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We discuss results of theoretical studies of rotationally resolved photoelectron spectra of the CH, NH, and OH radicals, including predictions of zero-kinetic-energy (ZEKE) spectra for molecular ions produced via resonance enhanced multiphoton ionization. The influence of dynamical features such as Cooper minima, orbital evolution, and 1 mixing are examined for photoionization of the 3σ Rydberg orbital in these radicals. We also make specific predictions for the photoelectron spectra for each molecule within the context of current experimental capabilities.

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.

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.

We present results of measurements and calculations of elastic electron scattering from pyrimidine in the energy range 3-50 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique. The measured cross sections are compared with results of calculations using the well-known Schwinger variational technique and an independent-atom model. Agreement between the measured differential cross sections and the results of the Schwinger calculations is good at lower energies but less satisfactory at higher energies where inelastic channels that should be open are kept closed in the calculations. © 2011 American Physical Society., The Support of the Australian Research Council, The International Science Linkage program of the Australian Government, the Spanish Ministerio de Ciencia e Innovacion´ (Project FIS2009-10245), and the EU Framework Programme (COST Action MP1002) is gratefully acknowledged. The work of V.M. and C.W. was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy under Grant No. DE-FG02-97ER14814, and employed the Supercomputing and Visualization Facility at the Jet Propulsion Laboratory.

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Recent advances in experimental techniques now make it possible to readily achieve rotational resolution in molecular photoelectron spectra. Here we discuss results of our recent theoretical studies of rotationally resolved photoelectron spectra at low and very near-threshold energies for molecules such as HBr, NH, OH, NO, CO, H2_O, and H_2S. These studies serve to reveal the rich dynamics of quantum-state specific studies of molecular photoionization and, where possible, provide a robust description of key spectral features of interest in related experimental studies.

A “new” non-dipole electronic transition at 7.7 eV in CF_3Cl has been observed using momentum-transfer-resolved electron energy loss spectroscopy at 2.5 keV impact energy. The generalized oscillator strength (GOS) of this transition has been determined and found to have a shape characteristic of a quadrupole transition with a maximum at ≈1 au of momentum transfer. Using the results from an ab initio GOS calculation and a single-excitation configuration interaction calculation, we show that this transition corresponds to the ^1E ← ^1A_1 (LUMO ← HOMO) transition with a very small computed dipole transition moment. In particular, this transition is related to an electronic transition from a non-bonding Cl 3p orbital (HOMO) to an antibonding p-σ* C-Cl orbital (LUMO). The nature of the orbitals involved in the transition, together with its small calculated dipole transition moment, strongly suggests that this transition is dominated by quadrupole interaction. The calculated potential energy diagram along the C-Cl bond direction also shows that a vertical transition from the ^1A_1 ground-state potential energy curve accesses the repulsive region of the ^1E excited-state curve, suggesting that this quadrupole transition may lead to dissociation of the C-Cl bond.

Combined theoretical and experimental studies of He I photoelectron spectra for photoionization of the X^1Σ^+ (v″ = 0) ground state HF leading to the X^2Π (v^+ = 0–2) and A ^2Σ^+ (v^+ = 0–3) ionic states, including detailed analyses of the rotational bandshapes, are reported. Agreement between the measured bandshapes and the calculated spectra, convoluted with the experimental resolution function (22 meV FWHM), is excellent. Ionization potentials of 16.046(1) eV and 19.116(2) eV are obtained for formation of the X and A ionic states, respectively. The vibrational spacings, deduced from the measured data by careful bandshape analysis, are in very good agreement with spectroscopic values.

Single‐photon threshold photoionization spectra for jet‐cooled NO have been measured for the v^+=0 and 1 vibrational levels of the X ^1Σ^+ ground state of NO^+. The NO^+ rotational state distribution for the v^+=0 level is shown to be perturbed by nearby autoionizing levels, whereas the v^+=1 level exhibits a cation rotational distribution which is in near quantitative agreement with calculated spectra near threshold. Only small changes in total angular momentum are observed (‖ΔJ‖=‖J^+ − J‘‖≤5/2) even though a wide range of photoelectron angular momenta (l=0–3) are predicted to contribute to the near‐threshold photoelectron continua. The present results are also discussed in light of recently published two‐photon threshold photoionization spectra of NO which exhibit nearly identical NO^+ rotational state distributions.

