Photoelectron Asymmetry Parameters Research Articles

Photoionization Dynamics of the Tetraoxo Complexes OsO4 and RuO4.

The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17–90 eV photon energy range. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom.

Communication: Photoionization of degenerate orbitals for randomly oriented molecules: The effect of time-reversal symmetry on recoil-ion momentum angular distributions.

The photoelectron asymmetry parameter β, which characterizes the direction of electrons ejected from a randomly oriented molecular ensemble by linearly polarized light, is investigated for degenerate orbitals. We show that β is totally symmetric under the symmetry operation of the point group of a molecule, and it has mixed properties under time reversal. Therefore, all degenerate molecular orbitals, except for the case of degeneracy due to time reversal, have the same β (Wigner-Eckart theorem). The exceptions are e-type complex orbitals of the Cn, Sn, Cnh, T, and Th point groups, and calculations on boric acid (C3h symmetry) are performed as an example. However, including those point groups, all degenerate orbitals have the same β if those orbitals are real. We discuss the implications of this operator formalism for molecular alignment and photoelectron circular dichroism.

Interference effects in photoelectron asymmetry parameter (β) trends of C 2s−1 states of ethyne, ethene and ethane

Photoelectron asymmetry parameters (β) of the gerade and ungerade C 2s−1 derived states of ethyne, ethene and ethane as a function of photon energy have been calculated and experimentally measured, to extend the search of interference effects on angular distributions to polyatomic molecules. The calculations cover the electron energy range from 0 to 1100 eV while the experimental measurements cover the electron energy range from 30 to 220 eV. Clear oscillations are interpreted in terms of interference of the photoelectron wave emitted from the two possible C 2s centres, or equivalently from the gerade and ungerade states associated with them. This is a microscopic analog of Young’s double-slit experiment. The effect is however quite small and requires very high experimental accuracy to be detected. It is best evidenced in the behaviour of β difference between the two channels. The connection between β trends and structural parameters shows the expected inverse correlation between oscillation period and distance between the carbon atoms, but do not simply parallel the analogous behaviour found in cross sections.

The role of Rydberg states in photoionization of NO2 and (NO+, O−) ion pair formation induced by one VUV photon

We report a combined experimental and theoretical study of photoionization (PI) of the NO2 molecule into the NO2(+) (X (1)Σg(+)) ground state and the photodissociation of NO2 into the NO(+)((1)Σ(+)) + O(-)((2)P) ion pair. These processes were induced by 10.9 eV-13 eV synchrotron radiation and the products were detected using electron-ion or O(-)-NO(+) coincident momentum spectroscopy. The results demonstrate the strong influence of [R(∗)(4b2)(-1), nlα(i), v2(')] Rydberg states vibrationally resolved in the v2(') bending modes for both processes. In particular, we emphasize two regions around 11.5 eV and 12.5 eV that were studied in more detail for their relevance to 400 nm multiphoton ionization induced by femtosecond pulses. The photoelectron energy spectra and asymmetry parameters support the existence of two PI mechanisms, as probed with the help of fixed-nuclei frozen-core Hartree-Fock calculations. We found significant deviations from Franck-Condon ionization predictions which may be assigned to vibronic coupling of NO2(∗) states such as that induced by a conical intersection. The limited agreement between theory and experiment, even for the non-resonant processes, indicates the need for calculations that go beyond the approximations used in the current study. Ion pair formation leads to strong vibrational and rotational excitation of the NO(+)((1)Σ(+),v) product, with an ion fragment angular anisotropy depending on both the v2(') bending quantum number of the excited parent molecule and the v vibrational level of the fragment.

Laboratory-frame electron angular distributions: Probing the chemical environment through intramolecular electron scattering

Carbon $1s$ photoelectron asymmetry parameters $\ensuremath{\beta}$ for the chlorinated and the methyl carbon atom of CH${}_{3}$CH${}_{2}$Cl, CH${}_{3}$CHCl${}_{2}$, and CH${}_{3}$CCl${}_{3}$ have been measured using synchrotron radiation in the 340--600 eV energy range. We provide experimental evidence that the intramolecular scattering strongly affects $\ensuremath{\beta}$ values, even far from the ionization threshold. The results are in agreement with B-spline density functional theory calculations, making it possible to single out the behavior of the various continuum partial waves. We conclude that the intramolecular scattering makes electron angular distributions sensitive to the chemical environment, even in isolated gas phase molecules.

