Measurements Of Relative Cross Sections Research Articles

Measurements of electron impact K-shell ionization cross sections of Cr21+ ions

We present relative cross section measurements of the inner-shell ionization of highly charged chromium ions by high-energy (7–30 keV) electrons. The measurements use a technique based on high-resolution X-ray spectroscopy, which correlates ionization events with Kα decay X-rays. Moreover, the measurements show that inner-shell ionization only affects the strength of the heliumlike 1s2s 3S1 → 1s2 1S0 forbidden line. The cross sections inferred for Li-like Cr21+ agree well with distorted wave calculations.

Photoionization of the3s23p43Pand the3s23p41D,1Sstates of sulfur: Experiment and theory

Photoionization of neutral atomic sulfur in the ground and metastable states was studied experimentally at a photon energy resolution of 44 meV FWHM. Relative cross section measurements were recorded by using tunable vacuum ultraviolet (VUV) radiation in the energy range 9 -- 30 eV obtained from a laser-produced plasma and the atomic species were generated by photolysis of molecular precursors. Photoionization of this atom is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The cross-section measurements are compared with state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb R-matrix method. Resonances series in the spectra are identified and compared indicating similar features in both the theoretical and experimental spectra.

Low‐energy electron interactions with tungsten hexacarbonyl – W(CO)6

Low-energy secondary electrons are formed when energetic particles interact with matter. High-energy electrons or ions are used to form metallic structures from adsorbed organometallic molecules like W(CO)(6) on surfaces. We investigated low-energy electron attachment to W(CO)(6) in the gas phase to elucidate possible reactions during surface modification. Two crossed electron/molecular beam setups were utilised: (i) a high-resolution electron monochromator combined with a quadrupole mass spectrometer which was used for the measurement of relative cross sections as a function of the electron energy, and (ii) a double focusing mass spectrometer used for measurements of metastable decays of anions. The study was performed in the electron energy range between ~0 and 14 eV. W(CO)(6) efficiently decomposed upon attachment of a low-energy electron and no stable W(CO)(6)(-) anion was observed on mass spectrometric time scales. The transient negative ion formed lost instead sequentially CO ligands. The fragment anions W(CO)(5)(-), W(CO)(4)(-), W(CO)(3)(-), and W(CO)(2)(-) were observed. However, no W(-) was detectable. Dissociative electron attachment (DEA) to W(CO)(6) led to strong dissociation but a complete loss of all CO ligands was not observed in DEA. Deposit contaminations might be a direct result of DEA reactions close to the irradiation spot in beam deposition techniques.

Multielectron processes in collisions of Xe23+ ions with Ar atoms

We report the measurements of relative cross sections for multielectron processes in collisions of Xe23+ ions with argon atoms in the velocity range of 0.65–1.32 a.u. By means of the coincidence time-of-flight (TOF) technique, the final charge states of both the projectile and target ions for each collision event are determined. The present experimental data are compared with the scaling law by Selberg et al. [Phys. Rev. A 54, 4127 (1996)] and the extended classical over-barrier (ECB) model.

Single- and double-electron processes in collisions of Xe23+ions with helium

We report the measurements of relative cross sections for single capture (SC), double capture (DC), single ionization (SI), double ionization (DI), and transfer ionization (TI) in collisions of Xe${}^{23+}$ ions with helium atoms in the velocity range of $0.65\ensuremath{-}1.32$ a.u. The relative cross sections show a weak velocity dependence. The cross-section ratio of double- (DE) to single-electron (SE) removal from He, ${\ensuremath{\sigma}}_{\mathrm{DE}}/{\ensuremath{\sigma}}_{\mathrm{SE}}$, is about $0.45$. Single capture is the dominant reaction channel which is followed by transfer ionization, while only very small probabilities are found for pure ionization and double capture. The present experimental data are in satisfactory agreement with the estimations by the extended classical over-barrier (ECB) model.

Formula omitted] differential cross sections at low energies

Differential cross sections for π−p and π+p elastic scattering were measured at five energies between 19.9 and 43.3 MeV. The use of the CHAOS magnetic spectrometer at TRIUMF, supplemented by a range telescope for muon background suppression, provided simultaneous coverage of a large part of the full angular range, thus allowing very precise relative cross section measurements. The absolute normalisation was determined with a typical accuracy of 5%. This was verified in a simultaneous measurement of muon proton elastic scattering. The measured cross sections show some deviations from phase shift analysis predictions, in particular at large angles and low energies. From the new data we determine the real part of the isospin forward scattering amplitude.

