Absolute Photoabsorption Cross Sections Research Articles

The 4 d–4 f dipole resonance of the Pr atom in an endohedral metallofullerene, Pr@C 82

The photoion yield spectra of an endohedral metallofullerene Pr@C 82 were measured in the photon energy range of 100–150 eV by using time-of-flight mass spectrometry. Parent ions Pr@C 82 +, Pr@C 82 2+ and Pr@C 82 3+ were observed in the mass spectra. The photoion yield spectra of Pr@C 82 2+ showed a broad peak at 120–140 eV that was assigned to the 4 d–4 f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C 82 were evaluated from the photoion yield spectra to be 37±12 Mb at 110 eV (off-resonance) and 52±13 Mb at 130 eV (on-resonance). These cross sections of Pr@C 82 were compared with the results of Ce@C 82, the only metallofullerene whose photoionization properties have ever been studied near the 4 d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C 82 and Ce@C 82, but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms.

The electronic states of cyclopropane studied by VUV absorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption cross-section of cyclopropane has been measured between threshold and 30 eV using monochromated synchrotron radiation. The spectrum is dominated by prominent broad peaks arising from electronic transitions into valence-excited states. Much weaker structure ascribed to Rydberg states associated with excitation from the 3e′ or 1a″2 orbitals is also discernible. Some of these Rydberg states display vibrational progressions which can be correlated with similar structure observed in the corresponding photoelectron spectrum. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the observed absorption bands.

Theoretical evaluation of PAH dication properties

Aims. We present a systematic, theoretical study of40 polycyclic aromatic hydrocarbon dications (PAHs ++ ) containing up to 66 carbon atoms. Methods. We performed our calculations using well-established quantum-chemical techniques in the framework of (i) the density functional theory (DFT) to obtain the electronic ground-state properties and of(ii) the time-dependent DFT (TD-DFT) to evaluate the excited-state properties. Results. For each PAH ++ considered, we computed the absolute visible-UV photo-absorption cross-section up to about 30 eV. We also evaluated their vibrational properties and compared them to those of the corresponding neutral and singly-ionised species. We estimated the adiabatic and vertical second ionisation energy ΔI through total energy differences. Conclusions. The ΔI values obtained fall in the energy range 8-13 eV, confirming that PAHs could reach the doubly-ionised state in HI regions. The total integrated IR absorption cross-sections show a marked increase upon ionisation, being on the average about two and five times larger for PAHs ++ than for PAHs + and PAHs, respectively. The visible-UV photo-absorption cross-sections for the 0, +1 and +2 charge-states show comparable features but PAHs ++ are found to absorb slightly less than their parent neutral and singly ionised species between ∼7 and ∼12 eV. Combining these pieces of information, we found that PAHs ++ should actually be stabler against photodissociation than PAHs and PAHs + , if dissociation thresholds are nearly unchanged by ionisation.

Ion-pair formation mechanisms in chloromethane, bromomethane and dichlorodifluoromethane

The first absolute photoion-pair cross sections have been derived from measurements of the absolute photoabsorption cross section and the photoion-pair quantum efficiency. The results for CH3Cl, CH3Br and CF2Cl2 show that direct transitions into ion-pair states play a significant role. In each molecule, the structure in the photoion-pair spectrum is different to that in the photoabsorption spectrum and often does not resemble the strong bands, due to Rydberg transitions, prominent in the latter spectra. At photon energies associated with excitations into Rydberg states, and consequently with peaks in absorption, the photoion-pair quantum efficiency usually exhibits a local minimum. This indicates that the Rydberg state decays preferentially by predissociation into a state yielding neutral fragments rather than by predissociation into a state evolving into ion pairs.

High-resolution absolute photoabsorption cross sections for Ne in the 1s2s and 1s2p resonant double excitation

The high-resolution absolute photoabsorption cross sections with an absolute photon energy scale for Ne 1s2p/1s2s double excitation in the energy region of 900–940 eV have been measured using a multi-electrode ionization chamber and monochromatized synchrotron radiation. The Ne [1s2p](3,1P)nln′l′ doubly excited states observed in the energy range of 900–920 eV are analysed with the aid of theoretical calculations based on the multiconfiguration Dirac–Fock method. The second strongest peak in this energy region is assigned to be [1s2p](3P)3p4p.

