The role of torsional modes in the electronic absorption spectrum of acetone

The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8 eV at a resolution of ～4 meV at 4 eV and ～10 meV at 10 eV using synchrotron radiation. The absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd) converging to the first ionization potential of acetone at 9.708 eV. Careful consideration of hot band contributions from torsional modes and symmetry selection rules have resulted in an improved set of vibronic assignments as compared to earlier room temperature work. Revised quantum defect values for some of the Rydberg transitions and a few new assignments in the nd series are also reported in this paper.     

Two-photon vibronic spectroscopy of allene at 7.0–10.5 eV: experiment and theory

2?+?1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5?eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π*?←?π, 3p?←?π, 4s?←?π, 4p?←?π, and 4d?←?π transitions. Vibrational progressions related to the CH₂ twisting (ν₄ ～770?cm^?1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH₂ twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.     

An ab initio molecular orbital study of the electronically excited and cationic states of the ozone molecule and a comparison with spectral data

A number of valence and Rydberg, singlet and triplet excited states for ozone in the excitation energy range 1–12eV have been calculated by large scale CI methods preceded by MCSCF studies. A comparison of the theoretical intensity envelope with the VUV + EELS spectrum has been made. The present work supports the assignments for the Huggins + Hartley bands as having two electronic origins, 2 ¹A₁ and 1 ¹B₂. The experimental ～ 9.3eV and ～ 10.2eV bands of the VUV spectrum must have adventitious superposition of valence states on Rydberg transitions, because the high oscillator strengths of the valence states cannot be attributed to the 8.8eV broad band. A number of new valence and Rydberg states have been calculated, and these lead to the conclusion that the experimental 9–11 eV VUV spectral range in particular must yield more experimental states than the few so far identified. This suggests a major need for more sophisticated methods of experimental study for the excited state manifolds. The use of various MCSCF/CI studies of the vertical cationic states, supports the IP order as ²A₁ < ²B₂ < ²A₂. A re-analysis of the 12–13.4eV range of the UV-photoelectron band has been performed, with a view to determining the adiabatic IPs more accurately. The present work suggests that the adiabatic IP₂ lies at 12.86eV, slightly lower than has been assumed, with consequential effect on the analysis of the VUV spectrum near 9.4eV.     

The Rydberg spectroscopy of methyl chloride

The absorption spectrum of methyl chloride has been studied in the energy region from 68 600 to 94 760 cm^?1. With the exception of a valence-shell transition, all transitions were interpreted as molecular Rydberg transitions. These assignments were made utilizing the Rydberg formula. The similarity of the vibrational fine structures observed in the photo-electron spectrum of methyl chloride and its VUV spectroscopy is shown to further support the Rydberg assignments.     

Vacuum Ultraviolet Absorption Spectrum of Difluoromethane Reinvestigated

The vacuum ultraviolet (VUV) absorption spectrum of difluoromethane (CH₂F₂) was studied using synchrotron radiation from the storage ring Indus-1, Indore, India. Spectra were recorded in the spectral region 1050–1500 Å (～8.3–11.8 eV) at a resolution of 1.5 Å. Three absorption band systems were observed in this region. Overall features observed are in good agreement with previously published work. Some discrepancies in assignments of the observed vibronic bands carried out by previous workers have been resolved. The observed bands have been classified in terms of Rydberg series.     

