Relative Partial Cross Sections Research Articles

The electron affinity of rubidium: a state selective measurement

Negative ions, which are formed when an electron is attached to a neutral system, are unique quantum systems. The lack of a long-range Coulomb force causes the inter-electronic interactions to become relatively more important. As a consequence, the independent particle model, which adequately describes atomic structure under normal conditions, breaks down. The alkali negative ions, with a closed valence s-shell, are among the simplest anionic systems. Hence, they can favorably be used to benchmark atomic theory. In this work, we have determined the electron affinity of 85Rb by measuring the relative partial photodetachment cross section of the negative ion, leaving the residual atom in the 5p 2P3/2 excited state. Resonance ionization spectroscopy allows for state selectivity and the ability to measure the Wigner s-wave threshold onset of the photodetachment process. The electron affinity of 85Rb was determined to be 485.887(6) meV.

Total Electron Detachment and Induced Cationic Fragmentation Cross Sections for Superoxide Anion (O2-) Collisions with Benzene (C6H6) Molecules.

In this study, novel experimental total electron detachment cross sections for O2− collisions with benzene molecules are reported for the impact energy range (10–1000 eV), as measured with a transmission beam apparatus. By analysing the positively charged species produced during the collision events, relative total ionisation cross sections were derived in the incident energy range of 160–900 eV. Relative partial ionisation cross sections for fragments with m/z ≤ 78 u were also given in this energy range. We also confirmed that heavier compounds (m/z > 78 u) formed for impact energies between 550 and 800 eV. In order to further our knowledge about the collision dynamics governing the fragmentation of such heavier molecular compounds, we performed molecular dynamics calculations within the framework of the Density Functional Theory (DFT). These results demonstrated that the fragmentation of these heavier compounds strongly supports the experimental evidence of m/z = 39–42, 50, 60 (u) cations formation, which contributed to the broad local maximum in the total ionisation observed from 550 to 800 eV. This work reveals the reactivity induced by molecular anions colliding with hydrocarbons at high energies, processes that can take place in the interstellar medium under various local conditions.

Dissociative electron attachment to benzoic acid (C7H6O2)

The dissociative electron attachment (DEA) to benzoic acid (C6H5COOH) has been studied using an experimental crossed beam setup of a quadrupole mass spectrometer and a trochoidal electron monochromator. Relative partial cross sections for the DEA to produce negative ion fragments show the main channels for dissociation. The comparison of the present results with the ultraviolet photoelectron spectrum of benzoic acid [J. Meeks, A. Wahlborg, and S. P. McGlynn, J. Electron Spectrosc. Relat. Phenom. 22, 43 (1981)] implies that most DEA bands in the high energy range are due to Feshbach resonances with double occupation of diffuse Rydberg-like orbitals. The measurements are supported by density functional theory calculations of the threshold energies.

Investigation of discrete states and quasidiscrete structures observed in Sm150 and Sm152 using the (p,tγ) reaction

New levels and $\ensuremath{\gamma}$-ray transitions were identified in $^{150,152}\mathrm{Sm}$ utilizing the $(p,t)$ reaction and particle-$\ensuremath{\gamma}$ coincidence data. A large, peak-like structure observed between 2.3--3.0 MeV in excitation energy in the triton energy spectra was also investigated. The orbital angular-momentum transfer was probed by comparing the experimental angular distributions of the outgoing tritons to calculated distorted wave Born approximation curves. The angular distributions of the outgoing tritons populating the peak-like structure are remarkably similar in the two reactions and are significantly different from the angular distributions associated with the nearby continuum region. Relative partial cross sections for the observed levels, angle averaged between 34 and 58 degrees, were measured. In $^{150}\mathrm{Sm}$, 39(4)% of the strength of the peak-like structure could be accounted for by the observed discrete states. This compares with a value of 93(15)% for $^{152}\mathrm{Sm}$.

Kinetic energy distribution for the ionization and dissociation process of C2H4 by electron impact

The dissociative ionization of ethylene molecules following impact by electrons in the energy range of 20 to 200 eV has been studied with a cold target recoil-ion momentum spectrometer (COLTRIMS). The relative partial cross sections for the ionic fragments C2Hn+ (n=0~3) with respect to C2H4+ were plotted as a function of the incident energy. By measuring the time of flight and position of the fragment ions, the kinetic energy distribution has been deduced. Taking advantage of the supersonic jet expansion, the thermal motion contributing to the kinetic energy distribution of the recoil-ions was reduced. The average kinetic energy of the recoil ions as a function of the incident electron energy has also been plotted. Furthermore, the average kinetic energy release for the dissociation to C2H3+ and C2H2+ were estimated as 0.76 eV and 0.73 eV respectively.

