Helium Collisions Research Articles

Tandem mass spectrometry of bilin tetrapyrroles by electrospray ionization and collision-induced dissociation.

Bilins are metabolic products of hosts and bacteria on porphyrins, and are markers of health state and human waste contamination. Although bilin tandem mass spectrometry reports exist, their fragmentation behavior as a function of structure has not been compared, nor has fragmentation been examined as a function of collision energy. The fragmentation of bilins generated by positive ion mode electrospray ionization is examined by collision-induced dissociation (CID). CID on a quadrupole ion trap and on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer as a function of collision energy is compared. Methyl esterification was used to deduce which product ions contain the inner pyrrole rings. FT-ICR high mass accuracy measurements were used to determine the formulas of the resultant product ions. The central carbon's bonding to the inner pyrrole rings influences fragmentation. Bilirubin is unique because fragmentation adjacent to the central methylene group between innermost rings predominates, and loss of a terminal pyrrole is observed only with helium collision gas. The other bilins lose the terminal pyrroles first; as CID energy is increased, additional fragmentation due to neutral losses of small molecules such as H(2)O, CO, CO(2), and methanol occurs. Based on these observations, fragmentation schemes for the bilins are proposed that are strongly dependent on the molecular structure and collision energy; only bilirubin fragmentation is influenced significantly by the collision gas used. This report should have value in identification of this class of molecules for biomarker detection.

Post-collision interactions and the polarization effect in (e, 2e) collisions of helium

A modified distorted-wave Born approximation (DWBA) method is used to calculate the triple differential cross sections (TDCSs) in a coplanar asymmetric geometry for the electron impact single ionization of a He (1s2) atom at intermediate and lower energies. The post-collision interaction and the polarization effect in (e, 2e) collisions of helium are considered in the calculations. The polarization potentials from the damping method and density functional theory (DFT) are compared. Theoretical results are compared with the recent experimental data.

Projectile scattering in one- and two-electron transitions

Projectile angular distributions in one- and two-electron transitions in collisions of helium with energetic bare projectile impacts are discussed within the framework of Born and continuum distorted wave approximations. Characteristic critical scattering angles, due to projectile–electron interactions, are investigated under certain kinematical conditions in the projectile angular distributions. Only a single critical deflection, characteristic to the Thomas double-collision mechanism, is observed when a one-electron transition is considered, which is at variance with the recent experimental finding of an additional peak in the measured angular differential cross sections for single-electron capture in H+-He collisions (Gudmundsson et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 185209). Multiple scattering theories predict more than one projectile critical deflection only when two or more electrons participate in the processes. The analyses are performed both for ionization and capture reactions, where scattering quantities for the latter transitions are derived from the former ones using continuity of transition amplitudes across the ionization limit.

Four-body charge transfer processes in proton–helium collisions

Recent advancements in experimental techniques now allow for the study of fully differential cross sections (FDCS) for four-body collisions. The simplest four-body problem is a charged particle collision with a helium atom, in which both atomic electrons change state. This type of collision can result in many different outcomes, such as double excitation, excitation ionization, double ionization, transfer excitation, transfer ionization and double charge transfer. In this paper, we compare absolute experimental proton–helium FDCS for transfer excitation with the fully quantum mechanical 4BTTE (four-body transfer with target excitation) model. This model was previously used to study TTE for proton energies between 25 and 75 keV and reasonable agreement was found with the experimental data for large scattering angles, but not small angles. Since this is a first-order model, which contains contributions from all higher order terms, one would expect improved agreement with increasing energy and the purpose of this work was to look at higher energies. We found that the agreement with the magnitude of the experimental data became worse with increasing energy while the agreement with the shape of the data was reasonably good. Consequently, we conclude that the model contains the physical effects that determine the shape but not the magnitude of the cross section.

