Relative Partial Photoionization Cross Sections Research Articles

Photoionization of iodine atoms: Angular distributions and relative partial photoionization cross-sections in the energy region 11.0–23.0 eV

Relative partial photoionization cross-sections and angular distribution parameters, beta, have been measured for the first, I(+)((3)P(2))<--I((2)P(3/2)), and fourth, I(+)((1)D(2))<--I((2)P(3/2)), (5p)(-1) photoelectron (PE) bands of atomic iodine, by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands in the photon energy range 11.0-23.0 eV. Three Rydberg series, two ns and one nd series, which converge to the I(+) (3)P(1) limit at 11.33 eV and four Rydberg series, two ns and two nd series, which converge to the I(+) (1)D(2) limit at 12.15 eV were observed in the first PE band CIS spectra. The fourth band CIS spectrum showed structure in the 12.9-14.1 eV photon energy range, which is also seen in the first band CIS spectra. This structure arises from excitation to ns and nd Rydberg states that are parts of series converging to the I(+) (1)S(0) limit we reported on earlier, as well as 5s-->5p excitations in the photon energy range 17.5-22.5 eV. These atomic iodine CIS spectra show reasonably good agreement with the equivalent spectra obtained for atomic bromine. The beta-plots for the first PE band recorded up to the I(+) (3)P(1) and I(+) (1)D(2) limits only show resonances corresponding to some of the 5p-->nd excitations observed in the first band CIS spectra scanned to the I(+) (1)D(2) limit (12.15 eV). These plots are interpreted in terms of an angular momentum transfer model with the positive values of beta obtained on resonances corresponding to parity allowed j(t)=1 and 3 channels and the off-resonance negative beta values corresponding to parity unfavored channels, where j(t) is the quantum number for angular momentum transfer between the molecule, and the ion and photoelectron. The beta-plots recorded for iodine are significantly different from those obtained for atomic bromine. Comparison of the experimental CIS spectra and beta-plots with available theoretical results highlights the need for higher level calculations which include factors such as configuration interaction in the initial and final states, relativistic effects including spin-orbit interaction, and autoionization via resonant Rydberg states.

ChemInform Abstract: Variable Photon Energy Photoelectron Spectroscopy of OsO4 and Pseudopotential Calculations of the Valence Ionization Energies of OsO4 and RuO4.

Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetraoxide in the ionization energy range 12-18 eV. The photon energies used ranged between 24 and 115 eV. The ionization cross sections of the 2t2, 1e, and 2a1 orbitals show evidence of substantial metal character. By use of an ab initio many-body Green's function formalism that takes into account the effect of electron correlation and relaxation, ionization energies of OsO4 and RuO4 have been calculated. Considerations of the spectral features and the predictions of the calculation lead to an assignment of ion state ordering of 2T1 < 2T2 < 2A1 < 2E < 2T2. The presence of a substantial p-d resonance feature in the cross section of the upper 2T2 and 2E ion states shows that the 2t2 and le orbitals have significant Os 5d character. The lower 2T2 ion state (arising from ionization from the 3t2 orbital) undergoes a spin-orbit splitting of 0.4 eV resulting from an Os 6p contribution to the 3t2 molecular orbital.

Photoionization of iodine atoms: Rydberg series which converge to the I+(S1)←I(P23/2) threshold

Relative partial photoionization cross sections and angular distribution parameters beta have been measured for the first and fourth (5p)(-1) photoelectron (PE) bands of atomic iodine by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands between the (1)D(2) and (1)S(0) (5p)(-1) ionic thresholds in the photon energy region of 12.9-14.1 eV. Rydberg series arising from the 5p-->ns and 5p-->nd excitations are observed in both the first PE band, I(+)((3)P(2))<--I((2)P(3/2)), and the fourth PE band, I(+)((1)D(2))<--I((2)P(3/2)), CIS spectra. For each Rydberg state, the resonance energy, quantum defect, linewidth, line shape, and photoelectron angular distribution parameter beta have been determined. For the beta-plots for each PE band, only resonances corresponding to 5p-->nd excitations are observed; no resonances were seen at photon energies corresponding to the 5p-->ns resonances in the CIS spectra. The beta-plots are interpreted in terms of the parity unfavored channel with j(t)=4 being the major contributor at the 5p-->nd resonance positions, where j(t) is the quantum number for angular momentum transferred between the molecule, and the ion and photoelectron. Comparison of the results obtained with those published for bromine shows reasonably good agreement for the CIS spectra but poor agreement for the beta-plots. It appears that parity unfavored channels are playing a greater role in the valence (np)(-1) ionization of atomic iodine than in the corresponding ionization of atomic bromine.

