Energy Electron Impact Ionization Research Articles

Metastable Evaporation of Molecules from Water Clusters.

We probe the stability of water clusters by means of their metastable decay probability extracted from two-dimensional reflectron time-of-flight mass spectra. Two different methods are used to ionize and potentially excite the clusters and trigger the evaporation: (i) attachment of electrons with near-zero energies, producing negatively charged clusters, and (ii) electron impact ionization, producing protonated (H2O)nH+ clusters. The electron attachment is a soft ionization and therefore provides information about the size distribution of the neutral clusters in the beam due to a very limited amount of post-ionization loss of water molecules. A dependence of metastable fractions on the conditions of neutral clusters production prior to the electron attachment is reported. For the cations, the higher energy electron impact ionization leads to a more extensive metastable loss of water molecules. The results are discussed in the light of neutral cluster excitation energy distributions and, for negative clusters, also in terms of binding energies. The experiments demonstrate clearly the role of the excess electron vs the excess proton in the two different charge states of the clusters around sizes N = 50-55, for which binding energies of the anions are derived from the data.

Research and Application of Mass Spectrometry with Low Energy Electron Impact Ionization Source

Abstract The changes of concentrations of O2 and CO2 in tail gas of fermentation industry reflect some important physiological and metabolic parameters in the fermentation process. Monitoring of the tail gas of fermentation has important significance for guiding the fermentation process. However, traditional monitoring methods are time-consuming with low precision and poor versatility, and are not suitable to monitor fermentation tail gas accurately in real time. In the process mass spectrometry, the traditional 70 eV electron impact ionization source will produce a large number of fragment ions, which may overlap with different substances, thus increasing the difficulty of qualitative analysis and causing inaccurate monitoring results. In order to realize real-time, on-line accurate monitoring of fermentation tail gas, a quadrupole process mass spectrometry based on electron impact ionization was developed for on-line analysis in this study. Through investigation of the relationship between emission current of filament and the electron receiving pole at low ionization energy, the ionization efficiency of the electron impact ionization source under low ionization energy mode was improved, and CO2, O2 and N2 were tested. The results showed that under the conditions of electron energy of 22 eV, electron receiving electrode of −22 V and filament emission current of 20–25 mA, the relative errors of quantitative detection of CO2 with volume fraction of higher than 0.023% and O2 with volume fraction of higher than 0.021% were all less than 1%. The instrument and the developed method had ideal application prospect in the rapid real-time, on-line monitoring of industrial processes.

Development of electron impact excitation and ionization in noble gases and liquids in strong electric fields

This paper reports on establishing the electron energy distribution function and electron impact excitation and ionization rates in noble gases and liquids in strong electric fields. The method of the unsteady electron kinetic Boltzmann equation is used allowing for all the dense effects typical of the electron scattering at spatially correlated heavy particles. Account is also taken of the influence of the electron number density growth on the relaxation process. Some special features of the excitation and ionization processes in noble media are revealed. The respective electron impact rates have been calculated and the delay times for each the process determined. A paradoxical conclusion has been drawn about the electron multiplication rates in the media with extremely sharp increase in the momentum transfer frequency.

A theoretical study on collision mechanisms for low energy electron impact ionization of helium in the perpendicular geometry

Under the condition of ten different incident energies ranging from 3 eV to 80 eV above the ionization potential of helium and the outgoing electrons having equal energies, by making use of 3C model and modified 3C model, the triple differential cross sections of electron-impact single ionization of the ground state of helium in the perpendicular geometry are calculated. The result is compared with corresponding experimental result to systematically investigate the influences of various screening effects on the triple differential cross sections for helium. The collision mechanisms of the triple differential cross sections are explored. The result shows that the effects of dynamic screening in the final state can directly affect the structures of the triple differential cross sections at lower incident energy, which will unavoidably affect the angular distribution and relative amplitude of side peaks at angles =90 and =270. The screening effects of residual electron in the final state of He+ have a similar significant effect on the amplitude of triple differential cross section of helium and angular distributions and relative amplitudes of side peaks at angles =90 and =270. When the incident energy is over 84.6 eV, the screening effect of residual electron in the final state of He+ has a slight effect on the amplitude of triple differential cross section, which can be overlooked. But the effects of dynamic screening in the final state on side peaks at angles =90 and =270 need considering. In addition, by taking advantage of DS3C-Z model, the results of collision mechanism of various peaks at angles =180, =90 and =270 show that the middle peak at angle =180 is produced by a process called triple scattering mechanism and then the side peaks at angles =90 and =270 are produced by a process called double scattering mechanism. Such a collision mechanism has a direct influence on the generation and variation law of triple differential cross sections.