Experimental and theoretical asymmetry parameters for photoionization of H2 showing interference from the Q1 and Q2 doubly excited states

The photoelectron asymmetry parameter for single photon ionization of H2 between hν = 31 eV and 35 eV has been measured. At these photon energies the doubly excited Q1 and Q2 states are energetically accessible and lie within the Franck-Condon absorption range. For electron energies greater than ~6 eV, the β parameter oscillates strongly, mainly due to interferences in autoionization decay of the Q1 1Σu+ and Q2 1Πu doubly excited states. The experimental results are compared with close coupling calculations.

Observation of Interference between Two Distinct Autoionizing States in Dissociative Photoionization ofH2

Dissociative photoionization (DPI) of randomly oriented H(2) molecules has been studied using linearly polarized synchrotron radiation at selected photon energies of 31, 33, and 35 eV. Large amplitude oscillations in the photoelectron asymmetry parameter β, as a function of electron energy, have been observed. The phase of these β oscillations are in excellent agreement with the results of recent close coupling calculations [Fernández and Martín, New J. Phys. 11, 043020 (2009)]. We show that the oscillations are the signature of interferences between the 1Q(1) (1)Σ(u)(+) and 1Q(2) (1)Π(u) doubly excited states decaying at different internuclear distances. The oscillations thus provide information about the classical paths followed by the nuclei. The presence of such oscillations is predicted to be a general phenomenon in DPI.

A study of the valence shell photoionisation dynamics of pyrimidine and pyrazine

The complete valence shell photoelectron spectra of pyrimidine and pyrazine have been recorded with synchrotron radiation and the observed structure has been interpreted with the aid of vertical ionisation energies and relative spectral intensities calculated using time-dependent density functional theory. The theoretical predictions for the single-hole ionic states due to outer valence shell ionisation agree satisfactorily with the experimental results. Ionisation from the inner valence orbitals is strongly influenced by many-body effects and the intensity associated with a particular orbital is spread amongst numerous satellites. Photoelectron angular distributions and partial cross sections have been determined both experimentally and theoretically, and demonstrate that shape resonances affect the valence shell photoionisation dynamics. In addition to shape resonances occurring a few eV above the ionisation threshold, the calculations indicate that many of the orbitals are influenced by shape resonant processes at much higher energies. Some of these higher energy resonances have been confirmed through a comparison between the relevant theoretical and experimental photoelectron asymmetry parameters. The spectral behaviour of asymmetry parameters associated with π-orbitals has been shown to differ from that of asymmetry parameters associated with σ-orbitals. These differences provide a means of distinguishing between the two types of orbitals even in heavily congested regions of the photoelectron spectrum suffering from band overlap.

Asymmetry in the Strong-field Photodetachment of H− by Linearly Polarized Few-cycle Pulses

Photodetachment of H− irradiated by linearly polarized few-cycle laser field is investigated by time-dependent Schrödinger equation numerically. The photo-electron left-right asymmetry parameter as a function of carrier-envelop (CE) phase of few-cycle pulses is attained. We confirm the asymmetry of photoelectron distribution in H− photodetachment and find that the maximal asymmetry parameter of H− is equal to that of H atom under the same conditions but the corresponding CE phases are quite different. Thus a CE phase shift appears. Compared to that of H atom and field free electron, the zero asymmetry CE phase shift is sensitively affected by Coulomb field. The Coulomb effect on the asymmetry of H− photodetachment mainly behaves in the CE phase shift of H− instead of the amplitude of asymmetry parameter curve.

Cross sections and asymmetry parameters for photoionization of [formula omitted] in the VUV region

Calculated cross sections and asymmetry parameters for photoionization of the three outermost valence orbitals of CH 3 F for photon energies from near threshold to 50 eV are reported. The photoelectron continuum orbitals are obtained using the Schwinger variational method at the level of static-exchange plus correlation potential. Our results are compared with other theoretical and experimental results showing a good quantitative agreement. Correlation effects do not significantly affect the computed total cross sections and photoelectron asymmetry parameters. The broader structure that appears in the experimental cross section are well described. However, the narrow resonant features near threshold are not found in the present one-electron treatment of the photoionization process.

