Crossed Beam Technique Research Articles

Elastic electron scattering from formamide molecule

Elastic electron scattering from gaseous formamide (H2NCHO) has been investigated. Absolute elastic differential cross sections (DCSs) were determined both experimentally and theoretically. The measurements were performed using a cross beam technique, for the incident energies of 100, 150 and 300eV and scattering angles from 20° to 110°. Relative elastic DCSs were measured as a function of the angle and the absolute DCSs were determined using the relative flow method. The calculations of electron interaction cross sections are based on a corrected form of the independent-atom method, known as the SCAR (screen corrected additivity rule) procedure and using an improved quasifree absorption model. Calculated results agree very well with the experiment.

Electron-impact single ionization of Xe10 + ions

Absolute cross sections for electron-impact single ionization of Xe10 + ions have been measured by employing the crossed-beam technique in the electron–ion collision energy range from threshold up to 1000 eV. In addition, fine-step energy-scan measurements of the cross-section function have been performed revealing resonant structures associated with the excitation of an electron from inner electron subshells. For better understanding of the relative importance of direct and indirect ionization contributions to the measured cross section, configuration-averaged distorted-wave calculations have been performed. Significant contributions of indirect processes involving inner-shell excitation with subsequent autoionization could be identified. Furthermore, the calculations reveal that an ≈1% fraction of metastable ions present in the parent Xe10 + ion beam has been analysed.

Fragmentation of the glycine molecule by low-energy electrons

Using a crossed-beam technique with the mass-spectrometric analysis of the collision products, the processes of dissociative ionization-induced fragmentation of the glycine (C2H5NO2) molecule have been studied. The main emphasis has been given to the production of the CH4N+ fragment. A special procedure applied gave the values of the appearance potential for this fragment of 10.1 ± 0.1 eV as well as the ionization threshold for the parent glycine molecule (8.1 ± 0.1 eV), which are in fairly good agreement with the known results of other studies.

Crossed Beams Study on the Dynamics of the F-Atom Reaction with Ammonia

The F-atom reaction with NH(3) and ND(3) has been studied using the universal crossed beams technique. Angular resolved time-of-flight spectra were measured for the HF and DF reaction products. Product angular distribution and product kinetic energy distribution in the center-of-mass frame were determined from the experimental TOF spectra. Experimental results show that the HF and DF products are largely forward-scattered relative to the F-atom beam direction with a considerable amount of product at sideway and backward scattering directions. High-level ab initio calculation on the reaction energy pathway suggests that the forward-scattered products are mainly produced via a direct abstraction mechanism at large impact parameters, whereas sideway- and backward-scattered products are likely due to a long-lived complex formation mechanism.

Aktuelle Entwicklungen in der vaskulären Sonographie

Ultrasound plays a central role in the diagnostic imaging of venous and arterial vessels, especially for the assessment or exclusion of arteriosclerotic vessel obstructions as well as venous or arterial thrombosis. Due to its excellent patient acceptance and its broad availability, ultrasound is considered the standard method of choice for vascular imaging. New techniques and methods have greatly enhanced its diagnostic accuracy, the most notable of which are the B-flow technique, a variant of Doppler signal read-out for reduction of artifacts in duplex sonography, as well as other techniques, such as tissue harmonic imaging, the cross-beam technique and the speckle-reduction technique, which employ different echo processing methods for contrast improvement and enhanced delineation of body structures adjacent to the vessels. The introduction of contrast enhanced ultrasound represents an important advancement and has brought a substantial improvement in sensitivity. This article describes and discusses these new techniques and methods of vascular ultrasound diagnostics with respect to their diagnostic value.

Dynamics of Formation of Products D2CN+, DCN+, and CD3+ in the Reaction of N+ with CD4: A Crossed-Beam and Theoretical Study

The formation of D(2)CN(+) in the reaction of N(+) ((3)P) with CD(4) was studied using the crossed beam technique at collision energies of 3.66 and 4.86 eV. The experiments were complemented by calculations of stationary points on the triplet hypersurface of the system. The scattering data showed that the reaction proceeds by the formation of two intermediate complexes having different lifetimes: a long-lived statistical intermediate and a short-lived complex (mean lifetime about one period of an average rotation) with more energy in translation than the statistical complex. Comparison with theoretical calculations suggests that the long-lived complex leads the CDND(+) isomer of the product ion, whereas the short-lived complex leads prevailingly to the CD(2)N(+) isomer. The product DCN(+) results from further decomposition of the primary product D(2)CN(+), whereas CD(3)(+) is formed both by a hydride-ion transfer and a long-lived complex decomposition.

