M.A. MacDonald

Double photoionization of CO2, OCS, C2H2, CF4, and C6H6 studied by threshold photoelectrons coincidence (TPEsCO) spectroscopy

Abstract Double photoionization of CO 2 , OCS, C 2 H 2 , CF 4 and C 6 H 6 molecules has been studied in an electron-electron coincidence experiments, where nearly zero kinetic energy (⩽20 meV) electrons are detected. This first application of TPEsCO spectroscopy to polyatomic molecules confirms the ability of this experimental technique to determine accurate double ionization potentials and locate excited electronic states. The spectrum of benzene reveals what appears to be the first observation of vibrational structure in a polyatomic dication.

A study of the absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of oxygen from the ionization threshold to 490 Å

Abstract The absolute photoabsorption, photoionisation and photodissociation cross sections and the photoionisation quantum efficiency of sulphur dioxide have been measured using a double ion chamber and monochromated synchrotron radiation. The absorption spectrum exhibits extensive vibrational structure extending from the ionisation threshold to approximately 750 A. Many of the features can be arranged into vibrational progressions with spacings characteristic of the symmetrical stretching mode, in agreement with previous interpretations. The photoionisation quantum efficiency increases rapidly to reach a value of approximately unity around 880 A, but at shorter wavelengths local minima are observed around 760, 820 and 870 A. Some of the variations in the photoionisation quantum efficiency appear to be correlated with the absorption vibrational structure. A sum rule analysis has been carried out by combining the present absolute photoabsorption measurements with similar data covering the remaining wavelength regions.

A study of the spectroscopic and thermodynamic properties of furan by means of photoabsorption, photoelectron and photoion spectroscopy

Abstract Three experimental techniques (photoabsorption, photoelectron, and photoion spectroscopy) have been used to study the spectroscopic and thermodynamic properties of furan. The absolute photoabsorption cross-sections of furan-h4 and furan-d4 have been measured using a double ion chamber and a new Rydberg series converging onto the G 2 A 1 ionisation threshold has been observed. HeI-excited photoelectron spectra of the X 2 A 2 , the A 2 B 1 and the G 2 A 1 states of furan-d4 have been recorded. Vibrational structure has been observed in all three bands and has allowed the energies of the ν3, ν4, ν6 and ν8 vibrational modes to be determined. Time-of-flight mass spectra have been recorded using monochromatic synchrotron radiation, and appearance energies have been measured for 19 fragment ions and the doubly charged parent ion. The fragmentation processes leading to the production of several high-intensity fragment ions have been modelled using ab initio and semi-empirical methods.

AN EXPERIMENTAL STUDY OF THE VALENCE SHELL PHOTOELECTRON SPECTRUM OF HYDROGEN SULPHIDE

Abstract The complete valence shell photoelectron spectrum of hydrogen sulphide has been studied in the 10 to 25 eV binding energy region using He I, He II and synchroton radiation. Using monochromated synchroton radiation, photoelectron angular distributions and branching ratios have been determined in the 14 to 120 eV photon energy range. The results suggest that most of the prominent inner valence features should be associated with ionisation from the 4a 1 molecular orbital. High resolution spectra have been obtained over a similar binding energy range using He I and He II excitation. Several narrow photoelectron bands have been observed in the region between 22.5 and 24.5 eV, and are associated with vibrational progressions belonging to two different electronic states. The He I excited spectrum has allowed a detailed analysis to be made of the vibrational and rotational fine structure exhibited in the photoelectron bands associated with ionisation from the 2b 1 and the 5a 1 orbitals. For the A 2 A 1 state, comparisons are made with the rovibronic structure predicted by Duxbury et al.

An experimental study of the valence shell photoelectron spectrum of the NO2 molecule

Abstract The complete valence shell photoelectron spectrum of the NO 2 molecule has been studied in the binding energy range between 10 and 50 eV. HeI and HeII resonance radiation as well as monochromated synchrotron radiation have been used for the ionisation. Assignments have been made for most of the photoelectron bands by comparison with previous theoretical predictions. Photoelectron angular distributions and branching ratios for the bands associated with the outer valence shell have been determined for photon energies between 18 and 120 eV. The HeI and HeII excited spectra have allowed detailed studies to be made of the vibrational fine structure. For the X 1 Σ + g ground ionic state, close lying vibrational lines associated with combined excitations of the symmetric ν 1 and the bending ν 2 modes have been observed for the first time. Excitation of the asymmetric stretch ν 3 mode, in one quantum, is found in the A 1 A 2 photoelectron band, and its presence is explained in terms of vibronic coupling.

