K-shell excitation of CH4, NH3, H2O,CH3OH,CH3OCH3 and CH3NH2 by 2.5 keV electron impact

The regions around the respective carbon, nitrogen and oxygen K-edges of CH₄, NH₃, H₂O, CH₃OH, CH₃OCH₃ and CH₃NH₂ have been investigated by electron energy loss spectroscopy using a beam of 2.5 keV electrons. All spectra show a number of discrete peaks just below the K-shell ionization threshold. These discrete structures have been interpreted as being associated with the promotion of a K-shell electron to Rydberg orbitals which converge to the K-shell ionization threshold.     

Lifetimes of high Rydberg states of methyl halides

The lifetimes of high Rydberg states of the methyl halides CH₃I, CH₃Br and CH₃Cl have been studied. A pulsed dye laser tuned near half the ionization energy was used, with preprogrammed field pulses, and the high Rydberg states of the compounds in selected ranges of n values were ionized after a variable delay. Plots of the electron signal versus delay yielded information about the Rydberg state lifetime. The clearest trend was found for around n = 120–160: the lifetimes of the states decreased by more than an order of magnitude from CH₃I (τ ≈12.3 μs) through CH₃Br to CH₃C1. Within the range studied, the theoretically predicted increase in lifetimes (n ⁵ or n ³) was not found, and in fact the highest group of states studied (with n > 215) exhibited for all three compounds relatively short lifetimes (≈2-3 μs), possibly due to collisional processes.     

The electronic structures of the core and valence ion states of metal oxides

SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO₆^10?, AlO₄^5? and SiO₄^4? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO₆^10?, CrO₆^9? and TiO₆^8? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e _g than for t_2g orbitals and increases as the separation of the e_g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO₂ (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr₂O₃ and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e_g bonding → e_g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO₂. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO₆^9? to FeO₆^10? to TiO₆^8?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO₂ may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr₂O₃ arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO₆^10?, CrO₆^9? and TiO₆^8? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO₆^10? Fe 2p core hole state to Co₃O₄) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e_g → 3e_g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.     

Ab initio study of the ground and excited states of hydrogen sulphide using SCF and CI calculations

Results from ab initio SCF and CI calculations on the ground state and low-lying valence and Rydberg states of H₂S are reported. A double ξ basis of contracted gaussian functions augmented by polarization and diffuse 3d, 4s and 4p functions is used for the calculations. The geometries of various excited states are studied by means of SCF calculations. The first observed band in the absorption spectrum is predicted to arise from the overlapping of transitions from the 2b₁ orbital to a Rydberg 4s and strongly bent valence upper state. The calculations support the assignment of other spectral features to transitions from the 2b₁ to components of the Rydberg 3d and 4p orbitals.     

Inner-shell excitation and EXAFS-type phenomena in the chloromethanes

Small angle inelastic scattering of 2.5 keV electrons was used to study the inner-shell excitation of CH₄, CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄ and C₂H₅Cl in the regions of carbon 1s, chlorine 2p and chlorine 2s excitation. Structure observed below the carbon 1s ionization threshold in each molecule is assigned to promotions of a carbon 1s electron to unoccupied valence and Rydberg orbitals. Trends in the distribution of spectral intensities through the series of chloromethane carbon 1s spectra are discussed in terms of the growth of a potential barrier. Broad features are observed in the chlorine 2p continua of CH₂Cl₂, CHCl₃ and CCl₄ and the carbon 1s continuum of CCl₄ which are assigned as the energy loss equivalent of extended X-ray absorption fine structure (EXAFS).     

Carbon K-shell excitation spectra of linear and branched alkanes

The electron energy loss spectra of ethane, propane, n-butane, n-pentane, n-hexane, isobutane, isopentane and neopentane in the region of carbon K-shell excitation have been recorded under dipole-dominated conditions (2.8 ke V impact energy, small angle). The spectra are dominated by transitions to unoccupied valence π¹(CH₂, CH₃) and σ¹(C-C) levels. Additional weak features are assigned to Rydberg transitions. The position of the main continuum feature in each spectrum is consistent with the predictions of an empirical relationship with bond length. Systematic variations of spectral intensities are observed which support our assignments. The dominant feature in the K-shell spectrum of ethane, which was previously assigned to C 1s → 3p Rydberg transitions, is reassigned to excitation to a 3p/π¹(CH₃ ), mixed Rydberg/valence orbital (of antibonding σ-¹(C-H) character), in comparison to the other alkane spectra. An improved calibration value of 290.74(5) eV for the energy of the C 1s → π¹ transition in CO₂ is also obtained.     