High resolution core and valence band XPS spectra of non-conductor pyroxenes

High resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of the non-conductor pyroxene minerals, bronzite ((Mg_0.8,Fe_0.2)₂Si₂O₆) and diopside (Ca(Mg_0.8Fe_0.2)Si₂O₆) have been obtained with the Kratos magnetic confinement charge compensation which minimizes differential charge broadening. Observed Si 2p, O 1s, Mg 2p and Ca 2p total linewidths are all about 1.3 eV, very similar to those observed previously with the same instrument for SiO₂ and olivines ((Mg,Fe)₂SiO₄); and we consider that these widths are within 0.05 eV of the minimum room temperature linewidths for these samples with the experimental resolution of this instrument of 0.35 eV. These linewidths are all determined by vibrational broadening due to the M-O symmetric stretch in the ion states. The Si 2p binding energies (BE) are intermediate between the quartz and olivine Si 2p binding energies; but the O 1s spectra resolve the bridging oxygen (BO) and non-bridging oxygen (NBO) in the unit, with the NBO O 1s very close in BE to the O in olivine, and the BO very close to the BO in SiO_2. Indeed in both diopside and bronzite, it is possible to separate the three structurally inequivalent O atoms in the O 1s spectra: the BO at a BE of about 532.6 eV, a NBO peak from the MgOSi moiety (Mg in the M1 site) at about 531.3 eV, and a NBO peak at 531 eV from the CaOSi or the FeOSi moieties (Ca and Fe in the M2 site). The O 1s BE increases with the increase in the electronegativity Ca < Mg < Fe < Si. Moreover, the linewidths of these peaks increase when Fe and Mg are both present in either M1 (diopside) or M2 (bronzite) sites.The valence band spectra for the two pyroxenes are rather similar, and quite different from the VB spectra of quartz and olivines. The dispersion of the pyroxene VB spectra is intermediate between the VB spectra of quartz and olivines; the valence band spectrum of pyroxenes are more dispersed than in olivines by about 1.5 eV but less dispersed than quartz by about 1.5 eV. These VB spectra can be assigned using the previous olivine VB spectra and high quality pseudopotential density functional theoretical calculations in the generalized gradient (GGA) approximation. As for the olivine VB spectra, the Fe 3d t_2g and e_g orbitals in M1 and M2 sites of the pyroxene are located at the top of the pyroxene valence band, and the BE of the Fe 3d peaks from M1 are about 0.7 eV smaller than the Fe 3d peaks in M2. The theoretical XPS valence band spectra using the theoretical density of states and the Gelius intensity approximation are is in good semi-quantitative agreement with the experimental spectra. This intermediate dispersion of pyroxenes is due to the partial polymerization of the Si-O units in pyroxenes, and the intermediate charge on the Si atoms as indicated by the Si 2p BE.     

New interpretations of XPS spectra of nickel metal and oxides

A current interpretation of XPS spectra of Ni metal assumes that the main 6 eV satellite is due to a two hole c3d⁹4s² (c is a core hole) final state effect. We report REELS observation in AES at low voltages of losses (plasmons and inter-band transitions) corresponding to the satellite structures in Ni metal 2p spectra. The satellite near 6 eV is attributed to a predominant surface plasmon loss. A current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignments of the main peak at 854.6 eV and the broad satellite centred at around 861 eV to the cd⁹L and the unscreened cd⁸ final-state configurations, respectively (L is a ligand hole). Multiplet splittings have been shown to be necessary for assignment of Fe 2p and Cr 2p spectral profiles and chemical states. The assignments of Ni 2p states are re-examined with intra-atomic multiplet envelopes applied to Ni(OH)₂, NiOOH and NiO spectra. It is shown that the free ion multiplet envelopes for Ni²⁺ and Ni³⁺ simulate the main line and satellite structures for Ni(OH)₂ and NiOOH. Fitting the NiO Ni 2p spectral profile is not as straightforward as the hydroxide and oxyhydroxide. It may involve contributions from inter-atomic, non-local electronic coupling and screening effects with multiplet structures significantly different from the free ions as found for MnO. A scheme for fitting these spectra using multiplet envelopes is proposed.     

The electron impact spectra of the fluoromethanes

The electron impact spectra of methane, methyl fluoride, methylene fluoride, fluoroform, and tetrafluoromethane have been obtained out to 22 eV energy loss using electrons incident at 400 eV and scattered through small angles (0–2°). Combining the major features of these spectra with the molecular ionization potentials determined by photoelectron spectroscopy allows consistent assignments of the bound transitions to be made involving the uppermost three or four filled molecular orbitals and the lowermost two or three Rydberg levels. The assignments follow naturally from the constancy of Rydberg term values and considerations of intensity.     

