Relative Partial Photoionization Cross Sections Research Articles

Electron localization in the bis-arene complexes [(η-C 6H 6) 2Cr] and [(η-C 6H 5Me) 2Mo]: an investigation by photoelectron spectroscopy with variable photon energy

Relative partial photoionization cross sections have been measured for the valence bands of [(η-C 6H 6) 2Cr] and [(η-C 6H 5Me) 2Mo] over the photon energy ranges 24–80 eV ([(η-C 6H 6) 2Cr]) and 24–100 eV ([(η-C 6H 5Me) 2Mo]). Both compounds show pronounced intensity increases of the metal-based ionizations (a −1 1g and e −1 2g) in the region of the np subshell ionization potentials, attribute to nd resonant photoemission following np→ nd giant resonant absorption. Shape resonances are also observed in the cross sections of these bands. For [(η-C 6H 5Me) 2Mo], the two resonance processes are almost coincident in photon energy, but can be distinguished more clearly in the [(η-C 6H 6) 2Cr] cross sections. Evidence for metal d—carbon 2pπ covalency in the e 1g set of molecular orbitals is found, with features in the cross sections of the metal-based bands correlating with those in the e −1 1g ionizations. The e −1 1u bands show an essentially monotonic cross section decrease with photon energy.

Variable photon energy photoelectron spectroscopy of osmium tetroxide and pseudopotential calculations of the valence ionization energies of OsO4 and ruthenium tetroxide

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.

Angle-resolved photoelectron spectroscopy of atomic oxygen

Using synchrotron-radiation-based angle-resolved photoelectron spectroscopy, the relative partial photoionization cross sections for the production of the $^{4}$${\mathit{S}}^{\mathit{o}}$ and $^{2}$${\mathit{D}}^{\mathit{o}}$ ionic states in atomic oxygen, as well as the corresponding asymmetry parameters, are measured from threshold at 13.62 to about 30 eV. The cross sections are placed on an absolute scale using previous data obtained with an electron spectroscopy modulation method. Attention is focused on the numerous autoionization resonances below the 2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$ $^{2}$${\mathit{D}}^{\mathit{o}}$, 2${\mathit{p}}^{\mathrm{\ensuremath{-}}1}$ $^{2}$${\mathit{P}}^{\mathit{o}}$, and 2${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ $^{4}$${\mathit{P}}^{\mathit{e}}$ limits. The behavior of the asymmetry parameters across these resonances is observed for the first time. The 2s2${\mathit{p}}^{4}$${(}^{4}$${\mathit{P}}^{\mathit{e}}$)3p${(}^{3}$${\mathit{S}}^{\mathit{o}}$${,}^{3}$${\mathit{P}}^{\mathit{o}}$${,}^{3}$${\mathit{D}}^{\mathit{o}}$) resonances are fitted by a Fano-type profile to obtain accurate values for the position, width, and q parameter.

An investigation of the electronic structure of osmium tetroxide by photoelectron spectroscopy with variable photon energy

Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetroxide in the ionization energy range 12 to 18 eV. The photon energies used ranged between 24 and 100 eV. The ionization cross sections of the 2t 2 and 2a 1 orbitals show evidence or substantial metal character establishing strong Os−O σ-bonding. Very little evidence is found for π-bonding.

ChemInform Abstract: Investigation of the Electronic Structure of Tetrakis(trifluorophosphine)nickel by Photoelectron Spectroscopy with Variable Photon Energy.

Photoelectron spectra of [Ni(PF3)4] have been measured using synchrotron radiation. Relative partial photoionization cross-sections and photoelectron branching ratios of the valence bands (binding energy 9–21 eV) of [Ni(PF3)4] are reported over the photon-energy range 21–100 eV. The cross-sections of the d bands show both a shape resonance between 30 and 50 eV and a p–d resonance between 70 and 100 eV. Intensity variations are interpreted in terms of the atomic orbital contributions to the molecular orbital from which ionization is taking place. The occurrence of the p–d resonance for a d10 compound is attributed to covalent mixing of d character into unoccupied molecular orbitals.

Investigation of the electronic structure of tetrakis(trifluorophosphine)nickel by photoelectron spectroscopy with variable photon energy

Photoelectron spectra of [Ni(PF3)4] have been measured using synchrotron radiation. Relative partial photoionization cross-sections and photoelectron branching ratios of the valence bands (binding energy 9–21 eV) of [Ni(PF3)4] are reported over the photon-energy range 21–100 eV. The cross-sections of the d bands show both a shape resonance between 30 and 50 eV and a p–d resonance between 70 and 100 eV. Intensity variations are interpreted in terms of the atomic orbital contributions to the molecular orbital from which ionization is taking place. The occurrence of the p–d resonance for a d10 compound is attributed to covalent mixing of d character into unoccupied molecular orbitals.