Approximate SCF calculations on the [Mn(H2O)6]2+ complex with a minimal basis set

In the restricted Hartree-Fock scheme approximate SCF-LCAO calculations have been performed for the [Mn(H₂O)]₆ ²⁺ complex using a minimal basis set consisting of nine Slater-type orbitals of the manganese ion and four Slater-type orbitals of the water molecule. The 1s, 2s and 2p orbitals of the manganese ion and the 1s orbital of the oxygen atom are treated as frozen core orbitals. In evaluating the different parts of the Hartree-Fock operators we used a two-centre approximation for the multicentre integrals. A new aspect of the calculations is the use of the Hartree-Fock orbital energies of the free water molecule as a first approximation for the corresponding orbital energies of the complex. The calculations have been done for the ground states and six excited states of the complex with symmetry T _h and also for the ground states of two distorted complexes. From the resulting eigenvectors we calculated the hyperfine interaction of the valence electrons of the central ion with the protons of the water molecules for three different geometries. The excited states give two different ways of finding values of 10 Dq and the Racah parameters B and C. The results are encouraging.     

CH3NO2在Cu(111)面吸附态的研究

用CNDO/2半经验量化计算方法对CH₃NO₂分子在Cu(111)面四个吸附位上25种吸附态进行了优化计算,得到以CH₃NO₂分子中的-NO₂取向吸附在Cu(111)面的桥位上,且CH₃NO₂分子中的ONO面与Cu-Cu键成60°时为最稳吸附态。计算得到的这一稳定吸附态的吸附取向和吸附体系的态密度结果与我们的实验结果是一致的;从吸附态的轨道成分分析表明, 关键词：     

Inner-shell excitation of formaldehyde,acetaldehyde and acetone studied by electron impact

Excitation and ionisation of the carbon and oxygen 1s electrons of formaldehyde, acetaldehyde and acetone have been examined by small-angle inelastic scattering of 2.5-keV electrons, with energy resolutions in the range 0.20–0.50 eV FWHM. Features in the electron energy loss spectra below the inner-shell ionisation thresholds have been assigned to transitions to unoccupied valence (π*) and Rydberg orbitals. Broad maxima in the inner-shell continua of all three molecules have been interpreted as resonances associated with transitions to σ*(CO) orbitals. The assignments of the acetaldehyde and acetone spectra have been supported by comparison with those for CH₄ and H₂ CO. The adiabatic first ionisation potential of H₂ NO and H₂ CF have been estimated from the inner-shell spectra of formaldehyde using the equivalent-core analogy.     

Photoelectron and electronic absorption spectra of SCl2, S2Cl2, S2Br2 and (CH3)2S2

Photoelectron and electronic absorption spectra of SCl₂, S₂Cl₂, S₂Br₂, and (CH₃)₂S₂ have been measured and analyzed. Quantum chemical calculations (CNDO/ 2 and MWH (Mulliken-Wolfsberg-Helmholtz) have been carried out and the electronic structures have been described in terms of molecular orbital theory. The variation in differential photoionization cross-section as a function of incident photon energy and results of MO computations are used to identify ionization bands and assign ground state MO configurations. Suggested ground state electronic structures coupled with computed virtual MO's are used to interpret the visible and near-ultraviolet electronic absorption spectra. The low energy excited states are described as molecular states followed by the initial members of Rydberg series. Calculated oscillator strengths for molecular transitions are in good agreement with those observed experimentally. Quantum defects, δ, for the Rydberg states have been calculated from the Rydberg equation using the adiabatic first ionization potential.     

Rydberg states of carbon dioxide and carbon disulfide

The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and (npσ) and (npπ) Rydberg series have been assigned. For both molecules, the lowest excited ¹Π_g and ¹Π_u states are identified as Rydberg states. The lowest ${}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state is mainly Rydberg for CO₂, but contains some valence character for CS₂, There is no evidence for transitions to additional valence states of these symmetries.It is shown that $\text{LCAO}\text{MO}$ predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO₂ and CS₂ predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.     

