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.     

Analysis of chemical bonding in C2 using Dyson orbitals

Multireference configuration interaction wave functions with single and double excitations were calculated for the ¹Σ⁺_g ground state of the C₂ molecule and the excited states of C⁺₂ with symmetries ²Σ⁺_g, ²Σ^-_u, ²Π_u, and ²Π_g. The corresponding σ_g, σ_u, π_u, and π_g valence Dyson orbitals were calculated. Most of the density due to the valence electrons is accounted for by three σ_g, one σ_u, and one degenerate pair of π_u Dyson orbitals. Electron correlation plays an important role in the bond strength of C₂ by increasing the occupation of the σ_g valence orbitals and decreasing the occupation of the σ_u and π_u valence orbitals. © 1996 John Wiley & Sons, Inc.     

Ground state correlations in H2 measured by the (e,2e) technique

The relative intensities for exciting the 1sσ_g, 2pσ_u, 2pπ_u, and 2sσ_g states of H₂ are measured in a 1200 eV noncoplanar symmetric (e,2e) experiment on H₂. Momentum distributions are obtained at separation energies corresponding to the various transitions. The ratio of transition probability to the excited states relative to the ground state is strongly dependent on the ion recoil momentum q, having a minimum value of approximately 2% at small q. The excited state cross sections are sensitive to electron correlation effects and the data are compared with calculated cross sections using a configuration interaction wavefunction for H₂.     

Formation of C2(e3πg, C1πg) in the photolysis of CO(X1ϵg+) at 193 nm with an ArF laser

CO(X¹?_g⁺) was irradiated with an ArF laser (193 nm) at room temperature at pressures of 2–20 mbar. C(2 ¹D) atoms were formed by two-photon absorption and observed by the 247.8 nm C(3 ¹P^o→2 ¹S) fluorescence. Fluorescence of C₂ (e ³π_g → a ³π_u, C ¹π_g → A ¹π_g and d ³π_g → a ³π_g) was detected. The emission time dependence led to the conclusion that C₂¹ is formed by recombination of electronically excited carbon atoms.     

Photoionization of carbon disulfide: theoretical studies of outer-valence-shell partial cross sections

Studies in vertical-electronic, static-exchange approximation are reported of the outer-valence-shell (2π_g⁻¹, 2π_u⁻¹, 5σ_u⁻¹, 6σ_g⁻¹) partial-channel photoionization cross sections of CS₂. The origins of strong features present in the calculated profiles, and in corresponding experimental cross sections, are attributed to the 6σ_u(σ¹) and 7σ_g (σ¹) orbitals of Mulliken on basis of Stieltjes orbital diagnostics.     

Configuration interaction calculation of the potential curves for the C3 molecule in its ground and lowest-lying Πu states

Ab initio SCF and Cl calculations are reported for the C₃ molecule using a basis set of double-zeta plus polarization quality. Potential curves are obtained for the symmetric stretch and bending and antisymmetric stretch vibrational coordinates for the ground and 3σ_u → l π_g^3,1Π_u excited states of this system in order to calculate the intensity distributions for the associated electronic transitions. The calculated T₀ value for the ¹Π_u ← X?¹ ∑⁺_g transition of 3.03 eV is in quite good agreement with the location of the origin of the 4050 Å (3.06 eV) band system in C₃, confirming its previous assignment to this electronic transition; the lifetime of the ¹H_u upper state is also obtained in the CI treatment. A value of 2.04 eV is calculated for the corresponding ³Π_u ← X?¹ ∑⁺_g origin, which result in turn suggests that the weak feature starting at 2.10 eV (5900 Å) should be assigned thereto.     

On Excited States of Alkyl Bridged [14] Annulenes

The linear dichroic absorption spectrum of 1,3,6,8-trans-15, 16-hexamethyl-dihydropyrene has been measured in stretched polyethylene at 77K, and CNDO-CI calculations with inclusion of singly and doubly excited configurations have been carried out on a series of alkyl bridged [14] annulenes with pyrene- and anthracene-shaped perimeters. Transitions to e_3g → e_4u type ¹B states are well described, and the results indicate that additional low-energy excited states originate from e_3g→ e_5g and e_2u → e_4u type configurations interacting strongly with doubly excited configurations of the e_3g, e_3g → e_4u, e_4u type. The second excited singlet state responsible for the weak transition observed between the ¹L bands may be assigned to one of these additional states, but it is probably of complex nature, similar to the ‘phantom’ state in linear polyenes.     