Exchange interaction effects in NO core-level photoionization cross-sections

The effect of the exchange interaction in the photoionization continuum is investigated, using N 1s photoionization of NO into the 1s−12π (1Π) and (3Π) final states as an example. The separation in energy of these two final states is 1.41 eV. Significant differences in their partial photoionization cross-sections are observed over a wide range of energies and cannot be accounted for by the different multiplicity of the states. We suggest that the deviation of the 3Π/1Π cross-section ratio from the statistical weighting at intermediate energies is dominated by the difference in the final-state potential experienced by the photoelectron and at asymptotically high energies by the multiplet-dependent amount of intensity going into multi-electron (shake-up) processes. Calculations underpinning this point are presented. We also show supporting measurements of the 3Π/1Π cross-section ratio for O 1s ionization and the absolute photoabsorption cross-section for NO over a wide energy range covering the core level region.

A study of the valence shell photoelectron and photoabsorption spectra of CF3SF5

The outer valence shell photoelectron spectrum of CF3SF5 has been studied experimentally and theoretically. Synchrotron radiation has been used to record angle-resolved outer valence shell photoelectron spectra of CF3SF5 in the photon energy range 18–60 eV. These spectra have allowed photoelectron asymmetry parameters and branching ratios to be derived. The Outer Valence Green's Function approach has been employed to calculate the molecular orbital configuration and associated binding energies. A charge distribution analysis has also been obtained. Assignments have been proposed for the peaks observed in the photoelectron spectrum. The absolute photoabsorption cross section of CF3SF5 has been measured from threshold to 40 eV, and strongly resembles that of SF6. Assignments, involving intravalence transitions, have been proposed for some of the principal features appearing in the photoabsorption spectrum of CF3SF5.

Photoabsorption and resonant photoemission in the region of Ne 1s double excitations

The absolute photoabsorption cross-sections of Ne in the regions 865–873 (1s Rydberg states) and 900–940 eV (double excitations) have been measured. Several new double excitation states were detected, including a very weak member of the series converging to the 1s2p 53s conjugate shake-up ion state. States were also observed above the 1s2p 5 double ionisation potentials, and are assigned to the 1s2snsn′p series. At the first 1s2p 53p 2 double excitation state, the partial ionisation cross-section for the 1s −1 ion state resonates. The behaviour of the cross-section and Fano profile are analysed by a theoretical calculation using the time-independent scattering formalism.

Theoretical electron affinities of PAHs and electronic absorption spectra of their mono-anions

We present theoretical electron affinities, calculated as total energy differences, for a large sample of polycyclic aromatic hydrocarbons (PAHs), ranging in size from azulene (C10H8) to dicoronylene (C48H20). For 20 out of 22 molecules under study we obtained electron affinity values in the range 0.4-2.0 eV, showing them to be able to accept an additional electron in their LUMO π � orbital. For the mono-anions we computed the absolute photo-absorption cross-sections up to the vacuum ultraviolet (VUV) using an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD-DFT), an approach which has already been proven to yield accurate results for neutral and cationic PAHs. Comparison with available experimental data hints that this is the case for mono-anions as well. We find that PAH anions, like their parent molecules and the corresponding cations, display strong π ∗ ← π electronic transitions in the UV. The present results provide a quantitative foundation to estimate the fraction of specific PAHs which can be singly negatively charged in various interstellar environments, to simulate their photophysics in detail and to evaluate their contribution to the interstellar extinction curve.

Absolute photoabsorption cross section of C 60 in the extreme ultraviolet

The absolute photoabsorption cross section curve of C 60 has been determined by means of mass spectrometry with the photon source of monochromatized synchrotron radiation of hν = 24.5–150 eV. Description has been made on a high-temperature source of gaseous fullerenes and an efficient time-of-flight mass spectrometer. The cross section was estimated by assuming an approximate expression of the number density of C 60 in the ionization region. The resultant values were 762, 241, and 195 Mb at hν = 24.5, 90, and 110 eV, respectively, with about 10% errors. The cross section curve was then normalized at hν = 25 eV to the absolute photoabsorption cross section reported by Jaensch and Kamke [R. Jaensch, W. Kamke, Mol. Mater. 13 (2000) 143], the most reliable data so far available in the valence excitation region of C 60. Accordingly, the present cross section data were altered to 407, 144, and 114 Mb at hν = 25, 90, and 110 eV, respectively.

