Vibrational branching ratios and shape resonant photoionization dynamics in N_2O

Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N_2O(7σ^(−1)) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong non‐Franck–Condon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a σ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, N–N or N–O. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from Franck–Condon values observed in the symmetric than in the antisymmetric mode. The σ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes

Elastic electron scattering by fullerene, C60

We report cross sections for elastic scattering of low-energy electrons by fullerene, C60, calculated within the static-exchange approximation. The calculations are carried out via the Schwinger multichannel (SMC) method, equivalent in this case to the standard Schwinger variational principle. Combining the high parallel efficiency of the SMC method with a quadrature specially adapted to the high symmetry of C60 facilitates the most demanding step of the calculation and so permits the use of a large basis set. We analyze the structure of the cross section with reference to a simple spherical-shell model, and we compare our results to prior measurements and calculations

Rotationally resolved photoelectron angular distributions in resonance enhanced multiphoton ionization of NO

We report calculated ionic rotational branching ratios and associated photoelectron angular distributions for (1+1′) resonance enhanced multiphoton ionization (REMPI) via the R_(21)(20.5), P_(21)+Q_(11)(25.5), and P_(11)(22.5) branches of the A ^2 Σ^+(3sσ) state of NO. The branching ratios are dominated by even angular momentum transfer peaks, in agreement with the ΔN+l=odd (ΔN≡N+−Ni ) selection rule. Whereas the calculated photoelectron angular distributions are very branch dependent alignment, the ionic branching ratios are found to be less so. The present calculated results agree well with the experimental results of Allendorf et al

Cooper minima and rotationally resolved resonance enhanced multiphoton ionization spectroscopy

We demonstrate that a Cooper minimum, close to threshold, in photoionization via an excited molecular Rydberg state can have a dramatic influence on the ionic rotational branching ratios. It is also shown that this behavior can be exploited to produce ions selectively in a specific rotational level. To illustrate this effect we present the results of ab initio calculations for (2+1′) resonance enhanced multiphoton ionization via the O_(11) (23.5) branch of the H ^2Σ^+(3d,4s) state of NO, where a Cooper minimum is found in l=3 of the kσ and kπ continua at photoelectron kinetic energies of 2.6 eV and 2.9 eV, respectively

Shape resonance behavior in 1π_g photoionization of O_2

We report calculations of vibrationally resolved cross sections and photoelectron angular distributions for photoionization of O_2 leading to the X^ 2 Π_g (ν^+ =0–4) states of O^+_2 using Hartree–Fock continuum photoelectron orbitals. These studies were motivated by recent results which show that a σ_u shape resonance plays a dominant role in producing non‐Franck–Condon vibrational distributions in resonant multiphoton ionization of O_2 via the C ^3Π_g (1π_g3sσ_g) Rydberg state. In the present study, we investigate how this shape resonance influences photoionization dynamics in single‐photon ionization. Below 21 eV photon energy, we find significant non‐Franck–Condon effects in the vibrational branching ratios as well as in the vibrationally resolved photoelectron angular distributions. Substantial autoionization hinders a direct comparison between theory and experiment

Multiplet-specific shape resonant features in vibrationally resolved 3σ_g photoionization of O_2

We report multiplet‐specific vibrationally resolved photoionization cross sections and photoelectron angular distributions for the 3σ_g orbital of O_2 leading to the v^+=0–3 levels of the b^4Σ^−_g and B^2Σ^−_g states of O^+_2. These studies were motivated by recent work which shows significant nonstatistical behavior in the vibrationally unresolved spectrum at low photoelectron energies arising from the sensitivity of the kσ_u shape resonance to the multiplet‐specific exchange potentials. In addition to the anticipated non‐Franck–Condon vibrational distributions arising from the kσ_u shape resonance, we also find substantial nonstatistical effects in our vibrationally resolved cross sections and particularly in our photoelectron angular distributions over a broad energy range. Extensive electronic autoionization due to Rydberg levels leading to the c^ 4Σ^−_u (2σ^(−1)_u) ion makes it difficult to assess these effects in the available experimental data

Theory of resonantly enhanced multiphoton processes in molecules

In this paper we formulate a theory for the analysis of resonant enhanced multiphoton ionization processes in molecules. Our approach consists of viewing the (n+m) photon ionization process from an isotropic initial state as m‐photon ionization out of an oriented, excited state. The orientation in this resonant state, which is reached by n‐photon excitation from the initial state, is nonisotropic, and is characteristic of this absorption process. The ionization simply probes this anisotropic population. The calculation of the REMPI process thus consists of determining the anisotropy created in the resonant state and then coupling this anisotropic population to ionization out of it. While the former is accomplished by the solution of appropriate density matrix equations, the latter is done by coupling these density matrix elements to angle‐resolved ionization rates out of this state. An attractive feature of this approach is that the influence of saturation effects, and other interactions, such as collisions, on the photoelectron properties is easily understood and incorporated. General expressions are derived for photoelectron angular distributions. Based on these, several properties of the angular distributions that follow purely on symmetry considerations are discussed. One of the new features that emerge out of this work is the saturation induced anisotropy in REMPI. In this effect higher order contributions to the angular distributions appear since saturation influences different ionization channels differently thereby creating an additional anisotropy in the excited state

Photoionization of the valence orbitals of OH

We report the results of studies of the photoionization cross sections and asymmetry parameters for the 3σ and 1π levels of OH, corresponding to the production of the A ^3Π, c ^1Π, a ^1Δ, b ^1Σ+, and X ^3Σ− molecular ions. The calculations employed multiplet‐specific Hartree–Fock potentials and numerical photoelectron continuum orbitals, obtained using the iterative Schwinger variational method. Noticeable nonstatistical behavior of the cross sections is seen, mainly for the 1π level, although deviations are not as pronounced as in other open‐shell systems. Comparison with fragmentary experimental data is encouraging, although synchrotron radiation studies are needed to fully assess the accuracy of the calculated cross sections

Absolute angle-differential elastic cross sections for electron collisions with diacetylene

We report measured and calculated differential elastic cross sections for collisions of low-energy electrons with diacetylene (1,3-butadiyne). A generally satisfactory agreement between theory and experiment has been found. The calculated cross sections provide interesting insight into the underlying resonant structure