Search

We report results from a novel fully ab initio method for simulating the time-resolved photoelectron angular distributions around conical intersections in CS_2. The technique employs wavepacket densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of the calculated molecular-frame photoelectron angular distributions with measured values for CS_2 demonstrates that this approach can successfully illuminate, and disentangle, the underlying coupled nuclear and electronic dynamics around conical intersections in polyatomic molecules.

We examine the use of local orbitals to improve the scaling of calculations that incorporate target polarization in a description of low-energy electron-molecule scattering. After discussing the improved scaling that results, we consider the results of a test calculation that treats scattering from a two-molecule system using both local and delocalized orbitals. Initial results are promising.

We report computed cross sections for low-energy elastic collisions of electrons with the methyl esters of phosphoric acid, monomethyl, dimethyl, and trimethyl phosphate, and with phosphoric acid itself. For phosporic acid and monomethyl phosphate, polarization effects are included in the calculation, while the two larger molecules are treated in the static-exchange approximation, that is, with polarization neglected. The integral elastic cross sections exhibit broad shape resonances above 5 eV that give rise to strong variations in the differential cross section with energy. However, no shape resonances are evident below 5 eV. We compare our results to previous calculations and measurements and discuss their relevance to electron-induced damage to the DNA backbone.

The dissociation of a wide range of oxygen-containing molecules following impact with electrons of carefully controlled energy is critically reviewed. Molecules considered range from diatomics, like O_2 and CO, to large molecules of biological and technological interest. Dissociation mechanisms are discussed and, where possible, quantitative data for the various possible processes, ionization, attachment, dissociation, excitation, emission etc., are presented. Gaps and discrepancies in our current data base are highlighted. Both graphical and tabular data are presented.

We report a combined experimental and theoretical investigation of dissociative electron attachment (DEA) to the nucleobase uracil. Using ion momentum imaging experiments employing a DEA reaction microscope we have measured 3-dimensional momentum distributions of specific anionic fragments following DEA to uracil by 6 eV electrons. From the measured anion fragment kinetic energy we determine the possible dissociation pathways and the total kinetic energy release. We employ electronic structure and electron scattering calculations to determine the probability for electron attachment in the molecular frame. Combining these calculations with the imaging measurements, we reveal several key features of the coupled electronic and nuclear dynamics of DEA.

We describe a theoretical approach to molecular photoionization that includes first-order corrections to the dipole approximation. The theoretical formalism is presented and applied to photoionization of H_2 over the 20- to 180-eV photon energy range. The angle-integrated cross section σ, the electric dipole anisotropy parameter β_e, the molecular alignment anisotropy parameter β_m, and the first-order nondipole asymmetry parameters γ and δ were calculated within the single-channel, static-exchange approximation. The calculated parameters are compared with previous measurements of σ and β_m and the present measurements of β_e and γ+3δ. The dipole and nondipole angular distribution parameters were determined simultaneously using an efficient, multiangle measurement technique. Good overall agreement is observed between the magnitudes and spectral variations of the calculated and measured parameters. The nondipole asymmetries of He 1s and Ne 2p photoelectrons were also measured in the course of this work.

In homonuclear diatomic molecules such as N_2, the inversion symmetry of the system causes non-local, coherent behavior of the otherwise localized core holes. The non-locality of the electron emission and the remaining core hole changes in a continuous way into partially localized behaviour if a gradual breakdown of the inversion symmetry is induced by isotope substitution. This is reflected by a loss of interference and a parity mixing of the outgoing photoelectron waves. Our results represent the first experimentally observed isotope effect on the electronic structure of a diatomic molecule.

Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction ('gerade' and 'ungerade') are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites. Here we observe the coherence of core electrons in N(2) through a direct measurement of the interference exhibited in their emission. We also explore the gradual transition to a symmetry-broken system of localized electrons by comparing different isotope-substituted species--a phenomenon analogous to the acquisition of partial 'which-way' information in macroscopic double-slit experiments.

