Shape resonances in molecular photoionization are shown to induce strong coupling between vibrational and electronic motion over a spectral range several times broader than the resonances half-width. This coupling is manifested by large deviations from Franck-Condon intensity distributions and strong dependence of photoelectron angular distributions on the vibrational state of the residual ion. These effects are illustrated for the 3a g photoionization channel of N 2 .

The angular distribution of Auger electrons emitted in the decay of molecular iC-shell vacancies created by photoabsorption is predicted to be a direct probe of the anisotropy of molecular photoabsorption. The cr -*-n discrete absorption and the cr -*• cr /-wave shape resonance in N 2 and CO are given as examples.PACS numbers: 33.80.Eh Atomic #-shell vacancies are isotropic and hence electrons emitted by Auger decay of such vacancies have an isotropic angular distribution. 1 " 4 Molecules are different, however, for two reasons. First, molecular photoabsorption is highly anisotropic, both in the discrete, because excited states have definite symmetries but generally different energies, and in the continuum, where shape resonances can enhance photoabsorption along characteristic directions of the molecule. 5 " 9 As a result, molecular photoabsorption produces neutral-molecular excited states with definite, characteristic orientations and shape-resonant photoionization produces molecular ions with energy-dependent orientations characteristic of each resonance. Second, subsequent decay of the K-shell vacancy is a fast process compared with molecular rotation, so that the orientation of the molecular excited state/ ion will be reflected in the angular distribution of the emitted Auger electrons. For example, in , CO and N 2 there is a very intense discrete absorption of 7r symmetry about 1 Ry below the K-shell ionization threshold and a broad a-symmetry shape resonance centered about 1 Ry above threshold. 6 ' 7 ' 10 The photoabsorption to the 77 excited states preferentially creates excited molecules perpendicular to the electric vector of the light while photoionization within the a shape resonance yields molecular ions preferentially parallel. The angular distribution of a particular Auger electron will mirror these alternative orientations as one scans through the photoabsorption spectrum, Thus, molecular Auger-electron angular distributions, as a function of photoexcitation energy, provide us with a spectroscopy of the anisotropy of molecular photoabsorption. This spectroscopy permits an identification of the symmetry of discrete excited states as well as a symmetry decomposition of continuum states c For randomly oriented molecules in the gas phase, the molecular orientation induced by photoabsorption is given by Eq 0 (13) of Ref. 11 asin terms of the azimuthal and polar angles Q, m = (0 m ,

Articles you may be interested inMultipletspecific shape resonant features in vibrationally resolved 3σ g photoionization of O2 J. Chem. Phys. 91, 150 (1989); 10.1063/1.457493 Multipletspecific shape resonant features in 3σ g photoionization of O2Calculations of core and valence level photo ionization spectra of NO are presented and compared with available experimental data. A low-lying continuum shape resonance is identified in the (J' photoionization channel, which is the analog of similar states found in other first-row diatomic molecules. Both partial cross sections and photoelectron angular distributions are discussed, and the effect of nuclear motion on these observables is treated.

Eigenchannel wave functions are identified as the continuum analog of discrete-state eigenfunctions familiar from molecular structure calculations. As examples, eigenchannel wave functions are plotted for shape-resonant and nonresonant eigenchannels of N2. Both types of functions show characteristic nodal patterns. The penetration over a narrow energy range of the resonant wave function through a potential barrier into the molecular interior, the key feature of shape-resonant states, is clearly seen for the example of the N2 f-wave-dominated σu shape resonance at 1.2 Ry electron kinetic energy. This visualization of continuum electronic amplitudes should prove to be a significant new tool in the study of continuum electron–molecule dynamics.

Articles you may be interested inElectronic transition dipole moments and dipole oscillator strengths within Fock-space multi-reference coupled cluster framework: An efficient and novel approach J. Chem. Phys. 138, 094108 (2013); 10.1063/1.4793277 Ozone: Unresolved discrepancies for dipole oscillator strength distributions, dipole sums, and van der Waals coefficients J. Chem. Phys. 135, 074303 (2011); 10.1063/1.3626523 Experimental determination of the moments of the generalized oscillator strength distribution of HeThe complete dipole oscillator strength distribution for N 2 has ~ee~ cal.culated using , the multiple scattering model. From this, the moments of the dIstnbutIon Sip) and Lip) have been computed for -6 ~f.L ~ 2. The results are analyzed by decomposing the total moments according to the contributions from various subshells and also according to the contributions from various spectral ranges, e.g., discrete ~s ionizi~g excitations. Since many physical properties depend on these moments, thIS analysIs can pinpoint the spectral ranges with the greatest influence. Special attention is devoted to the distinctly molecular etTects on the dipole oscillator strength distribution and its moments, particularly for those channels atTected by shape resonances. The Rydberg series, for example, are significantly atTected. This analysis otTers an explanation for the weakness of certain dipole allowed Rydberg series and suggests a tentative new assignment for Worley's third series.