Absolute Partial Cross Sections Research Articles

Electron-recoil ion and recoil ion-projectile coincidence techniques applied to obtain absolute partial collision cross sections

We present in detail an alternative experimental set-up and data analysis, based on the electron-recoil ion and recoil ion-projectile coincidence techniques, that enable the measurement of partial pure ionization and partial charge exchange cross sections for an effusive gas jet set-up, where the absolute target density and recoil ion efficiency cannot be measured directly. The method is applied to the ionization of helium atoms due to collision with partially stripped C(3 +) projectiles. In order to check the method, the results are compared to data available in the literature where the target density and recoil ion detection efficiency were measured directly. The pure ionization channel is compared to the electron capture channel.

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.

Experimental and theoretical investigations on photoabsorption and photoionization of trimethylphosphate in the vacuum-ultraviolet energy range

In this work, we report a joint experimental-theoretical investigation on interaction of vacuum-ultraviolet radiation with trimethylphosphate (TMP) molecule (C(3)H(9)O(4)P) in gas phase. This species together with tetrahydrofuran (THF) are model compounds of deoxyribose nucleic acids (DNA)/ribose nucleic acids (RNA) backbone. Absolute photoabsorption cross sections (σ(a)) and ionization yields (η) are measured using the double-ion-chamber technique in the 11.0-21.45 eV energy range. Photoionization (σ(i)) and neutral-decay (σ(n)) cross sections in absolute scale are also derived. Moreover, theoretical photoabsorption cross sections are calculated using the time-dependent density functional theory from the excitation threshold up to 16 eV. Good agreement between the present calculated and experimental photoabsorption cross sections in the 11.0-14.5 eV range is encouraging. Also, the present measured data of σ(a) and σ(i) for TMP are about 1.3 and 1.5 times of those of THF, respectively. Thus, the experimental evidences that the majority of strand breaks being located at sugar rings in the irradiated DNA/RNA backbone moiety may be induced by a possible migration of the hole, initially created at phosphate group, to the linked sugar groups. Finally, absolute partial photoionization cross sections are derived from the experimental time-of-flight mass spectra.

Near-threshold photoionization from the excited mp5 (m+l)p J = 0 − 3 levels of Ar, Kr, and Xe (m = 3 − 5)

Using the configuration interaction Pauli-Fock including core polarization (CIPFCP) approach we have calculated absolute total and partial cross sections for photoionization of the excited mp5(m+1)p J = 0 − 3 levels in Ar, Kr and Xe (m = 3 − 5) near threshold, including the region of the odd nl' (l' = 0, 2, 4) autoionizing resonances. For several intermediate levels of Ar and Kr, the computed cross sections are compared with laser spec-troscopic measurements; the theoretical and experimental lineshapes and branching ratios for different resonances show good mutual agreement.

Multiple Ionization of Ne in coincidence with partially stripped B2+ ions in the intermediate energy regime

A projectile ion-recoil ion and electron-recoil ion coincidence technique has been used to study the multiple ionization process in collisions of 0.75-4.0MeV B2+ with Ne atoms. Absolute total and partial pure ionization cross sections are reported for the first time at these impact energies with the present setup. The relative contribution of each recoil charge state has been investigated. The experimental data are compared to CDW-EIS calculation and with He2+ projectiles. The data throw further light on the effect of projectile screening in the ionization of multi-electronic targets and provide a support for the validity of the CDW-EIS model in this intermediate-to low velocity regime.

Electron ionization of NF3

Experimental and theoretical study on electron ionization of NF3 is carried out in terms of the absolute partial and total ionization cross sections. The measurements are done in crossed electron beam-molecular beam geometry for electron incident energies ranging from 0 to 500eV using a newly built time of flight mass spectrometer with emphasis on complete collection, transmission and detection without mass and energy discrimination, while maintaining reasonable mass resolution. The cross sections are put on the absolute scale using the relative flow technique. The partial cross sections and the thresholds for the formation of NF3+, NF2+, NF+, F+, and N+ are compared with the previous results. Our cross section values for lower mass ions N+, F+, NF+ are found to be considerably higher as compared to previous measurements. We also have calculated the total scattering and total ionization cross sections using the spherical complex optical potential (SCOP) formalism and compared with existing data. While the total scattering calculations agree with the existing experimental results, the total ionization cross sections do not agree well with our experimental data. The present experimental results agree with those estimated using “Deutsch–Mark formulation with new additivity rules”, which were not in agreement with the existing experimental results for the case of NF3.