Measurements of photoelectron angular distributions for carbon K-shell ionization of fixed-in-space CO2 molecules with the molecular axis oriented along, perpendicular and at 45 degrees to the electric vector of the light are reported. The major features of these measured spectra are fairly well reproduced by calculations employing a relaxed-core Hartree-Fock approach. In contrast to the angular distribution for K-shell ionization of N-2, which exhibits a rich structure dominated by the f-wave (l = 3) at the shape resonance, the angular distribution for carbon K-shell photoionization of CO2 is quite unstructured over the entire observed range across the shape resonance.

The photoionizationdynamics of rotationally hot CO, photodissociated from OCS, have been studied using laser photoelectron spectroscopy via the intermediate B^1Σ^+Rydberg state leading to the X^2Σ^+ of the ion. The photodissociation of OCS near 230 nm produces rotationally hot, but vibrationally cold CO (X^1Σ^+,N″,v″=0,1) fragments along with S (^1D) atoms. These high rotational levels show photoelectron spectra with a very strong ΔN=0 transition and weaker ΔN=±1, ±2, and ±3 transitions. Agreement between measured and calculated spectra is good and suggests that there is significant angular momentum coupling in the photoelectron orbital. In the ionization step not only Δv=0, but also off-diagonal, non-Franck–Condon (Δv≠0) transitions are observed. The intensities of these transitions vary strongly within the region studied and can be explained by the excitation of superexcited Rydberg states with an A^2Π core.

Normalized experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from isobutanol (C_4H_9OH)are presented. The differential cross sections are measured at incident energies from 1 to 100 eV and scattering angles from 5° to 130°. These cross sections are compared to earlier experimental and theoretical results for isobutanol and n-butanol, as well as to results for smaller alcohols and for alkanes. Further comparisons are made with calculated cross sections for isobutylamine (C_4H_9NH_2)and for smaller amines, including ethylamine (C_2H_5NH_2), dimethylamine (CH_3NHCH_3)the two C_3H_7NH_2 isomers n-propylamine and isopropylamine, and ethylene diamine (NH_2C_2H_4NH_2). The calculated cross sections are obtained using the Schwinger multichannel method. The comparisons illuminate the role of molecular structure in determining the angular distribution of resonantly scattered electrons.

We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame photoelectron angular distributions around conical intersections in ${\mathrm{CS}}_{2}$. The technique employs wave packet densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of these results with measured molecular-frame photoelectron angular distributions for ${\mathrm{CS}}_{2}$ demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.

We present experimental and theoretical results on rotational distributions of CO^+ (B^2Σ^+) photoions. Rotational distributions were determined for both the v^+=0 and v^+=1 vibrational levels following photoionization of cold (T_0≈9K) neutral CO target molecules. Data were generated using dispersed ionic fluorescence over a wide range of photoelectron kinetic energies, 0⩽E_k⩽120eV, which allows one to interrogate the ionization dynamics. This wide spectral coverage permits illustrative comparisons with theory, and calculated spectra are presented to interpret the data. In particular, the comparison between theory and experiment serves to identify the strong continuum resonant enhancement at hν_(exc)≈35eV in the l=3 partial wave of the 4σ→kσ ionization channel, as this feature has profound effects on the ion rotational distributions over a wide range of energy. Second, there are differences between the rotational substructure for the v^+=0 and v^+=1 vibrational levels. All of the experimentally observed features and trends are reproduced by theory, and the consequences of these comparisons are discussed.

We have incorporated a classical treatment of molecular rotation into our formulation of energy- and angle-resolved pump–probe photoelectron spectroscopy. This classical treatment provides a useful approach to extracting the photoelectron signal primarily associated with vibrational dynamics in cases where rotational motion is slow and the coupling between rotational and vibrational motion is weak. We illustrate its applicability with pump–probe photoelectron spectra for wave packets on the ^1Σ^+_u double-minimum state of Na_2.