Electronic Structure of M(BH4)4, M: Zr, Hf, and U, by Variable Photon‐Energy Photoelectron Spectroscopy and Density Functional Calculations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electronic Structure of M(BH4)4, M = Zr, Hf, and U, by Variable Photon-Energy Photoelectron Spectroscopy and Density Functional Calculations

Photoelectron (PE) spectra have been obtained for the M(BH(4))(4) (M = Zr, Hf and U) molecules in the 20-60 eV photon-energy range, and for M = U, also in the 90-120 eV region. Derived branching ratios (BR) and relative partial-photoionization cross sections (RPPICS) of the valence bands are used to confirm band assignment and demonstrate d-orbital covalency for all three compounds and f-orbital covalency for U(BH(4))(4). Core ionizations are identified and used to confirm resonance features in the RPPICS. The absorption spectrum of Zr(BH(4))(4) between 20 and 60 eV shows 4p absorption at 35.5 eV, coincident with the 4p-4d resonance in the RPPICS of the 1e and 2t(2) ionization bands of Zr(BH(4))(4). Less intense absorption bands at 32.5 and 33.8 eV correspond with shape resonance features in the 1a(1) and 1t(2) PE bands. The RPPICS of the f band of U(BH(4))(4) shows two strong resonant features between 95 and 120 eV. Direct photoemission of the 5f electrons from U(BH(4))(4) results only in the observation of the (2)F(5/2) ion state of [U(BH(4))(4)](+), but in the 5d-5f resonant region, a weak band corresponding to the (2)F(7/2) ion state is also observed. The splitting of the 1t(2) band of U(BH(4))(4) is attributed to a small contribution of U 6p semi-core electrons to this MO. Density functional calculations give a good estimate of the pattern of ionization energies, although the calculated absolute values are lower than the experimental values, the first IE by 0.5 eV for Zr(BH(4))(4) and Hf(BH(4))(4) and 1.0 eV for U(BH(4))(4). The MO compositions are in very good agreement with the deductions made from the BR and RPPICS analyses.

Valence satellite photoionization of atomic scandium in the region of the 3p → 3d giant resonance

A determination of the angular distributions, β, and relative partial photoionization cross sections, σ, of the valence satellite lines of atomic scandium in the region of the 3p → nd, ms excitations has been carried out using electron spectrometry in conjunction with monochromatized synchrotron radiation. This work is complementary to our earlier study of the partial cross sections of the Sc valence satellites but now includes angular distribution measurements of the most intense satellite lines. A correct theoretical description of the experimental satellite σ and β values presented herein should help in rectifying present discrepancies between theory and experiment for the 3d and 4s mainline σ and β values.

Photoelectron spectrometry of atomic chromium in the region of the 3p → 3d giant resonance

A determination of the angular distributions and the relative partial photoionization cross sections of the 3d and 4s mainlines and the largest satellite line of atomic chromium in the region of the 3p → nd, ms excitations has been carried out using electron spectrometry in conjunction with monochromatized synchrotron radiation. We observe strong deviations of β from expected values of 2.0 for the 4s mainline at many of the 3p → nd, ms resonances. This indicates the importance of relativistic effects in the autoionization of this half-filled subshell atom. High resolution constant-ionic-state spectra are comparable with previous high resolution absorption measurements. A comparison of our 3d β values with recent spin-polarized random phase approximation calculations generally shows good accord with some exceptions.

Electronic structure of ReO3Me by variable photon energy photoelectron spectroscopy, absorption spectroscopy and density functional calculations.

