S 2p photoabsorption of the SF5CF3 molecule: Experiment,theory and comparison with SF6

The S 2p core excitation spectrum of the SF₅CF₃ molecule has been measured in the total ion yield mode. It resembles a lot the analogous spectrum of SF₆, also recorded in this study, displaying intense transitions to the empty molecular orbitals both below and above the S 2p ionization potential (IP) and weak transitions to the Rydberg orbitals. The S 2p photoabsorption spectra of SF₆ and SF₅CF₃ have been calculated using time-dependent density functional theory, whereby the spin–orbit coupling was included for the transitions below the S 2p IP. The agreement between experiment and theory is good for both molecules, which allows us to assign the main S 2p absorption features in SF₅CF₃.     

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.     

Inner shell excitation of SO2 by high-energy electron impact: a comparison with multichannel quantum defect theory

High-resolution electron energy loss spectra of the sulfur 2p, 2s and oxygen 1s inner shells of SO₂ have been obtained under conditions of high impact energy (1–3 keV) and zero-degree scattering angle. The results are also compared with photoabsorption spectra in the sulfur 1s and 2p regions. Multichannel quantum defect theory calculations of energies and oscillator strengths for S 2p, 2s, O 1s and S 1s are in excellent quantitative agreement with the measurements and are used to assign the spectra.     

High resolution gas phase photoabsorption spectra of SiCl4 and Si(CH3)4 at the silicon l edges: characterization and assignment of resonances

High resolution gas phase photoabsorption spectra of SiCl₄, and Si(CH₃)₄ have been measured using synchrotron radiation in the 100–225 eV photon energy range. This range encompasses the Si 2p, Si 2s and Cl 2p edges. Comparison with previous spectra shows that the high resolution is essential for characterizing all Si 2p features - both the narrow intense pre-edge features, and the broader above edge shape resonances. The Si 2s spectra, along with MS Xα calculations, enable assignment of the more complex Si 2p shape resonances. There is good agreement between theory and experiment for the Si 2p and Si 2s SiCl₄ shape resonance positions. The general shape of the shape resonance structure above the Si 2p and Si 2s edges of Si(CH₃)₄ is well reproduced by the Xα calculations.     

A theoretical and experimental study of the near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectra (XPS) of nucleobases: Thymine and adenine

The core level electron excitation and ionization spectra of thymine and adenine have been investigated by photoabsorption and photoemission spectroscopy, and the results interpreted by means of ab initio calculations using the second-order algebraic–diagrammatic construction (ADC(2)) method for the polarization propagator and the fourth-order ADC method (ADC(4)) for the one-particle Green’s function. The photoabsorption spectra are dominated by transitions from core levels to unoccupied π states, but also show clear structures due to Rydberg transitions. The calculated spectra are in good agreement with the experimental results, and many of the observed structures are assigned.     

Cationic Rydberg states observed in resonantly enhanced electron spectra of CS2

The inner valence electron spectrum of the CS₂ molecule has been investigated in the binding energy range between 18.6 and 26.3 eV using synchrotron radiation for ionisation. Photon energies in the range from 67 to about 167 eV have been used, with particular focus on 166.70, 166.89 and 167.09 eV for which S2p electrons are resonantly transferred into Rydberg orbitals close to the ionisation threshold. From there, autoionisation takes the molecule into various cationic states characterized by two valence holes and a Rydberg spectator electron. Many new bands are observed which contain vibrational progressions with spacings around 120 meV in most cases. These are assigned as excitations of the totally symmetric stretching ν₁ mode in the cationic state. The new bands reflect states in the cation that are close to the electronic states of the dication and assignments are made by comparison to double ionisation electron spectra.     

