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.

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.

It is demonstrated theoretically in the random phase approximation (RPA) that due to the intershell many-electron correlations the sigma(*) shape resonance in the photoionization of K shells of the N2 molecule appears not only in the 1sigma(g)-->varepsilonsigma(u) channel as it was believed earlier on the basis of single particle calculations, but in both 1sigma(g)-->varepsilonsigma(u) and 1sigma(u)-->varepsilonsigma(g) channels. As a confirmation of this phenomenon we show that the experimental angular distributions of photoelectrons ejected from fixed-in-space N2 molecules can be reproduced theoretically only after taking into account many-electron correlations.

Understanding hydrogen-bonding networks in nanocrystals and microcrystals that are too small for X-ray diffractometry is a challenge. Although electron diffraction (ED) or electron 3D crystallography are applicable to determining the structures of such nanocrystals owing to their strong scattering power, these techniques still lead to ambiguities in the hydrogen atom positions and misassignments of atoms with similar atomic numbers such as carbon, nitrogen, and oxygen. Here, we propose a technique combining ED, solid-state NMR (SSNMR), and first-principles quantum calculations to overcome these limitations. The rotational ED method is first used to determine the positions of the non-hydrogen atoms, and SSNMR is then applied to ascertain the hydrogen atom positions and assign the carbon, nitrogen, and oxygen atoms via the NMR signals for 1 H, 13 C, 14 N, and 15 N with the aid of quantum computations. This approach elucidates the hydrogen-bonding networks in l -histidine and cimetidine form B whose structure was previously unknown.

The dynamical information (ten dipole matrix elements and eight phase differences) has been deduced from the measured angular distributions of photoelectrons from O K shell of oriented CO molecules near the ionization threshold in the region of a sigma(*) shape resonance. Light polarization parallel and perpendicular to the molecular axis has been used. An important contribution of six lsigma partial waves with 0

Angular distributions of photoelectrons (ADPs) from the 2σg shell of a fixed-in-space N2 molecule have been measured for left- and right-elliptically polarized, as well as for linearly polarized, light. From these data a set of dipole matrix elements and phase shift differences characterizing the process has been determined taking into account the acceptance angles of both electron and ion detectors, i.e. the complete experiment has been performed. Good agreement between the experimental and the relevant theoretical values calculated in the random phase approximation is obtained. Based on the results of the complete experiment, three-dimensional ADPs and ions are predicted for different light polarizations.

No abstract

Angular distributions of photoelectrons from a 2sigma(g) shell of fixed-in-space N2 molecules have been measured for left- and right-elliptically polarized and for linearly polarized light at several photon energies in the region of sigma(*) shape resonance. That allowed the determination of a set of dipole matrix elements and phase shift differences characterizing the process. These data clearly show the enhancement of the fsigma(u) partial cross section in the resonance simultaneously with an abrupt increase of the corresponding phase shift by pi, which is the first experimental demonstration of the nature of the sigma(*) shape resonance in homonuclear diatomic molecules.