Polyatomic molecules formed with a Rydberg atom in an ultracold environment

We investigate properties of ultralong-range polyatomic molecules formed with a Rb Rydberg atom and several ground-state atoms whose distance from the Rydberg atom is of the order of n²a₀, where n is the principle quantum number of the Rydberg electron. In particular, we put emphasis on the splitting of the energy levels, and elucidate the nature of the splitting via the construction of symmetry-adapted orbitals.     

CASPT2 and CCSD(T) calculations of dipole moments and polarizabilities of acetone in excited states

The acetone molecule is investigated in its ground state and valence ^1,3n-π*, ^1,3π-π*, and ^1,3σ-π* excited states and Rydberg ^1,3n-3s, ^1,3π-3?, ^1,3n-3p_y and ^1,3π-3p_y states using the CASSCF, CASPT2, and CCSD(T) methods. Equilibrium geometries of excited states are obtained and their changes with respect to the ground state are discussed. For most excited states the C_2v symmetry of the ground state is lowered to the C_s symmetry. A series of valence vertical and adiabatic excitation energies is presented along with excitation energies for Rydberg states. The main body of the paper contains Finite-Field Perturbation Theory (FFPT) calculations of electric properties of the vertically as well as geometry relaxed excited states. Dipole moments of valence excited states decrease significantly upon excitation, being about one half of the ground state dipole moment. Polarizabilities usually change upon excitation much less (increase by about 30%) but hyperpolarizabilities are enhanced up to one or two orders of magnitude. The orientation of the dipole moment is reversed in some vertically excited Rydberg states. Properties of the ground and excited states are discussed considering alterations of the electronic structure and shifts in the geometry.     

Probing the molecular orbitals of FePc near the chemical potential using electron energy-loss spectroscopy

We have studied the electronic structure of iron phthalocyanine (FePc) films at low temperature using electron energy-loss spectroscopy. The electronic excitation spectrum of FePc is rather complex and comprises both π-π^* transitions of the phthalocyanine ligand and transitions that involve the Fe 3d orbitals. The C 1s core excitations provide so far unidentified information on the molecular orbitals. They demonstrate that the Fe 3d orbital with e_g symmetry is energetically located in between the highest occupied and the lowest unoccupied ligand state and that it is not fully occupied.     

Satellite peaks in the high energy photoelectron spectra of some small first row molecules

The gas phase high energy photoelectron spectra of CH₄, NH₃, H₂O, N₂, O₂, CO and CO₂ have been recorded, and in all cases weak satellite peaks to high binding energy of the main ionization peak are observed. These peaks are assigned to transitions to ionic states in which valence electron excitation as well as core ionization has occurred. The intensity and position of these peaks, relative to the main ionization peak have been estimated from ab initio UHF calculations on the core hole states, which in general allow assignment of the satellite peaks in terms of orbital transitions of the core hole ion.     

The 4s Orbital of Sulfur

The 4s orbital of sulfur has been studied by semiempirical calculations on the H₂S molecule. Overlap population, overlap energy, dipole moment, and oscillator strengths are computed as functions of the orbital exponent of the Slater type 4s orbital, with and without 3d orbitals. The 4s orbital contracts from what is expected for the free atom, but remains essentially Rydberg in character.     

Molecular structure and IR spectra of bromomethanes by DFT and post-Hartree-Fock MP2 and CCSD(T) calculations

