Total Single Ionization Cross Section Research Articles

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.

Electron-impact ionization of silicon tetrachloride (SiCl4)

We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on silicon tetrachloride (SiCl4) using two different experimental techniques, a time-of-flight mass spectrometer (TOF-MS) and a fast-neutral-beam apparatus. The energy range covered was from the threshold to 900 eV in the TOF-MS and to 200 eV in the fast-neutral-beam apparatus. The results obtained by the two different experimental techniques were found to agree very well (better than their combined margins of error). The SiCl3(+) fragment ion has the largest partial ionization cross section with a maximum value of slightly above 6x10(-20) m2 at about 100 eV. The cross sections for the formation of SiCl4(+), SiCl+, and Cl+ have maximum values around 4x10(-20) m2. Some of the cross-section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV followed by a broad second maximum at around 100 eV. This is similar to what has been observed by us earlier for another Cl-containing molecule, TiCl4 [R. Basner, M. Schmidt, V. Tamovsky, H. Deutsch, and K. Becker, Thin Solid Films 374 291 (2000)]. The maximum cross-section values for the formation of the doubly charged ions, with the exception of SiCl3(++), are 0.05x10(-20) m2 or less. The experimentally determined total single ionization cross section of SiCl4 is compared with the results of semiempirical calculations.

Partial cross sections for positive and negative ion formation following electron impact on uracil

We report absolute partial cross sections for the formation of selected positive and negative ions resulting from electron interactions with uracil. Absolute calibration of the measured partial cross sections for the formation of the three most intense positive ions, the parent C4H4N2O+2 ion and the C3H3NO+ and OCN+ fragment ions, was achieved by normalization of the total single uracil ionization cross section (obtained as the sum of all measured partial single ionization cross sections) to a calculated cross section based on the semi-classical Deutsch–Märk formalism at 100 eV. Subsequently, we used the OCN+ cross section in conjunction with the known sensitivity ratio for positive and negative ion detection in our apparatus (obtained from the well-known cross sections for SF+4 and SF−4 formation from SF6) to determine the dissociative attachment cross section for OCN− formation from uracil. This cross section was found to be roughly an order of magnitude smaller, about 5 × 10−22 m2 at 6.5 eV, compared to our previously reported preliminary value. We attribute this discrepancy to the difficult determination of the uracil target density in the earlier work. Using a reliably calculated cross section for normalization purposes avoids this complication.

Correlation effects in two-photon single and double ionization of helium

We study the effects of two-photon absorption in the region of the double continuum of helium, with extreme ultraviolet (xuv) laser fields. The nonrelativistic time-dependent Schroedinger equation is solved for the two active electron systems, interacting with strong laser pulses. The dynamic calculations include electronic correlations. The final double continuum states are calculated by treating the electronic term 1/r{sub 12} within the zero- and first-order perturbation theory. We calculate the total single and double ionization probabilities and cross sections for photon energies of 45 and 57 eV, as well as the electron energy distributions in the double continuum. The effects of electronic correlations are discussed in the context of ultrashort pulses.

Dissociation of multiply ionized carbonyl sulfide due to electron impact

The dissociation of singly to triply ionized carbonyl sulfide has been studied through electron impact ionization at an electron energy of 200 eV. Two- and three-dimensional covariance mapping techniques are used, accompanied by a focusing time-of-flight mass spectrometer. The absolute cross sections for the various dissociation channels of up to triply ionized carbonyl sulfide (OCS) have been obtained. The unstable OCS dications dissociate mostly into ion pairs, while the OCS trications dissociate into ion triple or ion pair with comparable probabilities. The total single ionization cross section agrees with the result of the binary-encounter-Bethe method. A metastable decay trace corresponding to OCS2+→CO++S+ is observed on the covariance map.