The photoionizationdynamics of rotationally hot molecular nitrogen are studied employing resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy.Photodissociation of N_2O at ∼203 nm results in highly rotationally excited N2 fragments in X ^1∑^+_g (N″,v″=0,1) states and O atoms in the excited ^1D_2 state. Photoelectron detection of the rotationally hot N_2 states is performed by a two-photon excitation to the lowest a″ ^1∑^+_g Rydberg state followed by one-photon ionization. The large number of observed rotational levels, from N′=49 up to N′=94, results in improved rotational parameters for a″ ^1∑^+_g (v′=0). In addition, experimental and theoretical rotationally resolved photoelectron spectra of the a″ ^1∑^+_g (v′=0,1;N′) state are presented. In these spectra only ΔN = N^+ − N′ = even transitions are observed, with a dominant ΔN=0 peak and rather weak ΔN = ±2 peaks. The one-photon ionization is dominated by ejection of electrons in p and f partial waves. The agreement between experimental and calculated spectra is excellent.

Femtosecond energy- and angle-resolved pump-probe photoelectron spectra of wavepackets in a double-minimum state of Na 2 are studied. These studies illustrate that a robust description of the photoionization dynamics significantly enhances the utility of time-resolved photoelectron spectroscopy as a probe of wavepackets and of the evolution of electronic structure, particularly in regions of avoided crossings, and that the dependence of these angular distributions on the relative polarizations of the pump and probe lasers can be exploited to monitor molecular alignment.

Molecular electronic structure calculations involving unbound or scattering states occupy a small but important territory adjacent to the broader field of computational chemistry. Besides providing fundamental scientific insights, such calculations can provide essential data for technological applications, provided that reliable calculations can be carried out for the relatively large molecules of interest. In this article, we describe one approach to the electron–molecule collision problem and discuss how that approach has been adapted to parallel computers – from monolithic supercomputers to workstation clusters – in order to achieve the levels of performance necessary to address collisions with larger polyatomic targets.

We present a formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets and results of its application to the ^1Σ^+_u double-minimum state of aligned Na_2. The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump–probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.

Several recent studies have demonstrated how well-suited femtosecond time-resolved photoelectron spectra are for mapping wavepacket dynamics in molecular systems. Theoretical studies of femtosecond photoelectron spectra which incorporate a robust description of the underlying photoionization dynamics should enhance the utility of such spectra as a probe of wavepackets and of the evolution of electronic structure. This should be particularly true in regions of avoided crossings where the photoionization amplitudes and electronic structure may evolve rapidly with geometry. In this paper we present the results of studies of energy- and angle-resolved femtosecond photoelectron spectra for wavepackets in the diatomic systems, Na2 and NaI. Both cases involve motion through regions of avoided crossings. In Na2, however, wavepacket motion occurs on a single adiabatic potential with an inner and outer well and a barrier between them, while in NaI wavepackets move on the nonadiabatically coupled covalent (NaI) and ionic (Na+I−) potentials. Results of these studies will be used to illustrate the insight into wavepacket dynamics that time-resolved photoelectron spectra provide. For example, in the case of NaI these angle-resolved photoelectron spectra seem to offer some promise for probing real-time dynamics of intramolecular electron transfer occurring in the crossing region of the ionic and covalent states.

We report cross sections for collisions of low-energy electrons with carbon dioxide obtained by the Schwinger multichannel variational method.Elastic cross sections are obtained in the static-exchange-plus-polarization approximation. We pay particular attention to the position of the ^2Π_u resonance and to the strong enhancement in the integral cross section near zero energy, both prominent features whose accurate treatment requires an accounting for polarization effects. To include such effects in the resonant symmetry, we use an objective and physically motivated criterion to construct a set of configurations that accurately accounts for polarization while avoiding overcorrelation. In addition, we study the origin of the nonisotropic behavior of the elastic differential cross section at very low energies and conclude that it is caused by significant contributions from vibrationally excited CO_2. Cross sections from threshold to 50 eV for excitation of the ^3Σ^+_u, ^1Δ_u, ^3Δ_u, ^3Σ^−_u, and ^1Σ^−_u states that arise from the (^1π_g → ^2π_u) transition are presented for the first time.

We present a formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets and results of its application to the ^1Σ^+_u double-minimum state of aligned Na_2. The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump–probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.

