Near-threshold Electron Energy-loss Spectroscopy Research Articles

ChemInform Abstract: Electronic States of Some Simple Ethers Studied by Vacuum UV Absorption and Near-Threshold Electron Energy-Loss Spectroscopy.

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The electronic states of but-2-yne studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio configuration interaction methods

The photoabsorption spectrum of but-2-yne in the range 5.5–11 eV (225–110 nm) has been recorded using a synchrotron radiation source. The spectrum is dominated by three d-type Rydberg series, converging to the first ionisation energy (IE) (π −1, 9.562 eV). Origins of the π3d members are 7.841, 7.977 and 8.018 eV, respectively. Transitions of low intensity, arising from excitation of the π3s state (origin, ∼6.35 eV) and two π3p Rydberg states (7.38 and 7.51 eV, respectively) have also been identified in the spectrum. Near-threshold electron energy-loss spectra reveal valence excited triplet states at about 5.2 and 5.8 eV, respectively. Electronic excitation energies for valence and Rydberg-type states have been computed using ab initio multi-reference multi-root CI methods. These studies used a triple zeta + polarisation basis set, augmented by diffuse (Rydberg) orbitals, to generate the theoretical singlet and triplet energy manifolds. The correlation of theory and experiment shows the nature of the more intense Rydberg state types, and identification of the main valence and Rydberg bands. Calculated energies for Rydberg states are close to those expected, and there is generally a good correlation between the theoretical and experimental envelopes. It was possible to generate singlet Rydberg states which relate to the 5-lowest IEs of but-2-yne; furthermore, the separation of these sequences shows that the IE order (under D 3h symmetry) is: 2 e ′ < 5 a 1 ′ < 1 e ″ < 1 e ′ < 4 a 2 ″ , also supported by direct calculation of the IEs by CI. The lowest valence singlet states are ππ ∗, optically forbidden, and calculated to lie near 7.3 and 7.6 eV. The states which contribute strongly to the observed spectrum are πσ ∗ ( 1 E ′ + 1 A 2 ″ ) near 7.9 eV having 2 e ′ 6 a 1 ′ ∗ excitation, followed by several ππ ∗ and πσ ∗ states ( 1 E ′ + 1 A 2 ″ ) between 10.0 and 10.5 eV; an 1E′ antisymmetric combination(2e′2e″ − 2e′2e″) is by far the strongest in intensity. A further group of symmetry-allowed valence states are calculated to lie near 12.3 and 12.9 eV. The two lowest triplet states, both of E′ symmetry (ππ ∗), have vertical excitation energies of 5.7 and 6.2 eV, but are strongly bent with a trans-CCCC unit (C S and C 2h). The theoretical work confirms that, on intensity grounds, valence excited states do not contribute significantly to the spectrum. CI calculations of the ionic states give the ionisation energy sequence ( D 3 h ) : 2 E ′ < 2 A 1 ′ < 2 E ″ < 2 E ′ < 2 A 2 ″ . Adiabatic structures for the first cation, two triplets, and a singlet (C 2h) were obtained; these show shortening of C–C, and lengthening of C C, in a trans-CCCC, as is found with ethyne.

Photoabsorption and near-threshold electron energy-loss spectroscopy of OClO

The spectroscopy of chlorine dioxide, OClO, has been investigated using the techniques of dissociative electron attachment (DEA), near-threshold electron energy loss (EEL, 0-10 eV) and optical absorption (2.5-10.8 eV). Mass and energy analyses of the ions formed in the DEA processes show that at energies above about 3 eV, the product ions are predominantly near-thermal , suggesting that the accompanying oxygen atoms are excited electronically. All features observed in the EEL experiments are in regions of optical absorption, indicating that the optically dark doublet and quartet states have energies which overlap optically allowed transitions. The optical spectrum itself is in good qualitative agreement with previous measurements, but there are differences in relative absorption cross sections. Alternative interpretations of some spectral features are proposed.

