Photoabsorption Oscillator Strengths Research Articles

R-Matrix calculations for opacities: II. Photoionization and oscillator strengths of iron ions Fe xvii, Fe xviii and Fe xix

Iron is the dominant heavy element that plays an important role in radiation transport in stellar interiors. Owing to its abundance and large number of bound levels and transitions, iron ions determine the opacity more than any other astrophysically abundant element. A few iron ions constitute the abundance and opacity of iron at the base of the convection zone (BCZ) at the boundary between the solar convection and radiative zones and are the focus of the present study. Together, Fe xvii, Fe xviii and Fe xix represent 85% of iron ion fractions, 20%, 39% and 26% respectively, at the BCZ physical conditions of temperature T ∼ 2.11×106 K and electron density Ne = 3.1×1022 c.c. We report the most extensive R-matrix atomic calculations for these ions for bound–bound and bound–free transitions, the two main processes of radiation absorption. We consider wavefunction expansions with 218 target or core ion fine structure levels of Fe xviii for Fe xvii, 276 levels of Fe xix for Fe xviii, in the Breit–Pauli R-matrix (BPRM) approximation, and 180 LS terms (equivalent to 415 fine structure levels) of Fe xx for Fe xix calculations. These large target expansions, which include core ion excitations to n = 2,3,4 complexes, enable accuracy and convergence of photoionization cross sections, as well as the inclusion of high lying resonances. The resulting R-matrix datasets include 454 bound levels for Fe xvii, 1,174 levels for Fe xviii, and 1,626 for Fe xix up to n⩽ 10 and l = 0–9. Corresponding datasets of oscillator strengths for photoabsorption are: 20 951 transitions for Fe xvii, 141 869 for Fe xviii, and 289 291 for Fe xix. Photoionization cross sections have been obtained for all bound fine structure levels of Fe xvii and Fe xviii, and for 900 bound LS states of Fe xix. Selected results demonstrating prominent characteristic features of photoionization are presented, particularly the strong Seaton photoexcitation-of-core resonances formed via high-lying core excitations with Δn=1 that significantly impact bound–free opacity.

High-resolution study of oscillator strengths and predissociation rates for13C16O and12C18O

Models of astronomical environments containing CO require accurate molecular data to reproduce and interpret observations. We conduct experiments at the SOLEIL synchrotron facility to acquire data to model CO photochemistry in the vacuum ultraviolet. The improvement in UV spectroscopic instrumentation in both sensitivity and resolution provides more accurate laboratory spectroscopic determinations. We continue our work on photoabsorption oscillator strengths and predissociation rates for 12 C16 O; we report new measurements for two additional isotopologues, 13 C16 O and 12 C18 O, of four bands from X 1 Σ+ (v′′ = 0) to the v′ = 0–3 vibrational levels of the core-excited W 1 Π Rydberg state and for four overlapping bands (three resolved and one diffuse) between 92.97 and 93.35 nm. Some of the latter include unidentified perturbations. Absorption spectra were recorded using the vacuum-ultraviolet Fourier-transform spectrometer that is installed on the DESIRS beamline at SOLEIL which provides a resolving power R = 350 000. This resolution allows the analysis of individual line strengths and widths in the electronic transitions and the identification of a previously unobserved perturbation in the W(1) level. Gas column densities in the differentially-pumped system were calibrated using the B 1 Σ+ – X 1 Σ+ (v′ = 0,v′′ = 0) band. Absorption bands are analyzed by synthesizing line and band profiles and fitting them to the measured spectra. These considerably improved results are compared with earlier determinations.

Redistribution of Ce 4d oscillator strength observed in photoionization of endohedral Ce@C+82 ions

Cerium atoms and ions with a filled 4d subshell are expected to have a 4d photoabsorption oscillator strength of 10 according to the number of equivalent electrons in that shell. While this expectation was confirmed for free atomic cerium ions by observing the single and double photoionization channels, measurements with Ce ions encapsulated within a C82 Fullerene cage showed that the single and double ionization channels contain far less than 50% of the Ce 4d oscillator strength.