Two-photon dissociation of HD+ by the 1sσg → 2pσu electronic transition

Two-photon dissociation (TPD) of HD⁺ is studied for the 1sσ_g → 2pσ_u transition using a direct perturbation technique. Non-resonant TPD cross sections from the ν = 6, J = 0 level of the ground 1sσ_g state are calculated at different wavelengths of linearly polarised infrared photons, considering all possible bound and free intermediate states. The minimal coupling form (A · p) of the interaction Hamiltonian is used. The cross sections are found to have an oscillatory behaviour in the wavelength range (λ = 16000–17000 A) considered.     

The valence shell spectra of acetylene and hydrogen excited by Zr Mζ x-rays. An example of the advantages provided by an ultra-soft x-ray source

Excitation of the valence shell molecular orbital spectrum of acetylene with Zr M_ζ (151.4 eV) radiation shows clearly all four expected bands 1π_u (11.4 eV), 3σ_g (16.8 eV), 2σ_u (18.8 eV) and 2σ_g (23.5 eV) with approximately equal intensity in contrast to He or Mg K_α excited spectra. A spectrum of molecular hydrogen can also be obtained with this radiation.     

On the lower excited electronic states of biacetyl

An ab initio SCF and CI study has been carried out for the ground and electronically excited states of biacetyl (CH₃COCOCH₃). The second absorption band in the 4.40 eV region has been assigned to a ¹A_g → ¹B_g nπ^* transition The character of the lower-lying states has been analyzed in terms of the CI wavefunctions.     

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.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD‐DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, ¹A_u, which is conventionally regarded dark due to the dipole‐forbidden ¹A_u←¹A_g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* ¹B_3u and ππ* ¹B_2u states through vibronic coupling. The scans on potential‐energy surfaces further indicated that the ¹A_u state intersects with the ¹B_2u states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.     

Electronic band structure of solid CO2 as determined from the hv-dependence of photoelectron emission

Photoelectron energy distribution curves from solid CO₂ have been determined for excitation energies from hv = 14 up to 40 eV using synchrotron radiation. A 1:1 correspondence to the gas-phase photoelectron spectrum is observed for the occupied molecular orbitals. The vertical binding energies E_B^v (E_VAC = 0) and widths (fwhm) of the valence bands of solid CO₂ are determined to be 13.0 and 0.95 eV (1π_g); 16.7 and 1.1 eV (1π_u); 17.6 and 0.85 eV (3σ_u) and 18.8 and 0.8 eV (4σ_g) for the individual bands respectively. The partial photoemission cross sections differ importantly from those of the gas phase in exhibiting pronounced maxima at 5.2 eV (1π_g), 4.4–5.3 eV (1π_u + 3σ_u) and 4.2 eV (4σ_g) above the vacuum level, which is attributed to effects of high density of final (conduction-band) states. Further weaker maxima are observed at higher photon energies. Contrary to the case for the gas phase, the resonances are unperturbed in the solid by degenerate autoionizing molecular Rydberg states. The molecular origin of the resonances in the continuum is discussed and related to X-ray absorption spectra, electron-scattering data and to theoretical cross-section calculations. It is shown that the same set of resonances is observed in the different experiments. The resonances occur however at different energies due to different Coulomb interactions. The photoemission results presented provide also a key to the hitherto unexplained optical spectrum of solid CO₂ in the VUV range, making possible an assignment of the structures observed to Frenkel-type excitons (hv ≤ 15 eV) and interband transitions (hv ? 15 eV).     

A CI non-empirical pseudopotential calculation of the vertical valence excited states of the iodine molecule

The non-empirical atomic pseudopotential proposed by Durand and Barthelat has been used, together with the CIPSI algorithm for large scale CI, to calculate the vertical transition energies of the iodine molecule, in a valence extended (double-zeta + d) basis set. All the valence excited states were considered. The mixing of configurations is very important especially for the Σ⁺_g, Π_g and Π_u symmetries. The experimentally known transition energies are calculated within a 1 eV error, despite the lack of diffuse orbitals and spin-orbit interaction. Some qualitative Mulliken's estimates are discussed. A new ³Σ⁺_g state from the 10 σ_u → 11 σ_u single excitation is predicted in the 9 eV region.     

Valence-shell photoabsorption by CO2 and its connections with electron-CO2 scattering

Photoabsorption cross sections and photoelectron asymmetry parameters, calculated with the multiple-scattering model (MSM), are reported for the 4σ_g, 3σ_u, 1π_u and 1π_g valence levels of CO₂. The results are discussed in the context of photoabsorption and electron energy-loss measurements and other theoretical calculations. Further comparisons are made with previously reported MSM calculations of elastic electron-CO₂ scattering. The close connection between the sets of shape resonances in the electron-scattering and photoabsorption by CO₂ is emphasized with plots of continuum eigenchannel wavefunctions for shape-resonant and non-resonant eigenchannels of σ_u symmetry.     

