Main Photoelectron Lines Research Articles

Energy-Dependent Relative Cross Sections in Carbon 1s Photoionization: Separation of Direct Shake and Inelastic Scattering Effects in Single Molecules.

We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range allows us to study and disentangle shake processes and intramolecular inelastic scattering effects. In this gas-phase study, relative intensities of the carbon 1s photoelectron lines from chemically inequivalent carbon atoms in the same molecule have been measured as a function of the incident photon energy in the range of 300-6000 eV. We present relative cross sections for the chemically shifted carbon 1s lines in the photoelectron spectra of ethyl trifluoroacetate (the "ESCA" molecule). The results are compared with those of methyl trifluoroacetate and S-ethyl trifluorothioacetate as well as a series of chloro-substituted ethanes and 2-butyne. In the soft X-ray energy range, the cross sections show an extended X-ray absorption fine structure type of wiggles, as was previously observed for a series of chloroethanes. The oscillations are damped in the hard X-ray energy range, but deviations of cross-section ratios from stoichiometry persist, even at high energies. The current findings are supported by theoretical calculations based on a multiple scattering model. The use of soft and tender X-rays provides a more complete picture of the dominant processes accompanying photoionization. Such processes reduce the main photoelectron line intensities by 20-60%. Using both energy ranges enabled us to discern the process of intramolecular inelastic scattering of the outgoing electron, whose significance is otherwise difficult to assess for isolated molecules. This effect relates to the notion of the inelastic mean free path commonly used in photoemission studies of clusters and condensed matter.

Photoelectron angular distribution in two-pathway ionization of neon with femtosecond XUV pulses

We analyze the photoelectron angular distribution in two-pathway interference between non\-resonant one-photon and resonant two-photon ionization of neon. We consider a bichromatic femtosecond XUV pulse whose fundamental frequency is tuned near the $2p^5 3s$ atomic states of neon. The time-dependent Schr\"odinger equation is solved and the results are employed to compute the angular distribution and the associated anisotropy parameters at the main photoelectron line. We also employ a time-dependent perturbative approach, which allows obtaining information on the process for a large range of pulse parameters, including the steady-state case of continuous radiation, i.e., an infinitely long pulse. The results from the two methods are in relatively good agreement over the domain of applicability of perturbation theory.

X-Ray photo- and resonant Auger-electron spectroscopy studies of liquid water and aqueous solutions

It has only recently become possible to use photoelectron spectroscopy (PES) to study the electronic structure of highly volatile aqueous surfaces. Here, we review current X-ray PES and related resonant Auger-electron decay and intermolecular Coulomb decay investigations in solution, which aim at understanding the solute–water, water–water, and solute–solute interactions at the microscopic level. Systems that will be discussed include neat liquid water, and aqueous solutions of hydroxide, hydronium, salts and amino acids. From observed chemical shifts in the main photoelectron lines or from the occurrence of spectator Auger-electron peaks, information on electron dynamics and/or energy transfer processes, and at times on the structure of the hydration complex, may be accessed. Furthermore, we discuss PES in conjunction with variable energy incident X-ray photons; with the resulting change of the (photo)electron kinetic energy, ion spatial distributions at the liquid–vapor interface can be determined.

Auger rates of second-row atoms calculated by many-body perturbation theory

Many-body perturbation theory is applied to calculate the KLL Auger rates of all terms of atoms C, N, F and Ne. Theoretical rates are compared with experiment and other theories for Ne and O. A new set of the Auger satellites, corresponding to the main photoelectron line binding energy is proposed. The intensities of these satellites are calculated and some of these satellites are identified in experimental KLL Auger spectra of Ne.