Valence photoelectron (PE) spectra have been measured for ReO(3)Me using a synchrotron source for photon energies ranging between 20 and 110 eV. Derived branching ratios (BR) and relative partial photoionization cross sections (RPPICS) are interpreted in the context of a bonding model calculated using density functional theory (DFT). Agreement between calculated and observed ionization energies (IE) is excellent. The 5d character of the orbitals correlates with the 5p --> 5d resonances of the associated RPPICS; these resonances commence around 47 eV. Bands with 5d character also show a RPPICS maximum at 35 eV. The RPPICS associated with the totally symmetric 4a(1) orbital, which has s-like character, shows an additional shape resonance with an onset of 43 eV. The PE spectrum of the inner valence and core region measured with photon energies of 108 and 210 eV shows ionization associated with C 2s, O 2s, and Re 4f and 5p electrons. Absorption spectra measured in the region of the O1s edge showed structure assignable to excitation to the low lying empty "d" orbitals of this d(0) molecule. The separation of the absorption bands corresponded with the calculated orbital splitting and their intensity with the calculated O 2p character. Broad bands associated with Re 4d absorption were assigned to (2)D(5/2) and (2)D(3/2) hole states. Structure was observed associated with the C1s edge but instrumental factors prevented firm assignment. At the Re 5p edge, structure was observed on the (2)P(3/2) absorption band resulting from excitation to the empty "d" levels. The intensity ratios differed from that of the O 1s edge structure but were in good agreement with the calculated 5d character of these orbitals. An absorption was observed at 45 eV, which, in the light of the resonance in the 4a(1) RPPICS, is assigned to a 4a(1) --> ne, na(2) transition. The electronic structure established for ReO(3)Me differs substantially from that of TiCl(3)Me and accounts for the difference in chemical behavior found for the two complexes.

Photoelectron spectroscopy of atomic oxygen using the Elettra synchrotron source

Atomic oxygen has been studied using angle resolved photoelectron spectroscopy (PES) and constant-ionic-state (CIS) measurements using radiation from the Elettra synchrotron as the photon source. Relative partial photoionization cross-sections and angular distributions for the O+(4S) ← O(3P) and O+(2D) ← O(3P) ionizations have been measured as a function of photon energy from threshold (13.6 eV) to 19.0 eV. Comparison of the results obtained with recent experimental work performed at lower resolution reveals a number of differences and comparison with results of recent calculations shows the need for the inclusion of coupling intermediate between the j-j and L-S limits in future calculations of photoionization cross-sections and angular distributions. This work has demonstrated the feasibility of and results to be expected from angle resolved PES and CIS measurements on reactive intermediates at Elettra, a third-generation synchrotron source, and further studies on small molecular radicals are proposed.

Interference effects in the Auger decay of theAr 2p−13dresonances

Using two-dimensional photoelectron spectroscopy, relative partial photoionization cross sections of all accessible Auger decay channels have been measured in the vicinity of the $\mathrm{Ar} {2p}^{\ensuremath{-}1}3d$ resonances. Significant asymmetric resonance profiles are observed, which mirror each other in their asymmetric features, giving symmetric Lorentzian profiles for the total photoionization cross section. This indicates that interference between direct photoionization and resonant photoexcitation autoionization needs to be taken into account in order to accurately describe partial photoionization cross sections, despite the absence of asymmetric features in the total cross section.

Photoelectron spectrometry of atomic scandium in the region of the3p→3dgiant resonance

A determination of the relative partial photoionization cross sections of the $3d$ and $4s$ main lines and associated satellite lines of atomic scandium in the region of the $3\stackrel{\ensuremath{\rightarrow}}{p}nd,ms$ excitations has been carried out using electron spectrometry in conjunction with monochromatized synchrotron radiation. The superior resolution inherent in these measurements has revealed additional structure not previously seen in earlier experimental work, highlighting the rich and complex structure of this relatively simple open-shell atom. Relative intensities and binding energies of the $3p$ photolines are also reported. A comparison with recent many-body perturbation theory calculations in LS coupling shows only marginal agreement. Discrepancies between theory and experiment are thought to arise from the use of LS coupling and neglect of satellite channels in the calculation.