Inner-shell photoionization of atomic chlorine

Relative partial cross sections have been measured following photoexcitation of atomic chlorine near the Cl 2p and Cl 1s ionization thresholds. In addition, Breit-Pauli R-matrix calculations have been carried out in the region of the 2p thresholds, and the results are compared with experiment. Owing to angular-momentum considerations, it was found that the resonances associated with the higher 2p(-1) thresholds should be significantly wider than the lower ones, and this is borne out in both the experimental and the theoretical results. It is shown that a large number of resonance series contribute to the cross section, which make it difficult to untangle, and suggestions for further work to better understand the spectra are presented.

Electron ionisation of sulfur dioxide

Relative precursor-specific partial ionisation cross sections for the fragment ions formed following electron ionisation of sulfur dioxide (SO2) have been measured for the first time, from 30 to 200 eV, using time-of-flight mass spectrometry coupled with two-dimensional ion coincidence detection. These data quantify the yields of O(2+), O(+), SO(2+), S(+), O2(+), and SO(+) ions, relative to the formation of SO2(+), via single, double, and triple electron ionisation of SO2. Formation of O(2+), following electron-SO2 collisions, has been quantified for the first time. The data allow a first experimental estimate of the triple ionisation potential of SO2 (69.0 ± 3.6 eV), an energy in good agreement with a value derived in this study via computational chemistry. The triple ion combination S(+) + O(+) + O(+) is clearly detected following electron collisions with SO2 at electron energies markedly below the vertical energy for forming SO2(3 +). This observation is accounted for by the operation of a stepwise pathway to the formation of S(+) + 2O(+) which does not involve the formation of a molecular trication.

Microscopic two-nucleon overlaps and knockout reactions from12C

The nuclear structure dependence of direct reactions that remove a pair of like or unlike nucleons from a fast $^{12}$C projectile beam are considered. Specifically, we study the differences in the two-nucleon correlations present and the predicted removal cross sections when using $p$-shell shell-model and multi-$\hbar\omega$ no-core shell-model (NCSM) descriptions of the two-nucleon overlaps for the transitions to the mass $A$=10 projectile residues. The NCSM calculations use modern chiral two-nucleon and three-nucleon (NN+3N) interactions. The $np$-removal cross sections to low-lying $T$=0, $^{10}$B final states are enhanced when using the NCSM two-nucleon amplitudes. The calculated absolute and relative partial cross sections to the low energy $^{10}$B final states show a significant sensitivity to the interactions used, suggesting that assessments of the overlap functions for these transitions and confirmations of their structure could be made using final-state-exclusive measurements of the $np$-removal cross sections and the associated momentum distributions of the forward travelling projectile-like residues.

Electron ionization of W(CO)6: Appearance energies

The electron ionization (EI) of W(CO)6 has been studied in detail using a high resolution crossed electron-molecular beams technique. We have detected singly (W+, CW+, (CO)xW+ (x=1–6), (CO)yCW+ (y=1–3), CO+) and doubly charge positive ions (W2+, CW2+, (CO)xW2+ (x=1–6) and (CO)yCW2+ (y=1–3)). The relative partial cross sections for formation of particular ions have been measured and the breakdown curves constructed. The ionization energies (IE) for single (8.47±0.06eV) and double ionization (22.90±0.08eV) of the molecule, as well as the appearance energies (AEs) for the dissociative ionization channels for singly and doubly charged ions have been estimated from experimental ion efficiency curves. On the basis of the IEs and AEs values we have calculated sequential bond dissociation energies (BDEs) of different bonds in the positive ions formed from W(CO)6.

Electron Ionization of Dimethylphenylsilane - Appearance Energies of Selected Ionic Fragments

Electron ionization (EI) to dimethylphenylsilane (DMPS) is studied in crossed electron-molecular beams experiment. Using this technique with improved energy resolution of the electron beam, the positive mass spectra and the relative partial cross sections for EI to DMPS are obtained. The ionization energy of DMPS of 9.04 ± 0.06 eV and the threshold energies for dissociative ionization channels are estimated. The bond dissociation energies for single bond cleavage channels are also estimated on the basis of experimental observations. The detection of doubly charged ions of DMPS2+ (AE = 26.3 ± 1 eV) as well as several other doubly charged fragments is discussed in detail.

