Dan Dill

Electron‐molecule scattering and molecular photoionization using the multiple‐scattering method

We adapt the multiple‐scattering method to treat unbound electronic states of molecules in the independent electron approximation. An inhomogeneous linear system is derived whose solution yields the K matrix for the electron‐molecule interaction. Using the K matrix, we derive continuum electronic wavefunctions by imposing boundary conditions corresponding to electron‐molecule scattering and molecular photoionization, i.e., the wavefunctions satisfy the so‐called outgoing‐wave and incoming‐wave normalization, respectively. These wavefunctions are then used to obtain expressions for elastic scattering and photoionization differential cross sections.

Molecular effects on inner‐shell photoabsorption. K‐shell spectrum of N2

K‐shell photoabsorption spectra of the first‐row diatomic molecules N2, CO, and NO are known to depart drastically from the behavior characteristic of K‐shell excitation in atoms. Below the K‐shell thresholds these spectra are dominated by a single, very intense peak rather than normal Rydberg structure, and the first 10–20 eV of the continuum exhibit a broad band of enhanced absorption, rather than a monotonic decrease. We use the multiple‐scattering model to compute the discrete part and the first 100 Ry of the photoionization continuum for K‐shell photoionization of N2. This calculation accounts for both novel features described above and shows that they arise from centrifugal barrier effects manifested as shape resonances in high‐l components of the final state wavefunctions. These effects are molecular in origin, resulting from the interaction between the photoelectron and the anisotropic molecular field. We also discuss the energy dependence of the photoelectron angular distributions, Kronig structu...

Fixed‐molecule photoelectron angular distributions

Expressions are obtained for the angular distribution of electrons ejected by electric dipole interaction from molecular targets fixed in the laboratory coordinate frame. The analysis is geometrical and independent of any particular dynamical description of the photoionization process. Thus the results will serve as the framework for study of the dynamics of particular processes, such as photoionization of molecules oriented on surfaces or by molecular beam techniques.

Spectral variation of fixed‐molecule photoelectron angular distributions

We have computed the angular distribution of electrons ejected by electric‐dipole interaction from the K shells of CO and N2 molecules held fixed in space, e.g., by adsorption onto a surface. The predicted distributions are very rich in structure, manifesting directly the spectral variation of the orbital momentum composition of the photocurrent. Experimental determination of such distributions will provide a sensitive, detailed probe of molecular photoionization dynamics.

Pictures of unbound molecular electrons, including shape‐resonant states. Eigenchannel contour maps

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.

Shape‐resonance‐induced non‐Franck–Condon effects in the valence‐shell photoionization of O2

Non‐Franck–Condon effects in the valence‐shell photoionization spectrum of O2 are studied using the multiple scattering model. Calculations for the 3σg, 1πu, and 1πg levels indicate that the enhanced sensitivity of dipole strengths to internuclear separation, induced by shape resonances, reduces and broadens the resonant cross section; nonresonant channels are negligibly affected. The resulting cross sections and asymmetry parameters show fair overall agreement with line and continuum source measurements.

Shape resonances in e–SF6 scattering

Elastic scattering cross sections for e‐SF6 are calculated. The spectrum exhibits resonances of ag, t1u, t2g and eg symmetries between 0 and 40 eV.(AIP)

Shape resonant features in the photoionization spectra of NO

Calculations of core and valence level photoionization spectra of NO are presented and compared with available experimental data. A low‐lying continuum shape resonance is identified in the σ 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.

Elastic electron scattering by CO2, OCS, and CS2 from 0 to 100 eV

The integrated elastic electron scattering cross sections for CO2, OCS, and CS2 were calculated from 0 to 100 eV using the continuum multiple‐scattering model with the Hara exchange approximation. For each molecule, a strong π‐type shape resonance occurs between ∼1–4 eV, followed by multiple, weakly resonant features at higher kinetic energy. The latter are only marginally observable in the integrated cross section, but, in some cases, should be clearly observable in vibrational excitation spectra. Agreement with available experimental information is good for all three cases, except for a gross underestimation of the background (nonresonant) scattering cross section in OCS resulting from our omission of dipole scattering in this work.

Shape resonance effects in the photoabsorption spectra of BF3

Multiple‐scattering model calculations of cross sections for dipole transitions from all occupied orbitals of BF3 to excited bound states and continuum states of electron kinetic energy <30 eV are presented. The photoelectron angular distribution asymmetry parameters β are also given for all occupied orbitals. The boron and fluorine K‐shell calculations are in qualitative agreement with, and provide a clear interpretation of, the measured spectra. Two shape resonances are found in the low energy continuum: one of a′1 symmetry, the other of e′ symmetry. The resonances are found to be due to trapping of p waves on the fluorine atoms. This atomic localization as well as the dominance of low‐l partial waves outside the molecule put these shape resonances in a class distinct from those observed in diatomic molecules.