Covalency in bis([8]annulene)uranium from photoelectron spectroscopy with variable photon energy

Relative partial photoionization cross sections and photoelectron branching ratios of the valence bands (6-22-eV binding energy) of bis((8)annulene)uranium(IV) are reported over the photon energy range 24-125 eV. The f{sup {minus}1} band shows cross-section features attributable to 5f resonant photoemission in the vicinity of the 5d {yields} 5f giant resonant absorption (hv = 101 and 110 eV). The e{sub 2g}{sup {minus}1} and e{sub 2u}{sup {minus}1} bands also show small cross-section maxima at these energies, that for the e{sub 2u}{sup {minus}1} ionization being the more intense. The f{sup {minus}1} band shows a large maximum at 39 eV, which is assigned to a delayed maximum due to the centrifugal barrier. The three highest ionization energy bands (from ligand-based orbitals) show essentially monotonic cross section decrease with photon energy. The mapping of the intensity changes of the f{sup {minus}1} band by the e{sub 2u}{sup {minus}1} band provides strong evidence for f orbital covalency in this molecule.

Near-threshold photoionisation studies of N2

Photoionisation of N2 to the nu =0, 1 and 2 vibrational levels of the X 2 Sigma g+ state of N2+, over the photon energy range from 15.5 to 18.0 eV has been studied in two ways. Firstly the threshold photoelectron spectrum has been obtained with a threshold resolution of approximately 3 meV, using a new threshold analyser. Secondly the relative partial photoionisation cross sections for the nu =0, 1 and 2 vibrational levels of the X 2 Sigma g+ state have been measured, using a technique in which the photon energy is varied continuously while the collection energy of the photoelectron analyser is changed in synchronism. The partial photoionisation cross sections show much structure due to the existence of autoionising states, and give information on the decay of these autoionising states into the vibrational levels of the X 2 Sigma g+ ion state.

Relative partial photoionization cross-sections and photoelectron branching ratios of the valence-bands of the group VIII metallocenes of iron, ruthenium and osmium

Relative partial photoionization cross-sections have been measured for the valence-bands of bis-η5-cyclopentadienyl iron, ruthenium and osmium over the photon energy range 16–115 eV. In each case the a′1 -1 and the e′2 -1 bands show cross-section features attributable to nd resonant photoemission in the vicinity of np → nd giant resonant absorption. For Ru(η-C5H5)2 a large intensity increase is observed, while Fe(η-C5H5)2 and Os(η-C5H5)2 show cross-section minima due to interference effects, followed by much weaker maxima. In the case of Ru(η-C5H5)2 all the ligand ionization bands exhibit resonance features very similar to the metal based orbitals, with those observed for e″1 -1 ionization being the most intense. Even the e″1 -1 orbital, which by symmetry has no metal d character, exhibits such a resonance in its cross-section. For ferrocene and osmocene the e″1 -1 bands also exhibit the greatest degree of cross-section correlation with the d bands. Significant differences are found between the a′1 -1 and the e′2 -1, and between the e′1 -1 and e″1 -1 bands. These are attributed to the varying amounts of metal character and the different symmetries of the corresponding molecular orbitals. The implications of the relative intensity changes with regard to the use of He I/He II intensity ratios as an aid to spectral assignment are examined. Other intensity maxima in the d orbital cross-sections are tentatively assigned to shape resonances in one or more of the f-wave ionization channels. The cross-sections of the ruthenocene e″1 -1 bands cannot be adequately described by linear combinations of the e′1 -1 and metal band cross-sections. Relative partial photoionization cross-sections for the 4f ionization bands of Os(η-C5H5)2 are reported over the photon energy range 75–115 eV.

Decay channels of the 4p resonances in atomic Sr

The photoelectron spectra and the relative partial photoionisation cross sections of atomic Sr have been determined in the photon-energy range of the 4p to nl, in l excitations. Below the 4p54d(3P)5s ionisation limit the Sr 4p65s2 to 4p6nl in l' partial cross sections are approximately equal to the Sr+ ion yield obtained by Nagata et al. Above the 4p ionisation threshold the 4p cross section closely resembles the Sr2+ ion yield and the two-step autoionisation Auger processes comprise more than 70% of all decay channels of the 4p5nln'l' resonances.

Relative partial photoionization cross-sections and photoelectron branching ratios of ethylene

Relative partial photoionization cross-sections and photoelectron branching ratios of the valence bands (~10–25 eV binding energy) of ethylene are reported over the photon energy ranges 18–100 eV and 21–100 eV, respectively. The four lowest ionization energy bands (1 b −1 1u, 1 b −1 1g, 3 a −1 g, and 1 b −1 2u) show monotonic cross-section decreases with photon energy from 33 eV, the 1 b −1 1u CC π band showing the least rapid decline. In contrast, the 2 b −1 3u and 2 a −1 g bands show almost constant cross-sections up to ~50 eV photon energy, followed by similar, although slower, monotonic decreases. This is attributed to the substantial carbon 2 s character of the 2 b 3u and 2 a g orbitais. The cross-section behaviour of all bands is interpreted with the aid of SCF-LCAO-MO calculations on the neutral molecule using the Gaussian-80 series of programs.