Partial-photoelectron-wave analysis in diatomic molecule photoionization by fluorescence polarization experiments

Parameters $\ensuremath{\beta}1$ and $\ensuremath{\beta}2$ describing the angular distribution of circularly polarized fluorescence photons emitted by randomly oriented diatomic molecules excited by circularly polarized radiation are derived. We prove theoretically that measuring these parameters allows a partial-wave analysis of the emitted photoelectrons in the case of closed-shell diatomic molecules. The determination of the relative partial cross sections for emission of the $\ensuremath{\varepsilon}\ensuremath{\sigma}$, $\ensuremath{\varepsilon}\ensuremath{\pi}$, and $\ensuremath{\varepsilon}\ensuremath{\delta}$ photoelectrons is of fundamental importance for a sensitive test of corresponding calculations.

Specific formation of negative ions from leucine and isoleucine molecules

Dissociative electron attachment (DEA) to gas phase leucine (Leu) and isoleucine (Ile) molecules was studied using experimental and quantum-chemical methods. The relative partial cross sections for DEA have been measured using crossed electron/molecular beams technique. Supporting ab initio calculations of the structure, energies of neutral molecules, fragments, and negative ions have been carried out at G3MP2 and B3LYP levels in order to interpret the experimental data. Leu and Ile exhibit several common features. The negative ionic fragments from both molecules are formed in the electron energy range from 0 to approximately 14 eV via three resonances (1.2, 5.5, and 8 eV). The relative partial cross sections for DEA Leu and Ile are very similar. The dominant negative ions formed were closed shell negative ions (M-H)(-) (m/z=130) formed preferentially via low electron energy resonance of 1.23 eV. Additional negative ions with m/z=115, 114, 113, 112, 84, 82, 74, 45, 26, and 17 have been detected.

A mystery solved? Photoelectron spectroscopic and quantum chemical studies of the ion states of CeCp3+

The electronic states of CeCp(3)(+) have been studied experimentally by variable photon energy photoelectron spectroscopy, and computationally using multi-configurational ab initio methods. Relative partial photoionisation cross section and branching ratio data are presented to confirm our previous conclusion that bands A and D in the valence photoelectron spectrum, despite their 3.2 eV separation, are produced by ionization of the single 4f electron of CeCp(3) [M. Coreno, M. de Simone, J. C. Green, N. Kaltsoyannis, N. Narband and A. Sella, Chem. Phys. Lett., 432, 2006, 17]. The origin of this effect is probed using the CASSCF/CASPT2 approach. While configurations based on the canonical CASSCF orbitals are found to be an unreliable description of the ground and excited states of CeCp(3)(+), the state-specific natural orbitals and their occupations yield greater insight, allowing us to characterize ion states in terms of the presence or otherwise of a Ce 4f-localised electron. Neither the CeCp(3)(+) ground state (assigned to band A), and two excited states ((1)A' and (1)A'', associated with band D), possess such a metal-based electron, as expected of f ionization. The (1)A' and (1)A'' states differ from the ground state in having a significant Ce 5d population, arising from Cp --> Ce charge transfer, which accompanies f ionization, and which is responsible for the energetic separation of bands A and D in the valence photoelectron spectrum.

A systematic study of photoionization of free lanthanide atoms in the 4d giant resonance region

Charge-separated photoion-yield spectroscopy using monochromatized synchrotron radiation was conducted on free Ce, Nd, Gd, Dy, and Er lanthanide atoms in the 4d giant resonance region to measure the relative partial cross-section curves σ(M n+ ) for the formation of M n+ ( n = 1–4) ions. In addition, photoelectron spectroscopy was conducted on free Nd, Dy, and Er atoms to measure the cross-sections σ( nl) for photoionization of the nl (=4d, 5s, 5p and 4f) subshell electron. Drawing upon the obtained spectra and the results of previous photoion-yield studies on Xe, Cs, Ba, Sm, Eu, and Yb atoms, we discuss the character of photoionization processes in the 4d region. The obtained charge-state distribution shows that as the atomic number Z increases, the 4d giant resonance changes gradually, rather than suddenly, from a shape resonance to an autoionization resonance. One possible explanation for this change is that it is due to the dependence of the potential for the excited 4f electron and the corresponding 4f radial wavefunction on terms in the 4d 94f n+1 configuration. For σ(M +) and σ(4f), which exhibit an asymmetric Beutler–Fano autoionization profile, a curve fitting analysis was conducted to obtain the profile parameters E r, Γ, and q. It was found that for the Gadolinium atom, which has a configuration [Xe]4f 75d6s 2 characterized by a half-filled 4f 7 subshell plus a 5d electron, both the asymmetry parameter q and the charge-state distribution varied peculiarly with the atomic number Z.

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.