Antiprotonic helium atomcules

About 3% of antiprotons (p) stopped in helium are long-lived with microsecond lifetimes, against picoseconds in all other materials. This unusual longevity has been ascribed to the trapping of p on metastable bound states in pHe+ helium atom-molecules thus named atomcules. Apart from their unique dual structure investigated by laser spec-troscopy - a near-circular quasi-classical Rydberg atom with l ∼ n - 1 ∼ 37 or a special diatomic molecule with a negatively charged p nucleus in high rotational state with J = l - the chemical physics aspects of their interaction with other atoms or molecules constitute an interesting topic for molecular physics. While atomcules may resist to million collisions in helium, molecular contaminants such as H2 are likely to destroy them in a single one, down to very low temperatures. In the Born-Oppenheimer framework, we interpret the molecular interaction obtained by ab initio quantum chemical calculations in terms of classical reactive channels, with activation barriers accounting for the experiments carried out in He and H2. From classical trajectory Monte Carlo simulations, we show that the thermalization stage strongly quenches initial populations, thus reduced to a recovered 3 % trapping fraction. This work illustrates the pertinence of chemical physics concepts to the study of exotic processes involving antimatter. New insights into the physico-chemistry of cold interstellar radicals are anticipated.

Effective reduction of polyatomic interferences produced by high chloride and carbon concentrations in determination of Cr(vi) by FPLC-ICP-MS

Determination of Cr(VI) in aqueous samples including carbonate matrix solutions was performed by fast protein liquid chromatography (FPLC) coupled to inductively coupled plasma mass spectrometry (ICP-MS). NaCl was used for separation of Cr species representing a high burden of total dissolved solids for plasma. In addition, detection of Cr species in the NaCl and carbonate matrix was severely compromised also by chlorine and carbon polyatomic interferences at m/z 52 and 53. The influence of high amounts of chloride and carbonate ions on detection of Cr species was investigated using ICP-MS with a High Matrix Introduction (HMI) system, which reduced the sample aerosol transport and enabled direct analysis of separated Cr species in the presence of up to 4% NaCl. The potential of Helium Collision mode (He mode) and High Energy Collision Mode (HECM) to reduce polyatomic interferences from chlorine and carbon on m/z 52 and 53 was also evaluated. It was experimentally proven that for efficient removal of chlorine and carbon polyatomic interferences HECM instead of commonly used He mode should be applied. By applying HMI and HECM, determination of Cr(VI) in carbonate buffer matrices using NaCl as eluent is possible, following the most abundant Cr 52 and 53 isotopes. Despite the use of HMI and HECM, which significantly reduce the analyte transport to plasma and detector, low LODs were obtained for Cr(VI): 0.01 ng mL−1 at m/z 50, 0.06 ng mL−1 at m/z 52 and 0.02 ng mL−1 at m/z 53. The repeatability of the measurement (5 ng Cr(VI) mL−1) was better than ±1.5%. The simple solution of efficient lowering of the blank at low pH arising from a stainless steel injection needle was also demonstrated by the use of the reverse injection of samples.

Structural Characterization of Drug-like Compounds by Ion Mobility Mass Spectrometry: Comparison of Theoretical and Experimentally Derived Nitrogen Collision Cross Sections

We present the use of drug-like molecules as a traveling wave (T-wave) ion mobility (IM) calibration sample set, covering the m/z range of 122.1-609.3, the nitrogen collision cross-section (Ω(N(2))) range of 124.5-254.3 Å(2) and the helium collision cross-section (Ω(He)) range of 63.0-178.8 Å(2). Absolute Ω(N(2)) and Ω(He) values for the drug-like calibrants and two diastereomers were measured using a drift-tube instrument with radio frequency (RF) ion confinement. T-wave drift-times for the protonated diastereomers betamethasone and dexamethasone are reproducibly different. Calibration of these drift-times yields T-wave Ω(N(2)) values of 189.4 and 190.4 Å(2), respectively. These results demonstrate the ability of T-wave IM spectrometry to differentiate diastereomers differing in Ω(N(2)) value by only 1 Å(2), even though the resolution of these IM experiments were ∼40 (Ω/ΔΩ). Demonstrated through density functional theory optimized geometries and ionic electrostatic surface potential analysis, the small but measurable mobility difference between the two diastereomers is mainly due to short-range van der Waals interactions with the neutral buffer gas and not long-range charge-induced dipole interactions. The experimental RF-confining drift-tube and T-wave Ω(N(2)) values were also evaluated using a nitrogen based trajectory method, optimized for T-wave operating temperature and pressures, incorporating additional scaling factors to the Lennard-Jones potentials. Experimental Ω(He) values were also compared to the original and optimized helium based trajectory methods.