Angular distribution measurements for spin-orbit-state-resolved S2pphotoelectrons ofSF6in the shape-resonance region

The angular distributions of the spin-orbit state resolved S $2p$ photoelectrons of ${\mathrm{SF}}_{6}$ have been measured in the vicinity of the shape resonances. Relative partial photoionization cross sections of ${\mathrm{SF}}_{6}$ into the (S ${2p}_{3/2}{)}^{\ensuremath{-}1}$ and (S ${2p}_{1/2}{)}^{\ensuremath{-}1}$ states, and the asymmetry parameters of the S ${2p}_{3/2}$ and S ${2p}_{1/2}$ photoelectrons are presented. Enhancement of the photoionization cross section into the (S ${2p}_{3/2}{)}^{\ensuremath{-}1}$ state at the (S ${2p}_{1/2}{)}^{\ensuremath{-}1}$ ${2t}_{2g}$ resonance has been observed. The excitation photon energy dependence of the asymmetry parameter shows a broad dip at about the photon energies of the resonances. Continuum multiple-scattering calculation of the asymmetry parameters for the (S ${2p}_{3/2}{)}^{\ensuremath{-}1}$ and (S ${2p}_{1/2}{)}^{\ensuremath{-}1}$ states of ${\mathrm{SF}}_{6}$ reproduces this behavior.

Structure and dynamics of the4p→ns,mdautoionizing resonances between the3Pand1Sthresholds in atomic bromine

The relative partial photoionization cross sections ${\ensuremath{\sigma}}_{i}$ and photoelectron angular distribution parameters ${\ensuremath{\beta}}_{i}$ are measured for all possible final ionic states of ${\mathrm{Br}}^{+}$ between the ${}^{3}{P}_{2}$ and ${}^{1}{S}_{0}$ thresholds. The decay patterns of the autoionizing ${4p}^{4}{}^{3}{P}_{1,0}n\mathcal{l}$, ${4p}^{4}{}^{1}{D}_{2}n\mathcal{l}$, and ${4p}^{4}{}^{1}{S}_{0}n\mathcal{l}$ Rydberg series arising from the $4\stackrel{\ensuremath{\rightarrow}}{p}ns,md$ excitations are observed at the fine-structure level in all available channels. For each Rydberg series, the energies, quantum defects, and photoelectron angular distribution parameters are determined, as well as the widths and shape parameters of the low-lying members. Relative total cross sections are derived from the partial cross sections and from ion-yield measurements over the lowest members of the ${}^{3}{P}_{1,0}$ and ${}^{1}{D}_{2}$ series. All major spectroscopic and dynamic properties of these series are reported for this open-shell atom. Conclusions are drawn from the comprehensive data sets in comparison with other halogen atoms and the neighboring closed-shell rare gas atoms. Our results, encompassing the entire autoionization regime, are compared with other experimental data and theoretical calculations, where available.

Efficiency of High-n Rydberg-State Stabilization in Pulsed-Field Ionization Zero-Kinetic-Energy Photoelectron Spectroscopy

To examine the issues concerning Rydberg-state stabilization in pulsed-field ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy, spectra at the first and second ionization thresholds of argon have been taken under different ion-density conditions. These provide a contrast between the signals from autoionizing and nonautoionizing Rydberg series. A careful comparison between the PFI-ZEKE spectra and relative partial photoionization cross sections provides the first measurement of the efficiency of stabilization in PFI-ZEKE spectroscopy. This efficiency or quantum yield varies at the second ionization threshold with changing ion density and principal quantum number, introducing ion-density-dependent line-shape distortions. These changes may be ascribed to the competition between stabilization and autoionization of the Rydberg states and its variation with principal quantum number.

Investigation of the bonding in methyl titanium trichloride by variable-energy photoelectron spectroscopy and density functional calculations

Preparation of an authentically pure sample of [TiMeCl 3 ] has enabled measurement of its photoelectron spectrum with photon energies varying between 20 and 50 eV. Study of the relative partial photoionisation cross-sections of the valence photoelectron bands has enabled a full assignment of the related ion states. The character of the ionising orbitals was also deduced from the cross-section variations. The titanium–carbon bond was shown to have a significant Ti 3d character. Density functional calculations at the non-local level, including estimates of the ionisation energies using Slater’s transition-state method, are in good agreement as to the ion state ordering. Fragment analysis of the one-electron eigenfunctions is fully consistent with the experimentally deduced orbital character.

