Photoelectron trapping in N2O 7sigma-->ksigma resonant ionization

Vibrationally resolved photoelectron spectroscopy of the N2O+(A 2Sigma+) state is used to compare the dependence of the photoelectron dynamics on molecular geometry for two shape resonances in the same ionization channel. Spectra are acquired over the photon energy range of 18< or =hv< or =55 eV. There are three single-channel resonances in this range, two in the 7sigma-->ksigma channel and one in the 7sigma-->kpi channel. Vibrational branching ratio curves are determined by measuring vibrationally resolved photoelectron spectra as a function of photon energy, and theoretical branching ratio curves are generated via Schwinger variational scattering calculations. In the region 30< or =hv< or =40 eV, there are two shape resonances (ksigma and kpi). The ksigma ionization resonance is clearly visible in vibrationally resolved measurements at hv=35 eV, even though the total cross section in this channel is dwarfed by the cross section in the degenerate, more slowly varying 7sigma-->kpi channel. This ksigma resonance is manifested in non-Franck-Condon behavior in the approximately antisymmetric v3 stretching mode, but it is not visible in the branching ratio curve for the approximately symmetric v1 stretch. The behavior of the 35-eV ksigma resonance is compared to a previously studied N2O 7sigma-->ksigma shape resonance at lower energy. The mode sensitivity of the 35-eV ksigma resonance is the opposite of what was observed for the lower-energy resonance. The contrasting mode-specific behavior observed for the high- and low-energy 7sigma-->ksigma resonances can be explained on the basis of the "approximate" symmetry of the quasibound photoelectron resonant wave function, and the contrasting behavior reflects differences in the continuum electron trapping. An examination of the geometry dependence of the photoelectron dipole matrix elements shows that the ksigma resonances have qualitatively different dependences on the individual bond lengths. The low-energy resonance is influenced only by changes in the end-to-end length of the molecule, whereas the higher-energy resonance depends on the individual N-N and N-O bond lengths. Branching ratios are determined for several vibrational levels, including the symmetry-forbidden bending mode, and all of the observed behavior is explained in the context of an independent particle, Born-Oppenheimer framework.     

Mass spectrometry study of the fragmentation of valence and core-shell (Cl 2p) excited CHCl3 and CDCl3 molecules

The dissociative photoionization of the chloroform and chloroform-d molecules has been studied in the valence region and around the chlorine 2p edge. Time-of-flight mass spectrometry in the coincidence mode-namely, photoelectron-photoion coincidence (PEPICO)-was employed. He I lamp and tunable synchrotron radiation were used as light sources. Total and partial ion yields have been recorded as a function of the photon energy. Singly, doubly, and triply ionized species have been observed below (195 eV), on (201 eV), and above (230 eV) the Cl 2p resonances. A definite degree of site-selective fragmentation was observed at the Cl 2p resonance as the relative contributions of several ionic species were seen to go through a maximum at 201 eV. At the same time all stable doubly charged ions were also observed at 198 eV (below the 2p resonances), resulting from direct ionization processes. Isotopic substitution is shown to provide a very efficient means of improving the mass resolution and assignment of unresolved peaks in spectra of CHCl(3), particularly for those fragments differing by a hydrogen atom. It is suggested that ultrafast fragmentation of the system following 2p excitation to a strongly antibonding state contributes to the large amount of Cl(+) observed in the PEPICO spectrum measured at 201 eV. Kinetic energy distributions were determined for the H(+), D(+), and Cl(+) fragments.     

Shape resonance and electronic autoionization in vibrationally resolved partial photoionization cross sections of O2

Vibrationally resolved partial photoionization cross sections of O₂ leading to the b⁴Σ^?_g and B²Σ^?1_g(3σ^?1_g) tonic states have been measured at high resolution in a wide photon energy range from 0.5 eV above threshold to 24.5 eV. The σ_u shape resonance is observed around 21.5 eV, in fairly good agreement with “one-electron” theoretical predictions. The series of resonances from 19 to 20 5 eV result from an autoionization process with a strong vibrational selectivity, explained by the similar geometries of the resonance states and the b ionic state. The large resonance widths originate from a strong Rydberg-valence orbital mixing Calculations using multichannel quantum defect theory reproduce the most important features.     