Spectroscopic study of a pulsed argon rf ICP discharge: stepwise excitation in the afterglow and its application in optical spectroscopy

It is shown experimentally in a noble-gas, pulsed rf ICP at low pressure (∼20 mTorr) that stepwise excitation may cause a maximum in the time dependence of the optical emission during the power-free stage following the rf pulse. During this stage, fast electrons are created by chemi-ionization of metastable atoms and collisions of second kind between metastables and slow electrons. These fast electrons have energies much greater than the overall average electron energy, which is typically ∼0.1 eV for the power-free stage. If the flux of fast electrons exceeds the ambipolar flux of ions to the chamber walls, a dramatic increase in the near-wall potential drop occurs, from a few tenths of a volt to several volts, and a corresponding increase in the near-wall electric fields. In this case, a fraction of the fast electrons are reflected by the near-wall potential drop and accumulate in the plasma volume, leading to an increase in density as a function of time. Eventually, the fast electrons have sufficient density to produce significant excitation of the metastable states, increasing with time and creating a maximum in the optical emission. In the opposite case, when the flux of fast electrons is less than the flux of ions to the wall, stepwise excitation in the afterglow is usually negligible. In this case, a maximum can be caused by recombination, but generally, only at higher gas pressures. These effects should be taken into account in spectroscopy of post-discharge plasmas of various types and may be useful for measurements of relative cross-sections for stepwise excitation of atoms and molecules by monoenergetic electrons. The latter is possible because the collisional energy relaxation time of the fast electrons at these pressures is on the order of milliseconds, leading to the existence of a near monoenergetic energy spectrum for a significant time.

Phenol, chlorobenzene and chlorophenol isomers: resonant states and dissociative electron attachment.

This paper reports a study of resonant dissociative electron attachment (DEA) to the phenol, chlorobenzene, p-, m-, and o-chlorophenol molecules. On the basis of spectroscopic and thermochemical approaches the resonant states of the molecular negative ions (NIs) and the structures of some dissociative decay products are assigned. In the electron energy range up to 3 eV, DEA processes are determined by the two 2[pi*]-shape resonances resulting mainly in formation of [M-H]- and/or Cl- ions. At higher electron energies the energy correlation between peaks in the negative ion effective yield curves and bands of UV spectra allowed identification of the core-excited resonances. The peculiarities of Cl- ion formation and the vibrational fine structure on the effective yield curves of the [M-H]- ions are discussed. The mass spectrometric procedures for measurement of relative cross sections for NI formation are described.

Absorption cross-section measurements of water vapor in the wavelength region 181–199 nm

Abstract Laboratory measurements of the relative absorption cross-sections of H 2 O at the temperature 295 K have been made throughout the wavelength region 181–199 nm. Laboratory measurements of the absolute absorption cross-sections of H 2 O at 295 K have been made at 15 different wavelengths, and these absolute values have been used to put the relative cross-section measurements of H 2 O at 295 K on an absolute basis, In combination with our previous measurements [Chem. Phys. 211 (1996) 387; 215 (1997) 429], the absorption cross-section of H 2 O are available in the wavelength region 118–199 nm at 295 K.

Absolute cross section for forward recoiling hydrogen with 1.0–12.5 MeV 4He

The differential cross-section for forward recoiling hydrogen with 4He ions has been determined for the energy range of 1.0–12.5 MeV at a laboratory angle of 30°. Relative measurements of the cross-section were determined from a magnetron sputtered diamond-like carbon thin film deposited on a polished molybdenum substrate. Polystyrene (C 8H 8) thin films were used as a standard to re-normalize the relative cross-section measurements through the Rutherford cross-section of carbon. This technique allowed the determination of all data points to 5% absolute error over the entire energy range. A polynomial fit of the absolute cross section is also presented.

XUV-laser spectrometer for direct absorption measurements: application to CO in the wavelength range 95–97 nm

A narrowband and tunable XUV-laser source, based on harmonic generation of visible laser light, is developed for sensitive and direct absorption measurements. A novel method to monitor and eliminate the fluctuating intensity of this source is developed, which increases the sensitivity for absorption by an order of magnitude. This procedure allows for accurate relative cross-section measurements, demonstrated on selected bands of CO in the wavelength range 95–97 nm.

Absorption cross section measurements of carbon dioxide in the wavelength region 118.7–175.5 nm and the temperature dependence

Laboratory measurements of the relative absorption cross sections of CO 2 at the temperatures 195 and 295 K have been made throughout the wavelength region 118.7–175.5 nm. Laboratory measurements of the absolute absorption cross sections of CO 2 at the temperature 195 and 295 K have been made at 12 different wavelengths in the region 118.7–175.5 nm. These absolute values have been used to put the relative cross section measurements of CO 2 at 195 and 295 K on an absolute basis throughout the region 118.7–175.5 nm. Almost no temperature dependence was observed in the peak cross sections of the strong bands at 130 nm region, but large temperature effects are seen in the other regions.

A hard shape potential model for atom—symmetric top inelastic cross sections

The quantum theory of hard shape collisions has been applied to atom—symmetric top collisions, with particular reference to HeNH 3. The hard shape radius function has been expressed by an expansion in angular functions and key cross section ratios have been calculated as functions of the coefficients of the leading terms. A hard shape function for HeNH 3 has been derived by optimising cross ections to previous calculations by Billing et al.; reported to be broadly in line with experimental measurements of relative cross sections by Schleipen et al. Energy and angular momentum propensity rules have been examined for the cross sections σ 11→ j′ k′ and σ 41→ j′ k′ . An exponential energy gap propensity rule is satisfactory but with different parameters for different ¦Δ k¦. However, the cross section σ 11→44 is unexpectedly large; it appears that transitions are favored in which the magnitude of the total angular momentum is changed but not its direction.