Low energy electron-impact ionization of hydrogen atom for coplanar equal-energy-sharing kinematics in Debye plasmas

Low energy electron-impact ionization of hydrogen atom in Debye plasmas has been investigated by employing the exterior complex scaling method. The interactions between the charged particles in the plasma have been represented by Debye-Huckel potentials. Triple differential cross sections (TDCS) in the coplanar equal-energy-sharing geometry at an incident energy of 15.6 eV for different screening lengths are reported. As the screening strength increases, TDCS change significantly. The evolutions of dominant typical peak structures of the TDCS are studied in detail for different screening lengths and for different coplanar equal-energy-sharing geometries.

Calculation of FDCS for the low and intermediate energy electron impact ioni-zation of water molecules

SynopsisFully differential cross sections (FDCS) results for the electron impact ionization of water molecules taking place from 1b1, 1b2 and 2a1 orbitals are presented. The FDCS calculations have been performed in second Born approximation.

Interference effects for intermediate energy electron-impact ionization of H2 and N2 molecules

We have studied electron impact ionization of H2 and N2 molecules at intermediate energies to look for possible two center interference effects experimentally and theoretically. Here we report a study of the interference factor I for 250 eV electron-impact ionization. The experimental measurements are performed using a crossed-beam-type electron-electron coincidence spectrometer and theoretical calculations are obtained using the Molecular Three Body Distorted Wave Approximation (M3DW). We found that the I-factor demonstrated strong evidence for two-center interference effects for both H2 and N2. We also found that the I-factor is more sensitive to projectile angular scans than to ejected electron energy scans which indicate that for the present set of kinematics the diffraction of the projectile from two scattering centers is more important than interference between electron waves emitted from two different centers.

Comparison of Experiment and Theory for Electron Impact Ionization of Isoelectronic Atoms and Molecules

Experimental and Theoretical Triply Differential Cross sections will be presented for low energy electron impact ionization of Ne, CH4, and NH3. The collision mechanisms responsible for the various structures found in the cross sections will be discussed.

Electron impact ionization and excitation studies of laser prepared atomic targets

A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.

Absolute triple-differential cross sections for intermediate energy electron impact ionization of neon and argon

Absolute triple-differential cross sections for intermediate energy electron impact ionization of neon and argon are reported. The data highlight ongoing challenges for theory in the description of this ionization problem and the immense value of absolute measurements over relative data in guiding the ongoing development of theory.

(e,2e) measurements using a magnetic angle changer

While measurements of triple differential cross sections (TDCS) for electron impact ionization have been performed for almost 40 years, the angular range of available data has generally been restricted due to physical constraints of the apparatus. We have overcome these constraints by incorporating a magnetic angle changer (MAC) into a conventional (e,2e) spectrometer. Use of a MAC in an (e,2e) experiment allows access to regions where as yet unobserved physical effects occur. One such example is where the scattered and ejected electrons emerge at the same angle, a region where the Coulomb repulsion between the two should be largest. This ‘post-collision interaction’ (PCI) is thought to play an important role in low energy electron impact ionization. We discuss recently published data for TDCS of the inner valence shell 3s ionization of argon measured over the backward scattering direction. We also present new data for outer shell 3p ionization of argon over the full angular range in the scattering plane, including the region where scattered/ejected electrons emerge at the same angle. To our knowledge, this is the first reported TDCS over the complete 360° angular range using a conventional (e,2e) spectrometer.

Rate Constant for the Recombination Reaction CH3 + CH3 → C2H6 at T = 298 and 202 K.

