M.A. Hayes

A study of the absolute photoabsorption cross section and the photoionization quantum efficiency of nitrous oxide from the ionization threshold to 480 Å

Abstract The absolute photoabsorption cross section and the photoionization quantum efficiency of nitrogen have been measured using a double ion chamber and monochromated synchrotron radiation. In the wavelength range encompassed by the present study, several well-established Rydberg series give rise to prominent structure in the absorption spectrum. A molecule excited into a Rydberg state may decay either by autoionization or by predissociation into neutral products, and this competition between de-excitation processes causes variations to be observed in the photoionization quantum efficiency. The results indicate that many of the Rydberg states undergo rapid predissociation. A sum rule analysis has been carried out by combining the present absolute photoabsorption cross section with similar data covering the remaining wavelength regions.

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 and theoretical study of the valence shell photoelectron spectrum of carbon disulphide

The complete valence shell photoelectron spectrum of carbon disulphide has been studied using HeI, HeII and synchrotron radiation. In addition to the photoelectron bands associated with the X 2Πg, A 2Πu, B 2∑u+ and C 2∑g+ single-hole ionic states, several satellite features have been observed which are due to multielectron processes. The high resolution HeI and HeII excited spectra have allowed a detailed analysis to be made of the vibrational structure exhibited in the photoelectron bands corresponding to the single-hole and satellite states. The photoelectron spectra recorded with synchrotron radiation demonstrate that the inner valence region contains much complicated structure, and that distinct features are discernible up to a binding energy of at least 34 eV. Many-body Greens function calculations have been performed to evaluate the ionization energies and pole strengths associated with the main lines and satellite lines distributed throughout the valence shell region, and an interpretation of some of the experimental features is proposed, based upon these predictions.

An experimental and theoretical study of the valence shell photoelectron spectrum of butadiene

The valence shell photoelectron spectrum of butadiene has been studied using He I, He II 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 function method have been employed to evaluate the ionization 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 measured using synchrotron radiation in the 12 - 120 eV range. Vibrational structure has been observed in several of the photoelectron bands excited with He I radiation and analyses have been performed for the , and ionic states.

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.

A study of the unimolecular decomposition of internal-energy-selected furan molecular ions by threshold-photoelectron-photoion coincidence spectroscopy

Abstract 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-h 4 and furan-d 4 have been measured from the ionisation threshold to 28 eV, and vibrational structure has been observed and assigned in the bands due to the X 2 A 2 , the A 2 B 1 and the G 2 A 1 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 G 2 A 1 state ionisation threshold.

An experimental and theoretical study of the spectroscopic and thermodynamic properties of toluene

Abstract Three experimental techniques – photoabsorption, photoelectron and photoion spectroscopy – have been combined with many-body Greens function calculations to investigate the spectroscopic and thermodynamic properties of toluene. The absolute photoabsorption cross section has been measured from the ionisation threshold to 350 A using a double ion chamber and monochromated synchrotron radiation. Some of the structure has been arranged into Rydberg series. He I excited photoelectron spectra of toluene-h8 and toluene-d8 have been recorded and vibrational progressions have been observed in three bands. Fragmentation processes have been studied by measuring time-of-flight spectra and appearance energies have been determined for many small fragments, and the doubly-charged parent ion. The many-body Greens function approach, specially adapted for the outer valence region, has been used to calculate ionisation energies and pole strengths.

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.

A photoelectron study of the inner valence molecular orbitals of N2O

Abstract Photoelectron spectra are reported for the inner and outer valence orbitals of carbonyl sulphide for several photon energies up to 110 eV. Photoionization of the inner valence 6σ and 7σ molecular orbitals results in the observation of a complicated structure extending to a binding energy of approximately 40 eV. In the inner valence energy region electron correlation leads to an extensive redistribution of intensity associated with single-hole configurations into multiple electron transition states. Some of the features can be interpreted with the aid of theoretical predictions. The experimental data are compared with a spectrum recorded using X-ray excitation, and a strong energy dependence is observed for one of the peaks.