Cross sections for ionization and ionic fragmentation of pyrimidine molecules by electron collisions

The electron impact mass spectroscopy and the total ion collection measurements were used to investigate the ionization and ionic fragmentation of the pyrimidine, C4H4N2, molecules in the gas phase. The cation mass spectra were measured in the 10−85 amu range and the observed mass peaks assigned to corresponding ionic fragments. The most abundant cation in the mass spectra is the parent cation, C4H4N \hbox{$_{\mathrm{2}}^{\mathrm{+}}$} + 2 , at 80 amu. The appearance energies of the ionic fragments were determined and the possible fragmentation processes are discussed. The electron impact absolute total and partial ionization cross sections in pyrimidine were obtained over the energy range from the respective ionization thresholds up to 150 eV.

Dissociative-ionization cross sections for 12-keV-electron impact on CO2

The dissociative ionization of a CO${}_{2}$ molecule is studied at an electron energy of 12 keV using the multiple ion coincidence imaging technique. The absolute partial ionization cross sections and the precursor-specific absolute partial ionization cross sections of resulting fragment ions are obtained and reported. It is found that \ensuremath{\sim}75% of single ionization, 22% of double ionization, and \ensuremath{\sim}2% of triple ionization of the parent molecule contribute to the total fragment ion yield; quadruple ionization of CO${}_{2}$ is found to make a negligibly small contribution. Furthermore, the absolute partial ionization cross sections for ion-pair and ion-triple formation are measured for nine dissociative ionization channels of up to a quadruply ionized CO${}_{2}$ molecule. In addition, the branching ratios for single-ion, ion-pair, and ion-triple formation are also determined.

Near-threshold photoionization from the excited mp5(m + 1)p J = 0−3 levels of Ar, Kr and Xe (m = 3−5)

Using the configuration interaction Pauli-Fock including core polarization (CIPFCP) approach we have calculated absolute total and partial cross sections for photoionization of excited mp 5(m + 1)p J = 0−3 levels in Ar, Kr and Xe (m = 3−5) near threshold. Particular emphasis is paid to the lineshapes of the odd mp 1/2 5 n(s/d)′ autoionizing resonances and their variation with the character of the intermediate excited J = 0−3 state. For selected intermediate levels of Ar and Kr the computed cross sections are compared with laser spectroscopic measurements; good agreement between the theoretical and experimental lineshapes and branching ratios for different resonances is observed.

Electron ionization of hydrogen sulfide

Precursor-specific relative partial ionization cross sections, for all the different fragment ions formed by electron ionization of hydrogen sulfide, have been measured using time-of-flight mass spectrometry coupled with a two-dimensional ion coincidence technique. Relative cross sections are reported for ionizing energies from 30 to 200 eV. These cross sections allow the contribution from single, double and triple ionization to the individual fragment ion yields, following ionization of hydrogen sulfide, to be quantified. To compare our data with the literature we reduce our precursor-specific cross sections, by summing them for each fragment ion, to generate relative partial ionization cross sections. Following this data reduction, good agreement is found between our data and one set of recently published absolute partial ionization cross sections, but discrepancies are observed with another set of recently published data. Our analysis shows that the contribution of double ionization to the total ion yield reaches a maximum of 20% at 100 eV. Given the lack of available information on the fate of the excited electronic states of H 2S 2+, we have extracted kinetic energy releases for the various dicationic fragmentation channels from our coincidence data. From these kinetic energy releases, estimates of the energies of the electronic states of H 2S 2+ which are responsible for the different fragmentation channels can be made. These estimates, in comparison with other data on the electronic states of H 2S 2+, reveal the population of excited states of H 2S 2+ at electron energies above 50 eV. At ionizing electron energies above 50 eV, a significant proportion of the major dissociation channels of H 2S 2+ appear to involve the population of excited electronic states.

