N. A. Cherepkov

Ultrafast Probing of Core Hole Localization in N2

Although valence electrons are clearly delocalized in molecular bonding frameworks, chemists and physicists have long debated the question of whether the core vacancy created in a homonuclear diatomic molecule by absorption of a single x-ray photon is localized on one atom or delocalized over both. We have been able to clarify this question with an experiment that uses Auger electron angular emission patterns from molecular nitrogen after inner-shell ionization as an ultrafast probe of hole localization. The experiment, along with the accompanying theory, shows that observation of symmetry breaking (localization) or preservation (delocalization) depends on how the quantum entangled Bell state created by Auger decay is detected by the measurement.

Interference effects in photoionization of noble gas atoms outer s-subshells

A strong influence of ns2np6 → ns2np5 ϵd transition on the outer ns subshell photoionization cross section of noble gas atomsis demonstrated.

Angular distribution of photoelectrons with many-electron correlations

Abstract The angular distribution of electrons in the noble gas photoionization is calculated with many-electron correlations taken into account in the random phase approximation with exchange.

K-shell photoionization of CO: II. Determination of dipole matrix elements and phase differences

Using the experimental angular distributions of photoelectrons from the K-shells of an oriented CO molecule reported in a companion paper, we have performed a so-called complete experiment and determined 18 dynamical parameters (ten moduli of transition moments and eight phase differences) for the O K-shell, and 16 dynamical parameters (nine moduli of transition moments and seven phase differences) for the C K-shell, and compared them with the results of our calculations in the relaxed-core Hartree-Fock (RCHF) approximation. The agreement between theory and experiment is only qualitative, therefore the model has to be improved by including electron correlations. From the analysis of experimental data we proved that the σ* shape resonance is due to not only the f-wave, as was widely believed earlier, but is due to approximately equal contributions of three partial waves with 1≤l≤3 for the C K-shell, and four partial waves with 0≤l≤3 for the O K-shell, with a rather substantial contribution of other partial waves with l≤5. From the analysis of the transition moments determined from the experiment it follows that several Cooper minima are likely to exist in partial photoionization cross sections, in particular, in the C 1sσ→esσ and in the O 1sσ→edσ transitions.

K-shell photoionization of CO: I. Angular distributions of photoelectrons from fixed-in-space molecules

Angular distributions of photoelectrons from both C and O K-shells of the fixed-in-space CO molecule have been measured using the angle-resolved photoelectron-photoion coincidence technique. The measurements have been performed at several photon energies from the ionization thresholds up to about 30 eV above them, where the σ* shape resonances occur. Experimental results are compared with the multiple-scattering calculations of Dill et al (1976 J. Chem. Phys. 65 3158) and with our new calculations in the relaxed-core Hartree-Fock approximation. Our calculations are in a better agreement with the experimental data though numerical discrepancies remain. The experimental angular distributions are fitted by the expansion in Legendre polynomials containing up to ten terms and the extracted parameters are compared with the corresponding theoretical values.

Relative intensities in x-ray photoelectron spectra. Part VI. The spectra of He(I), He(II), Y Mζ and Zr Mζ☆

The 2s- and 2p-electron photoionization cross-sections at photon energies up to 190 eV have been calculated, using the RPAE method for averaged configurations of the C, N, O and Ne atoms. The RPAE method ensures a more accurate relation between the cross-sections, 2s/2p, than that obtained using the Hartree—Fock method. Within the framework of the Gelius—Siegbahn model, but with the use of theoretical atomic cross-sections, we have calculated the photoionization cross-sections for He(I), He(II), Y Mζ, Zr Mζ for CH4, C2H6, C3H8, C2H4, C2H2, NH3, H2O, CN−, N2, CO, CO2, N2O and NO2− molecules. For CO, N2, CO2, N2O and H2O molecules, a comparison is made between the theoretical and experimental cross-sections for hν < 60 eV. The calculated absolute and relative values of the molecular-orbital cross-sections are in reasonable agreement with experiment, especially at hν ⩾ 40 eV. The calculations correctly reproduce the change in intensities under the transition He(I) → He(II). We have shown that our calculations have a significant advantage over those performed using the PW and OPW approximations. It is shown for NO, N2, CO, H2O, CH4, NH3 and N2O molecules that the total photoionization cross-section calculated taking into account the real structure of the molecular orbitals is in better agreement with the experimental photoabsorption cross-section than is the sum of the cross-sections for the atoms in a molecule.

Localization of inner-shell photoelectron emission and interatomic Coulombic decay in Ne2

We used cold target recoil ion momentum spectroscopy (COLTRIMS) to investigate the decay of Ne2 after K-shell photoionization. The breakup into Ne1+/Ne2+ shows interatomic Coulombic decay (ICD) occurring after a preceding atomic Auger decay. The molecular frame angular distributions of the photoelectron and the ICD electron show distinct, asymmetric features, which imply localization of the K-vacancy created at one of the two atomic sites of the Ne2 and an emission of the ICD electron from a localized site. The experimental results are supported by calculations in the frozen core Hartree–Fock approach.

Hartree-Fock deformed jellium model for metallicclusters

We have developed the Hartree-Fock jellium model for deformed metal clusters, which treats the quantized electron motion in the field of the spheroidal ionic jellium background in the Hartree-Fock approximation. Using this model, we have calculated single electron energy levels as a function of the cluster deformation parameter for a series of sodium clusters with the number of atoms N in a cluster ranging from 4 to 40. We have established that the cluster deformations corresponding to the minimum total energy of the oblate and prolate clusters are in a reasonable agreement with the experimental data and predictions of other theoretical models.

State-dependent gerade/ungerade intensity ratios in the Auger spectrum of N2

We have applied both high-resolution photoelectron and Auger spectroscopy to study the N 1s core ionization of N2. From the respective spectra we are able to show that the Auger decay involves delocalized description of core-hole states. Specifically, the Auger spectra of the N 1sσg,u core holes to the quasi-stable final states X1Σ+g and D1Σ+u are presented. They exhibit the gerade/ungerade splitting as well as vibrational progressions. The 1sσg/1sσu Auger intensity ratios are, in agreement with ab initio calculations, 2.1 and 0.8 for the X and the D states respectively. The large value for the X state can qualitatively be related to the gerade or ungerade symmetry of the Auger electron.

Photoelectron and ICD electron angular distributions from fixed-in-space neon dimers

We report on molecular frame angular distributions of 2s photoelectrons and electrons emitted by interatomic Coulombic decay from neon dimers. We found that the measured angular distribution of the photoelectron strongly depends on the environment of the cluster. The experimental results are in excellent agreement with frozen core Hartree–Fock calculations. The ICD electrons show slight variations in their angular distribution for different kinetic energies.