Limonene: electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, electron scattering, He(I) photoelectron spectroscopy and ab initio calculations

Electronic state spectroscopy of limonene has been investigated using vacuum ultraviolet photoabsorption spectroscopy in the energy range 5.0-10.8 eV. The availability of a high resolution photon beam (~0.075 nm) enabled detailed analysis of the vibrational progressions and allowed us to propose, for the first time, new assignments for several Rydberg series. Excited states located in the 7.5-8.4 eV region have been studied for the first time. A He(I) photoelectron spectrum has also been recorded from 8.2 to 9.5 eV and compared to previous low resolution works. A new value of 8.521 ± 0.002 eV for the ground ionic state adiabatic ionisation energy is proposed. Absolute photoabsorption cross sections were derived in the 10-26 eV range from electron scattering data. All spectra presented in this paper represent the highest resolution data yet reported for limonene. These experiments are complemented by new ab initio calculations performed for the three most abundant conformational isomers of limonene, which we then used in the assignment of the spectral bands.     

CH3I low-n Rydberg states in supercritical atomic fluids near the critical point

Vacuum ultraviolet photoabsorption spectra of CH₃I 6s and 6s′ Rydberg states doped into supercritical argon, krypton, and xenon perturbers were measured from low density to the density of the triple point liquid at noncritical temperatures and on an isotherm near the perturber critical temperature. A full line shape analysis of these spectra was performed using a single set of intermolecular potential parameters for each dopant/perturber system. The resulting perturber induced shift of the simulated adiabatic transition of the 6s and 6s′ Rydberg states is presented as a function of perturber number density, and this shift illustrates a perturber critical point effect on the excitation energies of the molecular low-n Rydberg states.     

VUV electronic state spectroscopy of 1,1-difluoroethene and difluorochloromethane by high resolution synchrotron radiation

High resolution VUV photoabsorption spectra of 1,1-difluoroethene (C(2)H(2)F(2)) and difluorochloromethane (CF(2)HCl) are reported in the wavelength range 115-200 nm (10.8-6.2 eV). New photoabsorption features are observed in C(2)H(2)F(2) and some Rydberg and vibrational assignments are proposed for the first time. Evidence for a weak vibrational pattern in CF(2)HCl is discussed here for the first time. Absolute cross section values have been obtained allowing photolysis lifetimes to be derived in the Earth's troposphere and stratosphere.     

Measurement of the partial photoionization cross sections and asymmetry parameters of S atoms in the photon energy range 10.0-30.0 eV using constant-ionic-state spectroscopy

The partial photoionization cross sections and asymmetry parameters of S atoms have been measured using constant-ionic-state (CIS) spectroscopy in the photon energy range 10.0-30.0 eV. The ionizations investigated in these CIS experiments are the (3p)(-1) ionizations S(+)((4)S)<--S((3)P), S(+)((2)D)<--S((3)P), and S(+)((2)P)<--S((3)P). For the first time Rydberg series which converge to the fourth ionization limit have been observed and assignments of these series have been proposed. These correspond to excitations to Rydberg states that are parts of series which converge to the fourth ionization limit, S(+)((4)P)<--S((3)P) (3s)(-1), and autoionize to the lower S(+)((4)S), S(+)((2)D), or S(+)((2)P) states. For each series observed in the CIS spectra photoelectron angular distribution studies, combined with other evidence, has allowed the angular momentum character of the free electron on autoionization to be determined.     

The electronic states of 1,2,3-triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy, and a comparison with ab initio configuration interaction methods

The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (～5.5-6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series.     

The VUV electronic spectroscopy of acetone studied by synchrotron radiation

The electronic state spectroscopy of acetone (CH3)2CO has been investigated using high-resolution VUV photoabsorption spectroscopy in the energy range 3.7-10.8 eV. New vibronic structure has been observed, notably in the low energy absorption band assigned to the 1(1)A(1) --> 1(1)A2 (ny --> pi*) transition. The local absorption maximum at 7.85 eV has been tentatively attributed to the 4(1)A1 (pi --> pi*) transition. Six Rydberg series converging to the lowest ionisation energy (9.708 eV) have been assigned as well as a newly-resolved ns Rydberg series converging to the first ionic excited state (12.590 eV). Rydberg orbitals of each series have been classified according to the magnitude of the quantum defect (delta) and are extended to higher quantum numbers than in the previous analyses.     

