 2s and F 1s excitation recorded under dipole dominated conditions. The electron energy loss spectrum of SF₄ in the region of valence electron excitation is also reported. All of these spectra are interpreted in terms of a common manifold of upper levels which includes valence levels whose term values reflect the different axial and equatorial SF bond lengths in this molecule. The spectra are good examples of the effects of potential barriers on electronic excitation spectra.     

Vibrational excitation cross section and V → T rate constants in molecular hydrogen

Vibrational excitation cross sections are computed for H₂ gas at various collision energies and between the lower-lying levels of the molecule. Only V → T processes are considered, while a quantum mechanical treatment via spherical potentials is implemented and applied. Various suggested potentials have been used and the corresponding results critically compared. The low-energy behaviour of the σ_{1 → 0} cross sections, and their orders of magnitude, appear to be in agreement with “experimental” deconvoluted data. Relaxation and excitation rates are computed and the K_1-0 (T) is examined over a wide temperature range to allow comparison with experiments. While the high temperature results are in fair agreement with the available data, the pure V - T mechanism of the present model seems to overestimate low-T rates as a consequence of the increase of (1 → 0) computed ross sections when bringing E_coll very near the lowest, υ = 1, threshold.     

Experimental and theoretical cross section data of deuteron induced nuclear reactions on platinum

Journal of Radioanalytical and Nuclear Chemistry - Additional experimental activation cross sections for deuteron induced reactions on platinum were measured in the 35–49 MeV energy...     

Vibrational excitation of deuterium fluoride and hydrogen fluoride by atomic and diatomic species

The vibrational excitation of HF and DF and the energy transfer efficiencies for various collision partners were investigated over the temperature and pressure ranges of 1400°K to 4100°K and 0.1 to 0.3 atm, respectively. The extent of excitation was determined as a function of time by continuously monitoring the infrared emission intensity at the center of the 1–0 vibration-rotation band of the molecule. Collisional efficiencies of HF, N₂, O₂, F, Cl, and DF in relaxing HF and of DF, HF, and N₂ in relaxing DF are reported. A comparison with relaxation data for pure HF taken at lower temperature suggests that long-range attractive forces are mechanistically of major importance in the relaxation process. The relatively high efficiency of atomic chlorine in relaxing HF, i.e., (τP)_HF–HF/(τP)_HF–C1 ≥ 5 at 3000°K is discussed in terms of our previous result for atomic fluorine, i.e., (τP)_HF–HF/(τP)_HF–F = 18.     

Experimental and theoretical analysis of the rotational Raman spectrum of hydrogen molecules in clathrate hydrates

The Raman spectra of H(2) and HD molecules in simple hydrogen and binary hydrogen-tetrahydrofuran clathrate hydrates have been measured at temperatures as low as 20 K. The rotational bands of trapped molecules in simple and binary hydrates have been analyzed, and the contributions originating from hydrogen molecules in the large cages have been separated from those in the small cages. A theoretical model, consisting in rigid cages enclosing interacting hydrogen molecules, has been exploited to calculate, on the basis of quantum mechanics, the Raman intensity of the rotational transitions for up to two interacting molecules in one cage. A comparison with experiment leads to a clear interpretation of sidebands appearing in the Raman rotational lines. The quantitative agreement between theory and experiment obtained in some cases clarifies the importance of the choice of the interaction potential, and of the proton disorder in the clathrate crystal.     

Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of pyrimidine molecules

Cross-section data for electron impact induced ionization of bio-molecules are important for modelling the deposition of energy within a biological medium and for gaining knowledge of electron driven processes at the molecular level. Triply differential cross sections have been measured for the electron impact ionization of the outer valence 7b(2) and 10a(1) orbitals of pyrimidine, using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and ejected electron energy of 20 eV, for scattered electron angles of -5°, -10°, and -15°. The ejected electron angular range encompasses both the binary and recoil peaks in the triple differential cross section. Corresponding theoretical calculations have been performed using the molecular 3-body distorted wave model and are in reasonably good agreement with the present experiment.     