Excitation Energies and Photoabsorption Oscillator Strengths of the Rydberg Series in CF3Cl. A Linear Response and Quantum Defect Study

Vertical excitation energies of the CF(3)Cl molecule have been obtained from a response function approach with a CC reference function to determine absolute photoabsorption oscillator strengths in the molecular-adapted quantum defect orbital formalism (MQDO). The present work covers more highly excited Rydberg states than have been experimentally reported. Assessing of the reliability of the present calculations is provided through a comparative analysis between the results of the molecule and the Cl atom. This can be used to allow for predictions of the same type of properties in other analogous systems.

Lower Rydberg series of methane: A combined coupled cluster linear response and molecular quantum defect orbital calculation

Vertical excitation energies as well as related absolute photoabsorption oscillator strength data are very scarce in the literature for methane. In this study, we have characterized the three existing series of low-lying Rydberg states of CH4 by computing coupled cluster linear response (CCLR) vertical excitation energies together with oscillator strengths in the molecular-adapted quantum defect orbital formalism from a distorted Cs geometry selected on the basis of outer valence green function calculations. The present work provides a wide range of data of excitation energies and absolute oscillator strengths which correspond to the Rydberg series converging to the three lower ionization potential values of the distorted methane molecule, in energy regions for which experimentally measured data appear to be unavailable.

High Resolution Inner‐Shell Spectroscopy and ab initio CI Calculations on TiCl4 and Isoelectronic Molecules.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Oscillator strength distribution in C 3H 6 isomers studied with the time-dependent density functional method in the continuum

We present photoabsorption oscillator strengths for C 3H 6 molecules with emphasis on the difference between isomers, cyclopropane and propylene. We use an iterative numerical method based on the time-dependent local density approximation with continuum, which we have recently developed. The oscillator strengths for the two isomers differ at photon energies above their ionization thresholds. The magnitude and the shape of the oscillator strength distribution are in good agreement with recent experiments. The differences between the isomers arise from difference in symmetry of electronic states and different behaviors of continuum excitations.

High resolution inner-shell spectroscopy and ab initio CI calculations on TiCl4 and isoelectronic moleculesElectronic supplementary information (ESI) available: All excitation energies and oscillator strengths for TiCl4, VOCl3, CrO2Cl2 and MnO3Cl, including Rydberg levels. See http://www.rsc.org/suppdata/cp/b3/b302805b/

Inner-shell excitation spectra of gas phase TiCl4, VOCl3 and CrO2Cl2 have been recorded in the Cl 2p and O 1s edge regions. The assignment of the core excitations is based on ab initio calculations in the relaxed 1h–1p CI scheme. The computational approach has provided excitation energy patterns and photoabsorption oscillator strengths in satisfactory agreement with the experimental spectra, indicating the reliability of the theoretical model. The calculations are extended also to the compound MnO3Cl and to the Cl 1s core excitation spectra of the whole series of molecules. The interpretation of the final excited states in terms of orbital composition is also discussed as well as the spectral variations along the series with the progressive substitution of the chlorine atom with the oxygen atom.

Quantitative photoabsorption of methyl propyl ether in the UV, VUV and soft X-ray regions (5–200 eV)

Absolute photoabsorption oscillator strengths (cross-sections) in the valence-shell discrete and continuum regions methyl propyl ether CH3OC3H7 have been measured from 5 to 32 eV using high-resolution (∼0.05 eV fwhm) dipole (e,e) spectroscopy. Wide-range spectra, spanning the UV, VUV and soft X-ray regions, have also been obtained at low-resolution (∼1 eV fwhm) from 5 to 200 eV for methyl propyl ether, and these wide-range spectra have been used to determine the absolute oscillator strength scales by employing valence-shell TRK (i.e., S(0)) sum-rule normalization. Evaluations of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data give a static dipole polarizability for methyl propyl ether in good agreement with previously reported polarizability values. Other dipole sums S(u), (u=−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L(u), (u=−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum-rules. The presently reported absolute photoabsorption oscillator strength data for methyl propyl ether (and all other atoms and molecules we have studied) are web-accessible at ftp.chem.ubc.ca/pub/cooper.