We report cross sections for electronically elastic electron scattering by CF4 from 1 to 40 eV, calculated within the static‐exchange approximation using the Schwinger multichannel method. Although the static‐exchange approximation does not give results that are accurate in detail below 20 eV, it is useful in understanding resonant features in the elastic and vibrationally inelastic cross sections. Above 20 eV, where the static‐exchange approximation is more reliable, we derive a dissociation cross section in fair agreement with experiment by subtracting our result from the measured total cross section. We compare our integral and differential cross sections with the results of recent elastic and vibrationally inelastic scattering experiments.

Experimental and calculated differential cross sections for elastic scattering of electrons by C_(60) molecules at collision energies of 100 to 500 eV are reported. The elastic differential cross sections were measured in a standard crossed-beam apparatus, while the calculations were performed employing the Schwinger multichannel technique at the static-exchange level. Diffraction effects, some due to the overall spherical-cage structure and some to scattering by individual C atoms, are observed in both the measured and calculated cross sections.

We report cross sections for the elastic scattering of electrons by the C3H6 isomers propene and cyclopropane from 5 to 40 eV. These results were calculated within the static‐exchange approximation, using the Schwinger multichannel method as implemented on the Touchstone DELTA distributed‐memory parallel computer. The integral elastic cross sections so obtained support recent observations of a substantial isomer effect in the total scattering cross section. Partial‐wave analysis indicates the presence of an unusual A’2 shape resonance in cyclopropane.

We report cross sections for electronically elastic electron scattering by CF4 from 1 to 40 eV, calculated within the static‐exchange approximation using the Schwinger multichannel method. Although the static‐exchange approximation does not give results that are accurate in detail below 20 eV, it is useful in understanding resonant features in the elastic and vibrationally inelastic cross sections. Above 20 eV, where the static‐exchange approximation is more reliable, we derive a dissociation cross section in fair agreement with experiment by subtracting our result from the measured total cross section. We compare our integral and differential cross sections with the results of recent elastic and vibrationally inelastic scattering experiments.

We have calculated cross sections for low‐energy electron impact excitation of the first triplet state (5a_1’→3a_2") of [1.1.1] propellane. Our results support the assignment of a recently observed [O. Schafer, M. Allan, G. Szeimies, and M. Sanktjohansen, J. Am. Chem. Soc. (submitted)] near‐threshold peak as a (5a_1’)(3a_2")2 core‐excited shape resonance.

Results of theoretical studies of rotationally resolved photoelectron spectra for (2 + 1’) resonance enhanced multiphoton ionization (REMPI) of the C ^2Σ^-, D ^2Σ^-, E ^2Σ^-, and F ^2Σ^- Rydberg states of ClO are presented. Cooper minima are predicted to occur in the electronic continua for photoionization of the D and F states and lead to unusual behavior in the ion rotational distributions. Strong partial wave I mixing is also predicted in the continua for the C and E states due to the nonspherical molecular ion potentials. These Cooper minima and l mixing make the associated photoionization dynamics quite nonatomiclike.

We report the results of theoretical studies of the time-resolved femtosecond photoelectron spectroscopy of quantum wavepackets through the conical intersection between the first two (2)A' states of NO(2). The Hamiltonian explicitly includes the pump-pulse interaction, the nonadiabatic coupling due to the conical intersection between the neutral states, and the probe interaction between the neutral states and discretized photoelectron continua. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Photoelectron angular distributions are seen to provide a clearer picture of the ionization channels and underlying wavepacket dynamics around the conical intersection than energy-resolved spectra. Time-resolved photoelectron velocity map images are also presented.