Electronic structure of ReO3Me by variable photon energy photoelectron spectroscopy,absorption spectroscopy and density functional calculations

Valence photoelectron (PE) spectra have been measured for ReO(3)Me using a synchrotron source for photon energies ranging between 20 and 110 eV. Derived branching ratios (BR) and relative partial photoionization cross sections (RPPICS) are interpreted in the context of a bonding model calculated using density functional theory (DFT). Agreement between calculated and observed ionization energies (IE) is excellent. The 5d character of the orbitals correlates with the 5p --> 5d resonances of the associated RPPICS; these resonances commence around 47 eV. Bands with 5d character also show a RPPICS maximum at 35 eV. The RPPICS associated with the totally symmetric 4a(1) orbital, which has s-like character, shows an additional shape resonance with an onset of 43 eV. The PE spectrum of the inner valence and core region measured with photon energies of 108 and 210 eV shows ionization associated with C 2s, O 2s, and Re 4f and 5p electrons. Absorption spectra measured in the region of the O1s edge showed structure assignable to excitation to the low lying empty "d" orbitals of this d(0) molecule. The separation of the absorption bands corresponded with the calculated orbital splitting and their intensity with the calculated O 2p character. Broad bands associated with Re 4d absorption were assigned to (2)D(5/2) and (2)D(3/2) hole states. Structure was observed associated with the C1s edge but instrumental factors prevented firm assignment. At the Re 5p edge, structure was observed on the (2)P(3/2) absorption band resulting from excitation to the empty "d" levels. The intensity ratios differed from that of the O 1s edge structure but were in good agreement with the calculated 5d character of these orbitals. An absorption was observed at 45 eV, which, in the light of the resonance in the 4a(1) RPPICS, is assigned to a 4a(1) --> ne, na(2) transition. The electronic structure established for ReO(3)Me differs substantially from that of TiCl(3)Me and accounts for the difference in chemical behavior found for the two complexes.     

Inner-valence photoionization of O((1)D): experimental evidence for the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition

Photoionization and autoionization of electronically excited atomic oxygen O((1)D) are investigated in the energy range between 12 and 26 eV using tunable laser-produced plasma radiation in combination with time-of-flight mass spectrometry. A broad, asymmetric, and intense feature is observed that is peaking at 20.53+/-0.05 eV. It is assigned to the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition, which subsequently autoionizes by a Coster-Kronig transition, as predicted by the previous theoretical work [K. L. Bell et al., J. Phys. B 22, 3197 (1989)]. Specifically, the energy of the unperturbed transition occurs at 20.35+/-0.07 eV. Its shape is described by a Fano profile revealing a q parameter of 4.25+/-0.8 and a width of gamma=2.2+/-0.15 eV. Absolute photoionization cross section sigma is derived, yielding sigma=22.5+/-2.3 Mb at the maximum of the resonance. In addition, weak contributions to the O((1)D) yield from dissociative ionization originating from molecular singlet oxygen [O(2)((1)Delta(g))] are identified as well. Possible applications of the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition as a state-selective and sensitive probe of excited oxygen in combination with photoionization mass spectrometry are briefly discussed.     

N 1s photoelectron angular distributions from fixed-in-space NO2 molecules: stereodynamics and symmetry considerations

Angular distributions of N 1s photoelectrons from fixed-in-space NO(2) molecules have been measured over the energy region of shape resonance and above. A multiple-coincidence velocity-map imaging technique for observation of molecular frame photoelectron angular distributions (MF-PADs) has been extended to nonlinear molecular targets. Density functional theory calculations have also been conducted to elucidate the photoionization dynamics and shape resonance in the N 1s photoionization of NO(2). Results show that the N 1s MF-PADs exhibit strong shape variation as a function of both photoelectron kinetic energy and symmetries of final states, whereas asymmetry parameters of laboratory frame PADs show a local minimum around the shape resonance region and increase monotonically as the photon energy increases. Over the shape resonance, the spatial shape of the photoelectron wave function with b(2)-symmetry closely resembles that of 5b(2)(?) unoccupied molecular orbital of NO(2), although the MF-PAD pattern for b(2)-symmetry does not correspond directly to the 5b(2)(?) orbital shape. At higher kinetic energy of 90 eV, MF-PADs become less structured, but still show a significant dependence on the symmetry of final states.     