Simple model for the ionization of helium atoms by fully and partially stripped ions in the strong interaction region

Theoretical calculations employing the modified over-barrier model to investigate single and double ionization cross sections of helium in collisions with highly charged ions are presented in the strong perturbation region. It shows the velocity and charge state dependences of cross sections, which coincides with the available experimental data. In order to determine the reliability of this model for partially stripped projectiles, the cross-section ratios have been measured for C Z p + , O Z p + – He ( Z p = 1 – 4 ) by the coincidence technique in the velocity range of 1–4.5 a.u. The calculated results are approximately in agreement with the experimental data.

Combining Helium Collision Mode, Aerosol Dilution and Discrete Sampling to Maximize Matrix Tolerance and Productivity in ICP-MS Analysis of Environmental Samples

By combining recent advances in inductively coupled plasma mass spectrometry (ICP-MS) technology including improved collision cell efficiency, aerosol dilution and discrete sampling, a rare synergy has been achieved resulting in a system that is not only much faster than previous instruments, but also shows improved ease of use and superior matrix tolerance. This work demonstrates the benefits of such a system by demonstrating United States Environmental Protection Agency (USEPA) compliant analysis of a range of high matrix certified reference materials for 26 elements in less than 2 minutes per sample.

Emission Spectroscopic Study on Gas-Gas Interactions in Glow Discharge Plasmas Using Several Binary Gas Mixtures

Emission spectra of constituent gas species from glow discharge plasmas using argon-helium, krypton-helium, argon-krypton, and krypton-argon gas mixtures were analyzed to elucidate collisional energy transfer between these gas species occurring in the plasma. In the argon-helium mixed gas plasma, the enhancement or quenching of particular Ar II lines was observed when helium was added to an argon-matrix glow discharge plasma, meaning that a redistribution in the population among the excited levels could be induced through argon-helium collisions. On the other hand, the krypton-helium plasma showed little change in the emission intensities of Kr II lines when helium was added to a krypton-matrix glow discharge plasma, meaning that energy exchanges between krypton and helium excited species occur inactively. These phenomena are principally because the excitation energy as well as the spin multiplicity between collision partners follow both the energy resonance conditions and the spin conservation rule in collisions of the second kind in the argon-helium system, but not in the krypton-helium system. In the argon-krypton and krypton-argon mixed gas plasmas, significant intensity changes of particular Ar II or Kr II lines could not be found; therefore, there were no dominant channels for energy exchanges between argon and krypton species in the mixed gas plasmas.

Four-body model for transfer excitation

We present here a four-body model for transfer-excitation collisions, which we call the four-body transfer-excitation (4BTE) model. Each two-body interaction is explicitly included in the 4BTE model, allowing us to study the effects of individual two-body interactions. We apply our model to fully differential cross sections for proton+helium collisions, and study the effect of the incident projectile-atom interaction, the scattered projectile-ion interaction, the projectile-nuclear interaction, and electron correlation within the target atom.

Imaging the electrons from transfer ionization collisions

The electrons emitted into the continuum in transfer ionization of He2+ on helium collisions in the energy range of 75 – 400 keV were imaged using reaction microscope. The electron emission patterns show big difference for projectile velocity lower and upper than 1 a.u. in the present studies.

Collisional effects on the HFS transitions of antiprotonic helium

Collisions of metastable antiprotonic helium with ordinary He atoms can induce transitions between hyperfine structure (HFS) sublevels and thus relaxation of the populations as well as shift and broadening of M1 microwave spectral lines. Previously we have considered these effects, using quantum close-coupling method for the collisions and the model interatomic interaction involving tensor term (Korenman and Yudin, J Phys B At Mol Opt Phys 39:1473–1484, 2006). In this paper we extend the consideration in two lines: (i) collisional transition rates, shifts and broadenings of M1-lines are considered in relation to the strength of the tensor interaction; (ii) time evolution of the density matrix of HFS states is considered using the generalized master equation with the calculated elementary characteristics. A good agreement with the recent experimental data on the time dependence of optimum signal-to-noise ratio and of linewidth is obtained using the strength of tensor interactions as a free parameter.