Excited state reaction dynamics of Ti(aF5J)+O2→TiO(A,B)+O studied by a crossed-beam technique

Oxidation reactions of the gas-phase titanium atom in its excited state with oxygen molecule, Ti(a(5)F(J)) + O(2) --> TiO(A(3)Phi,B(3)Pi) + O, were studied by a crossed-beam technique. Metastable excited Ti, Ti(a(5)F(J)), was generated by an optical pumping method and the reaction products were detected by the chemiluminescence spectroscopy. The chemiluminescence from TiO(A(3)Phi,B(3)Pi) was analyzed to determine vib-rotational state distributions of both excited states and their branching ratio. The vib-rotational state distribution of TiO(B) was represented by the statistical energy disposal and the branching ratio of TiO(A)/TiO(B) was also consistent with the statistical expectation. These results suggested the presence of long-lived intermediates in the course of the reactions of the excited Ti(a(5)F(J)) atom with O(2). Also observed was the significant deviation of the vibrational state distribution of TiO(A) from the statistical one and another reaction pathway which may not proceed via the long-lived intermediates was implied.

Crossed Molecular Beams Study on the Formation of Vinylacetylene in Titan’s Atmosphere

The reaction of ground-state ethynyl radicals, C(2)H(X(2)Sigma(+)), with d(4)-ethylene, C(2)D(4)(X(1)A(g)), was investigated at a collision energy of 20.6 +/- 0.4 kJ mol(-1) utilizing the crossed-beams technique. Combined with electronic structure calculations, our results elucidate that this reaction follows indirect reaction dynamics via a doublet radical complex. The reaction is initiated by a barrierless addition of the ethynyl radical to a carbon atom of the d(4)-ethylene molecule to form a C(4)HD(4) intermediate. The latter is long-lived compared with its rotational period and decomposes via a tight exit transition state to form the d(3)-vinylacetylene product (HCCC(2)D(3)) plus a deuterium atom while conserving the ethynyl group; the center-of-mass angular distribution suggests that the deuterium atom leaves almost perpendicularly to the rotational plane of the fragmenting C(4)HD(4) complex. The overall reaction is found to be exoergic by 94 +/- 20 kJ mol(-1); this value agrees nicely with a computational data of 103 +/- 5 kJ mol(-1). This study indicates that the analogous vinylacetylene molecule (HCCC(2)H(3)) can be synthesized in a low-temperature environment such as Titan's atmosphere via the neutral-neutral reaction of ethynyl radicals with ubiquitous ethylene. The similarity between this reaction and that of the isoelectronic cyano radical, CN(X(2)Sigma(+)), with ethylene-yielding vinylcyanide (C(2)H(3)CN) is also discussed.

Absolute differential cross sections for elastic scattering of electrons from pyrimidine

Differential cross sections (DCSs) for elastic scattering of electrons from pyrimidine $({\text{C}}_{4}{\text{H}}_{4}{\text{N}}_{2})$ are presented for incident energies from 50 to 300 eV. The measurements were performed using a cross beam technique, for scattering angles from $20\ifmmode^\circ\else\textdegree\fi{}$ to $110\ifmmode^\circ\else\textdegree\fi{}$. The relative DCSs were measured as a function of both the angle and incident energy and the absolute DCSs were determined using the relative flow method. The calculations of electron interaction cross sections are based on a corrected form of the independent-atom method, known as the screen corrected additivity rule procedure and using an improved quasifree absorption model. Calculated results agree very well with the experiment.

Elastic electron scattering by silver atom

The experimental and theoretical studies of elastic electron scattering by silver atom have been carried out. The experimental investigation was based on crossed beam technique with effusive atomic beam being perpendicularly crossed by electron beam. The measurements were performed at electron-impact energies (E0) of 10, 20, 40, 60, 80 and 100eV and for a range of scattering angles (θ) from 10° up to 150°. The absolute differential cross sections (DCSs) have been obtained from the elastic-to-inelastic (the unresolved silver resonant lines 4d105p2P1/2, 3/2) intensity ratio at θ=10° at each E0. Calculations have been performed using the parameter-free complex optical potential (OP) with the inclusion of spin-orbit interaction for the same E0. Comparison between present experiment and theory has been made.

Reaction dynamics of Si(PJ3)+O2→SiO(XΣ+1)+O studied by a crossed-beam laser-induced fluorescence technique

Oxidation reaction of the ground state Si atom was studied by using a crossed molecular beam technique at 13.0 kJ/mol of collision energy. The Si atomic beam was generated by laser vaporization and crossed with the oxygen molecular beam at right angle. Products at the crossing region were detected by the laser-induced fluorescence (LIF). The LIF of SiO(A 1 Pi-X 1 Sigma+) was used to determine the vibrational state distribution of the electronic ground state, SiO(X 1 Sigma+). The determined distribution was inverted with the maximum population at v"=4, and in good agreement with the recent quasiclassical trajectory calculation on the singlet potential energy surface. The agreement suggested that an abstraction mechanism is dominant at the collision energy studied here. One of the counterproducts, O(3PJ), was also observed by the vacuum ultraviolet LIF and the distribution of the spin-orbit levels were determined. The formation of O(3PJ) was consistent with the significant population of v"=7 and 8 states of SiO, which could be explained by the presence of the triplet product channel with higher exothermicity.