High resolution electron attachment to CO₂ clusters

Electron attachment to CO? clusters performed at high energy resolution (0.1 eV) is studied for the first time in the extended electron energy range from threshold (0 eV) to about 10 eV. Dissociative electron attachment (DEA) to single molecules yields O(-) as the only fragment ion arising from the well known (2)Π(u) shape resonance (ion yield centered at 4.4 eV) and a core excited resonance (at 8.2 eV). On proceeding to CO? clusters, non-dissociated complexes of the form (CO?)(n)(-) including the monomer CO?(-) are generated as well as solvated fragment ions of the form (CO?)(n)O(-). The non-decomposed complexes appear already within a resonant feature near threshold (0 eV) and also within a broad contribution between 1 and 4 eV which is composed of two resonances observed for example for (CO?)(4)(-) at 2.2 eV and 3.1 eV (peak maxima). While the complexes observed around 3.1 eV are generated via the (2)Π(u) resonance as precursor with subsequent intracluster relaxation, the contribution around 2.2 eV can be associated with a resonant scattering feature, recently discovered in single CO? in the selective excitation of the higher energy member of the well known Fermi dyad [M. Allan, Phys. Rev. Lett., 2001, 87, 0332012]. Formation of (CO?)(n)(-) in the threshold region involves vibrational Feshbach resonances (VFRs) as previously discovered via an ultrahigh resolution (1 meV) laser photoelectron attachment method [E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf and H. Hotop, Eur. Phys. J. D, 2000, 12, 125]. The complexes (CO?)(n)O(-) clearly arise from DEA at an individual molecule within the cluster involving both the (2)Π(u) and the core excited resonance.     

Vibrational branching ratios in photoionization of CO and N2

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 N2 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 N2.     

Core level (S 2p) excitation and fragmentation of the dimethyl sulfide and dimethyldisulfide molecules

Electronic excitation and ionic dissociation of dimethylsulfide (DMS) and dimethyldisulfide (DMDS) have been studied around the S 2p edge using synchrotron radiation and time-of-flight mass spectrometry techniques. Mass spectra were obtained for both molecules, below, on and above the well defined resonances observed in the S 2p photoabsorption spectrum and centered at approximately 166 eV photon energy. Ab initio IS-CASSCF calculations were performed for a better understanding of the photoabsorption spectra. Similar calculations were also performed for the H(2)S molecule, in order to establish a bench mark. For both molecules, a higher fragmentation degree is observed with increasing photon energy. In the DMDS case, selective fragmentation was observed in the formation of the [CH(n)S](+) ions at the first S 2p resonance (corresponding to excitation to a σ*SS state) and in the formation of the [S(2)](+) and [S](+) ions at the third S 2p resonance (corresponding to excitation to a σ*CS state). Previously unreported doubly charged ([S](2+), [CH(3)](2+)) are observed for DMS and DMDS.     

Molecular size effect in NCO and NCS dianion resonances

Cross sections for electron-impact detachment and electron-impact dissociation of NCO- and NCS- were measured from about 3 to about 40 eV. The former are found to follow a classical prediction with a threshold energy of 9.1 +/- 0.1 eV for NCO- and 8.9 +/- 0.2 eV for NCS-. When the incoming electron binds to the monoanion, a short-lived dianion complex is formed, which is revealed as a resonance in the cross section. For NCO- a resonance is evident at 9.3 +/- 0.2 eV, which implies that the dianion lies above the monoanion by this amount of energy. In the case of NCS- two resonances are evident at 8.4 +/- 0.2 and 19.0 +/- 0.5 eV, respectively. The low-energy NCS dianion is less unstable than the dianion of NCO, which in turn is less unstable than the CN dianion (10-eV resonance). Thus the resonance shifts down in energy with the increasing size of the anion, a fact which is attributed to a decrease in Coulomb energy between the spatially separated electrons.     

