The electronically excited singlet and triplet valence, and Rydberg states, in the excitation energy range 1-16 eV, have been determined for sulphur dioxide, by large scale CI methods. Theoretical values for the excitation energies and oscillator strengths for a range of Rydberg states converging on the first six IPs have been calculated. The sequence of ionic states have been re-determined by CI and TDA calculations; the results support the recent study by Li et al. [J. Chem. Phys., 120, 4677 (2004)], that the sequence of IPs should be reordered to (X) over tilde (2)A(1) < <(A)over tilde>B-2(2) < <(B)over tilde>(2)A(2) << (C) over tilde B-2(1) < <(D)over tilde>(2)A(1) < <(E)over tilde>B-2(2) << (F) over tilde (2)A(1) ionized state equilibrium structures were determined using CCSD(T) and CI methods. Several bases were used, including one containing the s,p,d-components of a cc-pV5Z basis set; however, an alternative quadruple zeta basis set augmented by diffuse and Rydberg functions provided better coverage of the lower excited states, owing to a wider s,p,d-range of virtual orbitals.The gap in the published experimental VUV spectrum between 992 and 1078 angstrom has been completed using both new data, and re-evaluation of the Price and Simpson [Proc. RoY. Soc. Ser. A, 165, 272 (1938)] photographic data. A number of new Rydberg states in both the gap and nearby regions have been identified. The experimental assignment of the electronic spectrum is critically assessed in some detail. Assignment of the lower singlet and triplet manifolds and comparison of the theoretical intensity envelope with the VUV absorption and photoionization spectra has been made.

Photoelectron-photoion coincidence spectra have been recorded using synchrotron radiation to study dissociative photoionization in molecular oxygen. The O-2(+) B (2)Sigma(g)(-) v(+) = 2 state has been produced either by direct photoionization or by autoionization from Rydberg states belonging to series converging onto the c (4)Sigma(u)(-) limit. Coincidences have been detected between the energy analysed photoelectron and the O+ fragment released in the dissociation of the O-2(+) B (2)Sigma(g)(-) v(+) = 2 state. A comparison between the observed and the computer simulated O+ time-of-flight peak profiles has enabled the ratio of the cross sections for the degenerate ionization channels to be determined from threshold 35 eV. The O+ peak shape changes in the vicinity of autoionizing resonances and this variation can be attributed to an enhancement in the photoionization cross section for perpendicular transitions.

The fluorescence yields and polarizations of the O-2(+) A 2 Pi(u) -> X (2)Pi(g) and the b (4)Z(g)(-) -> a (4)Pi(u) transitions have been measured between their thresholds and 25 eV using synchrotron radiation. The O-2(+) ions may be formed either through direct photoionization or through excitation into a Rydberg state followed by autoionization. The alignment of the resulting (A (2)Pi(u) or b (4 Sigma)(-)(g) state) ion has been determined by measuring the degree of linear polarization of the fluorescence emitted in the subsequent decay. The fluorescence yields and polarizations display prominent structure associated with autoionizing Rydberg states belonging to series converging onto the b (4)Sigma(g)(-), B (2)Sigma(g)(-) or the c (4)Sigma(u)(-) ionization thresholds. The structure in the fluorescence polarization data has enabled the symmetry of the super-excited states to be deduced. Although the interpretation of this symmetry information has been straight-forward in regard to the Rydberg series converging onto the c (4)Sigma(g)(-) limit, the situation is less clear for the Rydberg series converging onto the b (4)Sigma(g)(-) or the B (2)Sigma(g)(-) limits. The measured polarizations have been compared with calculated values and, even for the A (2)Pi(u) -> X (2)Pi(g) transition where the theoretical model incorporates resonant processes, the agreement is not particularly good.

A dispersed fluorescence study of the CO2+B˜(001)Σg+2→X˜(001)Πu2 transition has been performed in the photon energy range 18.2–19.9eV. The fluorescence excitation spectrum exhibits prominent structure ascribed to two electric dipole forbidden Rydberg series, both of which involve a single excitation of the asymmetric stretching mode, converging onto the C˜(001)Σu+2 ionisation limit. The series, whose presence can be attributed to vibronic coupling, may be due to 4σg→nsσg (δ∼0.93) and 4σg→nd (δ∼0.38) transitions.

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.

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∼2A1 or the G∼2A1 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.