We present an improved nonlocal resonance model for electron-HCl collisions. The short-range part of the model is fitted to ab initio electron-scattering eigenphase sums calculated using the Schwinger multichannel method, while the long-range part is based on the ab initio potential-energy curve of the bound anion HCl-. This model significantly improves the agreement of nonlocal resonance calculations with recent absolute experimental data on dissociative electron attachment cross sections for HCl and DCl. It also partly resolves an inconsistency in the temperature effect in dissociative electron attachment to HCl present in the literature. Finally, the present model reproduces all qualitative structures observed previously in elastic scattering and vibrational-excitation cross sections.

We report measured and calculated results for elastic scattering of low-energy electrons by ethyl vinyl ether (ethoxyethene), a prototype system for studying indirect dissociative attachment processes that may play a role in DNA damage. The integral cross section displays the expected π* shape resonance. The agreement between the calculated and measured cross sections is generally good.

Results of calculations of rotationally resolved photoelectron spectra for resonance enhanced multiphoton ionization (REMPI) of SiF via the B ^2Σ^+ (4sσ), C" ^2Σ^+ (4pσ), and C’ ^2Π (4pπ) Rydberg states are reported. In addition to the expected ΔN=even peaks, unusually strong ΔN=±1 transitions are predicted for photoionization of the B ^2Σ^+ state. These unusual transitions are due to even angular momentum components of the photoelectron matrix element and arise from the formation of Cooper minima in the ionization channels and strong l mixing in the electronic continuum induced by the nonspherical molecular ion potential. Unexpected ΔN=0,±2 transitions, due to odd wave contributions to the photoelectron matrix element, are also predicted for photoionization of the C" ^2Σ^+ state. Asymmetrical ion distributions with respect to ΔN=0 are also predicted for the C’ ^2Π state. Cooper minima are predicted to occur in the l=2 wave of the kπ photoelectron channel for the B state and in the l=4 wave of the kσ and kπ channels for the C‘ state. Photoelectron angular distributions provide further insight into the photoionization dynamics.

A three-state calculation of electron-impact excitation of formaldehyde to the a^3A_2 and A^1A_2 states is carried out using the Schwinger multichannel variational method. The integral and differential cross sections so obtained agree fairly well with theoretical results obtained using the complex Kohn method. Though agreement between the calculated integral cross section and the single available experimental measurement is qualitative, similar conclusions regarding the excitation mechanism are reached. A generalization of the selection rule for (Σ^+⇆Σ^−) electron-impact excitation of diatomic molecules is used to explain the shape of the differential cross sections for the a^3A_2 and A^1A_2 excitations.

Ion rotational distributions for single‐photon VUV photoionization of the 1b_1 orbital of the X̃ ^1A_1 ground state of the jet‐cooled water are reported. These spectra reveal significant type a transitions which are seen to arise from odd angular momentum components of the photoelectron matrix element. The resulting photoionization dynamics are quite nonatomic‐like.

We report the results of theoretical studies of the rotationally resolved photoelectron spectra of ground state O_2 leading to the X ^2Π_g state of O^+ 2 via the absorption of a single vacuum ultraviolet photon. These studies elaborate on a recent report [M. Braunstein e t a l., J. Chem. Phys. 9 3, 5345 (1990)] where we showed that a shape resonance near threshold creates a significant dependence of the rotational branching ratios on the ion vibrational level. We also showed that analysis of the rotational branches yields detailed information on the angular momentum composition of the shape resonance. We continue this analysis giving a comprehensive derivation of the rotationally resolved cross sections and photoelectron angular distributions. We discuss the selection rules implied by these expressions and present very high resolution cross sections (J→J^+) obtained using static‐exchange photoelectron orbitals and explicitly taking into account the internuclear distance dependence of the electronic transition moment. These cross sections illustrate the selection rules and show more explicitly the angular momentum composition of the shape resonance. We also present rotationally resolved photoelectron angular distributions which would be expected at low energy.