Absolute total and partial cross sections for ionization of nucleobases by proton impact in the Bragg peak velocity range

We present experimental results for proton ionization of nucleobases (adenine, cytosine, thymine, and uracil) based on an event-by-event analysis of the different ions produced combined with an absolute target density determination. We are able to disentangle in detail the various proton ionization channels from mass-analyzed product ion signals in coincidence with the charge-analyzed projectile. In addition we are able to determine a complete set of cross sections for the ionization of these molecular targets by 20--150 keV protons including the total and partial cross sections and the direct-ionization and electron-capture cross sections.

Studies of the fragmentation of the monocation and dication of methanol

Relative partial ionization cross sections and precursor-specific relative partial ionization cross sections for fragment ions formed by electron ionization of methanol have been measured using time-of-flight mass spectrometry coupled with a two-dimensional ion coincidence technique. Relative cross sections are reported for ionizing energies from 30 to 200 eV. Good agreement is found between our data and one set of recently published absolute partial ionization cross sections. Conversely, discrepancies are observed with another set of recently published data; we attribute these discrepancies to the loss of translationally energetic fragment ions. Our precursor-specific cross sections allow the contribution from single and double ionization to the individual fragment ion yields, following ionization of methanol, to be quantified for the first time. Our analysis shows that the contribution of double ionization to the total ion yield reaches a maximum of 20% between 150 and 200 eV.

Isotope effect in dissociative electron attachment to acetylene

Absolute partial cross sections for dissociative electron attachment to $\text{H}--\text{C}\ensuremath{\equiv}\text{C}--\text{H}$ and $\text{D}--\text{C}\ensuremath{\equiv}\text{C}--\text{D}$ have been measured using a time-of-flight ion spectrometer combined with a trochoidal electron monochromator. For ${\text{C}}_{2}{\text{H}}_{2}$ the present data are in good agreement with the total cross sections measured previously in our laboratory using the total ion-collection method. The present measurements on deuterated acetylene revealed a pronounced isotope effect at the first dissociative electron attachment band at 3 eV, with the cross section for ${\text{C}}_{2}{\text{D}}^{\ensuremath{-}}/{\text{C}}_{2}{\text{D}}_{2}$ being 14.4 times smaller than that for ${\text{C}}_{2}{\text{H}}^{\ensuremath{-}}/{\text{C}}_{2}{\text{H}}_{2}$. The dominant fragments at the second dissociative attachment band around 8 eV are the light fragments ${\text{H}}^{\ensuremath{-}}$ or ${\text{D}}^{\ensuremath{-}}$. These bands exhibit weaker isotope effects, in agreement with their assignment to Feshbach resonances with narrow autodetachment widths.

High-resolution electron attachment to the molecules CCl4 and SF6 over extended energy ranges with the (EX)LPA method

Using a variant of the laser photoelectron attachment (LPA) method with an extended energy range (EXLPA), we have studied low-energy electron attachment to the molecules CCl4 (Cl− and Cl−2 formation) and SF6 (SF−6 and SF−5 formation) in a diffuse gas target (TG = 300 K) from 0 eV up to 2 eV at energy widths down to 14 meV. In the EXLPA method, pulses of near-zero energy photoelectrons are produced in a guiding magnetic field, accelerated by a weak electric field, brought to the energy of interest prior to their traversal through the target region and subsequently accelerated and deflected onto a detecting plate. Anions due to electron attachment are extracted by a pulsed electric field, during which the photoelectron current is interrupted, and detected by a quadrupole mass spectrometer. The EXLPA anion yields are combined with absolute cross sections, obtained at very high resolution (≈1 meV) with the LPA method over the range 0–0.17 eV, to yield new recommended absolute partial and total attachment cross sections over the range 0–2 eV at the well-defined gas temperature TG = 300 K. Our cross sections show characteristic deviations from previously reported results. At least in part, these differences can be attributed to the fact that in the earlier electron beam experiments the gas temperature was higher than 300 K. For SF6, the branching fractions for SF−5 formation at electron energies 0.002–0.43 eV and for different initial rovibrational distributions are compared with those recently predicted from kinetic modelling within the framework of statistical unimolecular rate theory. Satisfactory agreement is observed, but our data provide evidence that an additional path for producing SF−6 and SF−5 ions is available at electron energies above about 0.3 eV.