A study of the valence shell electronic structure of the 5-halouracils

Valence shell photoelectron spectra of the 5-halouracils have been recorded using synchrotron radiation at photon energies of 40 and 80 eV. Photoelectron angular distributions have been determined and these provide an experimental means of deducing the extent to which the molecular orbitals possess predominantly σ, π or halogen np character. In chloro-, bromo- and iodouracil two peaks, associated with the essentially halogen atom lone-pair orbitals, have been observed. The separation between these peaks, and the value of their photoelectron asymmetry parameters at an excitation energy of 40 eV, are compared with results for the corresponding orbitals in the halobenzenes and halothiophenes. Vertical ionisation energies and spectral intensities have been evaluated using the many-body Green’s function approach, thereby enabling theoretical spectra to be derived. Assignments have been proposed for most of the structure observed in the valence shell photoelectron spectra based upon the available experimental and theoretical evidence.

Random-phase approximation with exchange for the photoionization of an atom with an inner open shell

A random-phase approximation with exchange method has been developed for the photoionization of atoms (ions) with an inner open shell. The method allows for the inclusion of both intrashell and intershell correlations in the calculation and has been used to study the photoelectron asymmetry parameter and the cross sections of the Sc $4s$ photoionization in which the Sc $3p\text{\ensuremath{-}}3d$ transitions have been successfully included. The results demonstrate that the inclusion of the $3p\text{\ensuremath{-}}3d$ transitions is extremely important in the intershell coupling, which significantly influences the calculated asymmetry parameter and the cross sections of the Sc $4s$ photoionization. Comparison of our calculated asymmetry parameter and the cross sections with the measured data and available calculation is made and discussed. Possible improvements to the present calculated data are offered as well.

A study of the valence shell electronic structure and photoionization dynamics of selenophene

The photoelectron spectrum of selenophene has been recorded using synchrotron radiation in the photon energy range 20–80 eV and the inner valence region has been studied in detail for the first time. Green's function methods have been employed to evaluate the energies and spectral intensities of all valence shell ionization transitions and the results have facilitated an interpretation of the experimental spectra. Strong configuration interaction results in a redistribution of the intensity associated with the low lying π1(1b1) orbital amongst several satellite states located in the outer valence region. The continuum multiple scattering approach has been used to calculate photoelectron asymmetry parameters for selenophene, thiophene and hydrogen sulphide, and these theoretical predictions have been compared with the corresponding experimental data to assess the influence of Cooper minima and shape resonances. The comparison indicates that the Se 4p and the S 3p Cooper minima have little effect on the valence shell photoionization dynamics of selenophene and thiophene, respectively. This outcome is discussed in connection with the closely related hydrogen selenide and hydrogen sulphide molecules where strong resonant phenomena are observed.

A study of the photoionisation dynamics of chloromethane and iodomethane

Angle resolved valence shell photoelectron spectra of chloromethane and iodomethane have been recorded using synchrotron radiation in the photon energy range 14–120 eV. These have allowed photoelectron angular distributions and branching ratios to be determined not only for the main bands associated with the single-hole states but also for the satellite structure due to many-electron effects. The continuum multiple scattering approach has been used to calculate photoelectron asymmetry parameters and branching ratios for the valence orbitals of CH 3Cl and CH 3I, and also for the I 4d subshell. A comparison between the experimental data and the theoretical predictions has enabled the influence of Cooper minima, shape resonances and intershell coupling to be assessed. The asymmetry parameters and branching ratio for the spin–orbit split components of the CH 3I + X ˜ 2 E state have been measured and exhibit a spectral behaviour almost identical to that of the corresponding data for the Xe 5p 3/2 and 5p 1/2 levels. The many-body Green’s function approach has been employed to evaluate the ionisation energies and spectral intensities of all valence states and the results have facilitated an interpretation of the satellite structure. The I 4d shake-up, shake-off and Auger spectra of iodomethane have been recorded and many of the Auger peaks have been assigned using previously determined ionisation energies of doubly charged valence states.

Symmetry- and vibrationally-resolved C K-shell photoionization studies of C 2H 2

The C K-shell photoelectron mainline of C 2H 2 molecules have been measured at photon energies 295–350 eV. The gerade and ungerade symmetries of the core-ionized states, 1 σ g and 1 σ u, are resolved. Resonance enhancement is found only in the 1 σ g photoionization cross-section and assigned to the σ u ∗ shape resonance. The photoelectron asymmetry parameter takes a minimum value at the shape resonance energy. The shape resonance energy decreases with increase in the vibrational quantum number. No correlational shape resonance appears in the 1 σ u photoionization channel, suggesting that the channel coupling is negligible.