The valence shell threshold photoelectron spectra and the photoionisation yield curves of the boron trihalides have been recorded using synchrotron radiation. The threshold photoelectron spectra demonstrate that spin–orbit coupling is important in the heavier trihalides and affects the bands associated with degenerate orbitals. In addition to the photoelectron bands associated with direct ionisation, features have been observed due to autoionising Rydberg and valence excited states. An analysis of the vibrational progressions occurring in the photoelectron bands has enabled an almost complete set of ionic vibrational energies to be determined. The continuous nature of the inner valence shell photoelectron bands and the absence of main-lines illustrate the importance of electron correlation in redistributing the intensity amongst numerous final states. In all four of the boron trihalides, new Rydberg series have been observed in the photoionisation yield curves. However, prominent broad features attributable to intervalence transitions dominate the photoion spectra. In some Rydberg series the intensity distribution amongst the members appears irregular, and this may be due to mixing between Rydberg and valence excited states. Several, rather weak, vibrational progressions involving the ν1′,ν2′ and ν4′ modes have been observed.

The fluorescence yields and polarisations of the CS2+ Ã2Πu→X̃2Πg and the B̃2Σu+→X̃2Πg transitions have been measured between their thresholds and 30 eV using synchrotron radiation. In the outer valence region the fluorescence yields display prominent structure due to autoionising Rydberg states. New features, associated with multi-electron transitions, have been observed between 16 and 30 eV, and most have been assigned through reference to the inner valence shell photoelectron spectrum. Some of the new features exhibit vibrational progressions involving excitation of the ν1′ mode. The influence of shape resonances on the photoionisation partial cross-sections has been considered. The polarisation measurements for the Ã2Πu→X̃2Πg and the B̃2Σu+→X̃2Πg transitions have been compared with predictions derived from theoretical photoionisation partial cross-sections. The predictions for the B̃2Σu+→X̃2Πg fluorescence polarisation display a rapid increase in the threshold region, in agreement with the experimental results. Resonant excitation and decay processes produce significant variations in the fluorescence polarisation, and these changes can be used to deduce the symmetry of the excited state. This information has allowed the assignments of some of the Rydberg series converging onto the B̃2Σu+ or the C̃2Σg+ ionisation thresholds to be clarified.

Synchrotron radiation has been used to form CO+ in the A (2)Pi state, either through direct photoionization or through excitation into a Rydberg state followed by autoionization. The alignment of the resulting ion has been determined by measuring the degree of linear polarization of the fluorescence emitted in the CO+ A (2)Pi nu(A)(+), = 0-4 --> X (2)Sigma(+) nu(X)(+) = 0 or 1 transitions. These vibrationally resolved polarization data, together with the corresponding fluorescence yields, have been recorded in the 16.5-20 eV excitation range. In this energy range the fluorescence yields exhibit prominent structure due to autoionizing Rydberg states belonging to series converging onto various vibrational levels of either the A (2)Pi or the B (2)Pi(+), ionization thresholds. Resonant excitation and decay processes produce substantial deviations in the fluorescence polarization and these changes have been used to deduce the symmetry of the excited state. The vibrationally resolved data are particularly useful in this respect because the observed variations in the polarization can be related to the branching ratio between a specific vibrational level in the Rydberg state and a particular level in the A (2)Pi state. This information has allowed the assignments of the Rydberg series converging onto the B (2)Sigma(+) ionization limit to be verified. In addition, the measured polarization has been compared with predictions derived from a theoretical approach in which only direct photoionization is taken into account.

A time-of-flight (TOF) mass spectrometry study has been carried out to investigate the fragmentation processes occurring in SiX4 (X=F, Cl or Br). Synchrotron radiation has been used to record spectra in the photon energy range 10–210eV, and appearance energies have been determined for 31 singly or doubly charged fragment ions. These have enabled upper limits for, previously unknown, heats of formation to be estimated for several doubly charged atomic and molecular fragments. The TOF spectra show that the peaks due to some of the small fragments change shape as a function of excitation energy, and that at high photon energy several of the peaks consist of two components, one of which is narrow and the other broad. The latter component is due to fragments possessing substantial initial kinetic energy. The peak shape is discussed in relation to the initial formation of a doubly or triply charged parent ion, and a subsequent Coulomb repulsion.

The molecular and dissociative photoionization of C2H6 has been studied using synchrotron radiation within the energy range 11-40 eV. Positive and negative photoion mass spectra and threshold photoelectron spectra have been obtained. These show that superexcited states, which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant and hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra.