L-shell ionization in Au/Ag foils by electron and positron impact

Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30–70 keV were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum in Giessen. The experimental equipment will be described shortly and the corresponding results will be presented. These results agree well with independently performed plane wave Born (PWBA) calculations accounting for electron exchange, and the deceleration or acceleration of the projectiles in the nuclear field of the target atom.

L-Shell Ionization by Positron and Electron Impact

Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30–70 keV are reported. The experiments were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum. This source and the experimental equipment will be described briefly and the experimental results will be presented in detail. In addition, plane wave Born (PWBA) calculations were performed. They include an electron exchange term and the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. The present experimental results agree well with these calculations; for L-shell ionization, they are at variance with a previous experiment where a different energy dependence was observed.

Absolute absorption cross-section measurements of ozone in the wavelength region 238–335 nm and the temperature dependence

Laboratory measurements of the absolute absorption cross-section of ozone at the temperatures 195, 228 and 295 K have been made at several discrete wavelengths in the region 238–335 nm. Our results for ozone at 295 K are in excellent agreement with those of Hearn (1961, Proc. phys. Soc. Land. 78, 932), who used a different technique. Our absolute cross-section measurements of ozone at 195 K have been used to put our recent relative cross-section measurements at that temperature (Freeman et al., 1984, Planet. Space Sci. 32, 239) on a firm absolute basis throughout the region 240–335 nm; these recalibrated cross-sections are available as numerical compilations on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.

Mass thickness determination by use of Bethe sum rule normalization in Eels

In analytical electron microscopy it is desirable to have a method of determining the local mass thickness of a sample. One area where this is of importance is in the experimental determination of the electron scattering cross-sections required in EELS micro-analysis. For several years, the Bethe sum rule has been used to place relative cross-section measurements from gases on an absolute scale. We have applied this method to the energy loss spectra recorded frcm thin films in the electron microscope.

Energy transfer as a function of collision energy. IV. State-to-state cross sections for rotational-to-translational energy transfer in HF+Ne, Ar, and Kr

State-to-state cross sections for rotationally inelastic collisions of HF (v,J) with Ne, Ar, and Kr have been measured. Laser pumping of the molecular beam to the initial states v = 1, J = 1–6, and v = 2, J = 2, followed by infrared fluorescence, permitted measurements of relative cross sections with ‖ ΔJ ‖⩽8. The collision energy was varied between 4 and 16 kcal/mol. These cross sections could be fitted well using an inverse-power dependence on the rotational energy gap [due to Pritchard and co-workers; J. Chem. Phys. 70, 4155 (1979)] for rotational energy transfers of up to 55% of the initial translational energy. The energy-corrected sudden approximation was used to determine an ’’effective’’ collision length for rotationally inelastic scattering. The scattering is thought to occur predominantly on the repulsive wall of the intermolecular potential, except for the J = 1→J′ = 0 transition, which is shown to be sensitive to the depth of the van der Waals attractive well.

Photolytic spectroscopy of simple molecules. I. The production of 5 2D atoms from Cs2

A two-photon technique is reported for the measurement of relative cross sections for the selective photolysis of simple molecules into particular product channels. In a demonstration of this method two independently tunable dye lasers were used to sequentially dissociate molecules of Cs2 for wavelengths in the visible range and then to excite the resulting products to Rydberg states which could be readily ionized for detection. With this system the spectra were determined for the selective photolysis of Cs2 into product atoms in the 5 2D3/2 and 5 2D5/2 states. Two relatively broad absorption bands were found, each of which led to the preferential population of a different fine structure components of the 5 2D products. Analysis of the photolysis spectra indicated that both dissociation and predissociation mechanisms were responsible and led to the identification of a new 1Σu and 3Σu state correlated with the 5 2D atomic states.

Structure in the total 27Al(n, α) 24Na cross section around 14 MeV

An excitation function for the 27Al(n, α) reaction has been measured in steps of about 15 keV throughout most of the energy range between 13.45 and 14.91 MeV in an effort to observe structure in the cross section. The insulated core transformer (ICT) neutron facility was used as the neutron source; a rotating, tritiated titanium target, capable of handling ion beams with currents up to 10 mA and with energies up to 450 keV, was employed. Using the activation method, a total of about 500 relative cross-section measurements were made at 87 different neutron energies. Fluctuations with magnitudes of about 1.5 % were observed in the cross section after all of the data had been correlated and corrected for the anisotropy and scattering effects that had been determined experimentally. A statistical treatment of the data yielded a compound nucleus level width of about 30 keV, which is reasonable for a 28Al nucleus excited to 20–22 MeV. The observed structure can be explained in terms of Ericson fluctuation theory.