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.

Electron-impact ionization of the SiCl 3 radical

We describe improvements to the fast-beam apparatus that has been used extensively for electron-impact ionization cross section measurements for atoms, molecules, and free radicals in our group for the past 15 years. A high-intensity, dispenser-type electron emitter capable of producing an electron beam of more than 2 mA at electron energies above 50 eV is used instead of a conventional indirectly heated, oxide-coated electron source. We also replaced the channel electron multiplier by a position-sensitive, triple multi-channel plate ion detector. Experiments using well-established ionization cross sections in conjunction with extensive ion trajectory simulations were carried out to verify the performance of the modified fast-neutral-beam apparatus. This apparatus was subsequently employed in the measurement of absolute partial cross sections for the formation of various singly charged positive ions produced by electron impact on SiCl 3 for impact energies from threshold to 200 eV. A comparison with calculations and with the previously reported ionization cross section for SiCl 4, SiCl 2, and SiCl is also made.

Negative ion formation in dissociative electron attachment to selected halogen derivatives of propane

Dissociative electron attachment (DEA) to halogenated derivatives of propane: 1-bromo-3-chloropropane, 2-bromo-1-chloropropane, 3-bromo-1,1,1-trichloropropane and 1,3-dibromo-1,1-difluoropropane was studied in the gas phase at ambient temperature using a high resolution crossed electron/molecule beams technique. The negative ions formed via DEA reaction were identified using mass spectrometric technique and the anion yields were measured in the electron energy range from 0 to 10eV. The absolute partial cross sections for DEA to the molecules were estimated using the relative flow technique.

Electron-impact ionization of SiCl3 using an improved crossed fast-neutral-beam - electron-beam apparatus

The fast-neutral-beam technique is a versatile approach to the determination of absolute cross sections for electron-impact ionization of atoms, stable molecules as well as free radicals and metastable species. A fast neutral beam of the species under study is prepared by charge-transfer neutralization of a mass-selected ion beam and the species are subsequently ionized by an electron beam. Mass- and energy-dispersive selection separates singly from multiply charged ions and parent from fragment ions and allows the determination of partial ionization cross sections. Here we describe some major improvements that were made recently to the fast-beam apparatus that has been used extensively for ionization cross section measurements for the past 15 years in our group. Experiments using well-established ionization cross sections in conjunction with extensive ion trajectory simulations were carried out to test the satisfactory performance of the modified fast-neutral-beam apparatus. We also report absolute partial cross sections for the formation of various singly charged positive ions produced by electron impact on SiCl3 for impact energies from threshold to 200 eV in the modified fast-beam apparatus.

Absolute partial cross-sections for the destruction of H 2 and H 3+ in collisions with helium atoms at 50 and 60 keV/amu

The first partial cross-section measurements are presented for the various channels of H 3 + and H 2 dissociation in collisions with helium atoms at 50 and 60 keV/amu. The experimental technique includes the use of a removable grid in front of a surface barrier detector to enable neutrals of mass 2 amu to be separated into H 2 and 2H channels.

Electron impact ionization of SiCl2 and SiCl

We measured absolute partial cross sections for the formation of all singly charged positive ions produced by electron impact on SiCl2 and SiCl from threshold to 200 eV using the fast-neutral-beam technique. Some of the cross section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV, which may be indicative of the presence of indirect ionization channels. Dissociative ionization channels are dominant for both species. The experimentally determined total single ionization cross sections for both species agree very well with calculated cross sections using the Deutsch-Mark (DM) formalism. A brief summary of the ionization cross sections determined for all four SiClx (x=1–4) species is given highlighting similarities and differences.