The molecular parameters (geometries, rotational constants, dipole moments) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of bromomethanes (CH₃Br, CH₂Br₂, CHBr₃, CBr₄) are predicted by a density functional theory with the hybrid Becke3-LYP functional (DFT) and post-Hartree-Fock methods (MP2, CCSD(T)) using a 6-311G(2d,2p)-type basis set. The MP2 calculations are carried out with different numbers of frozen core orbitals to find how the number of bromine orbitals used for electron correlation influences the predicted molecular parameters and IR spectra of the species in question. Three options were used: (a) all electrons (full), with both the core and valence orbitals considered; (b) partial frozen core option (pfc), when the orbitals up to 3p of bromine were frozen; and (c) full frozen core option (ffc), when all core orbitals up to 3d were frozen. The CCSD(T) calculations for geometric parameters were carried out with both the pfc and ffc options, while for the prediction of the IR spectra only the ffc option was used. In addition, the calculations at the DFT and MP2(pfc) levels with inclusion of f functions on carbon and bromine atoms in bromomethanes (and also the CCSD(T)(pfc) calculations for CH₃Br) were carried out to predict the changes in the geometric parameters and/or vibrational IR spectra of the molecules upon inclusion of f functions The geometries of bromomethanes (particularly the CBr bond lengths) are predicted better by the DFT and CCSD(T) calculations when the f functions (in particular on bromine atom) are included, while the MP2 calculations without f functions are good enough for correct predictions of the molecular geometries. The molecular parameters and vibrational IR spectra of bromomethanes in question and their deuterated species predicted by the DFT, MP2(ffc) and CCSD(T)(ffc) with the 6-311G(2d,2p) basis set agree well with the available experimental data.     

Simulating electron momentum spectra of iso-C2H2Cl2： A study of the core electronic structure

Electron momentum spectroscopy(EMS) has been used for the first time to study core electronic structure of isoC2H2Cl2. In the present work, the pronounced difference between ionization energies of two C1 score orbitals(2A1 and 3A1) is seen as a chemical shift of 3 eV, which is due to different chemical environments of the related carbon atoms. Both the calculated spherically averaged core electron momentum distributions(MDs) and three-dimensional electron momentum density maps show that these core molecular orbitals(MOs) 2A1and 3A1 exhibit strong atomic orbital characteristics in real and momentum space. However, the core states 2B2 and 4A1, which are almost degenerate and related to two equivalent atoms, exhibit notable differences between the momentum and position depictions. In contrast to the position space, the momentum density maps of these two core MOs highlight the interference effects which are due to the nuclear positions. The 2B2 orbital of iso-C2H2Cl2 is the antisymmetric counterpart of the 4A1 core orbital in real space. However, it relates to the 4A1 orbital by an exchange of maxima and minima in momentum space. Due to interference effects between electrons scattered from different atomic centers, modulations with a periodicity of 1.12 a.u. can be seen in the computed momentum densities, which tend to decay with increasing electron momenta. Accordingly, the EMS can not only effectively image the electronic structure of compounds by studying valence orbitals, but also provides direct information on the nature of the nuclear geometry by investigating the core states.     

Adsorption of 2-butyne on Si(0 0 1) at room temperature: A valence band photoemission study

We present an angle resolved ultraviolet photoemission spectroscopy study of the adsorption of 2-butyne (CH₃-CC-CH₃) on Si(0 0 1)-2 × 1 at room temperature. We recorded valence band photoemission spectra for two azimuthal positions of a vicinal silicon surface, where all the rows formed by the surface silicon dimers are parallel. The photoemission symmetry selection rules allow the determination of the orientation of the molecular orbitals. The photoemission signal of the HOMO is enhanced when the electric field is parallel to the dimer rows. This showed that the π orbital left intact after the cyclo-addition reaction of the molecule with one silicon dimer is parallel to the dimer rows. This indicates that each 2-butyne molecule adsorbs on one silicon dimer. In spite of the size of the system and the vicinity of the orbitals, the angle resolved study points out that no dispersion of the electronic bands occurs. Not all the surface dimers are reacted so some disorder still exists on the surface preventing the formation of Bloch states.     

Rydberg states of the HeH+ ion calculated by ab initio methods: potential energies and effective principal quantum numbers

More than 80 excited electronic states of the hydrohelium ion HeH⁺ of ^{1, 3}Σ⁺, ^{1, 3}Π, ^{1, 3}Δ, ^{1, 3}Φ and ^{1, 3}Γ symmetry have been calculated ab initio up to n = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to n = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm^?1. It is shown that with the exception of its ground state, HeH⁺ is an effective one-electron system having an overall electronic structure similar to H⁺₂. The interaction of the excited electron with the He⁺ 1s core electron causes small singlet–triplet splittings to appear and ?-mixing interactions to occur, that are not present in H⁺₂.