Photo-Absorption Studies on Formaldehyde Using Synchrotron Radiation at Indus I

Photo-absorption spectra of formaldehyde (HCHO) is recorded in the range of 6–11.5 eV at various pressures (<0.001–2 mbar) at an average resolution of 1.2 Å using Photophysics beam line at the 450 MeV Indus-1 synchrotron radiation facilities at RRCAT Indore, India. The spectrum is found to consist exclusively of n → Rydberg transitions converging to the ground state of HCHO⁺. The highest identified Rydberg states, observed up to the first ionization limit of HCHO, correspond to 7s, 11p, 9d, and 12f orbitals. Analyzed electronic spectrum along with the intensities and quantum defects are presented. To interpret the observed weak valence transitions instead of strong valence transitions, a theoretical study of Rydberg and valence electronic states of HCHO is performed in the framework of single configuration interaction (CIS) and time-dependent density functional theory (TDDFT) using different basis sets. Electronic transition energies of high-lying singlet and triplet valence states as calculated using TDDFT (B3LYP) level of theory are found to give fairly-good agreement with the experimental data.     

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.     

X-ray diffraction and photoelectron spectroscopy study of swift heavy ion irradiated Mn/p-Si structure

The electronic structure and interfacial chemistry of thin manganese films on p-Si (1 0 0) have been studied by photoelectron spectroscopy measurements using synchrotron radiation of 134 eV and from X-ray diffraction data. The Mn/p-Si structures have been irradiated from swift heavy ions (∼100 MeV) of Fe⁷⁺ with a fluence of 1 × 10¹⁴ ions/cm². Evolution of valence band spectrum with a sharp Fermi edge has been obtained. The observed Mn 3d peak has been related to the bonding of Mn 3d-Si 3sp states. Mn 3p (46.4 eV), Mn 3s (81.4 eV) and Si 2p (99.5 eV) core levels have also been observed which show a binding energy shift towards lower side as compared to their corresponding elemental values. From the photoelectron spectroscopic and X-ray diffraction results, Mn₅Si₃ metallic phase of manganese silicide has been found. The silicide phase has been found to grow on the irradiation.     

The VUV Absorption Spectrum of Lead Monoxide

The VUV absorption spectrum of PbO vapor down to 300 ? has been observed for the first time. Four ionization energies have been determined from Rydberg series at 8.73, 8.83, 12.81, and 26.28 eV corresponding to the X(2)Pi, A(2)Sigma(+), B(2)X(+), and C(2)Sigma(+) electronic states of the PbO(+) ion. Copyright 2000 Academic Press.     

Two-color three-photon excitation studies of thallium Rydberg states

We present new data on the even-parity Rydberg states of atomic thallium using two-step three-photon laser excitation technique in conjunction with a thermionic diode ion detector. Atoms are excited from the 6p ²P_1/2 ground state to the 7p ²P_1/2 intermediate state via two-photon excitation and subsequently promoted to the high lying ns ² S_1/2 and nd ²D_3/2 Rydberg states. The first ionization potential of thallium is determined as 49,266.66(1) cm^-1 using data for the ns ² S_1/2 (25 ≤ n ≤ 54) and nd ²D_3/2 (24 ≤ n ≤ 65) Rydberg series. This value is believed to be more accurate because the contribution due to the hyperfine structure splitting of the 7p ²P_1/2 state (0.07185 cm^-1) is much smaller as compared to that of the 6p ²P_1/2 ground state (0.711 cm^-1).     

Study of the 3s Rydberg state of pyrimidine by multiphoton ionization spectroscopy

The 3s Rydberg state of pyrimidine is seen as a two-photon resonant structure on its three-photon ionization spectrum. The vibrational structure is analyzed and compared with the VUV absorption and photoelectron spectra.     