The electronic states and molecular properties of 1,2,3-triazine studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction studies

Optical electronic absorption spectra, and electron energy-loss spectra have been obtained for 1,2,3-triazine. The valence and Rydberg excited states have been studied by multi-reference multi-root configuration interaction studies using MRD–CI techniques, and the results related to the experimental spectra. A number of Rydberg states, associated with five ionisation limits, have been identified along with valence states of types 1, 3 nπ * and 1, 3 ππ *. The UV-photoelectron spectrum has been reconsidered by means of MRD–CI, Green's function and the non-diagonal TDA method; the first five IPs are as previously determined. This leads to the ionisation order: 2A 1(11a −1 1)< 2B 2(7b −1 2)< 2B 1(2b −1 1)< 2A 2(1a −1 2)< 2A 1(10a −1 1) . The electronic properties of the molecule have been determined at the equilibrium geometry for a large basis set at both the SCF and MP2 levels.

The molecular and electronic states of 1,2,4,5-tetrazine studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction studies

The VUV electronic absorption spectrum of 1,2,4,5-tetrazine has been re-investigated, and together with electron energy-loss spectra has led to identification of a number of new excited states. The valence and Rydberg excited states have been studied by multi-reference multi-root configuration interaction studies using MRDCI techniques. Initial studies with the RPA and TDA methods gave almost identical results for the excitation energies, but there is a substantial energy-lowering in the MRDCI calculations, which improves agreement with experiment substantially; these differences are a result of the double, triple and quadruple excited reference configurations included in the reference set of the latter method. The 1ππ ∗ excitations are calculated rather higher than experiment, except for the lowest-lying (weak) 1B 2u state at 5.0 eV. The calculated order for the next three ππ ∗ states is 1B 1u (weak) followed by 1B 2u (strong) and 1B 1u (strong), the last two bands being responsible for the dominant absorption near 7.5 and 8.5 eV. All of this group of four bands involve excitations from the pair of MOs 1b 2g and 1b 1g into the 1a u ∗ and 4b 3u ∗ VMOs. The sequence of nπ ∗ stakes are in a similar order to the ππ ∗ excitations, with respect to the upper state, and the two lowest singlet states, 1B 3u and 1A u are reasonably well determined. The triplet states follow a similar order to the singlets, and again the dominance of the effect of the two lowest VMOs is demonstrated, but considerable differences between the weighting of leading configurations occurs between singlet and triplet manifolds. The non-diagonal TDA method has been used to reconsider the UV-photoelectron spectrum. The ionisation potentials for tetrazine are reinterpreted with the first three bands being regrouped into 1, 2, 2 ionisations respectively. The ground state properties of tetrazine suggest that the NQR spectrum will show a principal axis 14N quadrupole coupling constant larger than the other known azines. The decline in diamagnetic susceptibility anisotropy in tetrazine, relative to benzene and pyridine, is indicative of lower resonance energy.

The electronic states of the azines. VII. 1,2,4-triazine, studied by photon absorption, near-threshold electron energy loss spectroscopy and ab initio multi-reference configuration interaction calculations

The electronic states of the azines. VII. 1,2,4-triazine, studied by photon absorption, near-threshold electron energy loss spectroscopy and ab initio multi-reference configuration interaction calculations

The electronic states of furan studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

Abstract The results of large-scale ab initio CI calculations on the electronic states of furan, using a multi-reference multi-root CI method (MRD-CI), are related to new VUV absorption and low-energy electron energy-loss (EEL) measurements, leading to detailed spectral assignments. All of the expected 1,3 ππ ∗ states have been assigned, there being improved agreement between theoretical and experimental energies in all cases. A number of new Rydberg series has been identified by experiment and, for these also, the calculations perform well. In addition, cationic states and ground state molecular electronic properties have been computed.