Dissociation of multiply ionized alkanes from methane to n-butane due to electron impact

The dissociation of up to triply ionized alkanes from methane to n-butane has been studied through electron impact ionization at an electron energy of 200 eV. The ionic fragments are recorded by a covariance mapping technique using a focusing time-of-flight mass spectrometer. The absolute cross-sections for the numerous dissociation channels of all the alkane monocations and dications, as well as propane and n-butane trications have been obtained. All the alkane dications are unstable and dissociate mostly into ion pairs, in which the production of proton pairs is found to be a major dissociation channel. The ionic products from the dissociation of ethane, propane and n-butane dications have a similar distribution, in which the first few abundant ion species are H +, CH 3 +, C 2H 3 + and C 2H 2 +. The total single ionization cross-sections agree well with the results of the binary-encounter-Bethe method. The distortion of the ion coincidence islands has been explained as due to the deviation from the space focusing condition of the mass spectrometer. Metastable decays, such as C 3H 5 2+→CH 3 ++C 2H 2 + in propane and C 4H 10 +→C 3H 7 ++CH 3 in n-butane have been observed and identified using the initial and final angles of the decay traces.

Calculated absolute electron impact ionization cross-section for the molecules CF 3X (X=H, Br, I)

We report calculated total single ionization cross-sections for the three molecules—CFH 3, CF 3Br, and CF 3I using the Deutsch–Märk (DM) formalism. In the case of CF 3H, our calculated cross-sections are in good agreement with the BEB calculation of Kim et al. and with the recent measurement of Iga et al., but disagree with earlier experimental data. Our calculated cross-sections for CF 3Br and CF 3I are in good agreement with recent experiments, which reported measured data for these two molecules for the first time.

Electron impact ionization of the TiClx (x=1 – 3) free radicals

Titanium tetrachloride (TiCl4) is used for the plasma-assisted chemical vapor deposition of titanium nitride films. We studied the electron impact ionization of TiClx (x = 1 – 3) free radicals, which are abundant constitutents in TiCl4-containing processing plasmas, for electron energies from threshold to 100 eV. Absolute partial cross sections for the formation of all singly charged molecular ions were measured using the fast-beam technique. Dissociative ionization was found to be the dominant process for TiCl3 whereas the formation of the respective parent ions plays a more important role for TiCl2 and TiCl. Total single ionization cross sections for the three radicals could not be determined with reasonable accuracy because of problems in the determination of the cross sections for the formation of the atomic ions Ti+ and Cl+ for the three targets and, as a consequence, no comparison of the measured cross sections with calculated total single ionization cross sections is possible.

Ionization of NF 3 by electron impact

The ionization and dissociative ionization of NF 3 by electron impact has been measured by Fourier transform mass spectrometry (FTMS). The total ionization cross-section rises to a maximum value of 2.4±0.4×10 −16 cm 2 at 140 eV. Estimates of the total single ionization cross-section using ab initio energies with the binary encounter Bethe (BEB) [Y.K. Kim, M.E. Rudd, Phys. Rev. A 50 (1994) 3594] or Deutsch–Märk [Int. J. Mass Spec. 197 (2000) 37] models are roughly twice the measured values. The partial cross-sections creating NF x + ( x=0, 1, 2, 3), F +, and NF x 2+ ( x=1, 2, 3) are reported. Differences between the FTMS results and quadrupole data and fast atom beam results of Tarnovsky et al. [Int. J. Mass Spectrom. Ion Processes 133 (1994) 175] are discussed.

Absolute total and partial cross sections for the electron impact ionization of tetrafluorosilane (SiF4)

We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on SiF4 from threshold to 900 eV using a time-of-flight mass spectrometer. Dissociative ionization was found to be the dominant process, although we found evidence of the presence of the SiF4+ parent ion in our experiment. The SiF3+ fragment ion has the largest partial ionization cross section with a maximum value of 4.3×10−16 cm2 at 90 eV. All other singly charged fragment ion cross sections are about one order of magnitude smaller at this impact energy. The cross-section values of the doubly charged ions with the exception of SiF2++ are about two orders of magnitude smaller. A comparison is made with available previously measured data. Additional measurements using a sector-field mass spectrometer revealed that all fragment ions are formed with excess kinetic energy. The experimentally determined total single ionization cross section of SiF4 is compared with results of semiempirical and semiclassical calculations and reasonable agreement is found.