Photoionization of acetylene near the threshold

Abstract The photoionization of C 2 H 2 has been studied by threshold photoelectron (TPE) spectroscopy and constant photoelectron energy (CPE) spectroscopy in the energy region between 11.3 and 20eV. TPE spectroscopy has enabled a detailed observation of the first three electronic states of the ion as well as the non-Franck-Condon region. Further investigation by CPE spectroscopy at photoelectron energies between 0.2 and 3.7eV has established that the main process occurring in this region is the autoionization of superexcited states of C 2 H 2 . A dynamic effect in the autoionization process has been proposed to explain the excitation of a quasicontinuum of vibrational levels of the ionic ground state in the region of 13eV.

The influence of non-adiabatic effects on the outer valence shell photoionisation dynamics of boron trifluoride

Abstract The entire valence shell photoelectron spectrum of boron trifluoride has been studied using synchrotron radiation in the photon energy range 20–120 eV. Photoelectron angular distributions and branching ratios for the six outer valence orbitals have been measured and compared with theoretical predictions. The strongest evidence for the e′ shape resonance affecting the valence shell photoionisation dynamics appears in the asymmetry parameter associated with the 4a 1 ′ orbital. Strong vibronic coupling effects are observed in the vibrational structure exhibited by the E 2 A 1 ′ state and asymmetry parameters are reported for several vibrational components. The data demonstrate that the asymmetry parameters associated with the photoelectron peaks induced through vibronic coupling are characteristic of the ionic state through which they derive their intensity. Complex structure, associated with ionisation from the 2e′ and 3a 1 ′ inner valence orbitals, is observed in the photoelectron spectrum for binding energies between 26 and 50 eV. This structure is compared with pole strengths calculated using the many-body Greens function approach.

THRESHOLD PHOTOELECTRON SPECTROSCOPY OF HCL UP TO 40 EV

The threshold photoelectron spectrum of HCl has been recorded using synchrotron radiation over the energy range encompassing both outer and inner valence ionization. A large enhancement in the intensity of the v+ = 0–1 bands of the HCl+ 2∏32 subsystem was observed in comparison with the same vibrational bands of the 2∏12 subsystem which are attributed to autoionization processes. Resonance autoionization was also found to be responsible for the populating of upper vibrational levels (v+ = 2–14) of the X 2∏i system in the Franck-Condon gap region preceding the onset of the A 2∑+ band system. The observation of vibrational-band broadening for v+ = 7–10 of the A 2∑+ system is attributed to combined autoionization processes between repulsive neutral Rydberg states and bound vibrational levels of the A 2∑+ state and the continuum of repulsive ion states, the latter of which are responsible for predissociation in the A 2∑+ state of HCl+. In the inner valence ionization region (20.0–40.0 eV), pronounced resonance structure was observed which correlates well with the calculated energy positions of 4σ−1 satellite ions states of HCl. In addition, the direct populating of repulsive ion states in the 20.0–25.2 eV range is suggested by the observation of several broad, structureless band features, and the detection of three vibrational bands in this same energy range suggests the presence of a bound neutral Rydberg state that undergoes autoionization into the continuum of a repulsive ion state.

Energy-dependent valence photoelectron spectra of SF6. Ab initio calculations and measurements

Abstract Ab initio calculations of branching ratios and cross-sections for inner- and outer-valence levels of SF 6 have been carried out by means of the static exchange (STEX) approximation including full intrachannel interaction. The calculations cover an energy region up to 90xa0eV for the outer and 130xa0eV for the inner valence molecular levels and the results are compared with experimental data. It is shown that the molecular orbital static exchange method successfully reproduces the strong multiple resonances in the near continuum. The resonances and the energy dependence of the photoionization cross-sections have been used to settle some assignment problems in the valence photoelectron spectrum of SF 6 . Photoelectron spectra associated with the 2e g , 3t 1u and 4a 1g inner valence orbitals show significant variations in the electronic state branching ratios at electron kinetic energies around 60xa0eV. These features are well reproduced by the present calculations.

A study of resonant behaviour in the photoelectron band of sulphur hexafluoride

Angle-resolved photoelectron spectra encompassing the and the states of sulphur hexafluoride have been recorded in the 20 - 22 eV photon energy range using synchrotron radiation. The envelope of the photoelectron band displays a complicated structure which varies significantly both as a function of excitation energy and as a function of electron ejection angle. The data for the state have been analysed by dividing the band into eight regions, specified by their binding energies. The results show that, as the photon energy is varied, the branching ratio for each region exhibits an enhancement at an electron kinetic energy of about 2.5 eV. The photoelectron angular distributions also vary significantly across the band. Various interpretations for this resonant behaviour are discussed.