Quantitative photoabsorption of diethyl ether in the valence and carbon 1s inner shell regions (5–360 eV)

Abstract Absolute photoabsorption oscillator strengths (cross-sections) in the valence-shell discrete and continuum regions of diethyl ether C2H5OC2H5 have been measured from 5 to 32 eV using high-resolution (HR) (∼0.05 eV fwhm) dipole (e,e) spectroscopy. Wide-range spectra, spanning the UV, VUV and soft X-ray regions, have also been obtained at low-resolution (LR) (∼1 eV fwhm) from 5 to 360 eV which covers the valence-shell and C 1s inner-shell regions. The LR spectrum has been used to determine the absolute oscillator strength scale by employing valence-shell TRK (i.e., S(0)) sum-rule normalization. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for diethyl ether in good agreement with previously reported polarizability values obtained by other methods. Other dipole sums S(u), (u=−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L(u), (u=−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum-rules. The presently reported absolute photoabsorption oscillator strength data are web-accessible at ftp.chem.ubc.ca/pub/cooper.

Absolute photoabsorption cross-sections (oscillator strengths) for ethanol (5–200 eV)

Absolute photoabsorption oscillator strengths (cross-sections) in the valence-shell discrete and continuum regions of ethanol (C 2H 5OH) have been measured from 5 to 32 eV using high-resolution (∼0.05 eV fwhm) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low-resolution (∼1 eV fwhm), and this has been used to determine the absolute oscillator strength scale by employing valence-shell TRK (i.e., S(0)) sum-rule normalization. The presently reported high- and low-resolution absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for ethanol in excellent agreement (within 0.5%) with previously reported polarizability values. Other dipole sums S( u), ( u=−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L( u), ( u=−1 to -6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum-rules.

Electronic excitation and optical cross sections of methylamine and ethylamine in the UV–VUV spectral region

High resolution UV–VUV photon absorption spectra of methylamine and ethylamine have been recorded between 5.0–9.0 eV (250–140 nm) using synchrotron radiation. In methylamine, the energies of the absorption bands are confirmed as centered at 5.7, 7.2, and 8.7 eV, respectively. In ethylamine the band centers are 5.8, 7.0, and 7.9 eV, respectively; the last band is seen here for the first time. Most of the transitions exhibit rich fine structure dominated by vibrational progressions involving excitation of an amino wagging vibration. The absolute photoabsorption oscillator strengths have been measured by photon absorption over the 5–9 eV range and by dipolar electron energy loss spectroscopy from 5–14 eV (250–90 nm).

Measurement of absolute dipole oscillator strengths for pyridine: photoabsorption and the molecular and dissociative photoionization in the valence shell (4–200 eV)

Absolute photoabsorption oscillator strengths for the valence shell of pyridine have been measured using dipole (e,e) spectroscopy at high resolution (∼0.05 eV FWHM) and low resolution (∼1 eV FWHM) from 4 to 30 eV and from 5 to 200 eV, respectively. The absolute photoabsorption oscillator strength scale (±5%) has been determined using valence shell TRK (i.e. S(0)) sum-rule normalization. The accuracy of the absolute scale has been verified using the S(−2) sum rule and literature values of the dipole polarizability. The low resolution photoabsorption results have been combined with time of flight mass spectral data measured using dipole (e,e+ion) coincidence spectroscopy (∼1 eV FWHM), to determine the absolute oscillator strengths for the molecular and dissociative photoionization of pyridine at equivalent photon energies from the first ionization threshold up to 60 eV.