Absolute total and partial cross-sections for the electron impact ionization of TiCl4

Titanium tetrachloride (TiCl 4) is used for the plasma-assisted chemical vapor deposition (CVD) of titanium nitride films. We studied the electron impact ionization of TiCl 4 for electron energies from threshold to 500 eV. Absolute partial cross-sections for the formation of all singly charged positive ions and for four doubly charged positive ions were measured using a time-of-flight mass spectrometer (TOF-MS), which can be operated either in the linear mode or in the reflection mode. Dissociative ionization was found to be the dominant process. At lower impact energies up to 40 eV, the ion abundance varies drastically with impact energy, whereas at higher energies, two ionization channels dominate, the formation of the TiCl 3 + fragment ion with a maximum cross-section of 3.75×10 −16 cm 2 at 100 eV and the formation of the Cl + fragment ion with a maximum cross-section of 4×10 −16 cm 2 at 70 eV. All fragment ions with the exception of TiCl 3 + are formed with excess kinetic energy with the Cl + ion showing the broadest distribution of kinetic energies. The cross-section values of the doubly charged ions are approximately one order of magnitude smaller than those of the singly charged ions. The experimentally determined total single ionization cross-section of TiCl 4 is compared with results of semi-empirical calculations and good agreement is found.

Ionization of argon and krypton by positron impact

Total single and double ionization cross sections for argon and krypton by positron impact have been measured for projectile energies ranging from threshold to ~1 keV. The single ionization cross sections are seen to behave as expected, showing large contributions from positronium formation at low energies. In the double ionization cross sections there seems to be a lack of positronium formation at threshold, while a clear contribution from the process is seen at intermediate energies.

Ionization of helium, neon and xenon by positron impact

Single ionization of xenon and double ionization of helium, neon and xenon by positron impact have been studied at energies from threshold to ~1 keV. The total single ionization cross section of xenon is seen to behave as expected. In particular, single ionization with positronium formation is a strong channel at low energies. For double ionization we found that positronium formation is strongly suppressed and makes no contribution to the total cross section in the light noble gases, while some evidence for this channel is observed in xenon.

Theoretical determination of the absolute electron impact ionization cross-section function for silver clusters Agn (n=2–7)

A recently developed semiclassical approach (DM formalism) was used to calculate the absolute total electron-impact single ionization cross sections for silver clusters Agn (n=2–7) from threshold to 1000 eV. Similar to other cluster properties, a clear odd–even effect has been observed for the calculated cross sections and is ascribed to the effective electron numbers in the valence shell. The only available experimental data for comparison are partial cross sections for the production of the singly charged parent ions for the monomer up to the tetramer. No experimental data are available as yet for any larger clusters nor for the total ionization cross sections. Nevertheless, a comparison of the results of the present calculations with calculations using additivity rules applied to molecules and clusters gives valuable insight into the ionization behavior of clusters as a function of cluster size and thus elucidates the transition from a molecular-type situation where the cross sections of the constituents are additive to a cluster-type situation where geometric effects lead to a n2/3-dependence in the limit of large cluster sizes n.

Electron impact ionization of the SF5 and SF3 free radicals

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the SF5 and SF3 free radicals from threshold to 200 eV using the fast-neutral-beam technique. The total single ionization cross sections at 70 eV were found to be 5.1×10−16 cm2 (SF5) and 3.2×10−16 cm2 (SF3). The cross sections reach their maximum at about 100 eV for both radicals, with values of 5.4×10−16 cm2 (SF5) and 3.5×10−16 cm2 (SF3). Dissociative ionization is important only for SF5, where the SF5+ parent ionization cross section and the SF4+ fragment ionization cross sections have roughly the same value. By contrast, the ionization of SF3 is dominated by the formation of SF3+ parent ions. A comparison of the measured total single ionization cross sections with calculated cross sections, using a modified additivity rule, shows excellent agreement for both radicals in terms of the absolute cross section values and the cross section shapes. Total single ionization cross sections were also calculated for SF4, SF2, and SF.