Comprehensive simulations of the processing plasmas used in semiconductor fabrication will depend on the availability of basic data for many microscopic processes that occur in the plasma and at the surface. Cross sections for electron collisions, a principal mechanism for producing reactive species in these plasmas, are among the most important such data; however, electron-collision cross sections are difficult to measure, and the available data are, at best, sketchy for the polyatomic feed gases of interest. While computational approaches to obtaining such data are thus potentially of significant value, studies of electron collisions with polyatomic gases at relevant energies are numerically intensive. In this article, we report on the progress we have made in exploiting large-scale distributed-memory parallel computers, consisting of hundreds of interconnected microprocessors, to generate electron-collision cross sections for gases of interest in plasma simulations.

Experimental and theoretical results on the rotationally resolved photoelectron spectra of the [a ^1Δ]3dπ^2Φ(v'=0) and the [a^ 1Δ]5pπ^2Φ(v'=0) Rydberg states of the SH radical are presented. In particular, for the former state, ionization via a large number of members of different rotational branches is considered. The interplay between experiment and ab initio calculations, including alignment, allows for a detailed interpretation of the results. Asymmetries apparent in the rotational ionic distributions and the observed suppression of transitions involving a large change in angular momentum can be explained on the basis of the theory.

We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame photoelectron angular distributions around conical intersections in CS2. The technique employs wave packet densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of these results with measured molecular-frame photoelectron angular distributions for CS2 demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.

We report results from a combined experimental and computational study of low-energy electron interactions with acetaldehyde in the gas phase. Differential cross sections for elastic electron scattering were measured at selected incident energies from 1 to 50 eV, while corresponding first-principles calculations were carried out up to 30 eV. Integral and momentum-transfer cross sections were derived from the angle-differential data. The role of resonances in the scattering is examined and comparison is made to previous results for acetaldehyde and for its analogs, formamide and formic acid.

Results of calculations of the rotationally resolved photoelectron spectra of the V_2^+ ground state obtained by two-color excitation via the A ^3Π_u ← X^3 Σ_g^- system are reported and compared with recent pulsed-field-ionization zero-kinetic-energy measurements of these spectra. Agreement between the measured and calculated spectra is very encouraging. Both illustrate that the spin-orbit interaction plays an important role in photoionization of the A ^3Π_u state of V_2 leading to the X^4Σ_g^− state of V_2^+. Striking differences are seen in the ion rotational distributions for ionization via the A ^3Π_(1u) and A ^3Π_(2u spin-orbit components.

Absolute differential cross sections for electron-impact excitation of Rydberg states of CO have been measured from threshold to 3.7 eV above threshold and for scattering angles between 20° and 140°. Measured excitation functions for the b 3Σ+, B 1Σ+ and E 1π states are compared with cross sections calculated by the Schwinger multichannel method. The behaviour of the excitation functions for these states and for the j 3Σ+ and C 1Σ+ states is analysed in terms of negative-ion states. One of these resonances has not been previously reported.

We report results of experimental and theoretical studies of the vibrational branching ratios for CO 4sigma(-1) photoionization from 20 to 185 eV. Comparison with results for the 2sigma(u)(-1) channel of the isoelectronic N-2 molecule shows the branching ratios for these two systems to be qualitatively different due to the underlying scattering dynamics: CO has a shape resonance at low energy but lacks a Cooper minimum at higher energies whereas the situation is reversed for N-2.

We present absolute experimental cross sections for elastic scattering, vibrational excitation by electron impact and for dissociative electron attachment to 1,3-butadiyne, as well as calculations of the elastic cross sections.

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Here, we discuss and compare results of recent theoretical studies of rotationally resolved photoelectron spectra with measurements for molecules such as HBr, OH, NO, N2, CO, H20, H2CO, and CH3. These studies reveal the rich dynamics of quantum-state-specific studies of molecular photoionization and provide a robust description of key spectral features resulting from Cooper minima, autoionization, alignment, partial-wave mixing, and interference in related experimental studies.