Experimental and theoretical study on generalized oscillator strengths of the valence-shell electronic excitations in CF4

We report an angle-resolved electron energy loss spectroscopy (EELS) study on the valence-shell electronic excitations in CF(4). Experimentally momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles for low-lying electronic excitations at 12.6, 13.8, and 14.8 eV are derived from EELS spectra measured at an incident electron energy of 3 keV. We also calculate GOS profiles using theoretical wave functions at the equation-of-motion coupled cluster singles and doubles level. There are good agreements between experiment and theory except for a significant discrepancy at small momentum transfer for the 1t(l) → 3s Rydberg excitation at 12.6 eV. The experimental GOS profile for 1t(l) → 3s exhibits a shape that is typical of a dipole allowed transition, while the excitation is formally dipole forbidden. This symmetry breaking behavior is rationally accounted for by qualitatively analyzing the nature of vibronic coupling effects. For the excitation band at 13.8 eV, a shoulder and extrema are observed in the GOS profile and are then found to be mainly due to the 2(1)T(2) transition. Furthermore, the theoretical GOS profile for the 2(1)T(2) transition exhibits a remarkable oscillatory pattern; its origin is discussed by considering multicenter interference effects. For the 14.8 eV excitation band, the predominant nondipole nature of the underlying transitions are revealed and comparisons with the theoretical calculations show that major contributions to this band come from the 4t(2) → 3p excitation.     

Absolute dipole oscillator strengths for molecular and dissociative photoionization of hydrogen fluoride

The internal rotation in the free anion radicals and the ion pairs from 3-nitroacetophenone and 4-nitroacetophenone is studied on the basis of their dynamic EPR spectra in solution. Energy barriers and related thermodynamic parameters which govern internal rotation and rotational diffusion of the studied systems have been provided together with a discussion about the accuracy and the reliability of their determination.     

Pseudopotential studies of the water and hydrogen fluoride molecules

A model pseudopotential is used to calculate valence electron properties for H₂O and HF. The calculated geometries, force constants, and ionization potentials are in excellent agreement with the results of corresponding all-electron calculations.     

A new formulation of the dynamical response of many-electron systems and the photoabsorption cross section of small metal clusters

Static polarizabilities and photoabsorption cross sections of clusters Na ₇ ^– , Na₈, Na ₁₉ ^– Na₂₀ are calculated, based on the spherical jellium model including the self-interaction correction (SIC) of Perdew and Zunger. To this end, a new formulation of the theory of the linear response is presented, which is suitable for general, self-interaction corrected, many-electron systems. The results obtained display an overall agreement with available experimental data, offering a systematic improvement with respect to the standard TDLDA. Furthermore, the cross sections of the negatively charged clusters are found to be dominated by a broad peak in the visible region, whose line width can be related to the lifetime of the surface plasmon against electron detachment.On leave of absence from the University of Coimbra, Portugal     

Application of the “helium excitation standard” procedure to some hydrocarbon Hβ absolute emission cross section measurements

Absolute emission cross sections for H_β radiation are presented. The H_β emission is obtained by dissociative electron excitation of propane, propylene, n-butane and 1-butene under binary collision conditions. Optical excitation functions measured for these transitions are normalized by the “helium excitation standard” procedure, using the 4 ¹S-2 ¹P transition in He as a standard at an electron impact energy of 100 eV. The results obtained are determined with an accuracy of ± 15%. The investigated energy interval is 50–500 eV. The experimental results show that the emission cross sections for H_β radiation are not independent of the number of atoms in the parent molecule.     

Bromodifluoroacetyl fluoride,CF2BrC(O)F: Experimental and theoretical studies

Bromodifluoroacetyl fluoride, CF₂BrC(O)F, was prepared through the gas-phase reaction of bromotrifluoroethene, CF₂CFBr, with molecular oxygen initiated either by NO₂ or CF₃OF. The compound was experimentally studied by FTIR spectroscopy of the gas phase and also isolated in Ar and N₂ matrices at low temperature. The energy differences between the possible conformers were theoretically studied, as well as the vibrational spectra of the conformers.     