Dipole (e,e+ion) spectroscopic studies of benzene: absolute oscillator strengths for molecular and dissociative photoionization in the VUV and soft X-ray regions

The branching ratios for molecular and dissociative photoionization of benzene (C 6H 6) have been measured in the VUV and soft X-ray regions using dipole (e,e+ion) coincidence spectroscopy (∼1 eV FWHM) at equivalent photon energies from the first ionization threshold up to 80 eV. The absolute partial oscillator strengths (cross-sections) for molecular and dissociative photoionization have been determined by combining previously reported absolute photoabsorption oscillator strength data [J. Electron Spectrosc. Relat. Phenom. 123 (2–3) (2002) 211–223] together with the photoionization branching ratios and the (multi-dissociative-corrected) photoionization efficiency obtained from time-of-flight mass spectra reported in the present work. The fact that the photoionization efficiency has been measured as greater than unity above ∼25 eV indicates the existence of multi-dissociative products from Coulomb explosion of multiply charged ions. Appearance potentials of all ion products from benzene are also reported. The presently reported results are compared with the previously published data where possible.

Dipole (e,e) spectroscopic studies of benzene: quantitative photoabsorption in the UV, VUV and soft X-ray regions

Absolute UV and VUV photoabsorption oscillator strengths (cross sections) for the valence shell discrete and continuum regions of benzene (C 6H 6) have been measured from 4 to 56 eV using high resolution (∼0.05 eV fwhm) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low resolution (∼1 eV fwhm), and this has been used to determine the absolute oscillator strength scale by employing valence shell TRK [i.e., S(0)] sum-rule normalization. The presently reported high and low resolution absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for benzene in good agreement (within ∼3.4%) with published polarizability values obtained from optical measurements. Other dipole sums S( u), ( u=−1, −3, −4, −5, −6, −8, −10), and logarithmic dipole sums L( u), ( u=−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum-rules.

Ionic photofragmentation and photoionization of dimethyl ether in the VUV and soft X-ray regions (8.5–80 eV) – absolute oscillator strengths for molecular and dissociative photoionization

The branching ratios for molecular and dissociative photoionization of dimethyl ether (CH 3OCH 3, DME) have been measured in the VUV and soft X-ray regions using dipole (e,e+ion) coincidence spectroscopy (∼1 eV FWHM) at equivalent photon energies from the first ionization threshold up to 80 eV. The absolute partial oscillator strengths (cross-sections) for molecular and dissociative photoionization have been determined from recently published absolute photoabsorption oscillator strength data [R. Feng, G. Cooper, C.E. Brion, Chem. Phys. 260 (2000) 391] together with the photoionization branching ratios and the (multi-dissociative-corrected) photoionization efficiency obtained from time-of-flight mass spectra reported in the present work. No stable multiply charged molecular ion(s) from DME have been found in the present work. However, the fact that the photoionization efficiency has been measured as greater than unity above ∼30 eV indicates the existence of multi-dissociative products from Coulomb explosion of multiply charged ions. Appearance potentials of all ion products from DME are also reported. The presently reported results are compared with the previously published data where possible.

High-Resolution Inner-Shell Photoabsorption and Dissociation of Ozone

The total-ion-yield (TIY) spectrum of ozone has been recorded in the K-edge region for the first time with high photon energy resolution and without significant O2 contamination. Spectral features have been clearly observed in the high energy region below the two ionization thresholds. The assignment for the core excitation processes from the “terminal” and “central” oxygen atoms, given for the first time in the whole spectral range, is based on high level ab initio calculations. The QDPTCI (quasidegenerate perturbation theory configuration interaction) theoretical approach provided an excitation energy pattern and photoabsorption oscillator strengths that were found in good agreement with the experimental TIY spectrum. The decay dynamics of the core excited resonant states is shown to be strongly dependent on the σ/π antibonding character of the virtual MO involved in the excitation process.

