D.M.P. Holland

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.

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.

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.

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 photoabsorption, photodissociation and photoelectron spectroscopy study of NH3 and ND3

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 2A2?? state. New structure has been observed for ND3 in the 10.0-11.3 eV range, and vibrational progressions due to transitions into the , and 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 ? 2E ionization threshold, and interpretations for some of these features have been proposed based upon the corresponding photoelectron spectra. The He I excited NH3+(1a2??)-1 2A2?? photoelectron band has been studied experimentally at a resolution of 3 meV and two vibrational progressions, each involving excitation of the 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 2+ mode. The effective bond angle has been found to be 120? for v2+ = 0, and similar to that of the neutral ground state near v2+ = 6. The second progression, of weak lines, has been interpreted tentatively as being due to n2++4+. The 4+ mode is doubly degenerate and the excitation of a single quantum has been explained by vibronic coupling with the ? 2E state. In addition, He II excitation has been used to record the entire valence shell photoelectron spectrum.

The identification of the outer valence shell π-photoelectron bands in furan, pyrrole and thiophene

Abstract The valence shell photoelectron spectra of furan, pyrrole and thiophene have been studied using synchrotron radiation and particular attention has been paid to the bands associated with the π orbitals. In each of these molecules three doubly occupied π-type molecular orbitals (denoted π 1 , π 2 and π 3 ) are formed, and previous work has established that π 2 and π 3 constitute the two outermost orbitals in all three molecules. However, the location of the most tightly bound π 1 orbital within the electronic configuration remains uncertain. In the present work the photoelectron bands due to the π orbitals have been investigated through angular distribution measurements. Photoelectron asymmetry parameters and branching ratios have been determined using monochromated synchrotron radiation in the photon energy range 15–120 eV. The spectral behaviour of asymmetry parameters associated with π orbitals is expected to differ from that of asymmetry parameters associated with σ orbitals. These differences provide an experimental means of distinguishing between the two types of orbitals. The many-body Green’s function method has been employed to evaluate the ionisation energies and pole strengths of all valence states. These theoretical predictions have proved equally important in identifying the π 1 photoelectron bands because they indicate whether ionisation from a particular orbital should give rise to a main-line, or whether the single-particle model has broken down. Interpretations have been proposed for most of the features observed in the inner valence region.

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 allene

Abstract The photoelectron spectrum of allene has been studied using He I, He II and synchrotron radiation as sources for ionisation. In addition to the main bands associated with the single hole states, a complex satellite structure due to many-electron effects has been observed in both the outer and inner valence regions. Two variations of the many-body Greens function technique have been employed to determine the ionisation energies and pole strengths for all valence states, and the results have facilitated the interpretation of the experimental spectra. Photoelectron angular distributions and photoinisation branching ratios have been measured using monochromated synchrotron radiation in the 15–120 eV range. He I and He II excitation have been used to obtain highly resolved spectra, and vibrational structure has been studied in the X 2E, A 2E and B 2A1 ionic states. Excitations of Jahn-Teller active modes have been found to be important in the X 2E and the A 2E states.

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.