Absolute Partial Cross Sections Research Articles

Absolute partial cross sections for electron impact ionization of SO2 from threshold to 1000 eV

Absolute partial cross sections for the production of SO2+, SO+, (S+ + O2+ + SO22+), O+, and SO2+ from electron impact ionization of SO2 are reported for electron energies from threshold to 1000 eV. The product ions are analyzed using a time‐of‐flight mass spectrometer and detected with a position‐sensitive detector, making it possible to demonstrate complete collection of all product ions. The overall uncertainty in the absolute cross section values is ±3.5% for SO2+ ions and is slightly higher for fragment and multiply charged ions. While much of the previously published data are in reasonable agreement with the data presented here, the present O+ cross section is higher than earlier measurements by approximately a factor of 3. The cross section for production of SO2+ has not been previously reported. The sum of the partial cross sections is compared to previously measured total cross sections.

Absolute partial and total cross-section functions for the electron impact ionization of C60 and C70

Electron impact ionization of C60 and C70 was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C+44 in the case of C60 and down to C2+50 in the case of C70) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C+60/C60) is more than a factor of 30 larger than σ(C+58/C60)] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C2+60 amounts to 30% of the maximum C+60 cross section and that of C2+70 to about 50% of C+70. Moreover, for all fragment ions, the formation of the doubly charged fragment ions has a larger cross section than that of the respective singly charged fragment ion. These peculiar features of the kinetics of electron impact ionization of C60 and C70 are related to the specific electronic and geometric structures of these fullerenes. The present absolute cross-section data for the summed up partial cross sections are in good agreement with a recent semiclassical calculation from our laboratory.

Cross sections and angular distributions of the photoelectron correlation satellites of the the Xe atom

A high resolution electron spectrometry study of the sub-valence 5 s-photoelectron correlation satellites of Xe is presented. We report both absolute partial cross sections and angular distributions of several selected satellite lines recorded at photon energies from 40.8 eV up to 150 eV. Our high resolution results are compared to earlier lower resolution measurements and to our calculations which incorporate both configuration interaction and many-body perturbation theory techniques.

Electron impact ionization of tetramethylsilane (TMS)

We studied the electron impact ionization of tetramethylsilane (TMS), Si(CH 3) 4, which is utilized in plasma polymerization applications, using various mass spectrometric techniques. Absolute partial cross-sections for the formation of the parent ion and 15 fragment ions were measured in a high resolution double-focusing sector field mass spectrometer from threshold to 90 eV. Complementary measurements of mass spectral cracking patterns of TMS were also carried out using two quadrupole mass spectrometers. Dissociative ionization of TMS was found to be the dominant process. The Si(CH 3) 3 + ion is by far the most abundant fragment ion with a maximum cross-section of about 1 × 10 −15cm 2. In contrast, the maximum parent ionization cross-section is less than 2% of that value. On the basis of the measured partial ionization cross-sections, mass spectral cracking patterns and appearance energies of the various ionic products, we propose a collision-induced decomposition scheme for the TMS molecule. Three major decomposition channels were identified which all involve the total or partial removal of a methyl group. The agreement of our measured absolute partial ionization cross-sections with earlier data obtained by a different technique is generally poor.

Absolute partial and total cross sections for electron-impact ionization of argon from threshold to 1000 eV.

Absolute partial cross sections from threshold to 1000 eV are reported for the production of ${\mathrm{Ar}}^{\mathit{n}+}$ (n=1--4) by electron-impact ionization of argon. The total cross sections, obtained from an appropriately weighted sum of the partial cross sections, are also reported. These results are obtained with an apparatus incorporating a time-of-flight mass spectrometer with position-sensitive detection of the product ions. The simple apparatus design embodies recent developments in pressure measurement and particle detection and is believed to yield more reliable results than those previously reported. For singly charged ions, the overall uncertainty in the absolute cross section values reported here is \ifmmode\pm\else\textpm\fi{}3.5%. Previous measurements of absolute partial and total cross sections are reviewed and compared with the present results.