Electron impact ionization of the hydroxyl radical

We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10−16 cm2 at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10−16 cm2 at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D2O molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations.

Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

We studied the electron impact ionization of hexamethyldisiloxane (HMDSO), Si 2O(CH 3) 6, which is widely used in plasma-enhanced polymerization applications. Appearance energies and absolute partial cross sections for the formation of fragment ions with relative intensities >1% of the most abundant ion, the Si 2O(CH 3) 5 + fragment ion, were measured in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage for electron energies from threshold to 100 eV. Dissociative ionization was found to be the dominant process. The main ionization channel removes a complete methyl group to produce the fragment ion Si 2O(CH 3) 5 + with a cross section of 1.7 × 10 −15 cm 2 at 70 eV. The stoichiometric and isotope composition of the various fragment ions was determined by using the high resolution ( m/Δ m = 40,000) of the mass spectrometer used in these studies. The methyl ion is formed with considerable excess kinetic energy, whereas all other fragment ions are formed with essentially no excess energy. The experimental total single ionization cross section of HMDSO (2.2 × 10 −15 cm 2 at 70 eV impact energy) is in good agreement with the result of a semiempirical calculation (2.1 × 10 −15 cm 2 at the same energy).

Semiclassical calculations of the cross section for electron-impact ionization of

The semiclassical Deutsch - Märk (DM) formalism for the calculation of absolute cross sections for the single ionization of atoms has been extended to the calculation of the total single ionization cross section of the fullerene . The present calculation uses a Mulliken population analysis in which the outermost 120 orbitals are represented as linear combinations of the atomic C(2s) and C(2p) orbitals and no contributions from the more tightly bound C(1s) electrons have been considered. Detailed comparisons are made with available experimental data. Calculated cross sections are considerably larger (and the cross section curve slightly different in shape) than the measured ones. This indicates that the DM formalism (along with other calculation schemes based on an additivity concept) cannot without appropriate corrections be applied to targets with a complex structure such as .

Electron impact ionization of the SiDx (x=1–3) free radicals

We report measurements of absolute cross sections for the electron-impact ionization and dissociative ionization of the SiDx (x=1–3) free radicals from threshold to 200 eV using the fast-neutral-beam technique. The deuterated rather than the protonated target species were used in order to allow a better separation of the various product ions from a given parent in our apparatus. A common feature of all three radicals studied in this work is a dominant parent ionization cross section with essentially the same absolute value of roughly 3.7×10−16 cm2 at 70 eV. Dissociative ionization processes for all three targets are less significant with a single dissociative process dominating in each case, viz. the removal of a single D atom (SiDx+e−→SiD+x−1+D+2e−). The cross section for this dominant dissociative ionization channel also had the same maximum value of about 1.2×10−16 cm2 for all three targets. A comparison of the experimentally determined total single ionization cross sections with calculated cross sections using a modified additivity rule showed good to satisfactory agreement for all three targets in terms of the absolute values, but reveals some discrepancies in the cross section shapes.

Electron impact ionization of ( x = 1 - 4)

We report measurements of absolute cross sections for the electron-impact ionization and dissociative ionization of the deuterated methane molecule and the (x = 1 - 3) free radicals from threshold to 200 eV using the fast-neutral beam technique. We used the deuterated rather than the protonated target species to facilitate a better separation of the various product ions from a given parent molecule in our apparatus. Good agreement was found between our results and the most recent measurements in . Our results for and agree well with earlier measurements carried out under different experimental conditions. The present results for the CD free radical are the first ionization cross section measurements for this target. A common feature of all four targets studied in this work was a dominant parent ionization cross section with essentially the same absolute value of 1.6 - at 70 eV. A comparison of the experimentally determined total single ionization cross sections with calculated cross sections using a modified additivity rule showed very good agreement for all four targets.