We report cross sections for the elastic scattering of electrons by the C3H6 isomers propene and cyclopropane from 5 to 40 eV. These results were calculated within the static‐exchange approximation, using the Schwinger multichannel method as implemented on the Touchstone DELTA distributed‐memory parallel computer. The integral elastic cross sections so obtained support recent observations of a substantial isomer effect in the total scattering cross section. Partial‐wave analysis indicates the presence of an unusual A’2 shape resonance in cyclopropane.

A two‐state close‐coupling calculation of electron‐impact excitation of ethylene to the ã ^3B_(1u) state (π→π*) is carried out using a version of the Schwinger multichannel method developed for distributed‐memory parallel computers. The calculated integral cross section shows a sharp rise at threshold, in agreement with available experimental data. The integral and differential cross sections are useful in understanding the mechanism of this excitation process and indicate the possible presence of a core‐excited shape resonance near threshold.

Experimental and calculated differential cross sections (DCSs) for electron-impact excitation of the (010) bending mode and unresolved (100) symmetric and (001) antisymmetric stretching modes of water are presented. Measurements are reported at incident energies of 1–100 eV and scattering angles of 10°–130° and are normalized to the elastic-scattering DCSs for water determined earlier by our group. The calculated cross sections are obtained in the adiabatic approximation from fixed-nuclei, electronically elastic scattering calculations using the Schwinger multichannel method. The present results are compared to available experimental and theoretical data.

We report the results of measurements and calculations of differential and integral cross sections for elastic electron scattering from the CF_2 molecular radical. The energy range of the present investigation was 2–20 eV, while the angular distributions were measured at specific angles within the scattered electron range 20°–135°. The calculations employed the Schwinger multichannel method and were carried out both in the static-exchange and static-exchange plus polarization (SEP) approximations, with generally quite good agreement found between the data and the SEP level results. This was particularly true at lower energies, illustrating the importance of correctly accounting for the polarization in these collisions.

We report the results of measurements and calculations of differential and integral cross sections for elastic electron scattering from the CF2 molecular radical. The energy range of the present investigation was 2–20 eV, while the angular distributions were measured at specific angles within the scattered electron range 20°–135°. The calculations employed the Schwinger multichannel method and were carried out both in the static-exchange and static-exchange plus polarization (SEP) approximations, with generally quite good agreement found between the data and the SEP level results. This was particularly true at lower energies, illustrating the importance of correctly accounting for the polarization in these collisions.

Cooper minima are predicted to occur in the 3pσ→kσ(l=2) and 3pσ→kπ(l=2) channels in the resonance enhanced multiphoton ionization of NO via the D ^2Σ^+(3pσ) Rydberg state. The low energy kσ(l=2) Cooper minimum leads to the observed ΔN = N^+ − N’=0 photoelectron peak, in addition to the ΔN = ±1, ±3 peaks seen in the rotational spectra. The Cooper minima are accompanied by significant l mixing in the continuum due to the nonspherical molecular potential and result in a strong dependence of rotational branching ratios and angular distributions on photoelectron kinetic energy. A Cooper minimum is also predicted in the 3pπ→kδ(l=2) channel for photoionization of the C ^2Π(3pπ) Rydberg state. The effect of this Cooper minimum on photoelectron spectra and photoelectron angular distributions is also investigated.

In this paper we extend a previous formulation of molecular resonance enhanced multiphoton ionization (REMPI) photoelectron spectra to explicitly include multiplet‐specific final state wave functions and intermediate coupling schemes. The results of this formulation should be well suited and helpful in quantitative theoretical studies of rotationally resolved REMPI spectra in many diatomic molecules of interest. As an example, we use this formulation to study the rotational branching ratios and photoelectron angular distributions for (3+1) REMPI of NH via the 3 ^3Π Rydberg resonant state. The predicted anomalous rotational distributions are interpreted as arising from a Cooper minimum in the l=2 component of the kπ photoionization channel. A number of other results are obtained and discussed.