Disposal of reactant vibrational excitation in adiabatically endothermic reactions. I. H+D2(v″=1, j″=2)→HD(v′, j′)+D

We report absolute partial and total cross sections for the H+D2(v″=1, j″=2)→HD(v′, j′)+D reaction at Erel=1.3 eV. Addition of D2 reactant vibrational energy increases the total reactive cross section from 1.2 to 2.5 Å2. That a similar amount of increased collision energy does not increase the cross section to such an extent distinguishes reactant vibrational energy from reactant translational energy. The average rotational energy for the HD product increases from 0.25 to 0.44 eV, but the effect is caused entirely by increased rotational energy in the v′=0 vibrational ground state. Reactant vibrational energy does not enhance the rotational energy for v′=1 and only modestly enhances HD vibrational energy. The average vibrational energy 〈Ev〉 is 0.10 eV for the v″=0 reaction and 0.16 eV for the v″=1 reaction. These results contrast with those of the D+H2(v″=j″=1)→HD(v′,j′)+H reaction at ∼1.4 eV, in which the vibrational energy of the HD product is three times as great for the v″=1 reaction as for the v″=0 reaction. This difference in reactions may be explained by the reactant H2 vibrational energy, as opposed to the reactant D2 vibrational energy, exceeding one quantum of vibration of the product HD. There is no specific or selective channeling of reactant vibration into product rotation in the present case, but reactive trajectories that allow channeling into v′=0, high j′ quantum states are enhanced upon the addition of D2 vibrational energy.

Elastic scattering of electrons from ions

First experimental results are reported for high angle elastic scattering of electrons from an ion at collision energies below the inelastic threshold. Absolute partial cross sections have been measured for the angular range π 2−π for Ar + over the energy range 4.0–7.0 eV. The results are compared to predictions of the classical Rutherford formula, and to a Born calculation using a screened nuclear potential. It is found that allowance for electron screening of the ionic core gives much better agreement with experiment. The application of such measurements of low energy backscattering from ions of various charge states as a sensitive test of many-body theory is discussed.

Electron-impact ionization of atoms, molecules, ions and transient species

This topical review examines the current status of the electron impact ionization of atoms, molecules, ions and transient species such as free radicals and atoms/molecules in metastable states from the point of a view of the relevance of these processes for the physics and chemistry of low-temperature and moderate-temperature plasmas. The primary emphasis of this review is on recent developments in the measurement (and to a lesser extent also in the calculation) of absolute total and partial angle-integrated electron impact ionization cross sections. An attempt is also made to identify the most promising directions for future research in this field.

Electron Impact Ionization of C60.

Absolute partial and total cross sections for the electron impact ionization of ${\mathrm{C}}_{60}$ have been measured using a novel approach for the absolute calibration. The results obtained reveal not only an anomalous large parent ion cross section (as compared to the other ionization channels), but also anomalies for the production of multiply charged parent and fragment ions. This special behavior has its origin in the specific electronic and geometric structure of ${\mathrm{C}}_{60}$. Semiclassical calculations for singly charged ions support the measured data.

Redetermination of absolute partial photoionization cross sections of He and Ne atoms between 20 and 300 eV photon energy

Using photoelectron spectroscopy, we have measured some relative partial photoionization cross sections in He and Ne atoms. By compiling these results, along with previously published data obtained from photoabsorption, photoion and photoelectron spectroscopies, we have determined accurate absolute partial photoionization cross sections, up to 300 eV photon energy. The absolute scale was obtained by normalization to the photoabsorption cross sections. The best recommended values, determined by this self-consistent experimental procedure, are compared with the results of various theoretical determinations of the photoionization cross sections.

State-to-state dynamics of atom+polyatom reactions. III. The H+CDCl3→HD(v′,j′)+CCl3 reaction

