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.

We have measured high-resolution angle-resolved ion-yield spectra of the O and N 1s excited N2O molecules (Nt–Nc–O) and investigated them with the help of ab initio quantum chemical calculations. The peak width of the Nc 1s→π* transition is larger than that of the Nt 1s→π* one. This mainly arises from different populations of bending vibrations excited through the Renner–Teller effect, which breaks the degeneracy of the 1s→π* excited states by bending the linear molecule. The angular distribution of fragment ions emitted after the Auger decay is also affected by the Renner–Teller effect. That is, fragment ions significantly lose the information of the Π symmetry of the linear molecule. For the 1s→Rydberg excitations no Renner–Teller effect is observed and the angular distribution is directly related to the Σ and Π symmetries. Furthermore, it is found that the peak intensities of the O and Nt 1s→nsσ Rydberg transitions are unusually large. The N2O molecule has two σ* orbitals, σs*(8σ) and σp*(9σ), below and above the ionization threshold and the 1s→σs* excited states are mixed with the O and Nt 1s–nsσ Rydberg excited states; on the other hand, the Nc 1s→σs* transition is almost dipole-forbidden because of the orbital symmetry.

This paper reviews our recent progress on the angle-resolved photoion-yield spectroscopic (ARPIS) technique applied to the K-shell excitations of the simple linear molecules, N2, CO, C2H2, O2, CO2, N2O, OCS and CS2. This spectroscopic technique enables us to distinguish symmetries of electronic states; e.g., the ΔΛ = 0 parallel and ΔΛ = ±1 perpendicular transitions for the diatomic molecules can be distinguishable. We call this technique symmetry-resolved inner-shell excitation spectroscopy. The complete symmetry resolution is rationalized by the axial-recoil fragmentation. Moreover, the angular distribution of fragment ions is directly related to the molecular orientation upon the photoabsorption and the vibrations; therefore, the ARPIS measurement can be used to reveal complicated vibronic couplings in polyatomic molecules in terms of anisotropy in fragmentation. We have studied Rydberg-valence mixings, vibronically induced 1s → Rydberg excited states, and strong bending-mode couplings in Renner–Teller split 1s →π* states for the linear molecules through the ARPIS spectra.

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 → εsσ and in the O 1sσ → εdσ transitions.

A highresolution xray fluorescence spectrometer for nearedge absorption studies Rev.High-resolution electron-yield and "symmetry-resolved" ion-yield spectra have been measured at the Nand 0 K edges of a 2n open-shell molecule, NO. Several Rydberg transitions converging to the 3n and 1 n ionization thresholds are found. The core-to-valence and core-to-Rydberg transitions are interpreted with the help of ab initio self-consistent-field configuration-interaction and frozen-core calculations for the core-excited states with three open shells. It is found that the equivalent-core model (N*0=02) breaks down in discussing the state ordering of the three N Is-2p1T* excited states, 2 1::., 2~ -, and 2~ +. It is important to consider explicitly the core hole and the exchange repulsion between the core and 'IT* electrons.

Fragment ions energetically emitted following the perpendicular (ΔΛ=+1) transitions of C 1s→π* of CS2, OCS, and CO2 are observed not only in the perpendicular (90°) direction but also in the parallel (0°) to the linear polarization; that is, ions have a momentum orthogonal to the linear molecule. This arises in the Renner–Teller (RT) vibronic coupling of bending vibrations in the C 1s→ in-plane π* excited state with a bent equilibrium geometry, though the RT splitting between the C 1s→ out-of-plane π* state with a linear geometry and the C 1s→ in-plane π* state is not visible directly due to the lifetime broadening. The 0° ion yield is relatively small in CS2 but is comparable to the 90° yield in CO2; in the latter the peak maximum at 0° is 0.06 eV lower than at 90° and the anisotropy parameter β is heavily dependent on the photon energy. In CO2 a great number of unresolved bending vibrations are coupled. The half-width at half-maximum on the lower energy side of the π* peak is much more sensitive to the RT splitting; 0.08, 0.11, and 0.29 eV for CS2, OCS, and CO2, compared with the full-width at half-maximum, 0.17, 0.39, and 0.64 eV. In CS2 the π* peak and β value are sharp and symmetric, indicating that the zero-point vibrational levels are only involved. In OCS three fine structures observed with separations of 0.21 eV are assigned to the ν3 mode, which is comparable to the stretching mode in CO.

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Measurements and calculations of the contribution of the non-dipole terms in the angular distribution of photoelectrons from the K-shell of randomly oriented N2 molecules are reported. The angular distributions have been measured in the plane containing the photon polarization and the photon momentum vectors of linearly polarized radiation. Calculations have been performed in the relaxed core Hartree–Fock approximation with a fractional charge, and many-electron correlations were taken into account in the random phase approximation. Both theory and experiment show that the non-dipole effects are rather small in the photon energy region from the ionization threshold of the K-shell up to about 70 eV above it. From the theory, it follows that the non-dipole terms for the individual 1σg and 1σu shells are considerably large; therefore measurements resolving the contributions of the 1σg and 1σu shells are desirable.