The vibrational excitation of hydrogen fluoride behind incident shock waves

The vibrational excitation of HF occurring behind incident shock waves has been studied in the temperature range of 1400°K to 4100°K. The extent of excitation was determined as a function of time by continuously monitoring the emission intensity from the 1–0 band of HF centered at 2.5 μ. The data were interpreted in terms of the process and gave a value of for M = HF. The corresponding result for (τp)^?1_Ar was found to be insignificant in comparison to this result. Data were also obtained for the effect of F atoms upon the relaxation rate, i.e., it was found that      

Experimental and theoretical study of the electronic spectrum of BeAl

The electronic structure of BeAl was investigated by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. BeAl was formed by pulsed laser ablation of a Be/Al alloy in the presence of helium carrier gas, followed by a free jet expansion into vacuum. In agreement with recent ab initio studies, the molecule was found to have a (2)Pi(1/2) ground state. Transitions to two low lying electronic states, (2)(2)Pi(1/2)(v') <-- X (2)Pi(1/2) (v' = 0) and (1)(2)Delta(v') <-- X (2)Pi(1/2) (v' = 0,1), were observed and rotationally analyzed. An additional band system, identified as (4)(2)Sigma(+)(v') <-- X (2)Pi(1/2), was found in the 28 000-30 100 cm(-1) energy range. This transition exhibited an unusual pattern of vibrational levels resulting from an avoided crossing with the (5)(2)Sigma(+) electronic state. New multi-reference configuration interaction calculations were carried out to facilitate the interpretation of the UV bands.An ionization energy of 48 124(80) cm(-1) was determined for BeAl from photoionization efficiency (PIE) measurements. Fine structure in the PIE curve was attributed to resonances with Rydberg series correlating with vibrationally excited states of the BeAl(+) ion. Analysis of this structure yielded a vibrational frequency of 240(20) cm(-1) for the cation.     

Experimental and theoretical studies of the propargyl-allenylindium system

The transient organoindium intermediates formed in the reaction of propargyl bromide with indium in aqueous media and tetrahydrofuran were investigated by NMR spectroscopy and found to be allenylindium(I) and allenylindium(III) dibromide. The influence of solvent and methyl substitution on the propargyl-allenylindium system was also studied. The experimental observations were supported by theoretical calculations using the B3LYP/6-311+G* method.     

Partial cross sections and autoionization resonances in the valence-shell photoemission from solid acetylene

Photoelectron energy distribution curves from solid acetylene were measured for excitation energies up to 30 eV using synchrotron radiation. The partial cross sections are discussed in relation to theory and other experiments. The 1π_u valence-band cross section shows a pronounced autoionization resonance with a shoulder and a peak 2.5 and 5.0 eV above the vacuum level. This originates from the resonant decay of the 2σ _u → 1π_g valence excitation (discrete shape resonance).     

Experimental and theoretical investigation of the photoionization of hydrogen cyanide

Cryogenic matrices containing molecular aggregates of methyl nitrite or methanol have been irradiated with a cw CO₂ laser. At the irradiation frequency hole burning was observed by means of a grating infrared spectrophotometer. The results suggest that vibrational absorption lineshapes of aggregates in cryogenic matrices are dominated by inhomogeneous line-broadening.     

Determination of the H2-H2 potential from absolute integral cross section measurements

Absolute integral cross sections have been measured for n-H₂-n-H₂ at primary particle velocities between 1270 and 9650 m/sec with a 77 °K target. The number density of target particles was determined with an accuracy of 1% by the method of dynamic expansion. The velocity dependence of the integral cross section was also measured for the isotopic systems n-D₂-n-H₂ and n-D₂-n-D₂ at primary beam velocities between 280 and 1170 m/sec with a 15 °K target. All available integral cross section data are compared with literature potentials. A new potential is proposed, which exhibits one bound state for H₂-H₂.Dedicated to Professor H. Hartmann on his 60^th birthday.