We report measurement of the HD product quantum state distributions for the H+CDCl3→HD(v′,j′)+CCl3 reaction at a collision energy of 1.6 eV using coherent anti-Stokes–Raman scattering (CARS). We determine relative HD(v′,j′) population distributions and absolute partial and total cross sections. The total cross section is 0.55±0.10 Å2. HD products are observed in v′=0, 1, and 2, with 7% of the available energy in HD product vibration. As with previous light+light–heavy→light–light+heavy reactions, the total energy in rotation, 15%, is also small. HD formed in v′=1 and 2 have greater fractions of rotational energy in relation to the energy available to them than HD in v′=0, with gr=〈Erot〉/(Etot−Evib)=0.15, 0.20, and 0.19 for v′=0, 1, and 2, respectively. Linear surprisal analysis shows that the excited vibrational levels have less bias against rotational excitation than the ground state level, respectively, if the CCl3 fragment is regarded as structureless. Comparison with previous state-to-state dynamics results of kinematically similar H+HX→H2+X (X=Cl, Br, I) and H+HR/DR→H2/HD+R (R=CD3, C2H5, C3H7) reactions indicate that the positive (v′,j′) correlation may be intrinsic to polyatomic hydrogen atom abstraction reactions.

Absolute partial and total electron impact ionization cross sections for C 2H 6 from threshold up to 950 eV

Electron impact ionization of ethane (C 2H 6) was studied with a modified Nier type ion source and a double focussing mass spectrometer (reversed geometry). Using the ion-beam-deflection-technique and a SIMION based correction procedure it is possible to determine accurately parent and fragment ion currents produced. This allows the determination of relative partial ionization cross section functions from threshold up to 950 eV for the production of C 2H + 6, C 2H + 5, C 2H + 4, C 2H + 3 C 2H + 2, C 2H +, C + 2; Ch + 3, CH + 2, CH +, C +; C 2H 2+ 5 and C 2H 2+ 3, respectively. Absolute partial ionization cross section functions for these ions were obtained by charge-weighted summing of all the observed partial ionization cross sections and by normalizing to be absolute total ionization cross section at 100 eV reported recently by Djuric. [N. Djuric, I. Cadez and M. Kurepa, Int. J. Mass Spectrom. Ion Processes 108 (1991) R1]. Comparisons are made to earlier measurements.

First observation of a Fano profile following one step autoionization into a double photoionization continuum

We have measured the double photoionization cross section of sodium atoms between the first 2s 2 p 5 2 P double photoionization (52.4 eV) and 2s-single 1.3 S photoionization thresholds (71.0 eV). We have also observed a Fano profile into the double ionization continuum resulting from the interference between the one-step direct double ionization process and the resonant double Auger decay of core-excited neutral sodium in the 2s→3p resonance region. Profiles of absolute partial and total cross sections have been obtained in all important channels. The Fano and Starace parameters, in particular a width of 0.23 eV, have been determined, allowing full characterization of the resonance

Absolute cross sections for the dissociative electron impact ionization of the CF x ( x = 1−3) free radicals

We studied the dissociative ionization of the CF x ( x = 1−3) free radicals by electron impact and measured absolute partial cross sections and appearance energies for the formation of the various fragment ions. Fast beams (3–3.5 kV) of CF 3, CF 2 and CF were prepared by near-resonant charge transfer of CF + 3, CF + 2 and CF + with Xe under conditions where the neutral beams contained primarily ground state ions with little vibrational excitation (0.5 eV or less). Absolute cross sections for the various molecular fragment ions were measured from threshold to 200 eV with peak cross sections of up to 1.3 Å 2. The cross sections for the formation of the atomic C + and F + ions were found to be small (0.3 Å 2 or less) except for the F + cross section from CF 2. The measured appearance energies indicate that the fragment ions are formed with excess kinetic energies ranging from essentially zero to about 3 eV per fragment. The CF + cross section from CF 2 shows a second onset which corresponds to the double positive ion formation CF 2 → CF + + F +. Extensive ion trajectory calculations were carried out in order to quantify the collection efficiency of our apparatus for fragment ions formed with excess kinetic energy.

ChemInform Abstract: Absolute Partial Cross Sections for the Parent Ionization of the CFx ( x = 1‐3) Free Radicals by Electron Impact.