Reactions of C2(a 3πu) with CH4, C2H2, C2H4, C2H6, and O2 using multiphoton UV excimer laser photolysis

C₂(a ³π_u) disappearance rate constants of 1.44, 0.96, 0.0296, 0.0130 and < 10^?6(x10^?10cm³s^?1) are reported for reactions with C₂H₄, C₂H₂, O₂, C₂H₆, and CH₄, respectively at 298 K. C₂(a ³π_u) fragments are generated by multiphoton ArF excimer laser photodissociation at C₂H₂, and monitored by dye laser induced fluorescence. Arguments are presented which favor chemical reactions over the C₂(a ³π_u) → (X ¹σ⁺_g) quenching channel. C₂ + C₂H₂ represents the one possible exception to the reactive channel.     

Branching ratio for the production of I2Psol12) in the photodissociation of I2

The production of atomic iodine in the ground (²Pfrsol|3/2) and electronically excited (²P$\text{1}\text{3}$) states following laser-induced photodissociation of I₂ the region 425–498 nm was monitored directly by resonance spectroscopy. The branching ratio for iodine atom formation. R = [I(²P$\text{1}\text{2}$)]/[I(²P$\text{3}\text{2}$)], is above 0.5 in the region 495–498 nm in agreement with the recent observation of laser action on the atomic transition at 1315 nm following photolysis of I₂ using a dye laser. The present experiments permitted deconvolution of the I₂ continuous absorption spectrum below 498 into contributions from the B³ Π_o,u → X ¹Σ_g⁺ and ¹Π_tu → X¹σ_g^? transitions.     

Electronic rearrangements during chemical reactions. II. Planar dissociation of ethylene

The direct dissociation of ethylene into two methylenes is studied along the least motion reaction path by means of an ab initio multiconfiguration self-consistent-field (MCSCF ) calculation. All eight configurations arising from those valence orbitals that form the CC bonds, seven of them singlet coupled and one triplet coupled, are taken into account. The HCH bond angle is optimized along the entire reaction path. Separate MCSCF optimizations are carried through for the lowest two states of ¹A_g symmetry. The (¹A_gσ²π²) ethylene ground state dissociates into two (³B₁σπ) ground-state methylenes. The (¹A_gσ²π*²) excited state of ethylene dissociates into two (¹A₁σ²) excited methylenes. It is established that both these dissociations proceed without any barrier in the energy curve. In the ground state, where orbital symmetry is conserved, the π-bond breaks before the σ-bond, and the calculated heat of reaction agrees within 6 kcal/mol with the experimental value. In the excited state, where orbital symmetry is not conserved, the nonbonded repulsion between methylene σ₂ lone pairs is found to blend into the antibonding character of the excited ethylene, yielding an energy curve that is everywhere repulsive. However, the variation of the HCH angle during the dissociation process is not simple, initially it expands and subsequently it contracts. Quantitative analytical approaches are developed which furnish conceptual interpretations of the orbital changes and configurational changes along the reaction path.     

Unprecedented Bonding Situation in Viable E2(NHBMe)2 (E=Be,Mg; NHBMe=(HCNMe)2B) Complexes: Neutral E2 Forms a Single E−E Covalent Bond

Is it possible to facilitate the formation of a genuine Be?Be or Mg?Mg single bond for the E₂ species while it is in its neutral state? So far, (NHC^R)Be?Be(NHC^R) (R=H, Me, Ph) have been reported where Be₂ is in ¹Δ_g excited state imposing a formal Be?Be bond order of two. Herein, we present the formation of a single E?E (E=Be, Mg) covalent bond in E₂(NHB^Me)₂ (E=Be, Mg; NHB^Me=(HCN^Me)₂B) complexes where E₂ is in ³∑_u⁺ excited state having (nσ_g⁺)²(nσ_u⁺)¹((n+1)σ_g⁺)¹ (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron‐shared bonding with two NHB^Me radicals. The effects of bonding with nσ_u⁺ and (n+1)σ_g⁺ orbitals will cancel each other, providing the former E?E bond order as one. Be₂(NHB^Me)₂ complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg₂(NHB^Me)₂ → Mg + Mg(NHB^Me)₂ and Mg₂(NHB^Me)₂ → Mg₂ + (NHB^Me)₂, are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol^?1, respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E₂ moiety with a single E?E covalent bond.