High resolution gas phase photoabsorption spectra of SiCl4 and Si(CH3)4 at the silicon l edges: characterization and assignment of resonances

High resolution gas phase photoabsorption spectra of SiCl₄, and Si(CH₃)₄ have been measured using synchrotron radiation in the 100–225 eV photon energy range. This range encompasses the Si 2p, Si 2s and Cl 2p edges. Comparison with previous spectra shows that the high resolution is essential for characterizing all Si 2p features - both the narrow intense pre-edge features, and the broader above edge shape resonances. The Si 2s spectra, along with MS Xα calculations, enable assignment of the more complex Si 2p shape resonances. There is good agreement between theory and experiment for the Si 2p and Si 2s SiCl₄ shape resonance positions. The general shape of the shape resonance structure above the Si 2p and Si 2s edges of Si(CH₃)₄ is well reproduced by the Xα calculations.     

Electronically forbidden (5sigmau-->ksigmau) photoionization of CS2: mode-specific electronic-vibrational coupling

Vibrationally resolved photoelectron spectroscopy of the CS(2) (+)(B (2)Sigma(u) (+)) state is used to show how nontotally symmetric vibrations "activate" a forbidden electronic transition in the photoionization continuum, specifically, a 5sigma(u)-->ksigma(u) shape resonance, that would be inaccessible in the absence of a symmetry breaking vibration. This electronic channel is forbidden owing to inversion symmetry selection rules, but it can be accessed when a nonsymmetric vibration is excited, such as bending or antisymmetric stretching. Photoelectron spectra are acquired for photon energies 17kpi(g), influences the symmetric stretch branching ratio. All of the observed effects can be understood within the framework of the Chase adiabatic approximation, i.e., the Born-Oppenheimer approximation applied to photoionization.     

Near-threshold shape resonance in the photoionization of 2-butyne

Photoelectron velocity map imaging is combined with one- and two-photon ionization to study the near threshold photoionization of the 2-butyne molecule. In this region, the photoabsorption and photoionization cross sections display a very intense broad feature that is assigned to an l = 4, π(g) shape resonance. The effect of this shape resonance on the vibrational branching ratios and photoelectron angular distributions is explored. Theoretical calculations of the photoionization cross section and photoelectron angular distributions are in good agreement with the experiments. The results for 2-butyne are compared with those of acetylene, propyne, and 1-butyne, none of which show such significant enhancements near threshold, and the differences are rationalized in terms of the symmetries and orbital angular momenta of the highest occupied orbitals and the corresponding shape resonances. Expectations for larger alkynes and alkynyl radicals are also discussed. A preliminary measurement of the ionization energy of the 2-butyne dimer is also presented.     

Calculation of photoionization cross sections of Na2–8 and K2–8 clusters

Analysis of free metal clusters studied with photoionization mass spectrometry or photoelectron spectroscopy requires theoretical predictions of the photoionization cross sections to gain a deeper physical understanding. Calculated energy-dependent photoionization cross sections of Na_2–8 and K_2–8 clusters are presented in this study. The ground state electronic structure of the clusters are calculated using the Local Spin Density method (LSD) which is also the starting point for the cross section calculation with the continuum multiple scattering method. A basic analysis of the photoionization process is given within the independent electron picture. Strong resonances are predicted in the UV cross sections (5–10 eV) of K_3–8 but not for Na_3–8, interpreted as shape resonances, i.e. quasibound states in which electrons are trapped by a potential barrier. Unfortunately experimental data are only known close to the ionization threshold and a comparison between our values and experimental data in a broad energy range is not possible.     

An unusual pi* shape resonance in the near-threshold photoionization of S1 para-difluorobenzene

Previously reported dramatic changes in photoelectron angular distributions (PADs) as a function of photoelectron kinetic energy following the ionization of S1 p-difluorobenzene are shown to be explained by a shape resonance in the b(2g) symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual pi* antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed.     