Measurements of a new observable quantity in photoemission from oriented molecules are reported. Using circularly polarized radiation, the photoemission intensities for a special geometry were found to depend on photon helicity. The basic theoretical description of the effect is outlined, showing that it occurs for all spatially fixed molecules in the electric dipole approximation and does not depend on electron-spin effects. Experimental case studies for diatomics (CO, NO), benzene, and the “heavy” molecule CH_3I, all oriented by adsorption on Pd(111) or graphite (0001), reveal huge intensity asymmetries in all cases. Comparisons of these measured asymmetries are also made with the results of model calculations based on oriented CO, NiCO, and NO. The effect provides a sensitive probe of molecular orientation and of photoemission dynamics.

Elastic electron scattering cross sections from 5–30 eV are reported for the molecules C_2H_4, C_2H_6, C_3H_8, Si_2H_6, and GeH_4, obtained using an implementation of the Schwinger multichannel method for distributed‐memory parallel computer architectures. These results, obtained within the static‐exchange approximation, are in generally good agreement with the available experimental data. These calculations demonstrate the potential of highly parallel computation in the study of collisions between low‐energy electrons and polyatomic gases. The computational methodology discussed is also directly applicable to the calculation of elastic cross sections at higher levels of approximation (target polarization) and of electronic excitation cross sections.

We report measured and calculated cross sections for elastic scattering of low-energy electrons by the alcohols n-propanol and n-butanol in the gas phase. The measurements were carried out using the relative-flow method with an aperture source rather than a conventional tube or capillary-array source, eliminating the need to know molecular diameters. The calculations employed two different implementations of the Schwinger multichannel variational method and included polarization effects. The differential cross sections are dominated by strong forward scattering due to the molecules' large electric dipole moments, but near 10 eV, they display structure at intermediate angles that is probably associated with shape resonances, notably a pronounced f-wave scattering pattern. Overall agreement between the measured and calculated results is fair. We compare the cross sections of these larger alcohols to those of methanol and ethanol, as well as to those of alkanes.

Differential cross sections for elastic electron scattering from gaseous water are reported. The measurements are obtained using the relative flow method with He as the standard gas and a thin collimating aperture source of gas instead of a conventional needle source. Differential cross sections were measured at incident energies of 1, 2, 4, 6, 8, 10, 15, 20, 30, 50, and $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for scattering angles ranging from 5\ifmmode^\circ\else\textdegree\fi{} to 130\ifmmode^\circ\else\textdegree\fi{} and integrated over angles to obtain integral cross sections. Corresponding calculations of the differential cross section are carried out using the Schwinger multichannel method, employing extensive basis sets and considering polarization and dipole-scattering effects. Whereas excellent qualitative agreement with past measurements of differential cross sections is observed, our measurements are found to be consistently in significant quantitative disagreement with these measurements. The present calculations, on the other hand, generally agree in magnitude with previous results.

We review recent theoretical work on low-energy electron collisions with the constituents of DNA and RNA, including the nucleobases (uracil, thymine, cytosine, adenine, and guanine), deoxyribose and its analogue tetrahydrofuran, and larger subunits including deoxynucleosides and deoxynucleotides. A particular focus is on the identification and characterization of low-energy resonances that may play a role in electron attachment leading to DNA strand breaking. Comparison with experiment indicates that high-level calculations can yield rather accurate resonance positions. However, the higher-lying π* resonances of the nucleobases appear to be of mixed elastic and core-excited character. This mixing must be accounted for in the calculation to obtain reliable resonance energies and may affect the electron-induced dissociation mechanisms. Resonant channel mixing is illustrated for the pyrimidine base analogue pyrazine.

We point out that the assignment of pi(*) resonances to calculated features in a recent paper by Gianturco et al. [J. Chem. Phys.128, 174302 (2008)] cannot be correct.

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay.

In molecular double-slit experiments, the interference between emitted core electrons of diatomic molecules gives rise to oscillations in the observed electron intensity. Here, we explore this behaviour for photoelectrons emitted from CO and N_2 by soft X-ray ionization in the molecular frame, and we argue that in addition to the undisturbed emission process, intramolecular scattering can lead to electron interference between the scattered and unscattered wave in two ways: two-centre interference between two spatially coherent emitters and one-centre self-interference. The latter is the signature of a loss of spatial coherence. The spatial scale over which the transition from two-centre to one-centre coherence occurs is the de Broglie wavelength of the scattered photoelectron in units of the bond length. These results highlight the fact that the molecular double slit is based on two independent uncertainty principles, Δp_xΔx and ΔEΔt, the second of which causes ongoing tunnelling between the two centres, even after the collapse of the electron wavefunction in real space.