The recombination of methyl radicals is the major loss process for methyl in the atmospheres of Saturn and Neptune. The serious disagreement between observed and calculated levels of CH3 has led to suggestions that the atmospheric models greatly underestimated the loss of CH3 due to poor knowledge of the rate of the reaction CH3 + CH3 + M → C2H6 + M at the low temperatures and pressures of these atmospheric systems. In an attempt to resolve this problem, the absolute rate constant for the self-reaction of CH3 has been measured using the discharge-flow kinetic technique coupled to mass spectrometric detection at T = 202 and 298 K and P = 0.6-2.0 Torr nominal pressure (He). CH3 was produced by the reaction of F with CH4, with [CH4] in large excess over [F], and detected by low energy (11 eV) electron impact ionization at m/ z = 15. The results were obtained by graphical analysis of plots of the reciprocal of the CH3 signal vs reaction time. At T = 298 K, k 1(0.6 Torr) = (2.15 ± 0.42) × 10-11 cm3 molecule-1 s-1 and k 1(1 Torr) = (2.44 ± 0.52) × 10-11 cm3 molecule-1 s-1. At T = 202 K, the rate constant increased from k 1(0.6 Torr) = (5.04 ± 1.15) × 10-11 cm3 molecule-1 s-1 to k 1(1.0 Torr) = (5.25 ± 1.43) × 10-11 cm3 molecule-1 s-1 to k 1(2.0 Torr) = (6.52 ± 1.54) × 10-11 cm3 molecule-1 s-1, indicating that the reaction is in the falloff region. Klippenstein and Harding had previously calculated rate constant falloff curves for this self-reaction in Ar buffer gas. Transforming these results for a He buffer gas suggest little change in the energy removal per collision, -〈Δ E〉d, with decreasing temperature and also indicate that -〈Δ E〉d for He buffer gas is approximately half of that for Argon. Since the experimental results seem to at least partially affirm the validity of the Klippenstein and Harding calculations, we suggest that, in atmospheric models of the outer planets, use of the theoretical results for k 1 is preferable to extrapolation of laboratory data to pressures and temperatures well beyond the range of the experiments.

Characterization of lake-aquatic humic matter isolated with two different sorbing solid techniques: pyrolysis electron impact mass spectrometry

The thermal degradation of different kinds of aquatic humic-matter fractions by means of pyrolysis (Py) in combination with mass spectrometric (MS) analyses using low energy (14 eV) electron impact (EI) ionization is reported. According to the data obtained it is difficult to divide aquatic organic solutes into real humic and non-humic matters based solely on qualitatively examination of thermal degradation products. On the contrary, the quantities of thermal degradation products were strongly correlated with the various humic-matter fractions isolated/fractionated by means of different operations. The close similarities between the principal building blocks of various organic solute–fractions isolated indicates that the formation of aquatic humic solutes based on certain universal one–way precursor–product relationship is an oversimplification of a complex process. Accordingly, isolation/fractionation procedures being purely connected with strongly acidic treatments are somewhat man-made for modelling aquatic acidic humic solutes.

Rate constant and reaction channels for the reaction of atomic nitrogen with the ethyl radical

The absolute rate constant and primary reaction products have been determined at T=298 K for the atom–radical reaction N(4S)+C2H5 in a discharge flow system with collision-free sampling to a mass spectrometer. The rate constant measurements employed low energy electron impact ionization while the product study used dispersed synchrotron radiation as the photoionization source. The rate constant was determined under pseudo-first-order conditions by monitoring the decay of C2H5 or C2D5 as a function of time in the presence of excess N atoms. The result is k=(1.1±0.3)×10−10 cm3 molecule−1 s−1. For the reaction product experiments using photoionization mass spectrometry, products observed at 114 nm (10.9 eV) were CD3, D2CN and C2D4 for the N+C2D5 reaction. The product identification is based on the unambiguous combination of product m/z values, the shift of the m/z peaks observed for the N+C2D5 reaction products with respect to the N+C2H5 reaction products and the photoionization threshold measured for the major products. The observed products are consistent with the occurrence of the reaction channels D2CN+CD3(2a) and C2D4+ND(2c). Formation of C2D4 product via channel (2c) accounts for approximately 65% of the C2D5 reacted. Most, if not all, of the remaining 35% is probably accounted for by channel (2a). These rate constant and product results are compared with those for the N+CH3 reaction as well as other atom+C2H5 reactions. The role of the N+C2H5 reaction in the formation of HCN in the atmospheres of Titan and Neptune is briefly considered. In addition, the appearance energy for the formation of C2D+3 from C2D5 was determined from photoionization threshold measurements, AE0(C2D+3,C2D5)=239.5 kcal mol−1. From this, values are derived for the zero Kelvin heats of formation of C2D+3 (266 kcal mol−1) and C2D3 (71.6 kcal mol−1).