Relative partial cross sections for anions formed upon electron attachment to nitrotoluene

Free electron attachment to the three different isomers of mono-nitrotoluene molecules in the gas phase is studied using a crossed electron-molecule beams technique. For the molecule 2-nitrotoluene 77 products anions are measured. The ion yield of the different anions spans a range of more than six orders of magnitude. The total anion yield of the three different isomers of mono-nitrotoluene is compared and shows remarkable differences mainly resulting from the different contributions of the NO 2 − fragment anions. The anion efficiency curves show six resonances that we assign to different transient negative ions. Competition of decay reactions is forming a series of fragment anions from each isomeric transient negative anion.

Beryllium doubly excited autoionizing resonances between the2pand3pthresholds

We present results of high resolution photoelectron spectrometry measurements for the $\ensuremath{\sigma}(2s)$ relative partial photoionization cross section between the $2p$ and $3p$ ionization thresholds in atomic beryllium. This includes the $3snp$ autoionizing resonance series converging to the $3s$ threshold, the $3pns$, $nd$ series converging to the $3p$ threshold, and several members of the $3dnp$, $nf$ series. The results for the perturbed $3snp$ series are compared with the ion yield results that give the total photoionization cross section. We found very good agreement with calculations for the $2s$ partial cross section both in the $R$-matrix approximation and a hyperspherical calculation. We confirm the perturbation of the series by the $3p4s$ interloper with a resonance profile that is the same as for the total cross section. We did not find evidence of a second interloper $(3d4p)$ between the $3s10p$ and $3s11p$ resonances. We also find very good agreement between experiment and theory for the Shore parameters of these resonances. The data for the resonances between the $3s$ and $3p$ thresholds are compared with the results of a $R$-matrix $2s$ cross section calculation. Good agreement is found for the energy positions and overall resonance shapes once the calculated energies are referred to the experimental $3p$ threshold.

Dissociative photo-multiple-ionisation of CO and CO2

In a photoelectron–photoion coincidence experiment on CO and CO2, we have observed the formation and fragmentation of singly to triply charged CO2 and singly to quadruply charged CO at various energies. Doubly charged cations of both molecules are found to have unstable as well as stable states. Cations with higher charge are found to dissociate promptly. The energy dependence of the relative partial cross-sections in the energy range 125–310 eV are presented.

Determining the Partial Photoionization Cross-Sections of Ethyl Radicals

Using a crossed laser-molecular beam scattering apparatus, these experiments photodissociate ethyl chloride at 193 nm and detect the Cl and ethyl products, resolved by their center-of-mass recoil velocities, with vacuum ultraviolet photoionization. The data determine the relative partial cross-sections for the photoionization of ethyl radicals to form C2H5+, C2H4+, and C2H3+ at 12.1 and 13.8 eV. The data also determine the internal energy distribution of the ethyl radical prior to photoionization, so we can assess the internal energy dependence of the photoionization cross-sections. The results show that the C2H4++H and C2H3++H2 dissociative photoionization cross-sections strongly depend on the photoionization energy. Calibrating the ethyl radical partial photoionization cross-sections relative to the bandwidth-averaged photoionization cross-section of Cl atoms near 13.8 eV allows us to use these data in conjunction with literature estimates of the Cl atom photoionization cross-sections to put the present bandwidth-averaged cross-sections on an absolute scale. The resulting bandwidth-averaged cross-section for the photoionization of ethyl radicals to C2H5+ near 13.8 eV is 8+/-2 Mb. Comparison of our 12.1 eV data with high-resolution ethyl radical photoionization spectra allows us to roughly put the high-resolution spectrum on the same absolute scale. Thus, one obtains the photoionization cross-section of ethyl radicals to C2H5+ from threshold to 12.1 eV. The data show that the onset of the C2H4++H dissociative photoionization channel is above 12.1 eV; this result offers a simple way to determine whether the signal observed in photoionization experiments on complex mixtures is due to ethyl radicals. We discuss an application of the results for resolving the product branching in the O+allyl bimolecular reaction.

Many-electron effects in 2p photoionization and Auger decay of atomic aluminium

High-resolution 2p photoionization and subsequent Auger decay of atomic Al have been investigated both experimentally and theoretically. Fine-structure-resolved experimental main and satellite photoelectron spectra were reproduced using extensive relativistic multiconfiguration Dirac–Fock calculations. Emphasis was placed on the electron correlations and how they affect the coupling of the valence and semi-valence electrons. The Auger electron spectrum was used to probe the electron correlation between the three electrons above the ionized 2p orbital and a breakdown of the two-electron picture describing the Auger emission was observed. Continuum wave selection was used to expound the intensity behaviour seen in the Auger electron spectra. Direct double photoionization thresholds and behaviour of the relative partial cross section as a function of photon energy during 2p ionization were experimentally obtained from the Auger spectra.