Decay of the giant 4d photoabsorption resonance in atomic Cs and Sm

The photoelectron spectra and the relative partial photoionisation cross sections of atomic Cs and Sm have been determined in the photon energy range of 4d-4f, epsilon f excitations (Cs50-130 eV; Sm 120-160 eV). For Cs direct 4d ionisation dominates whereas the Sm '4d to 4f, epsilon f resonance' mainly autoionises via the emission of a 4f electron. The Sm 4d partial cross section goes through a maximum about 12 eV above threshold, where it becomes comparable with the 4f partial cross section. The 4f type character of the Sm to excited wavefunction at threshold turns into 4f, epsilon f type character about 7 eV above. Close to the 4d threshold the 5s, 5p and two-electron satellite photoemission lines are strongly enhanced for both atoms.

Non-Franck–Condon 2σ−1u vibrational distributions in N+2. An interchannel-coupled shape resonance observed by dispersed fluorescence

We report evidence for a molecular interchannel-coupled shape resonance in N2 photoionization, observable in the vibrational branching ratios for the N+2(B 2Σ+u) state. The vibrational branching ratios for the N+2(B 2Σ+u) state are determined from vibrationally resolved dispersed fluorescence measurements on the N+2(B 2Σ+u→X 2Σ+g) band system. The results demonstrate that the partial cross section for the N+2(B 2Σ+u,v′=1) level is selectively enhanced in the photon energy range 28 &amp;lt;hνexc&amp;lt;30 eV. A shape resonance occurs in this range for the 3σg→εσu channel, and the observed enhancement is attributed to coupling between this channel and the 2σu→εσg channel. Vibrational branching ratio curves for the excited state ion also show a pronounced peak at hνexc=22 eV, which shifts for the curves that sample the v′=1 and v′=2 levels of the ion, suggesting a few interpretations. The utility of coupling monochromatized fluorescence detection with synchrotron radiation excitation for extracting relative partial photoionization cross sections is demonstrated for the first time.

Observation of resonance effects in the relative partial photoionization cross sections of the d bands of M(CO) 6, M = Cr, Mo and W

Relative partial photoionization cross sections as a function of photon energy, over the range 20–110 eV, have been measured for the valence bands of Cr(CO) 6, Mo(CO) 6 and W(CO) 6. All three t 2g −1 bands show a very pronounced increase in intensity at photon energies ( hv) corresponding to np resonant absorption (Cr(CO) 6, hv = 52.5 eV, n = 3; Mo(CO) 6, hv = 48 eV, n = 4; W(CO) 6, hv = 44 and 53 eV, n = 5). The other valence bands show a small intensity increase at similar energies. Observation of such resonant photoemission provides an unambiguous method for assignment of nd bands in the photoelectron spectra of gas-phase molecules.

Photoeffect in the 4d subshell of atomic silver between 14 and 140 eV

Relative partial photoionisation cross sections and the photoelectron angular distribution parameter beta have been measured for the 4d electrons of atomic silver in the energy range 14<or=h nu <or=140 eV. Data are found to be in good agreement with the results calculated in the relativistic random-phase approximation (RRPA) for the neighbouring atom palladium. Comparison with solid-state data reveals a dominant atomic character of the 4d electrons in metals. This is especially evident for the beta parameter. However, differences occurring around the Cooper minimum at 130 eV show the influence of the metallic state on the properties of the 4d electrons.

Resonant satellite photoemission of atomic Mn

The photoelectron spectra and the relative partial photoionisation cross sections of atomic Mn have been determined for photon energies between 45 and 65 eV. Besides the main 3d54s2 6S to 3d44s2(5D) epsilon l, 3d54s(5,7S) epsilon l photoemission lines there are prominent 3d44snl epsilon l satellite photoemission lines which are strongly enhanced in the range of the giant 3p to 3d resonance and the 3p63d54s2 to 3p54s2nl Rydberg transitions. Near the 3p to 3d resonance the partial cross sections of these two-electron satellites behave similarly to that of the 3s54s2 to 3d44s2 epsilon l main line The interference between the 3p63d54s2 6S to 3p53d64s2 6P excitation and the continuum channels manifests itself in the asymmetric profile of the partial cross sections. In contrast to this, the 3d54s2 to 3d54s5L epsilon l partial cross sections can be approximated by the superposition of Lorentzians centred at the 3p63d54s26S to 3p53d64s26D, 6P, 4L excitation energies. The 3d55s epsilon l photoemission only undergoes a single symmetric resonance. All these lines, including the main lines, do not show an enhancement close to the 3p53d54s2 7P ionisation limits. When tuning the photon energy across these ionisation limits post-collision interactions give rise to further satellite lines, and to asymmetric M2,3M4,5N1 Auger lines which, in comparison with the normal Auger lines, are shifted toward higher energies.

Relative and absolute partial photoionization cross sections of the2sand2psubshells of neon

Relative and absolute partial photoionization cross sections of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>2</mml:mn><mml:mi>s</mml:mi></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>2</mml:mn><mml:mi>p</mml:mi></mml:math>subshells of neon

Angular Distribution of Auger Electrons Following Photoionization

It is shown that the photoionization process of inner-shell electrons by unpolarized radiation leads to an alignment of the ionized atoms. From the nonisotropic angular distribution of the following Auger electrons the relative partial photoionization cross sections can be determined.