Electronic and cationic states of 2-methylpropene (isobutene) studied by VUV absorption,scattered electron spectroscopy and AB initio multireference configuration interaction calculations

VUV (6.2–9 eV) and electron scattering spectra (1–9 eV) have been recorded for 2-methylpropene (isobutene). Also, electronic states of the molecule, including the ground state and cationic states, have been investigated using ab initio multi-reference configuration interaction calculations. Some Koopmans-type in the UV photoelectron spectrum are reassigned and a number of shake-up states computed. In the electronic spectrum, Rydberg excited have been assigned and a second valence excited state (σ π*) located within about 1 eV of the V(ππ*) state. The experiments show, and theory confirms, that the Rydberg R(π3s) state has a positive electron affinity. Some interesting correlations between ionisation energies, energies of shake-up state electronic excitation energies are identified.     

Projection of Si 1s photoexcited orbitals into resonant Auger electron spectra in KLL decays of Si(CH3)4 and SiF4

Spectator resonant KL(23)L(23) Auger electron spectra have been measured in the Si 1s photoexcitation region of Si(CH(3))(4) using monochromatized undulator radiation combined with a hemispherical electron spectrometer. The broad peak with high intensity in a total ion yield spectrum, coming mainly from excitation of a 1s electron into the 6t(2) vacant orbital, induces a spectator Auger decay in which the excited electron remains in its excited orbital. The component on the higher energy side of this peak through 1s excitation into a Rydberg orbital produces resonant Auger decays in which the excited Rydberg electron moves into a slightly higher Rydberg orbital, or is partly shaken up to a significantly higher Rydberg orbital. These findings of Si(CH(3))(4) indicate a clear contrast to those for SiF(4), in which the 1s excitation into a Rydberg orbital induces a shake-down phenomenon as well as a shake-up one. The results of these molecules exhibit a clear splitting effect among excited orbitals which are smeared out by overlapping due to lifetime widths and due to densely populated levels in the 1s electron excitation spectrum. This is consistent with the calculation on photoexcitation within the framework of density functional theory.     

3s Rydberg and cationic States of pyrazine studied by photoelectron spectroscopy

We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-1) cationic states of pyrazine [1,4-diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D0 and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0 and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1-D0 and 3s(pi-1)-3s(n-1) conical intersections were estimated. From the line width in the D1 <-- S0 spectrum, we estimated the lifetime of D1 to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(pi-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1 observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.     

Fine structure in the VUV absorption spectrum of neopentane at 16 eV

The photoabsorption of neopentane (C(CH₃)₄) has been measured for photon energies from 10 to 30 eV with synchrotron radiation. Sharp absorption bands are observed around 16 eV on top of a strong continuum. These can be grouped into a p-like Rydberg series with accompanying vibrational sub-bands. The nature of the Rydberg states is discussed.     

Rydberg series in the absorption spectra of H2O and D2O in the vacuum ultraviolet

The photoabsorption cross section of molecular H₂O and D₂O has been determined in the range from hν = 10eV to 20 eV with 0.03 Å resolution. A refined analysis of the Rydberg series including the rotational line shapes of several bands to locate the band origins and a comparison with recent ab initio calculations is given. In the region of continuous absorption we have assigned a p-type and an s-type Rydberg series leading to the ²A₁ and the ²B₂ state respectively with quantum defects δ ≈ 0.75 and δ ≈ 1.36.     

Double-electron excitation above Xe K-edge

The photoabsorption cross-section in Xe has been measured in order to determine the features of the shake processes resulting from multiple electron excitations as a function of photon energy. Double-electron transitions of [1s4d], [1s4p], [1s4s], and [1s3d] have been detected.