Photoelectron spectra of the nucleobases cytosine, thymine and adenine

The complete valence shell photoelectron spectra of cytosine, thymine and adenine have been investigated experimentally and theoretically. Vertical ionization energies and spectral intensities have been evaluated using the many-body Green's function method, thereby enabling theoretical photoelectron spectra to be derived. In cytosine, the influence of tautomers and rotational conformers has been investigated. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Photoelectron asymmetry parameters have been determined from angle resolved spectra recorded with synchrotron radiation. The experimental data show that the electronic configuration of the five outer orbitals in cytosine, thymine and adenine is π, σ, π, σ, π. Vertical ionization energies have been measured for all the outer-valence orbitals even though some of the associated bands overlap significantly.

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.

An experimental and theoretical study of the photoelectron spectrum of hydrogen selenide

The photoelectron spectrum of hydrogen selenide has been recorded using synchrotron radiation in the photon energy range 15–110 eV and the inner valence region has been studied in detail for the first time. Green’s function methods have been employed to evaluate the ionisation energies and spectral intensities of all valence states and the results have facilitated an interpretation of the experimental spectra. Two outer valence satellites have been observed at binding energies of ∼17 and ∼18.5 eV. The continuum multiple scattering (CMS-Xα) approach has been used to calculate photoelectron asymmetry parameters and branching ratios for the outer valence orbitals and these have been compared with experimental data. The Se 4p Cooper minimum strongly affects the photoionisation dynamics of the outermost 4b 1 orbital and its influence is evident in both the theoretically predicted and the measured asymmetry parameter for the X ˜ 2 B 1 state. The experimental data for the A ˜ 2 A 1 and B ˜ 2 B 2 state asymmetry parameters also display energy dependent variations in the vicinity of the Cooper minimum. However, for these two states the agreement between the predicted and observed asymmetry parameters is less satisfactory. The present results for H 2Se have been compared with similar data for the closely related H 2S molecule.

A study of the photoionisation dynamics of the cyanogen halides

The valence shell photoelectron spectra of BrCN and ICN have been studied using synchrotron radiation. In addition to the main bands associated with the single-hole states, complex satellite structure due to many-electron effects has been observed in the inner valence region. The Green’s function method has been employed to evaluate the ionisation energies and pole strengths of all valence states and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios for the valence shell, the Br 3d level in BrCN and the I 4d level in ICN have been measured in the photon energy range 14–120 eV. The continuum multiple scattering (CMS-Xα) approach has been used to calculate photoelectron asymmetry parameters and branching ratios for the valence orbitals of ClCN, BrCN and ICN. These have been used to assess the influence of Cooper minima and shape resonances on the photoionisation dynamics. In ICN, the experimental and theoretical results provide evidence of core-valence shell coupling at energies above the I 4d threshold. The experimentally determined asymmetry parameters and branching ratios for the spin–orbit split Br 3d5/2,3/2 and I 4d5/2,3/2 components have been compared to those for Kr and Xe, and a strong atomic-like behaviour is evident. The calculated spin–orbit averaged β-parameters for the Br 3d and I 4d levels confirm this interpretation. Hartree–Fock (HF) calculations have been carried out to study the evolution of the valence orbital characteristics through the series of halogen-substituted XCN (X=H, Cl, Br, I) molecules.

The influence of shape resonancephenomena on the valence shell photoionization dynamics ofsilicon tetrafluoride

The complete valence shell photoelectron spectrum of silicontetrafluoride has been studied both experimentally and theoretically.Photoelectron angular distributions and branching ratios have beenmeasured in the photon energy range 17-120 eV, using synchrotronradiation, and compared with previous theoretical predictions. Thespectra demonstrate that shape resonance phenomena affect thephotoionization dynamics of the outer valence shell orbitals, and thepredicted shape resonance in the a1 channel, close to threshold,has been confirmed experimentally. At higher energies, the effects ofthe e and t2 symmetry shape resonances have been observed, forthe first time, in the photoelectron asymmetry parameters. Partialphotoionization cross sections have been derived by combining theexperimental branching ratios with the total ion yield curve andnormalizing the results to the absolute photoabsorption crosssection. The partial cross sections so obtained have been comparedwith theoretical curves reported in two earlier studies. The verticalionization energies and spectroscopic factors for the entire valenceshell photoelectron spectrum have been computed using the third-orderalgebraic-diagrammatic construction approximation scheme for theone-particle Green function.