Resonant Electron Capture Research Articles

Resonant electron capture by orotic acid molecules

Resonant electron attachment by orotic acid molecules (6-COOH-uracil) are studied in the energy range of 0–14 eV via negative ion mass spectrometry. Molecular ions, whose lifetimes relative to electron autodetachment are found to be ~300 μs are recorded in the region of thermal electron energies; they form in the valence state through a vibration-excited resonance mechanism. Unlike unsubstituted uracil, most dissociative processes occur in the low-energy region of <4 eV and are due to carboxylic anions. An absolute cross section of 2.4 × 10−17 cm2 is found for the most intense fragment ions [M–H]– at an output energy of 1.33 eV. The kinetics of decarboxylation is considered for these ions. This could be a model reaction for the last stage of uridine monophosphate biosynthesis.

Resonance capture of electrons and positrons in the axial channeling mode at a crystal surface

The motion of channeled particles in a single crystal is determined by the continuous potential of the crystallographic axes. The transverse motion of particles in the axial channeling mode is characterized by a discrete energy spectrum. In this paper, the criteria for selection of the continuous potential and the conditions for quantization of the transverse energy for axially channeled particles are discussed, and the criterion for the resonance capture of particles in the axial channeling mode during particle entrance into a single crystal is formulated; it requires that the particle’s angular momentum with respect to the channel axis coincides with a quantity that is a multiple of Planck’s constant. The effect of resonance capture can be observed, for example, via an increase in the intensity of electromagnetic radiation of the beam of channeled particles in the optical range at frequencies corresponding to transitions between the allowed levels of transverse motion.

Dielectronic recombination of highly charged ions with spin-polarized electrons

Angular distribution and linear polarization of photon emission following dielectronic recombination of initially lithium-like ions with spin-polarized electrons are studied. In particular, a general expression is derived for the alignment parameter of the doubly excited states produced via the resonant capture of spin-polarized electrons. By means of the alignment parameter, moreover, the angular distribution and linear polarization of the subsequently emitted photons are further obtained. Detailed computations are performed for the 1s22sJ0=1/2+εe-→1s2s22p1/2J=1→1s22s2Jf=0+γ resonant electron capture and subsequent radiative decay of iodine ions. It is found that the spin polarization of the incident electrons changes only the q=±1 components of the alignment parameter A2q. As a consequence, the electron spin polarization contributes weakly to the γ photon angular distribution and linear polarization that are dominantly determined by the A20 parameter.

Estimating electron affinity from the lifetime of negative molecular ions: Cycloheptatriene derivatives

Cycloheptatriene derivatives are studied by means of resonance electron capture negative ion mass spectrometry (REC NIMS). The average lifetimes of molecular negative ions (NIs) are measured with respect to electron autodetachment. Using the Arrhenius approach, electron affinity EAa of the molecules under study is estimated, and the effective temperature of the resulting negative molecular ions is determined as a function of the electron energy. It is assumed that the dissociation of negative molecular ions in the ground electronic state is a process similar to that of the thermal degradation of molecules.

Local charge exchange of He+ ions at Aluminum surfaces

We report on experiments designed to observe the correlation between the autoionization of doubly excited helium atoms and the Auger decay of 2p vacancies in Al. The autoionizing states are formed when incident He+⁎ and He++ are neutralized by resonant electron capture at the surface. 2p excitation in Al occurs in dielectronic charge transfer during the close encounter of an excited helium ion and an Al atom. These results clarify the mechanism for Al-2p excitation in the case of singly charged ground state He+(1s) ion impact, where the dielectronic transition occurs after promotion of the 1s electron of incoming ions.

Development of the ultra-low background HPGe spectrometer OBELIX at Modane underground laboratory

A new ultra low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm3 was developed to investigate rare nuclear processes, such as resonant neutrino-less double electron capture (0νEC/EC) and double beta decay processes (2ν2β−, 2νβ+EC, 2νEC/EC) to the excited states of daughter nuclei. The spectrometer was installed at the Modane underground laboratory (LSM, France, 4800 m w.e.). Sensitivity of the spectrometer and its background were tested. A new method for the efficiency calibration in measurements of low-active samples was developed. The spectrometer was used for the measurements of low active materials and samples. Results obtained in 395 h investigation of resonant 0νEC/EC decay of 106Cd to the 2718 keV and 2741 keV excited states of 106Pd with ∼23.2 g of enriched 106Cd and 2ν2β− decay of 100Mo sample with a mass of 2588 g to the 0+, 1130 keV and 2+, 539.5 keV excited states of 100Ru are presented.

Resonant electron capture by aspartame and aspartic acid molecules

The processes for dissociative electron capture are the key mechanisms for decomposition of biomolecules, proteins in particular, under interaction with low-energy electrons. Molecules of aspartic acid and aspartame, i.e. modified dipeptides, were studied herein to define the impact of the side functional groups on peptide chain decomposition in resonant electron-molecular reactions. The processes of formation and decomposition of negative ions of both aspartame and aspartic acid were studied by mass spectrometry of negative ions under resonant electron capture. The obtained mass spectra were interpreted under thermochemical analysis by quantum chemical calculations. Main channels of negative molecular ions fragmentation were found and characteristic fragment ions were identified. The СООН fragment of the side chain in aspartic acid is shown to play a key role like the carboxyl group in amino acids and aliphatic oligopeptides. Copyright © 2016 John Wiley & Sons, Ltd.