We report on results of recent studies of collisions of low-energy electrons with nucleobases and other DNA constituents. A particular focus of these studies has been the identification and characterization of resonances that play a role in electron attachment leading to strand breaks in DNA. Comparison of the calculated resonance positions with results of electron transmission measurements is quite encouraging. However, the higher-lying π* resonances of the nucleobases appear to be of mixed elastic and core-excited character. Such resonant channel coupling raises the interesting possibility that the higher π*resonances in the nucleobases may promote dissociation of DNA by providing doorway states to triplet excited states.

We report the results of measurements and calculations for elastic electron scattering from 3-hydroxytetrahydrofuran (C4H8O2). The measurements are performed with a crossed electron-target beam apparatus and the absolute cross-sections are determined using the relative flow technique. The calculations are carried out using the Schwinger multichannel method in the static-exchange plus polarization (SEP) approximation. A set of angular differential cross-sections (DCS) is provided at five incident energies (6.5, 8, 10, 15 and 20 eV) over an angular range of 20–130°, and the energy dependence of the elastic DCS at a scattering angle of 120° is also presented. Integral elastic and elastic momentum transfer cross-sections have also been derived and calculated. The results are compared with those of recent measurements and calculations for the structurally similar molecule tetrahydrofuran (C4H8O).

Using a crossed electron-molecular beam experiment, featuring a skimmed nozzle beam with pyrolytic radical production, absolute elastic cross sections for electron scattering from the CF2 molecule have been measured. A new technique for placing measured cross sections on an absolute scale is used for molecular beams produced as skimmed supersonic jets. Absolute differential cross sections for CF2 are reported for incident electron energies of 30-50 eV and over an angular range of 20-135 deg. Integral cross sections are subsequently derived from those data. The present data are compared to new theoretical predictions for the differential and integral scattering cross sections, as calculated with the Schwinger multichannel variational method using the static-exchange and static-exchange plus polarization approximations.

Strong non‐Franck–Condon behavior is predicted to occur in the vibrationally resolved photoionization spectra of NH for (3+1) resonance enhanced multiphoton ionization processes via the 3 ^3Π Rydberg state. The non‐Franck–Condon effects are interpreted on the basis of rapid orbital evolution, Cooper minima, and internuclear distance dependence of the dipole transition moment and cross sections. A Cooper minimum occurs in the 5σ→kπ channel at small internuclear distances, where NH resembles its united atom, oxygen. The iterative Schwinger variational method and multiplet‐specific ion potentials are employed in the calculation of the photoelectron continuum wave functions. Cross sections and asymmetry parameters for photoionization of the NH ground state leading to the X ^2Π, a ^4Σ^−, and A ^2Σ^− ions are also reported.

In this paper we discuss the single‐photon ionization dynamics of the d ^1Π_g Rydberg state of O_2. Comparison is made with vibrationally resolved measurements of photoelectron spectra which employ (2+1) resonance enhanced multiphoton ionization (REMPI) through the d ^1Π_g state. A σ_u shape resonance near the ionization threshold leads to non‐Franck–Condon vibrational branching ratios and a substantial dependence of photoelectron angular distributions on the vibrational state of the X ^2Π_g ion. Significant differences exist between our one‐electron predictions and experiment. These are mainly attributed to electronic autoionization of repulsive ^1∑^−_u , ^1∑^+_u , and ^1Δ_u states associated with the 1π^3_u 1π^3_g configuration. A proposed singlet "K" ^1Π_u Rydberg state converging to the A ^2Π_u ion probably also contributes to autoionization in the d ^1Π_g state spectrum. We also show that autoionizing H and J ^3Π_u Rydberg states of O_2 converging to the a ^4Π_u and A ^2Π_u ionic thresholds, respectively, may play a previously unsuspected role in the C ^3Π_g state one‐color REMPI spectra. We discuss multiplet‐specific (spin‐dependent) effects via comparison of these results with recent experimental and theoretical studies of O_2 C ^3Π_g photoionization.

