A.W. Potts

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 Greens 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.

An experimental and theoretical study of the valence shell photoelectron spectra of purine and pyrimidine molecules

The valence shell photoelectron spectrum of purine has been studied experimentally and theoretically. Synchrotron radiation has been used to record spectra at photon energies of 45 and 85 eV. Photoelectron angular distributions have been determined and these provide an experimental means of distinguishing between σ-and π-type orbitals. Vertical ionization energies and photoelectron spectral intensities have been evaluated using the many-body Green function method. The calculated spectra agree well with the experimental results in the outer valence region and have proved to be indispensable for interpreting the structure at higher binding energies where the single particle model of ionization breaks down. The photoelectron spectrum of pyrimidine has also been studied and is compared to that of purine.

An experimental and theoretical study of the valence shell photoelectron spectra of thiophene, 2-chlorothiophene and 3-chlorothiophene

Abstract The valence shell photoelectron spectra of thiophene, 2-chlorothiophene (2-Cl-Th) and 3-chlorothiophene (3-Cl-Th) have been investigated theoretically and experimentally. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green’s function has been employed to evaluate the vertical ionisation energies and spectral intensities. The ground state geometrical parameters of the three molecules have been optimised at the level of the second order Moller–Plesset perturbation theory, and standard cc-pvDZ basis sets have been used throughout. The results for the outer valence region of thiophene agree well with available experimental and theoretical data. Very satisfactory agreements have been obtained between the theoretical predictions for the photoelectron spectra of 2-Cl-Th and 3-Cl-Th and the corresponding experimental data recorded in the present study. Assignments have been proposed for the major spectral structures. In all three molecules the breakdown of the molecular orbital ionisation picture associated with the π1 molecular orbital is discussed. Synchrotron radiation has been used to record the complete valence shell photoelectron spectra of 2-Cl-Th and 3-Cl-Th. Photoelectron angular distributions and branching ratios have been determined in the photon energy range 13–115 eV, and those for the chlorine and sulphur 3p orbitals show effects that can be attributed to Cooper minima. A high resolution photoelectron spectrum of the outer valence shell of 2-Cl-Th has been recorded using HeI radiation, and vibrational structure has been observed and analysed in the X 2 A ′′ , A 2 A ′′ , B 2 A ′ , C 2 A ′′ and I 2 A ′ state bands.

An experimental and theoretical study of the valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene

Abstract The valence shell photoelectron spectra of 2-bromothiophene and 3-bromothiophene have been studied, both experimentally and theoretically, in order to characterise the main bands due to single-hole states and to assess the importance of electron correlation in the formation of satellite states. Synchrotron radiation has been employed to measure photoelectron angular distributions and branching ratios in the photon energy range 13–115 eV, and the results indicate that the photoionisation dynamics of the (12 a ′ ) −1 B 2 A ′ , (2 a ″ ) −1 C 2 A ″ and ( 11 a ′ ) −1 D 2 A ′ states are affected by Cooper minima. The experimental data demonstrate that the 12a′ and 2a″ orbitals, corresponding to the Br 4p lone pairs, retain their atomic properties to a substantial degree, and that the same is true, although to a lesser extent, for the 11a′ orbital which is related to the S 3p subshell. The results are compared with similar measurements on chlorothiophene and bromobenzene. The vertical ionisation energies and spectral intensities of the entire valence shell photoelectron spectrum have been computed using the third-order algebraic–diagrammatic construction approximation scheme for the one-particle Greens function. These theoretical predictions have allowed assignments to be proposed for all the prominent structure observed in the experimental spectra. The outer valence Greens function method has also been used and the calculated ionisation energies show good agreement with the experimental values. Mulliken atomic populations have been computed for some of the outer valence orbitals, and the calculated Br 4p and S 3p content possessed by the 12a′, 2a″ and 11a′ molecular orbitals is in accordance with the strength of the Cooper minimum observed in the photoelectron asymmetry parameters associated with these orbitals.