UV, VUV and soft X-ray photoabsorption of dimethyl ether by dipole (e,e) spectroscopies

Absolute UV and VUV photoabsorption oscillator strengths (cross-sections) for the valence shell discrete and continuum regions of dimethyl ether (CH 3OCH 3, DME) have been measured from 5 to 32 eV using high resolution (HR) (∼0.05 eV f.w.h.m.) dipole (e,e) spectroscopy. A wide-range spectrum, spanning the UV, VUV and soft X-ray regions, from 5 to 200 eV has also been obtained at low resolution (LR) (∼1 eV f.w.h.m.), and this has been used to determine the absolute oscillator strength scale by employing valence shell Thomas–Reiche–Kuhn (i.e., S(0)) sum-rule normalization. The presently reported HR and LR absolute photoabsorption oscillator strengths are compared with previously published data from direct photoabsorption measurements in those limited energy regions where such data are available. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for dimethyl ether in excellent agreement (within 0.5%) with previously reported polarizability values. Other dipole sums S( u), ( u=−1,−3,−4,−5,−6,−8,−10), and logarithmic dipole sums L( u), ( u=−1 to −6), are also determined from the presently reported absolute differential photoabsorption oscillator strength data using dipole sum rules.

Dipole (e,e+ ion) coincidence studies of the ionic photofragmentation and photoionization of carbonyl sulfide in the valence shell and S 2p, 2s and C 1s inner shell regions (10–300 eV)

The branching ratios for molecular and dissociative photoionization of carbonyl sulfide (OCS) in the valence shell continuum regions and the inner shell (S 2p, 2s and C 1s) regions have been determined using dipole (e,e+ ion) coincidence spectroscopy (∼1 eV FWHM) at equivalent photon energies from the first ionization threshold up to 300 eV. The absolute partial oscillator strengths for molecular and dissociative photoionization have also been determined from recently published absolute photoabsorption oscillator strength data (R. Feng, G. Cooper, C.E. Brion, Chem. Phys. 252 (2000) 359) together with photoionization branching ratios and the (multi-dissociative-corrected) photoionization efficiency obtained from time-of-flight mass spectra reported in this work. The doubly charged molecular ion (OCS 2+) and its dissociation products (mainly OC +, S +, O + and C +) have also been studied in this work. Much higher relative abundances of these products have been found above the S 2p ionization edge (170.72 eV), which is due to Auger processes combined with Coulomb explosion. The presently reported results have been compared with previously published data where possible.

Quantitative studies of the photoabsorption of carbonyl sulphide in the valence-shell, S 2p, 2s and C 1s inner-shell regions (4–360 eV) by dipole electron impact spectroscopies

Electronic excitation spectra and absolute photoabsorption oscillator strengths (cross-sections) have been measured in the UV, VUV and soft X-ray energy regions for the valence- and inner- (S 2p, 2s, C 1s) shell photoabsorption of carbonyl sulphide (OCS) from 5 to 360 eV using low-resolution (∼1 eV fwhm) dipole (e, e) spectroscopy. The absolute oscillator strength scale has been determined using valence-shell TRK (i.e., S(0)) sum-rule normalization. The discrete structures, as well as the continuum structures, in the valence region also have been studied at high resolution (∼0.05 eV fwhm) from 4 to 32 eV. The presently reported high- and low-resolution absolute photoabsorption oscillator strengths are compared with previously published data (from the direct photoabsorption measurements) in those limited energy regions where such data exist. Evaluation of the S(−2) sum using the presently reported absolute differential photoabsorption oscillator strength data gives a static dipole polarizability for carbonyl sulphide in excellent agreement (within 2%) with previously reported polarizability values. Other dipole sums S( u) ( u=−1, −3 to −6, −8, −10) and logarithmic dipole sums L( u) ( u=−1 to −6) are also determined from the presently reported absolute differential photoabsorption oscillator strength data.