Absolute photoabsorption cross-sections (oscillator strengths) for valence and inner shell excitations in hydrogen chloride, hydrogen bromide and hydrogen iodide

Absolute photoabsorption cross-sections (oscillator strengths) have been measured for the valence shell and selected (Cl 2p, Br 3d and I 4d) inner shell excitations in HCl, HBr and HI, respectively at photon energies up to 280 eV using dipole (e,e) spectroscopy. Absolute scales are established using the S(0) and S(−2) sum rules.

Electronic absorption spectra of PAHs up to vacuum UV

We computed the absolute photo-absorption cross-sections up to the vacuum ultaviolet (VUV) of a total of 20 Polycyclic Aromatic Hydrocarbons (PAHs) and their respective cations, ranging in size from naphthalene (C10H8 )t o di- coronylene (C48H20). We used an implementation in real time and real space of the Time-Dependent Density Functional Theory (TD-DFT), an approach which was proven to yield accurate results for conjugated molecules such as benzene. Concerning the low-lying excited states of π ∗ ← π character occurring in the near-IR, visible and near-UV spectral range, the computed spectra are in good agreement with the available experimental data, predicting vertical excitation energies precise to within a few tenths of eV, and the corresponding oscillator strengths to within experimental errors, which are indeed the typical accuracies currently achievable by TD-DFT. We find that PAH cations, like their parent molecules, display strong π ∗ ← π electronic transitions in the UV, a piece of information which is particularly useful since a limited amount of laboratory data is available concerning the absorption properties of PAH ions in this wavelength range. Moreover, a detailed discussion of the UV-VUV properties of both neutral and cation species is presented. Concerning neutrals, the agreement with existing laboratory data is very good, the specific TD-DFT implementation used in this work apparently being able to reproduce the overall far-UV behaviour, including the broad absorption peak dominated by σ ∗ ← σ transitions, which matches well both in position and width. The implications of these results are discussed in conjunction with the contribution PAH-like molecules are expected to give to the interstellar extinction curve.

Total photoabsorption cross-section of Ar in the sub-keV energy region

The absolute photoabsorption cross-section of Ar in the energy region of 50eV through 1100eV has been measured to a high precision using a multi-electrode ion chamber and monochromatized synchrotron radiation. In order to improve the spectral purity of monochromatized synchrotron radiation, an electron storage ring was operated at an electron beam energy that was considerably lower than the normal mode. For most photon energies the obtained photoabsorption cross-sections are in close agreement with data previously reported. However, some discrepancies are noted in the L-shell ionization region.

A study of the valence shell spectroscopic and thermodynamic properties of thiophene by photoabsorption and photoion spectroscopy

Abstract The valence shell spectroscopic and thermodynamic properties of thiophene have been investigated through photoabsorption and ion fragmentation studies. The absolute photoabsorption cross-section has been measured between the ionisation threshold and 35 eV using a double ion chamber and synchrotron radiation. New Rydberg series converging onto either the B ∼ 2 A 1 or the G ∼ 2 A 1 limits have been observed. The photoabsorption spectrum is dominated by broad features due to intravalence transitions, and tentative assignments have been proposed based upon a term value analysis. Time-of-flight mass spectra have been recorded in the photon energy range 12–28 eV and these have allowed the appearance energies for 18 fragment ions and the doubly charged parent ion to be determined. Molecular orbital calculations have been performed to compute the energies of several neutral or ionic species relevant to the lowest energy fragmentation channel. These energies, when compared with the experimental data, help to define the ion formation mechanisms.

Temperature‐dependent photoabsorption cross sections in the VUV‐UV region: Ethylene

Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of ethylene (C2H4) in the region between 118 nm and 192 nm with a spectral bandwidth (full width at half maximum) of 0.06 nm and at five different temperatures: 370 K, 330 K, 295 K, 200 K, and 140 K. Significant temperature effects are observed in several spectral regions, including the threshold absorption region. From the Pc,370K curve we are able to identify and assign several hot bands. When the gas temperature decreases from 370 K to 140 K, the cross sections of the hot bands in the 190 nm region decrease by an order of magnitude. The line shapes of the Pc,T exhibit characteristic symmetric and asymmetric peaks, as well as the first derivative‐like and second derivative‐like curves associated with various absorption features. The results obtained in the present work provide the required data to the planetary atmospheres community and may have an important contribution to our understanding of the atmospheres of giant planets.