We present the first measurements of ion rotational distributions for photoionization over an extended range [0 ≤ E_K ≤ 200 eV for N_2 (2σ^(−1)_u ) and 3 ≤ E_K ≤ 125 eV for CO (4σ^(−1))]. The N_2 ion rotational distributions are seen to change dramatically over this energy range, indicating that characteristically molecular behavior of the photoelectron persists far from ionization threshold. In addition, the N_2 and CO results show a strikingly different dependence on energy. Although differences are expected due to the absence of a center of symmetry in CO, detailed calculations reveal that this behavior arises from the presence of Cooper minima in the 2σ_u →kσ_g continuum in the case of N_2 and from an f‐wave shape resonance in the 4σ→kσ channel in CO. Agreement between measured and calculated ion rotational distributions is excellent. The N_2 results are also compared with electron bombardment ionization data. This comparison demonstrates that previous interpretations of electron bombardment data are prone to errors.

We report absolute differential and integral cross sections for electronic excitation of ethanol, by low-energy electron impact. Cross sections for low-lying excited states were measured at incident electron energies from 9 to 20 eV and at scattering angles from 5° through 130°. Our results include cross sections for excitation of the 1^3A" and 1^1A" states as well as for the 2^3A"+ 1^3A' and 2^1A" + 2^1A'+ section sums. Corresponding calculations were also performed using the Schwinger multichannel method, within an 11-channel close-coupling scheme.

Elastic differential and momentum transfer cross sections are reported for electron scattering by H2O at impact energies ranging from 4 to 50 eV. The iterative Schwinger variational method in the fixed-nuclei, static-exchange approximation is used to calculate the low partial wave scattering amplitudes and the higher partial wave contributions are included via closure using the Born approximation for a point-dipole. Comparison of our calculated cross sections with recent experimental and other theoretical results is encouraging.

Results of calculations of single-photon pulsed-field ionization of the 1π orbital of the X ^1Σ^+ (v" = 0) ground state of CO leading to the A Π (v+ = 0-2) excited ionic state are reported. Agreement between the calculated and measured spectra is reasonable except for the A ^2Π_(1/2) (v^+ = 0) level which shows strong autoionization at larger positive ΔJ transitions. Our studies show that the direct photoionization of the In orbital is very atomiclike.

We present results of experiment and theory for the alignment of CO^+(B^2Σ^+) and N_2^+(B^2Σ^+_u) photoions over an extended energy range (0 ≤ E_k ≤ 210 eV for CO and 0 ≤ E_k ≤ 250 eV for N_2). The polarization of CO^+(B^2Σ^+ → X^2Σ^+) and N_2^+(B^2Σ^+ → X^2Σ^+_g) fluorescence is used to interpret the oscillator strength distributions for normally unresolved degenerate ionization channels. The results show the influence of a CO 4σ → kσ shape resonance clearly and agreement between theory and experiment is excellent. However, agreement between the calculated and measured values is less satisfactory for N_2. This behavior is somewhat surprising, as previous rotationally resolved fluorescence experiments have shown excellent agreement between theory and experiment. This comparison helps to illustrate the complementarity of alignment studies relative to alternative probes of ionization. For both N_2 and CO, the data indicate that the photoions retain significant alignment even at high energies. The results demonstrate that even well above threshold the spectral dependence of the alignment (i.e., polarization) is very sensitive to the molecular environment for photoejection. Such behavior provides useful insight into fundamental scattering phenomena in chemical physics.

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.

The study of collisions between electrons and molecules is an old field, almost as old as the discovery of the electron itself. Yet it has never been a large field; indeed, if we confine our interest (as we do here) to low-energy collisions, only a handful of research groups, experimental or theoretical, are active throughout the world today. It might therefore be appropriate to begin with two questions: Why study such collisions at all? And why is so little research being done?