A systematic investigation of the influence of Cooper minima on the photoionisation dynamics of the monohalobenzenes

Abstract The valence shell photoelectron spectrum of iodobenzene has been studied using HeI and 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. Two variations of the many-body Green’s function method have 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 have been determined using monochromated synchrotron radiation in the photon energy range 16–120 eV, and those for the lone-pair orbitals show effects that can be attributed to Cooper minima. However, the influences of Cooper minima have also been observed in the asymmetry parameters associated with other states, and the possible role of interstate mixing is discussed. The present data for iodobenzene are compared with similar measurements for benzene, chlorobenzene and bromobenzene to examine and contrast the effect of the halogen atom substituent. Vibrational structure has been observed in several of the photoelectron bands of iodobenzene excited with HeI radiation and analyses have been performed for the X 2 B 1 , C 2 B 1 and D 2 A 1 ionic states.

Photoelectron spectra of the valence d shells of Tl(I), In(I), Pb(II) and Sn(II) Halides

Abstract The elements Zn, Cd, Hg, In, Tl, Sn and Pb all possess d electrons which fall within the ionization energy range normally considered as the valence region but which are essentially atomic in nature. The valence d −1 ionized states of the halides of the first three of these elements have already received some attention in the literature and here ionization from the corresponding d shells of the vapour-phase halides of the remaining four elements is reported. Spectra are shown to be consistent with an atomic description for the orbitals and ionization energies are shown to be reasonably described by a simple electrostatic model. Changes in ionization energy which occur with change in substituent for a particular metal are consistent with the chemical shift model. For the Tl halides fine structure is observed in the 2 D multiplet. This is interpreted in terms of a ligand field model and an analysis is proposed on this basis.

UV angle-resolved photoelectron spectra of mixed methylene dihalides using synchrotron radiation

Abstract Photoelectron spectra for the dihalomethanes CH2BrCl, CH2ClI and CH2Br2 have been recorded with photons in the energy range 19–115 eV using Daresbury Laboratory Synchroton Radiation Source. Ionization energies have been measured for all valence falling within this energy including Br 3d and I 4d orbitals. Asymmetry parameters have been measured for all intense ionization processes and characteristic Cooper minima observed for halogen lone pair orbitals. For CH2ClI lone pair orbital β spectra show strong evidence of mixed halogen character although a similar situation is not observed for CH2BrCl. Partial photoionization cross sections are tabulated for the observed ionization processes. Detailed spectra are presented for the molecule CH2ClI since this appears to be the first photoelectron study of this molecule.

The photoelectron spectrum and valence shell structure of (CuX) 3 and (AgCl) 3

Abstract Gas-phase He(I) and He(II) spectra have been obtained of CuX and AgCl, where X = Cl, Br, and I. Spectra have been interpreted in terms of a cyclic trimer and shown to be markedly similar to those of the molecules in the solid state. Frequency-dependence of band intensity has been used to deduce the contribution of Cu 3 d and X valence p orbitals to molecular orbitals associated with the various regions of the spectrum.

UV angle resolved photoelectron spectra of fluoro and chloromethane using synchrotron radiation

Abstract Photoelectron asymmetry parameters (β) and partial photoionization cross-sections have been measured for ionization from the molecular orbitals of CH 3 F and CH 3 Cl using synchrotron radiation in the photon energy range 19 to 115 eV. Cooper minima are observed in the β spectra of CH 3 C1 for ionization from orbitals with Cl 3 p character. Several shake-up bands observed in the F 2 s and Cl 3 8 ionization energy region indicate a breakdown of the one-electron picture of ionization. The position and relative intensities of the satellite bands are compared with the results of Greens function calculations.

Angle-resolved photoelectron spectra of CF3Br and CF2Br2 in the photon energy range 29-115 eV

Abstract Photoelectron asymmetry parameters β and partial photoionization cross sections have been measured for ionization from the molecular orbitals of CF 3 Br and CF 2 Br 2 using synchrotron radiation in the photon energy range from 19 to 115 eV. Cooper minima are observed in the β spectra for ionizations from orbitals with substantial bromine 4p character. The measured positions of the minima have been compared to the results of non-relativistic calculations performed for different elements of the periodic system. Several shape resonances have been observed in β spectra associated with orbitals of predominantly F(2p) character. Ionization from the Br(3d) subshell has also been observed and studied in some detail. Auger transitions involving 3d electrons have been observed at higher photon energies.