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Absolute partial and total electron impact ionization cross sections for C3H8 from threshold up to 950 eV

Electron impact ionization of propane (C3H8) was studied using the ion beam deflection technique and a double focussing mass spectrometer in conjunction with a recently developed correction procedure that accounts for discrimination due to the initial kinetic energy of fragment ions. The relative corrected partial ionization cross sections for the production of C3H 8 + , C3H 7 + , C3H 6 + , C3H 5 + , C3H 4 + , C3H 3 + , C3H 2 + , C3H+, C 3 + ; C2H 5 + , C2H 4 + , C2H 3 + , C2H 2 + , C2H+, C 2 + ; CH 3 + , CH 2 + , CH+, C+; C3H 5 2+ , C3H 4 2+ , C3H 3 2+ and C3H 2 2+ were determined from threshold up to 950 eV. Absolute partial ionization cross sections were obtained by charge weighted summing of all the observed partial ionization cross sections and by normalizing to a recent accurate determination of the total ionization cross section at 100 eV by Djuric et al. 1991. The absolute total ionization cross section curve obtained is in excellent agreement in shape and magnitude with the low energy data (<200 eV) of Djuric et al. 1991 and the high energy data points of Schram et al. 1966.

Absolute partial cross sections for the parent ionization of the CFx (x=1–3) free radicals by electron impact

We report absolute partial electron-impact ionization cross sections from threshold to 200 eV for the formation of the parent CFx+ ions from the CFx free radicals (x=1–3). Fast (3–3.5 kV) beams of CF3, CF2, and CF radicals were prepared by near-resonant charge transfer of CF3+, CF2+, and CF+ with triethylamine (TEA), Xe, and NF3. The CF and CF3 neutral beams were found to contain primarily ground state neutral radicals with some evidence for the presence of a small amount of vibrational excitation (0.5 eV or less). Contributions to the CF2+ ion signal obtained at electron energies several electron volts below the 11.4 eV threshold for the ionization of ground state CF2 radicals indicate the presence of CF2 metastables (presumably in the 3B1 state) in the CF2 neutral beam. The level of CF2 metastable contamination was found to depend critically on the exact experimental conditions. At 70 eV, the absolute parent ionization cross sections are 0.38±0.07 Å2 (CF3→CF3+), 1.03±0.16 Å2 (CF2→CF2+), and 1.25±0.19 Å2 (CF→CF+), respectively.

Electron-Impact Dissociation of Methane into CH3 and CH2 Radicals II. Absolute Cross Sections

The electron energy dependence of partial cross sections for dissociation of methane into CH3 and CH2 radicals has been reported in part I of this paper. In order to assign absolute values to these relative cross sections, two additional experiments were carried out. One was cross-section measurement for electron-impact nitrogen dissociation (N2→N+N, N+N+) using threshold-ionization mass spectrometry. The other was the measurement of sticking coefficients of CH3, CH2, and atomic nitrogen. According to these measurements, the absolute partial cross sections for neutral dissociation were determined: maximum cross section is 1.4×10-20 m2 at 24 eV for CH3 radical and 0.72×10-20 m2 at 18 eV for CH2 radical.

Absolute partial photoionization cross sections of ethylene

Absolute partial photoionization cross sections for ionization out of the first four valence orbitals to the X 2B 3u, A 2B 3g, B 2A g and C 2B 2u states of the C 2H 4 + ion are presented as a function of photon energy over the energy range from 12 to 26 eV. The experimental results have been compared to previously published relative partial cross sections for the first two bands at 18, 21 and 24 eV. Comparison of the experimental data with continuum multiple scattering Xα calculations provides evidence for extensive autoionization to the X 2B 3u state and confirms the predicted shape resonances in ionization to the A 2B 3g and B 2A g states. Identification of possible transitions for the autoionizing resonances have been made using multiple scattering transition state calculations on Rydberg excited states.

Electron impact ionization cross sections of molecules: Part I. Experimental determination of partial ionization cross sections of SF 6: a case study

Electron impact ionization of SF 6 has been studied as a function of electron energy from threshold up to 180 eV using a Nier type ion source in combination with a double focusing sector field mass spectrometer (reversed geometry). Absolute partial ionization cross section functions for the production of SF + 5, SF + 4, SF + 3, SF + 2, SF +, S + , F +, SF 2+ 4 and SF 2+ 2 have been determined using an improved experimental method based on ion trajectory calculations. In addition, the total ionization cross section function of SF 6 has been obtained. The present data are compared with previous experimental and theoretical results in order to evaluate the reliability and accuracy of the method used.