A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in benzene as a result of valence shell photoionisation. Special emphasis has been placed on high energy reactions which lead to the formation of small energetic fragments. Synchrotron radiation has been used to record spectra in the photon energy range ∼14–38eV, and appearance energies have been determined for 20 fragment ions. The time-of-flight mass spectrometer has been designed such that the collection efficiency is independent of the fragment ion initial kinetic energy and mass. This feature has enabled reliable ion yield curves to be measured. Absolute photoionisation partial cross-sections for particular fragments have been obtained by combining the ion yield curves with the absolute photoionisation cross-section. The charge separation reaction: C6H62+→CH3++C5H3+ has been observed and the appearance energy has been measured as 27.8eV. Fragmentation rates have been determined by comparing the asymmetric time-of-flight peak shape of the C3H3+ fragment with the corresponding data obtained by ion trajectory modelling.

The unimolecular decomposition of internal-energy-selected furan molecular ions has been studied by means of threshold-photoelectron–photoion coincidence spectroscopy. Monochromatic synchrotron radiation was used as the ionisation source, and the molecular ion internal energy was established through the detection of a threshold electron. A pulsed electric field was applied to extract the ions from the interaction region and direct them towards a time-of-flight mass spectrometer. Breakdown curves were measured for photon energies up to 30 eV, and these have allowed appearance energies for a wide range of fragment ions to be determined. In the threshold region the breakdown curves have been measured for various ion residence times by introducing electronic delays between the detection of the threshold electron and the application of the ion extraction field. The breakdown curves have been modelled using the RRKM (Rice, Ramsperger, Kassel and Marcus)/QET (quasi-equilibrium theory) approach, and this has allowed activation energies and transition state geometries to be deduced. The threshold photoelectron spectra of furan-h4 and furan-d4 have been measured from the ionisation threshold to 28 eV, and vibrational structure has been observed and assigned in the bands due to the X2A2, the A2B1 and the G2A1 states. Vibrational progressions discernible between 16.2 and 17.3 eV have been attributed to autoionisation from a p-type Rydberg series converging onto the G2A1 state ionisation threshold.

The threshold photoelectron spectra and the photoionisation yield curves of SiF4, SiCl4 and SiBr4 have been recorded in the photon energy range 10–240 eV using synchrotron radiation. The outer valence shell photoelectron bands exhibit effects due to spin–orbit splitting and to Jahn–Teller interactions, and vibrational progressions associated with the Jahn–Teller active modes have been observed. The continuous nature of the inner valence shell photoelectron bands and the absence of main-lines demonstrates the importance of electron correlation in redistributing the intensity amongst satellite states. New Rydberg series have been observed in the photoionisation yield curves associated with excitation from the valence shell, and the improved resolution has allowed some earlier assignment discrepancies to be clarified. The threshold photoelectron spectra and ion yield curves recorded at higher energies display features attributable to the Si2p,2s, Cl2p and Br3p shells. The photoelectron spectra have enabled the following splittings to be measured: Si2p–2s, Si2p1/2–2p3/2, Cl2p1/2–2p3/2 and Br3p1/2–3p3/2, and the corresponding values are 51.21, 0.52, 1.62 and 4.6 eV, respectively. The interpretation of the features observed in the SiBr4 spectra has been based upon previously reported assignments of similar structure in SiF4.

The design, construction and performance of a threshold-photoelectron–photoion coincidence (TPEPICO) spectrometer for the study of unimolecular decomposition in polyatomic ions is described. The spectrometer incorporates a hemispherical electrostatic energy analyser and a time-of-flight (TOF) mass spectrometer. The entrance lens to the hemispherical analyser has been designed to have a high collection efficiency for low energy electrons but to discriminate strongly against energetic electrons. This arrangement has resulted in a resolution of about 3.5 meV being achieved for the threshold electron peak recorded at the krypton 2P3/2 ionisation limit. A pulsed electric field is used to extract the ions from the interaction region and propel them towards the TOF analyser. Computer modelling has been used to trace the electron and ion trajectories through their respective analysers. These simulations have enabled the effective interaction volume to be defined, and this has allowed the transmission efficiency of energetic fragment ions, formed through a process which also yielded a threshold electron, to be quantified. The ion TOF peak shape has been examined as a function of initial kinetic energy and as a function of ion residence time. The contribution of energetic fragments, having specific initial spatial and directional properties, to the TOF peak shape has been determined by tracing the paths of individual ions. The actual performance of the spectrometer is illustrated by a TPEPICO spectrum of the krypton isotopes. Experimental breakdown curves for furan are presented as an example of the use of the apparatus to study unimolecular decomposition in polyatomic ions. By introducing a delay between the detection of the threshold electron and the application of the ion extraction field, breakdown curves can be recorded as a function of ion residence time in the source region. The procedure for analysing the data is described, and the experimental factors that need to be taken into account to obtain a meaningful comparison with theoretical predictions are discussed.