We review and discuss some recent resonance enhanced multiphoton ionization (REMPI) studies of small molecules, emphasizing the role that shape resonances and Rydberg orbital evolution play in determining vibrational and rotational ion distributions. Strong non-Franck-Condon effects observed in ion vibrational distributions for REMPI of O_2 via the C ^3Π_g and d ^1Π_g Rydberg states are partially attributed to a σ_u shape resonance, previously observed in ground state photoionization of diatomic molecules. Autoionization of repulsive valence states also induces non-Franck-Condon effects in these REMPI spectra. Significant non-Franck-Condon effects in molecular REMPI spectra also arise from a mechanism associated with rapid evolution of the resonantly populated Rydberg orbital with changing inter-nuclear distance. These effects should be most pronounced in diatomic hydrides, and are illustrated by theoretical predictions of vibrational and rotational ion distributions for REMPI of OH and CH via the D ^2Σ^− and E' ^2Σ^+ Rydberg states, respectively.

The iterative Schwinger variational method is used to obtain cross sections and photoelectron asymmetry parameters for photoionization of the three outermost valence orbitals (1b1, 3a1, and 1b2) of H2O for photon energies from near threshold to 50 eV. A comparison of these calculated results with available experimental data is encouraging.

We report computed cross sections for the elastic scattering of slow electrons by the pyrimidine bases of DNA, thymine and cytosine, and by the associated nucleosides, deoxythymidine and deoxycytidine. For the isolated bases, we carried out calculations both with and without the inclusion of polarization effects. For the nucleosides, we neglect polarization effects but estimate their influence on resonance positions by comparison with the results for the corresponding bases. Where possible, we compare our results with experiment and previous calculations.

We report cross sections for low-energy elastic electron collisions with the diazabenzene molecule pyrazine, obtained from first-principles calculations. The integral elastic cross section exhibits three sharp peaks that are nominally shape resonances associated with trapping in the vacant pi* molecular orbitals. Although the two lowest-energy resonances do in fact prove to be nearly pure single-channel shape resonances, the third contains a considerable admixture of core-excited character, and accounting for this channel coupling effect is essential to obtaining an accurate resonance energy. Such resonant channel coupling has implications for electron interactions with the DNA bases, especially the pyrimidine bases for which pyrazine is a close analog. In the absence of data on pyrazine itself, we compare our elastic differential cross section to measurements on benzene and find close agreement.

Zero-kinetic-energy (ZEKE) photoelectron techniques have now become new tools of physical chemistry that go far beyond the measurement of the spectra of molecular ions. The unprecedented resolution of the ZEKE method [8, 9], which is essentially limited by the laser bandwidth, has already led to many new and exciting applications in chemical physics and chemistry within the last few years. The rapid expansion of this method to a number of very active laboratories clearly suggests that this trend can be expected to continue [24- 47].

Detailed investigation of the three low-energy resonances seen in electron scattering by the diazabenzene molecule pyrazine reveals that the first two are nearly pure single-channel shape resonances, but the third is, as long suspected, heavily mixed with core-excited resonances built on low-lying triplet states. Such resonant channel coupling is likely to be widespread in pi-ring molecules, including the nucleobases of DNA and RNA, where it may form a pathway for radiation damage.

We report theoretical as well as (normalized) experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from chloroethane, C_2H_5Cl, also known as ethyl chloride. The theoretical cross sections were computed using the Schwinger multichannel variational method in the single-channel approximation, with polarization effects included via virtual excitations. Cross section measurements were made at incident energies ranging from 1 to 30 eV and at scattering angles from 10° to 125°. We compare our data to previous results for C_2H_5Cl and for the related molecule chloromethane.