The electronic states of the azines. VI. 1,3,5-triazine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

The results of large-scale ab initio CI calculations on the electronic states of 1,3,5-triazine, using a multi-reference multi-root CI (MRD-CI) method are related to new VUV absorption and low-energy electron energy-loss (EEL) measurements, leading to detailed spectral assignments. Vertical transition energies are computed to within 0.6 eV for all and 0.3 eV for most of the low-lying electronic states. The computations are extended to cationic states and to the evaluation of ground state molecular electronic properties of the molecule.

The electronic states of the azines. IV. Pyrazine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

The present paper extends the number of known low-lying electronically excited states of pyrazine through comparison of VUV absorption and near-threshold electron energy-loss spectra. Assignments of these, and assessment of previously established states, have been carried out by all-valence electron CI studies using multi-reference multi-root approaches in a double zeta+Rydberg function basis. In general, the first members of Rydberg series are well-determined by calculation, as are the lowest ππ* and nπ* states. However, the 1B 1u/ 1B 2u states derived from the 1E 1u state in benzene are not well-calculated. It seems that the paucity of MOs of symmetry a u lies behind this and that an iterative natural orbital approach may be necessary to obtain a better fit of theory with experiment. In the assignment of cationic states, there are some differences between present large-scale CI and earlier Green function (GF) and Tamm-Dancoff (TDA) results; in particular, the CI calculations produce many more shake-up states and with a lower onset energy than the TDA method with the same basis set. One-electron properties of the pyrazine ground electronic state have also been computed, with a larger basis set.

Electronic states of some simple ethers studied by vacuum ultraviolet absorption and near-threshold electron energy-loss spectroscopy

Vacuum ultraviolet absorption spectra (250–105 nm, 5–11.8 eV) and near-threshold electron energy-loss spectra (0 eV–Ei,1) have been recorded for dimethyl ether (1), diethyl ether (2), tetrahydrofuran (3), 1,4-dioxane (4) and 3,4-dihydropyran (5). For the saturated compounds 1–4 the low-lying electronically excited states are predominantly Rydberg in nature, selectively mixed with valence-excited states. Also, the lowest triplet state in each of the saturated molecules is within 0.5 eV of the 1nO 3s state. The spectra of 5 resemble those of unsaturated hydrocarbons; its lowest triplet state (3ππ*) is ca. 4.2 eV.

The electronic states of the azines. III. Pyrimidine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration calculations

The electronic states of pyrimidine have been investigated by VUV absorption and near-threshold electron energy-loss spectroscopy, together with ab initio multi-reference configuration interaction calculations. This approach has allowed the experimental assignment of all the low-lying valence-excited nπ* and ππ* singlet states. In contrast to ππ* excitations, some of which are difficult to compute within 0.5 eV, the nπ* states are more accurately calculated, as are the lowest Rydberg excited states. The experimental identification of many low-lying Rydberg states is not yet possible. The UV-photoelectron spectral assignments by large-scale CI are similar to earlier Green's function results, except for the inner π level (1b 1) ionisation; many shake-up states are computed below 17 eV and it is clear that the TDA method, with the same basis set, leads to a much later onset.

The electronic states of the azines. II. Pyridine, studied by VUV absorption, near-threshold electron energy loss spectroscopy and ab initio multi-reference configuration interaction calculations

The electronic states of pyridine have been investigated experimentally, using VUV absorption and near-threshold electron energy loss spectroscopy, and theoretically, with ab initio multi-reference configuration interaction calculations. The theoretical energies relate well to the experimental observations, allowing firm identification of all low-lying ππ* and nπ* excited states, including an optically forbidden nπ* 1A 2 state, detected at 5.43 eV in electron energy loss. Fine structure in the VUV spectrum, associated with Rydberg-state excitation, matches that observed in high resolution photoelectron spectra, facilitating Rydberg-state assignments. Good energies are calculated for n = 3, 4 members of Rydberg series. The Cl and MC SCF results confirm that the first ionisation potential in pyridine is 2A 1 corresponding to removal of a non-bonding electron, in agreement with experiment. A triple zeta valence plus polarisation SCF + all-valence electron Cl has led to good agreement for various one-electron operators of the wavefunctions with experiment.