Moderne Ultraschalldiagnostik der Beinvenenthrombose bei unklarer Genese der Lungenembolie

We compared innovative ultrasound techniques such as tissue harmonic imaging (THI) and cross-beam technique with speckle reduction imaging (SRI) to conventional fundamental B scan in the diagnosis of deep vein thrombosis. We investigated a total number of 185 patients with clinical symptoms of acute vein thrombosis. We documented the thrombosis in the patients using multifrequency ultrasound probes (5-7 MHz, 6-9 MHz, 9-14 MHz, Logig 9, GE) and recorded ultrasound sequences in fundamental B scan, THI, and cross-beam technique with SRI (grade 2). Three blinded ultrasound investigators ranked the marking of the thrombosis in each of these image modalities and graded them with the numbers 5 = weak, 4 = moderate, 3 = satisfactory, 2 = good, and 1 = excellent. We calculated the median and a t-test for each of these image modalities. We diagnosed 115 thromboses (62%) in 185 investigated patients. This group could be divided as follows: 11 patients (6%) with three-level thrombosis, 37 patients (20%) with two-level thrombosis, and 67 (36%) with one-level thrombosis. The one-level thrombosis group included five (3%) patients with muscle vein thromboses, seven (4%) cases of thrombophlebitis without involvement of the deep vein system, and three (2%) cases of thrombophlebitis with involvement of the deep vein system. The t-test for unconnected samples showed significant differences (p <0.05) in iliac veins and highly significant differences (p <0.001) in the veins of the lower extremity due to the superior capabilities for detection of thrombosis using the cross-beam technique with SRI compared to THI and the fundamental B scan. The use of high-resolution linear ultrasound probes with the concomitant application of THI and cross-beam technique with SRI facilitates the diagnosis of deep vein thrombosis. The employment of these new ultrasound modalities is an advantage in distinguishing the veins from the surrounding tissue structures and helps in evaluating the compressibility of venous vessels.

Hydride transfer reaction dynamics of OD++C3H6

The hydride transfer reaction between OD+ and C3H6 has been studied experimentally and theoretically over the center of mass collision energy range from 0.21 to 0.92 eV using the crossed beam technique and density functional theory calculations. The center of mass flux distributions of the product ions at three different energies are highly asymmetric, with maxima close to the velocity and direction of the precursor propylene beam, characteristic of direct reactions. In the hydride transfer process, the entire reaction exothermicity is transformed into product internal excitation, consistent with mixed energy release in which the hydride ion is transferred with both the breaking and forming bonds extended. At higher collision energies, at least 85% of the incremental translational energy appears in product translation, providing a clear example of induced repulsive energy release. Compared to the related reaction of OD+ with C2H4, reaction along the pathway initiated by addition of OD+ to the C=C bond in propylene has a critical bottleneck caused by the torsional motion of the methyl substituent on the double bond. This bottleneck suppresses reaction through an intermediate complex in favor of direct hydride abstraction. Hydride abstraction appears to be a sequential process initiated by electron transfer in the triplet manifold, followed by rapid intersystem crossing and subsequent hydrogen atom transfer to form ground state allyl cation and HOD.

Plasma characterization of cross-beam pulsed-laser ablation used for carbon thin film deposition

The dynamics of the interaction between two delayed plasmas induced by cross-beam pulsed-laser ablation was analyzed by fast photography using narrow interference filters. In this configuration, two perpendicular rotating carbon targets were ablated by two synchronized laser beams generating two interacting plasma plumes. A Nd: yttrium–aluminum–garnet (1064 nm) laser beam is focused onto a target generating a highly directed plume; subsequently an excimer laser (248 nm) produces a second perpendicular plasma, which expands through the plume region generated by the first laser. In the cross-beam configuration, collision processes cause a reduction in the C II ion kinetic energy from ∼ 110 to 35 eV; moreover, the species of the second plasma which travel on the normal direction to the target surface (toward the substrate) are mainly C II. Interaction between plasmas has been compared with laser-induced plume propagation through a background gas in terms to the drag model. Carbon thin films were deposited by the cross-beam technique for different delays between lasers. Raman spectroscopy was employed to study the changes in the bonding carbon films as a function of the kinetic energy of ablated C ions.

Tetra‐tert‐butylethylene, Fantasy, Fake, or Reality?