Resonance electron capture by the molecules of α- and β-C(14)-methoxy isomers of 10,12-dehydro-8,9-seco-8,9-dioxolappaconine and its oxo derivative

The formation of negative ions of some diterpene alkaloid molecules having conjugated πС=С and πС=О bonds in their structure by resonance electron capture has been studied. The mass spectra of these compounds are due to electron capture onto the lowest unoccupied molecular orbital (LUMO), which is π*-С=С or π*-С=О in nature. It has been found that the partial conversion of the test compound molecules into the enol form is possible on transferring into a gas phase; for this purpose, the appearance potentials of the ions (M–H)– and (M–OCH3)– have been compared with the calculated thermodynamic thresholds of their appearance in the keto and enol forms.

Development of an underground low background instrument for high sensitivity measurements

The Center for Underground Physics has developed in collaboration with CANBERRA a low background instrument composed of 14 HPGe detectors divided in two arrays facing each other. The performance and the background of a single detector of the array have been studied in order to improve the array final configuration. An accurate material selection, through the measurements of building material samples and Monte Carlo simulations based on Geant4, has been performed to reach the lowest possible intrinsic background. Alternative materials and configurations have been considered for the final design of the array simulating the expected intrinsic background of the instrument considering the needed changes. The expected sensitivity of the improved array configuration, concerning the low background material selection for rare events physics experiments, has been evaluated through Monte Carlo simulations considering 232Th concentration in a Copper sample. Since the array can also be used for rare decays searches, the expected sensitivity on the 156Dy resonant double electron capture has thus been calculated.

Formation of negative ions via resonant low-energy electron capture by cysteine and cystine methyl esters

The processes of resonance low-energy free electron attachment to methyl esters of some sulfur-containing amino acids were studied. The long-lived molecular negative ions of cystine dimethyl ester formed in the valence state via the Feshbach nuclear excited resonance mechanism were detected by mass spectrometry. The reactions of disulfide bond dissociation were identified in an electron energy range of 0—1 eV. They can be considered as model reactions regarding processes of peptide decomposition due to the resonance interaction with low-energy electrons. Predissociation of short-lived molecular ions of cysteine methyl ester formed by capture of electrons with energies of ~1.6 eV is accompanied by the intra-ionic transfer of negative charge from the carbonyl group to the sulfur atom leading to the elimination from the latter of hydrogen atom.

The negative ions adsorption on the ion source surface at the resonant electron capture by molecules and measurements of the ion lifetime

Using a static sector magnetic mass spectrometer MI-1201B, modified to record negative ions for resonant electron capture by molecules, the ion extraction time from the ionization chamber combined with this apparatus was measured up to several hundred microseconds. It was shown that the values obtained are anomalously great and they arise from the fact that part of the formed in the ionization chamber ions are adsorbed onto the chamber's surface. Those of them that were produced initially in the gas phase, namely the transient negative ions, with a finite lifetime (∼1μs and greater), transform themselves, on the surface, into ‘infinitely living’ (stable) ions. As a result, the contribution of the stable ions to the total ion beam falsifies the ion's lifetime measurements performed with this instrument.

Gas phase ion chemistry of coumarins: ab initio calculations used to justify negative chemical ionization trends

The gas phase ion chemistry of coumarins using electron ionization (EI), positive chemical ionization (PCI) and negative chemical ionization (NCI) in a time of flight and quadrupole mass spectrometer (qMS) coupled to a gas chromatograph is outlined. The observations in NCI mode were complimented with Ab initio calculations. This was because NCI gave enhanced signals in contrast to EI and PCI and therefore could serve as a potential ionization method for quantitative analysis. For the two classes of analytes that were examined here, i.e. methyl and acetyl coumarin derivatives, experimental data showed that the methyl derivatives underwent dissociative electron capture to produce ions of the type [M–H]¯ while the acetyl derivatives underwent resonance electron capture to produce ions of the type M˙¯. The M˙¯ type of ions were more intense than the earlier. Ab initio calculations showed higher electron affinities in the acetyl than the methyl derivatives. KEY WORDS : Coumarins, Ab Initio calculations, Electron ionization, Positive chemical ionization, Negative chemical ionization Bull. Chem. Soc. Ethiop. 2015 , 29(3), 473-484. DOI: http://dx.doi.org/10.4314/bcse.v29i3.15

Spin Blocking in the Correlated Double-Electron Capture from Metal Surfaces.

The resonant capture of electrons from a metallic surface into the outer shell of a helium ion creates doubly excited states in a spin singlet (↑↓) or triplet (↑↑) configuration. Here it is shown that the capture of one or two electrons can be described in a simple quantitative model, and the capture of two electrons by He(++) proceeds in a single step. The double capture of electrons from the Fermi energy of the metal is dominated by the spin dependent electron correlation that blocks the occupation of triplet states in the ion, but creates a singlet two-hole final state at the surface, related to the concept of the exchange-correlation hole.