Photoelectron and electron impact spectrum of HCN

The electronic structures of HCN and DCN have been determined by examining high resolution He(I) photelectron spectra of HCN and DCN, He(II) photoelectron spectrum of HCN, and the electron impact energy loss spectra of HCN and DCN. The present investigation supports an earlier assignment of the orbital sequence in HCN. New vibrational data are presented and the Rydberg series and valence transitions are reinvestigated. The adiabatic ionization energies for the 1π and 5σ orbitals in HCN are found to be 13.607 ± 0.002 eV and 14.011 ± 0.003 eV respectively.As mentioned above the investigation of the Rydberg series indicated that the first IP at 13.607 eV is the 1π ionization and the second IP at 14.011 eV is the 5σ ionization. A comparison of the experimental and theoretical intensity ratio between the two first PES progressions also supports this assignment. It is further supported by the fact that in the second IP the ν₃ vibration frequency is not changed as much as it is in the first IP, which is in agreement with the PES of N₂ and CO. The analysis of the bending vibrations also supports this ordering of the orbitals.The same orbital assignment has recently been proposed by Frost et al.⁵, using a comparison with the HCP photoelectron spectrum. The present paper supports their assignment of orbitals and (00⁰0)-(00⁰0) transitions. There are, however, some disagreements concerning the vibrational analysis. This is probably due to the fact that the HCN spectrum of Frost et al.⁵ revealed less structure than ours. As indicated by Figure 5 there is possibly still more structure to be revealed.     

Electronic structure of HgGa2S4

The electronic structure and chemical bonding in HgGa₂S₄ crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa₂S₄ is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS₂ and SrGa₂S₄ compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa₂S₄ crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.     

Rydberg series of NO converging to the ionic states b3Π, A1Π, and w3Δ

Rydberg series of NO in the 600–1000-Åregion were investigated by using a 6.65-m high-dispersion vacuum spectrograph. The previous β, γ (v = 0), and γ (v = 1) Rydberg series were extended up to n = 31, 31, and 28, respectively. From the analysis of these Rydberg series, accurate ionization energies were obtained: 133 565 ± 3 (16.5596 eV) for b³Π (v = 0); 147 811 ± 3 (18.3258 eV) for A¹Π (v = 0); 149 372 ± 3 cm^?1 (18.5193 eV) for A¹Π (v = 1) of NO⁺. A new Rydberg series converging to one of the triplet components in b³Π was identified, and the coupling constant of b³Π was estimated to be 35 ± 8 cm^?1. Higher members of two Rydberg series were newly observed in the 700–725-Åregion. From their series limits, 139 926 ± 3 and 141 160 ± 3 cm^?1, they were assigned to be the Rydberg series converging to the v = 3 and 4 levels of ω ³Δ in NO⁺.     

High resolution XPS study of the large-band-gap semiconductor stibnite (Sb2S3): Structural contributions and surface reconstruction

Conventional X-ray photoelectron spectroscopy (XPS) and synchrotron radiation XPS (SRXPS) were used to probe the chemical state properties of stibnite (Sb₂S₃), a large-band-gap semiconductor of complex structure. The conventional spectra were obtained with a Kratos Axis Ultra XPS with magnetic confinement charge neutralization, which is very effective in minimizing both uniform charging and differential charging on this large-band-gap semiconductor. The narrow linewidths (much narrower than previously obtained) for single doublet fits (e.g. Sb 4d_5/2 of 0.57 eV and S 2p_3/2 of 0.63 eV) enabled the observation of a small peak on the low binding energy side of the Sb 3d and Sb 4d lines. With the aid of the very surface-sensitive Sb 4d SRXPS spectra, these low energy peaks are assigned to small Sb metal clusters at the surface after cleavage; the signal for these clusters increases with X-ray dose on the sample.A detailed analysis of the Sb 4d and S 2p linewidths concludes that the Sb 4d_5/2 linewidth is larger than expected based on the inherent linewidth of the instrument and the Sb 4d lifetime width, and on comparison with the As 3d linewidth (0.52 eV) for the analogous As₂S₃. Also, the S 2p_3/2 linewidth is substantially broader than the Sb 4d_5/2 linewidth. These larger than expected linewidths are due to two structurally distinct Sb atoms and three structurally distinct S atoms in the Sb₂S₃ crystal structure. Accordingly, the Sb 4d and S 2p spectra have been fitted to two and three doublets respectively, and the linewidth for all peaks is 0.53 eV. Using recent molecular orbital calculations, the doublets have been assigned to the different structural Sb and S sites.