Electron emitted in transfer ionization in He2+ on helium collisions

A complete experiment for He2+ on helium collisions was performed over a relatively broad energy range by using a reaction microscope in Lanzhou in order to clarify the emission properties. In the experiment the momenta of recoil ions, the emitted electron, and the charge state of the scattered projectiles were recorded in coincidence in event-by-event mode. By reconstruction of the momentum vector of electrons, the momentum distributions of the continuum electrons in the scattering plane were obtained. The distributions show characteristics of the electrons originated from transfer ionization in the collisions.

Electron angular distributions in He single ionization impact by H2+ ions at 1 MeV

For the first time we investigated in a kinematically complete experiment the ionization of helium in collisions with H2+-molecular ions at 1 MeV. Using two separate detectors, the orientation of the projectile H2+-molecular ions was determined at the instance of the collision. The electron angular distribution was measured by a "Reaction Microscope". The observed structures are found in agreement with theoretical calculations, indicating that the ionized electron of He shows a slight preferential emission direction parallel to the molecular axis.

Electron angular distributions in He single ionization impact by H2+ ions at 1 MeV

For the first time we investigated in a kinematically complete experiment the ionization of helium in collisions with H2+-molecular ions at 1 MeV. Using two separate detectors, the orientation of the projectile H2+-molecular ions was determined at the instance of the collision. The electron angular distribution was measured by a "Reaction Microscope". The observed structures are found in agreement with theoretical calculations, indicating that the ionized electron of He shows a slight preferential emission direction parallel to the molecular axis.

Charge and Energy Dependences of Ionization and Transfer for Helium in Collisions with Fast Charged Projectiles

The classical method within the independent electron model is employed to investigate (i) charge dependences of single and double ionization for helium by various charged ions Aq+ (q = 1 – 8) at impact energies of 0.64 and 1.44MeV/u, respectively, (ii) energy dependences of transfer ionization for helium by 0.5–3 MeV/u A8,9+ ions impact. The Lenz–Jensen model of the atom is applied instead of the Bohr model of the atom, and the impact-parameter dependences are also introduced into the calculations. Satisfactory agreement is found between theoretical and experimental data.

Charge transfer in collisions of protons with water molecule at high projectile energies and small scattering angles

The electron capture process induced by fast protons impinging upon water has been theoretically studied for very small scattering angles. Singly differential and total cross sections have been calculated for scattering angles ranging from 0 to 0.5 mrad and compared to the existing experimental data. For the sake of comparison, we have also presented the theoretical charge transfer cross sections for proton–helium collisions.

Double excitation of helium in collisions with proton and antiproton impact in the energy range 50–500 keV

Double-electron excitation processes of helium atoms by proton and antiproton impact have been theoretically investigated using the four-body formalism of boundary corrected continuum intermediate state (BCCIS-4B) approximation in the energy range of 50–500 keV. In this formalism, the presence of the projectile in the exit channels is described by distorting the final bound state wave functions with coulomb waves (associated with the projectile–electron interactions). The results are in good agreement with the other theoretical and experimental results. Reasonably better agreements have been found in the intermediate and high energy regions. Contributions to the cross section of the different magnetic sub-shells are also analysed.

Electron–electron interaction and transfer ionization in fast ion–atom collisions

Recently it was pointed out that electron capture occurring in fast ion–atom collisions can proceed via a mechanism which earlier was not considered. In the present paper we study this mechanism in more detail. Similarly as in radiative capture, where the electron transfer occurs due to the interaction with the radiation field and proceeds via emission of a photon, within this mechanism the electron capture is caused by the interaction with another atomic electron leading mainly to the emission of the latter. In contrast to the electron–electron Thomas capture, this electron–electron (E–E) mechanism is basically a first-order one having similarities to the kinematic and radiative capture channels. It also possesses important differences with the latter two. Leading to transfer ionization, this first-order capture mechanism results in the electron emission mainly in the direction opposite to the motion of the projectile ion. The same, although less pronounced, feature is also characteristic for the momenta of the target recoil ions produced via this mechanism. It is also shown that the action of the E–E mechanism is clearly seen in recent experimental data on the transfer ionization in fast proton–helium collisions.