Resonant electron capture by quercetin derivatives

According to available experimental data, the formation of long-lived (∼10−5 s) negative molecular ions (NMIs) due to resonant electron capture (REC) by polyatomic molecules is observed experimentally with the proviso that their electron affinity (EA) is on the order of several tenths of electronvolt or above. Such compounds are characterized by large NMI formation cross sections. High ability to effectively capture thermal electrons correlates with their ability to capture free radicals, in particular, to act as inhibitors of radical polymerization [1]. It is likely that these compounds will exhibit antioxidant activity as well. It is therefore of interest to use the REC technique for studying antioxidant compounds, of which the bioflavonoid quercetin (QU) (1) (Fig. 1) and its derivatives are prominent representatives [2, 3].

Electronically excited negative ion resonant states in chloroethylenes

The negative ion mass spectra of the resonant electron capture by molecules of 1,1-dichloroethylene, 1,2-dichloroethylene-cis, 1,2-dichloroethylene-trans, trichloroethylene and tetrachloroethylene have been recorded in the 0–12eV range of the captured electron energy using static magnetic sector mass spectrometer modified for operation in the resonant electron capture regime. As a result, several novel low-intensive dissociation channels were revealed in the compounds under study. Additionally, the negative ion resonant states were recorded at approximately 3–12eV, mostly for the first time. These resonant states were assigned to the electronically excited resonances of the inter-shell type by comparing their energies with those of the parent neutral molecules triplet and singlet electronically excited states known from the energy-loss spectra obtained by previous studies.

Three-body mechanisms in plasma recombination of H3+and D3+Ions

The low-temperature plasma recombination of H3 + and D3 + ions with electrons partially occurs by three-body assisted (ternary) mechanisms that need to be better understood. Intermediate high Rydberg states are expected to play an important role, but it remains unclear which of their interactions with ambient neutrals and electrons enhance their decay into neutral products. This contribution discusses several proposed models, collisional radiative, collisional dissociative recombination, resonant electron capture, and angular momentum l -mixing. It appears that none of them provides a satisfactory explanation. There is one observation that points to a possibly efficient route to long-lived collision complexes that may play a role: Several of the so far unassigned peaks in storage ring data occur at energies where rotational resonances have long lifetimes. A tentative model, based on complex formation and collisional stabilization, leads to qualitative agreement with experiments.

Internal conversion as the main stabilization mechanism for long-lived negative molecular ions

The temperature dependence of the mean lifetime of Ph-N=N-Ph− azobenzene negative molecular ions on the captured electron energy is studied with a static mass spectrometer by the method of resonance electron capture. A family of respective experimental dependences is calculated accurate to 2–10%. It is shown that the molecular anions in the epithermal electron energy range can be stabilized through internal conversion, namely, a series of fast radiationless transitions without change in the multiplicity.

Polarization of the strongest nf→3d (n = 4, 5, 6) radiative lines emitted from tungsten ions following EIE and DR processes

Electron-impact excitation and resonant electron capture cross sections to the specific magnetic sublevels of highly charged Ni-like to Ge-like tungsten ions have been calculated systematically by using a fully relativistic distorted-wave method. And these magnetic cross sections have further been employed to obtain the degrees of linear polarization of the corresponding strongest nf → 3d (n = 4, 5, 6) x-rays. We compare polarizations of the same lines but from the above two different processes for the first time. It has been found the polarizations following both the both processes are totally different. It is expected that the obvious differences between the polarizations can be used to distinguish the formation mechanism of the corresponding lines.

Polarization of the nf → 3d (n=4, 5, 6) x-rays from tungsten ions following electron-impact excitation and dielectronic recombination processes

Electron-impact excitation and resonant electron capture cross sections to the specific magnetic sublevels as well as the polarization of the strong nf→3d (n=4, 5, 6) x-ray emitted from Ni-like through Ge-like tungsten ions have been studied systematically by using a fully relativistic distorted-wave method. We compare the polarizations for the same x-ray but from the above two different processes for the first time. It is found that the polarization degrees of x-ray following both the electron-impact excitation and the dielectronic recombination are totally different. Because the electron-impact excitation dominates in hot and dense plasmas and the dielectronic recombination plays a key role in low-density plasmas which are cooled and recombining by capturing continuum electrons, we hope that the specific plasmas conditions could be assessed by using these x-ray polarizations.

Resonant dissociative electron capture by simple tripeptides

The processes of resonant electron capture by tripeptide molecules of diglycylalanine and trialanine were studied in the electron energy range 0–14 eV. The experimentally observed H-shift rearrangement reactions, metastable dissociations of fragmentation ions, and electron autodetachment processes by the fragmentation ions indicate that short-lived molecular negative ions are formed in vibrationally excited states in the whole energy range and their subsequent fragmentation occurs predominantly via nonradiative transitions. The characteristic reactions in negative ions of peptides were identified and considered to be model reactions in protein decomposition processes through electron-molecular interactions.