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Charge transfer in homonuclear collisions of heavy ions

Charge-changing collisions due to intra-beam scattering can have a high impact on modern high intensity accelerators and storage rings. Using the crossed-beams technique we have measured absolute cross sections for charge transfer in homonuclear collisions of Ar, Kr, Xe, Pb and Bi ions with charge states q = 2+, 3+ and 4+ and centre-of-mass energies between 2 keV and 94 keV. Typical cross section values are in the order of 2 × 10−17–3 × 10−16 cm2 depending on the Q-value of the reaction. For the heavier ions and higher charge states a significant influence of metastable ions is found.

Tetra-tert-butylethylene, fantasy, fake, or reality?

This paper reviews all recent attempts to prepare tetra-tert-butylethylene. Some of the synthetic pathways approached this molecule very close but failed during the last step. According to recent DFT calculations this alkene should be a stable molecule with a strain energy in the range of 93 kcal/mol. Since all synthetic approaches failed new methodologies have to be developed, that is either synthesis of the perfluoro derivative of this alkene or the reaction of the radical cation or the radical anion of di-tert-butylmethane with the carbene by a crossed beam technique in the gas phase.

Theoretical Investigation of the Decarbonylation of Acetaldehyde by Fe+ and Cr+

We report a comprehensive theoretical study on the decarbonylation of acetaldehyde by Fe+ and Cr+. Various intermediates, transition states, and products involved in the decarbonylation reactions are fully optimized at the B3LYP/6-311+G(2df,2pd) level of theory. The potential energy surfaces (PESs) corresponding to [M,O,C2,H4]+(M=Cr and Fe) are examined in detail using B3LYP and CCSD(T) methods, respectively. The validity of these theoretical methods is calibrated with respect to the available thermochemical data. Calculations suggest that the Cr+ mediated decarbonylation of acetaldehyde takes place in four steps on the sextet surface: encounter complexation, C-C activation, aldehyde H-shift, and nonreactive dissociation, in good accordance with the Co+ mediated decarbonylation of acetaldehyde [Zhao, Zhang, Guo, Wu, Lu, Chem. Phys. Lett. 2005, 414, 28], while for the Fe+/acetaldehyde system decarbonylation can occur on both the quartet and the sextet PESs. The quartet pathway, which experiences spin-orbit coupling between the two surfaces, is energetically more favorable; whereas along the sextet decarbonylation coordinate several high-energy barriers are revealed. The theoretical results are compared with the experimental product kinetic energy and angular distributions of decarbonylation of acetaldehyde by Fe+ and Cr+ measured using a crossed-beam technique [Sonnenfroh, Farrar, J. Am. Chem. Soc. 1986, 108, 3521].

Reaction dynamics of V(aFJ4)+NO→VO(XΣ−4)+N studied by a crossed-beam laser-induced fluorescence technique

The dynamics of the reaction, V(a 4FJ)+NO-->VO(X 4Sigma-)+N was studied by using a crossed-beam technique at 16.4 kJ/mol of collision energy. The V atomic beam was generated by laser vaporization and crossed with the O2 beam at a right angle. The laser-induced fluorescence (LIF) for the transition of VO(B 4Pi-X 4Sigma) was used to determine the rotational state distribution of the reaction product in the vibrational ground state. Almost pure V(a 4FJ) beam was obtained by using the mixture of NH3 with N2 as a carrier gas. Comparing the LIF spectra of VO measured for two carrier gases, i.e., NH3N2 and pure N2, it was concluded that the vibrational ground state of VO(X 4Sigma-) is formed almost entirely from the reaction of V(a 4FJ) and the contribution of the metastable V(a 6DJ) is negligible. The observed rotational distribution was similar to a statistical prior prediction, and suggested that the title reaction proceeds via a long-lived intermediate, which is consistent with an electron transfer mechanism.

Dynamics study of the reaction OH−+C2H2→C2H−+H2O with crossed beams and density-functional theory calculations

The proton transfer reaction between OH- and C2H2, the sole reactive process observed over the collision energy range from 0.37 to 1.40 eV, has been studied using the crossed beam technique and density-functional theory (DFT) calculations. The center of mass flux distributions of the product C2H- ions at three different energies are highly asymmetric, characteristic of a direct process occurring on a time scale much less than a rotational period of any transient intermediate. The maxima in the flux distributions correspond to product velocities and directions close to those of the precursor acetylene reactants. The reaction quantitatively transforms the entire exothermicity into internal excitation of the products, consistent with an energy release motif in which the proton is transferred early, in a configuration in which the forming bond is extended. This picture is supported by DFT calculations showing that the first electrostatically bound intermediate on the reaction pathway is the productlike C2H- H2O species. Most of the incremental translational energy in the two higher collision energy experiments appears in product translational energy, and provides an example of induced repulsive energy release characteristic of the heavy+light-heavy mass combination.