The valence shell photoelectron spectrum of cyanogen chloride has been studied using HeI and synchrotron radiation. In the outer valence region the molecular orbital model of ionization holds, and the main bands can be associated with single-hole states. However, in the inner valence region electron correlation effects become important, and these result in complex satellite structure being observed. Vertical ionization energies and spectral intensities have been computed using the Green's function approach, and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios have been measured and have been used to assess the bonding characteristics of the outer valence molecular orbitals. The experimental data for the 8 sigma orbital display an energy dependence which suggests that photoionization from this orbital may be influenced by the chlorine 3p Cooper minimum. The extent to which the 8s orbital can be considered as a chlorine atom lone-pair is discussed. Vibrational structure has been observed in the (X) over tilde (2)Pi, (A) over tilde (2)Sigma (+) and (B) over tilde (2)Pi photoelectron bands recorded with HeI radiation, and has been assigned to progressions involving the nu (+)(1) and nu (+)(3) modes.

HeI excited photoelectron spectra of the boron trihalides have been recorded with improved resolution, and several new vibrational progressions have been observed in the spectra of BCl3, BBr3 and BI3. The spectra illustrate, as would be expected, that spin-orbit coupling becomes important in the heavier trihalides, and affects the photoelectron band structure strongly. Progressions involving excitation of the symmetric stretching vibrational mode are observed in all four molecules. Some of the new structure revealed in the photoelectron spectra of BCl3, BBr3 and BI3 is assigned, and the molecular orbital configurations in BBr3 and BI3 are discussed, taking into account existing theoretical predictions and the present experimental results.

The absolute photoabsorption cross-section and the photoionisation quantum efficiency of allene have been measured from the ionisation threshold to 35 eV using a double ion chamber and monochromated synchrotron radiation. New structure has been observed in the 10.5-14.6 eV range and attributed to autoionisation from states belonging to Rydberg series converging onto the (A) over tilde(2)E ionisation threshold. Two prominent broad absorption features located at 11.3 and 14.1 eV have been associated, tentatively, with valence-shell transitions into the 3e orbital. It is noticeable that at excitation energies coinciding with these two valence transitions the photoionisation quantum efficiency exhibits local minima. This indicates that predissociation plays a prominent role in the decay of these super-excited states. (C) 1999 Elsevier Science B.V. All rights reserved.

The absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of ammonia and deuterated ammonia have been measured from the ionization threshold to 25 eV using a double ion chamber and monochromated synchrotron radiation. The photoabsorption spectrum displays extensive vibrational progressions associated with Rydberg series converging onto excited vibrational levels of the (X) over tilde (2)A(2)" state. New structure has been observed for ND3 in the 10.0-11.3 eV range, and vibrational progressions due to transitions into the (E) over tilde, (F) over tilde and (G) over tilde Rydberg states have been recorded with improved resolution. Features have been observed, for the first time, in the photoabsorption spectra of NH3 and ND3 due to Rydberg series converging onto the (A) over tilde E-2 ionization threshold, and interpretations for some of these features have been proposed based upon the corresponding photoelectron spectra.The He I excited NH3+ (1a(2)")(-1) (X) over tilde (2)A(2)" photoelectron band has been studied experimentally at a resolution of 3 meV and two vibrational progressions, each involving excitation of the nu(2)(+) mode, have been observed. The vibrational lines in the main progression show a complex structure associated with rotational excitations. This structure changes gradually in a way that can be explained by the variation of the H-N-H bond angle with the nu(2)(+) mode. The effective bond angle has been found to be 120 degrees for nu(2)(+) = 0, and simiiar to that of the neutral ground state near nu(2)(+) = 6. The second progression, of weak lines, has been interpreted tentatively as being due to n nu(2)(+)+nu(4)(+). The nu(4)(+) mode is doubly degenerate and the excitation of a single quantum hits been explained by vibronic coupling with the (A) over tilde E-2 state. In addition, He II excitation has been used to record the. entire valence shell photoelectron spectrum.

Both calcite and vaterite polycrystalline phases of LuBO3 doped with cerium and praseodymium have been prepared and their fluorescence properties investigated in the visible, near UV and VUV regions. The systems exhibit a strong and rapid near UV-blue fluorescence typical of the 5d --> 4f parity-allowed transitions of dopants. It is shown that the excitation transfers from excitons and electron-hole pairs to dopants are very efficient. The light yield can be three times ore efficient than that of the well-known Bi4Ge3O12 scintillator for the best samples.