Variable photon energy photoelectron spectroscopy of Mo2(O2CCF3)4

Abstract Relative partial photoionization cross sections, branching ratios and β measurements are reported for Mo2(O2CCF3)4 over the photon energy range 20 to 70 eV. The metal based δ and σ + π ionizations are strongly enhanced over the photon energy range 36–65 eV. The comparable size of the resonance in the two metal bands suggests similar d character for the associated MO. A smaller resonance is also found in the oxygen localized ionization bands. An additional resonance is identified for the δ band at 24 eV which is accompanied by a change in β.

Redetermination of absolute partial photoionization cross sections of He and Ne atoms between 20 and 300 eV photon energy

Using photoelectron spectroscopy, we have measured some relative partial photoionization cross sections in He and Ne atoms. By compiling these results, along with previously published data obtained from photoabsorption, photoion and photoelectron spectroscopies, we have determined accurate absolute partial photoionization cross sections, up to 300 eV photon energy. The absolute scale was obtained by normalization to the photoabsorption cross sections. The best recommended values, determined by this self-consistent experimental procedure, are compared with the results of various theoretical determinations of the photoionization cross sections.

Variable Photon Energy Photoelectron Spectroscopic, and Theoretical Investigations of the Electronic Structure of TiCl4

The photoelectron spectrum of TiCl{sub 4} has been measured using synchrotron radiation over the incident photon energy range 17-69 eV, and relative partial photoionization cross sections have been derived for the valence bands. Band A (1t{sub 1}{sup {minus}1}) and band B (3t{sub 2}{sup {minus}1}) display cross section variations typical of Cl 3p atomic orbital-localized levels. Bands C+D show cross section enhancement between 40 and 60 eV and are assigned to le{sup {minus}1} and 2t{sub 2}{sup {minus}1}. B and E (2a{sub 1}{sup {minus}1}) shows no such feature and has cross section behavior which is appreciably different from that of any of the other bands. The small cross section maximum at 40 eV is attributed to a molecular shape resonance. Discrete variational X{alpha} calculations support this orbital ordering, and quantitative agreement is found between the experimental and calculated ionization energies. Photoionization cross sections were calculated using the multiple-scattering X{alpha} method. Good agreement was found between experimental and theoretical branching ratios, confirming the assignment and suggesting that the resonances found in the 2t{sub 2} and 2a{sub 1} orbital cross sections are due to multiple scattering from the localized Cl electrons. The minimum in the 3t{sub 2} cross section is associated with the Coopermore » minimum of the Cl 3p orbital.« less

A study of valence level photoelectron cross sections for the metallocenes of vanadium, chromium, cobalt and nickel from 20 to 100 eV

Relative partial photoionization cross sections and photoelectron branching ratios have been measured for the valence bands of the bis-η 5-cyclopentadienyl compounds of vanadium, chromium, cobalt and nickel over the photon energy range 20 to 80 or 100 eV. The d bands show cross section features attributable to 3d resonant photoemission in the vicinity of the 3p → 3d giant resonance absorption. These features are stronger for V and Cr than for Co and Ni. Other intensity maxima are attributed to shape resonances in f ionization channels for the a 1′ and e 2′ ionizations those for the a 1′ being stronger than those for the e 2′ bands. The 2 e 1″ bands of Co(η-C 5H 5) 2 and Ni(η-C 5H 5) 2 show particularly strong shape resonances. Evidence is presented for d orbitals covalency in the 1 e 1″ orbitals. Previous assignments for the a 1′ ionizations are confirmed.

Angle-resolved photoelectron spectroscopy of Ca in the 3p-3d giant-resonance region.

Using monochromatized synchrotron radiation combined with angle-resolved photoelectron spectroscopy, we have measured the relative partial photoionization cross sections \ensuremath{\sigma} and photoelectron asymmetry parameters \ensuremath{\beta} for populating the ${\mathrm{Ca}}^{+}$ 4s, 3d, and 4p levels in the vicinity of the 3p-3d giant resonance of Ca.