2-methyl furan: An experimental study of the excited electronic levels by electron energy loss spectroscopy, vacuum ultraviolet photoabsorption, and photoelectron spectroscopy

The vacuum ultraviolet absorption spectrum of 2-methyl furan has been recorded between 5 eV (248 nm) and 9.91 eV (125 nm) and absolute photoabsorption cross sections measured. The electronic excited states of the molecule have also been probed using high resolution electron energy loss spectroscopy. Recorded under electric–dipole conditions, it has confirmed the magnitude of the photoabsorption cross section values and extended the optical oscillator strength values up to 12 eV. Measurements at several scattering angles have allowed the angular behavior of differential cross section ratios for some features in the 5–7.1 eV region to be measured, which in turn have helped in the assignments of electronic states to observed absorption bands. A high-resolution photoelectron spectrum was measured and allowed the two lowest ionization energies to be determined, these have been used in the identification of the related Rydberg states. Vibrational fine structure in the photoelectron spectrum has also been analyzed. The spectrum is dominated by intense π–π* transitions. Rydberg series associated with the first and second ionization energies have been identified. The effects of symmetry reduction induced on the furan ring by the methyl substitution are also discussed.

Temperature-dependent photoabsorption cross sections in the VUV-UV region. I. Methane and ethane

Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of methane (CH 4) and ethane (C 2H 6) from their respective absorption thresholds to 120 nm , with a spectral bandwidth (FWHM) of 0.06 nm and at three different temperatures, i.e., 370, 295, and 150 K . Only moderate temperature effects are observed in the changes of cross-section values of these two molecules and are attributed to their high vibrational frequencies of the ground electronic states and their repulsive potential surfaces of the excited electronic states. When the gas temperature decreases from 360 to 150 K , the percentage changes of cross sections amount to a maximum of ±30% in CH 4 at 142.5 nm and ±20% in C 2H 6 at 142.3 and 155 nm . The well-known vibrational progressions of C 2H 6 exhibit pronounced temperature effects in their band profiles which become narrower and sharper as the gas temperature decreases. The data presented are an extension of our effort to provide the required data to the planetary atmospheres community and will have an important impact on our understanding of the atmospheres of the giant planets.

Absolute photo-absorption cross sections and electronic state spectroscopy of selected fluorinated hydrocarbons relevant to the plasma processing industry

Photo-absorption cross sections have been measured for methyl iodide, CF 3I (310 nm> λ>110 nm) and dichlorodifluoromethane, CCl 2F 2 (225 nm> λ>110 nm) using synchrotron radiation. Electron energy loss spectroscopy was also used to probe the electronic and vibronic excitation of CF 3I. Electronic states have been assigned to each of the observed absorption bands incorporating both valence and Rydberg transitions. The measured VUV cross sections are used to derive the photolysis rates in the terrestrial atmosphere and hence determine the potential importance of each gas in global warming and ozone depletion.

Krypton3pexcitations and subsequent resonant Auger decay

Krypton $3p$ excitations and their subsequent resonant Auger decay have been studied. The absolute photoabsorption cross section was determined in the region of the $3p$ resonances, and some structures were detected in the absorption spectrum. Hartree-Fock calculations including correlation between ${3p}^{\ensuremath{-}1}\mathrm{nl}$ and ${3d}^{\ensuremath{-}2}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}\mathrm{nl}$ configurations were used to interpret the structures of the $3p$ resonant excitation series and of the corresponding decay spectra. Furthermore, the decay spectra confirmed the assignment of the main structures in the absorption spectrum.

Absolute photoabsorption cross-sections of Ne and Xe in the sub-keV X-ray region

Total photoabsorption cross-sections of Ne in the energy region of 50–1300 eV and of Xe in 85–1100 eV have been measured in an absolute scale using a multi-electrode ion chamber combined with monochromatized synchrotron radiation. In order to improve the spectral purity of monochromatized synchrotron radiation, an electron storage ring was operated at an electron beam energy lower than the normal mode, and a thin foil was inserted into the photon beam. The photoabsorption cross-sections obtained for Ne are close to the data previously reported in most photon energies. However, for Xe, there are some discrepancies between the present results and the literature data at some photon energies, in particular near 300, 700, and 950 eV.