Electron-induced chemistry of alcohols

We studied dissociative electron attachment to a series of compounds with one or two hydroxyl groups. For the monoalcohols we found, apart from the known fragmentations in the 6-12 eV range proceeding via Feshbach resonances, also new weaker processes at lower energies, around 3 eV. They have a steep onset at the dissociation threshold and show a dramatic D/H isotope effect. We assigned them as proceeding via shape resonances with temporary occupation of sigma orbitals. These low energy fragmentations become much stronger in the larger molecules and the strongest DEA process in the compounds with two hydroxyl groups, which thus represent an intermediate case between the behavior of small alcohols and the sugar ribose which was discovered to have strong DEA fragmentations near zero electron energy [S. Ptasińska, S. Denifl, P. Scheier and T. D. M?rk, J. Chem. Phys., 2004, 120, 8505]. Above 6 eV, in the Feshbach resonance regime, the dominant process is a fast loss of a hydrogen atom from the hydroxyl group. In some cases the resulting (M- 1)(-) anion (loss of hydrogen atom) is sufficiently energy-rich to further dissociate by loss of stable, closed shell molecules like H(2) or ethene. The fast primary process is state- and site selective in several cases, the negative ion states with a hole in the n(O) orbital losing the OH hydrogen, those with a hole in the sigma(C-H) orbitals the alkyl hydrogen.     

Near threshold photoionization of the ground and first excited states of C(2)

Calculations using the multichannel Schwinger configuration-interaction method are presented for the photoionization from the ground and the first excited states of the C(2) molecule. Both single channel and multichannel calculations are presented in a photon energy range from the threshold to about 50 eV of photon energy. For the ground state, inclusion of both intrinsic and dynamical correlation effects is seen to strongly alter the picture of the photoionization process inferred from single-channel frozen-core Hartree-Fock calculations. Furthermore, the photoionization study of the first excited state of molecular carbon has revealed the presence of strong interchannel coupling between the 3sigma(g)-->ksigma(u) channel and the photoionization channels leading to the A (4)Pi(g) and f (2)Pi(g) ionic states in the near threshold region.     

Dissociative electron attachment to Pt(PF3)4--a precursor for focused electron beam induced processing (FEBIP)

Experimental absolute cross sections for dissociative electron attachment (DEA) to Pt(PF(3))(4) are presented. Fragment anions resulting from the loss of one, two, three and four PF(3) ligands as well as the Pt(PF(3))F(-) and the F(-) ions were observed. The parent anion Pt(PF(3)) is too short-lived to be detected. The dominant process is loss of one ligand, with a very large cross section of 20?000 pm(2); the other processes are about 200× weaker, with cross sections around 100 pm(2), the naked Pt(-) anion is formed with a cross section of only 1.8 pm(2). The resonances responsible for the DEA bands were assigned based on comparison with electron energy-loss spectra and spectra of vibrational excitation by electron impact. Bands around 0.5 eV and 2 eV were assigned to shape resonances with single occupation of virtual orbitals. A DEA band at 5.9 eV was assigned to a core-excited resonance corresponding to an electron very weakly bound to the lowest excited state. An F(-) band at 12.1 eV is assigned to a core excited resonance with a vacancy in an orbital corresponding to the 2nd ionization energy of the PF(3) ligand. Implications of these findings for FEBIP are discussed.     

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.     

Angle-resolved photoelectron spectra of CF3Br and CF2Br2 in the photon energy range 29–115 eV

Photoelectron asymmetry parameters β and partial photoionization cross sections have been measured for ionization from the molecular orbitals of CF₃Br and CF₂Br₂ using synchrotron radiation in the photon energy range from 19 to 115 eV. Cooper minima are observed in the β spectra for ionizations from orbitals with substantial bromine 4p character. The measured positions of the minima have been compared to the results of non-relativistic calculations performed for different elements of the periodic system. Several shape resonances have been observed in β spectra associated with orbitals of predominantly F(2p) character. Ionization from the Br(3d) subshell has also been observed and studied in some detail. Auger transitions involving 3d electrons have been observed at higher photon energies.