ANALYSIS OF THE ATOMIC Fe3d AND 4s PARTIAL CROSS SECTIONS IN THE REGION OF THE 3p → 3d GIANT RESONANCE

Using the technique of photoelectron spectrometry in conjunction with synchrotron radiation, we have carried out a systematic study of the partial cross sections of the main photoelectron lines arising from the ionization of the 3d and 4s subshells of atomic Fe in the vicinity of 3p → 3d autoionizing resonances. Our results confirm the presence of two broad and intense resonances which have been observed earlier. However, our high resolution results also clearly indicate the presence of numerous other weaker resonances. Through a fitting procedure we were able to accurately determine both resonance widths and positions.

The influence of core hole relaxation on the main-line intensities in X-ray photoelectron spectra

The main photoelectron line spectroscopic factors that are currently used in photoelectron spectroscopy for quantitative analysis are calculated for a broad set of atoms. General trends of the behaviour of the spectroscopic factors of core atomic levels are discussed. The influence of the spectroscopic factors on the main line intensities and on concentrations of the elements, that are obtained using X-ray photoelectron spectra, is analysed.

Photon energy dependence of the high resolution C 1s photoelectron spectrum of CO in the threshold region.

The C 1s photoelectron spectrum of CO has been measured with high resolution in the threshold region. The data exhibit hitherto unresolved vibrational structure on the C 1s main photoelectron line as well as new features in the satellite region. The vibrational energy of the ${\mathrm{CO}}^{+}$ ion is 301\ifmmode\pm\else\textpm\fi{}3 meV. The vibrational structure changes dramatically with photon energy due to the influence of the ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ shape resonance. The new satellite lines which are not present at high photon energies are attributed to conjugate shakeup processes.

Krypton 4p, 4s, and 3d partial photoionization cross sections below a photon energy of 260 eV.

The photoionization of krypton 4p, 4s, and 3d subshells has been studied theoretically from the 4p threshold and experimentally from the 3d threshold up to a photon energy of 260 eV. In converting the measured count rates of the 4p, 4s, and 3d main photoelectron lines into partial cross sections, we have used recent photoion yield measurements to estimate the contribution of multiple-excitation processes to the total photoabsorption cross section. Theoretical partial cross sections and angular asymmetry parameters have been calculated using the multichannel Dirac-Fock method. Eighteen leading single-excitation channels related to the 4p, 4s, 3d, and 3p hole configurations of the final ionic state were included. Relaxation was taken into account by using separately optimized initial- and final-state orbitals and by including all the resulting overlap integrals in the transition amplitudes. The relaxation effect was found to depend strongly on the photon energy in the near-threshold regions. For the 3d subshell the relaxation reduces the 3d cross section by 35--40 % at the maximum, whereas it is less significant for the 4p and 4s cross sections.

Photoionization of atomic Ca in the 26-120-eV photon-energy range.

We present detailed experimental results for the resonance structure of atomic Ca in the region of the 3p threshold, from 26 to 40 eV, as manifested in the partial cross sections of the 4${s}^{\mathrm{\ensuremath{-}}1}$ and 3${p}^{\mathrm{\ensuremath{-}}1}$ main photoelectron lines and several of their satellites, including satellite--to--main-line branching ratios. We also present partial photoionization cross sections for the 4${s}^{\mathrm{\ensuremath{-}}1}$, 3${p}^{\mathrm{\ensuremath{-}}1}$, and 3${s}^{\mathrm{\ensuremath{-}}1}$ main lines up to 120 eV photon energy. Finally, we give a qualitative, theoretical discussion of the resonance structure and decay mechanisms, and compare the measured 4s, 3p, and 3s photoionization cross sections with theoretical results within the framework of random-phase approximation and many-body perturbation theory.

Satellite structure in the argon 2p X-ray photoelectron spectrum

The argon 2p photoelectron spectrum has been measured using Al K alpha radiation. A complex satellite spectrum within 35 eV of the main photoelectron lines is reported with an intensity of 9.6% relative to the main 2p lines. The spectrum has been interpreted with the aid of a theoretical study using correlated final-ion-state wavefunctions calculated with a configuration interaction technique. The major contributions to the satellite spectrum come from 'shake-up'-type processes which have their origin in relaxation effects and involve 3p to np excitations.