We measured differential cross sections for elastic (rotationally integrated) electron scattering on pyrimidine, both as a function of angle up to 180∘ at electron energies of 1, 5, 10, and 20 eV and as a function of electron energy in the range 0.1–14 eV. The experimental results are compared to the results of the fixed-nuclei Schwinger variational and R-matrix theoretical methods, which reproduce satisfactorily the magnitudes and shapes of the experimental cross sections. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. Resonant structures are observed at 0.2, 0.7, and 4.35 eV and calculations for different symmetries confirm their assignment as the X^2A^2, A^2B_1Ã^2B_1, and B˜2B1B̃2B1 shape resonances. As a consequence of superposition of coherent resonant amplitudes with background scattering the B^2B_1 shape resonance appears as a peak, a dip, or a step function in the cross sections recorded as a function of energy at different scattering angles and this effect is satisfactorily reproduced by theory. The dip and peak contributions at different scattering angles partially compensate, making the resonance nearly invisible in the integral cross section. Vibrationally integrated cross sections were also measured at 1, 5, 10 and 20 eV and the question of whether the fixed-nuclei cross sections should be compared to vibrationally elastic or vibrationally integrated cross section is discussed.

Cross sections for elastic scattering of low-energy electrons by tetrahydrofuran, a prototype for the furanose ring found in the backbone of DNA, have been measured and calculated over a wide energy range, with an emphasis on energies below 6 eV, where previous data are scarce. The measurements employ a thin-aperture version of the relative-flow method, while the calculations employ the Schwinger multichannel method with an extensive treatment of polarization effects. Comparisons with earlier results, both experimental and theoretical, are presented and discussed. A proper accounting for the strong permanent electric dipole of tetrahydrofuran is found to be essential to obtaining reliable cross sections, especially at energies below 5 eV.

We report results of a study of the electronic excitation of silane by low‐energy electron impact, including cross sections from 10 to 40 eV impact energy for excitation of the first triplet and singlet excited states (2t2→4sa1)1,3T2. Our results are calculated using the Schwinger multichannel method as implemented for massively parallel computers and include a correction for high‐impact‐parameter excitation of the optically allowed 1T2 transition. We also report values of the total dissociation cross section derived from our calculations and total scattering measurements, and we compare these with direct measurements of the dissociation cross section.

Combined theoretical and experimental studies of rotationally resolved photoelectron spectra for single‐photon ionization of the outermost valence orbitals of H2O, H2S, H2CO, and CH3 are reported. Agreement between calculated and measured spectra is very encouraging. Both show that photoionization dynamics is very molecular in origin for H2O, H2S, and H2CO but quite atomiclike for CH3. Parity selection rules and the angular momentum composition of the photoelectron are used to illustrate the dynamical aspects of photoionization of polyatomics as molecular symmetry changes in a group of structurally related systems.

We report both measured and calculated rotationally resolved threshold photoelectron spectra for single-photon coherent VUV ionization of the X ^1A_1 ground state of jet-cooled H_2O and H_2S. Agreement between the calculated and measured spectra is very encouraging. Both the H_2O and H_2S spectra show type a transitions which arise from odd angular momentum components of the photoelectron matrix element and are quite molecular in origin. These type a transitions, however, are much weaker in H_2S due to its more atomiclike character.

We present the first measurements of rotational distributions for photoionization over extended energy ranges [0 ≤ E_k ≤ 200 eV for N_2 (2σ^(-1)_u) and of 3 ≤ E_k ≤ 125 eV for CO (4σ^(−1))]. The N_2 and CO results show a strikingly unusual and different energy dependence. Although differences are expected due to the absence of a center of symmetry in CO, detailed calculations reveal that this behavior arises from the presence of Cooper minima in the photoelectron continuum (kσ_g) in the case of N_2 and from an f-wave shape resonance for 4σ^(−1) photoionization in CO.

Results of theoretical studies of rotational branching ratios and photoelectron angular distributions for (2 + 1) resonance-enhanced multiphonon ionization (REMPI) of NO via the Q_(21)(13.5) branch of the C ^2Π(3pπ) Rydberg state are reported and compared with recent measurements. Both calculated and measured spectra reveal that the ΔN=±2 peaks (ee line), which are very weak for the laser polarization parallel to the direction of photoelectron detection, have about 1/4 the intensity of the main ΔN=0 peak when the laser polarizaiton is rotated by 90°. This dependence on laser polarization is due to strong inteference among partial wave components of the photoelectron matrix element. This behavior is very evident in the photoelectron angular distribution. In addition, a Cooper minimum is predicted in the d wave of the 3pπ→Kδ ionization channel.