Infrared vibrational photodissociation spectra of Ar+2 ions

The infrared photodissociation spectra of Ar dimer ions have been obtained at several wavelengths with a line tunable CO2 laser. The dimer ions were produced by high energy electron impact ionization near or at the nozzle orifice of a supersonic expansion. When the electron beam is focused several millimeters from the nozzle, and the laser polarization is parallel to the dimer ion beam, the product Ar+ kinetic energy spectra exhibit structure, with a spacing of 117 cm−1. However, when the ions are produced by focusing the electron beam directly onto the nozzle, the structure largely disappears. Analysis of the results indicates that the transition is a bound to continuum transition, and that the observed spacing is associated with vibrational levels of the first excited Ar+2 I(3/2)g state.

Testing the performance of a VUV photoionization source on a double focussing mass spectrometer using alkanes and thiophenes

The performance of a newly developed photoionization source in combination with a high resolution mass spectrometer is tested. The total ion currents for several linear alkanes ( n-pentane to n-decane) and for some thiophenes (2-methylthiophene, 2,5-dimethylthiophene and 2-ethylthiophene) are measured at three fixed wavelengths: the Kr I (10.03 eV and 10.64 eV), Ar I (11.62 eV and 11.83 eV) and Ne I (16.67 eV and 16.85 eV) resonance lines. These total ion currents are compared with the data for the alkanes and thiophenes obtained with low energy (10.6 eV, 11.8 eV and 16.7eV) electron impact ionization on the same mass spectrometer. The loss in ion intensity at several positions throughout this instrument is determined for the photo-ionization source and for the electron impact ionization source. One out of every 1500 ions created in the photoionization source is measured by the detector. For the electron impact ionization source, one out of 180 ions is measured. The introduction of a five-element “Heddle” lens for the transfer of the ions from the photoion source to the mass analyser resulted in an approximately 1.5 fold loss in ion current. From the measured total ion intensities, the photoionization and electron impact ionization cross-sections at energies of 10.6 eV, 11.8 eV and 16.7 eV are calculated. The photoionization cross-section values of the linear alkanes are found to be in the range of 2.5 Mbarn to 355 Mbarn, and the electron impact ionization cross-section values are between 40 Mbarn and 735 Mbarn. The photoionization cross-section values of the thiophenes range from 4 Mbarn to 31 Mbarn, and the electron impact ionization cross-section values from 81 Mbarn to 760 Mbarn.

The detection of CH 3CO, C 2H 5, and CH 3CHO by rempi/mass spectrometry and the application to the study of the reactions H+CH 3CO and O+CH 3CO

The discharge flow reactor technique combined with molecular beam sampling and either resonance enhanced multi-photon ionization (REMPI)/mass spectrometry or low energy electron impact ionization (EI)/mass spectrometry has been used to study the specific and sensitive detection of radicals and molecules and the reaction of radicals with atoms. The acetyl radical CH 3 CO shows a REMPI mass spectrum at m/e =43 in the wavelength region 395–415 nm and at 430 nm. The ethyl radical C 2 H 5 is detected by multi-photon ionization at the wavelength 400–460 nm by the parent ion m/e =29. Acetyldehyde is monitored sensitively by a (2+1) Rydberg transition at 440.6 nm with ion fragmentation at m/e =15, 29, 43. The reaction mechanism of CH 3 CO radicals with H atoms at low pressure (0.01–0.1; 0.5–10 mbar) and temperature (293–300 K) is given by CH 3 CO+H→CH 3 +HCO (7a) →CH 3 CHO (7b) →CH 2 CO+H 2 (7c) Reaction channel (a) is the main route at low pressure (1 mbar) and the ratio of (a)/(b) is discussed in a model of chemical activation/collision stabilization. The rate constant k 1 was determined relative to the reaction C 2 H 5 +H leading to k 7 =(3.3+2)·10 13 cm 3 /mol· s The ratio of the rate constants of acetyl CD 3 CO and vinoxy CD 2 CHO radicals with D atoms was determined as k 7 dd (CD 3 CO+D)/ k 11− d (CD 2 CHO+D)=1.6+0.6 For the reaction CH 3 CO+O→CH 3 +CO 2 (12a) →OH+CH 2 CO (12b) the route (12a) dominates. The rate of this reaction (12) was measured using the reaction C 2 H 5 +O as a standard: k 12 =(1.2±0.4)·10 14 cm 3 /mol· s

Internal energy effects in photodissociation: relaxation of excited thiophenol ions formed by electron-impact ionization

The internal energy of thiophenol ions formed by moderate energy electron-impact ionization was measured as a function of the time after ionization using a visible laser probe. Immediately after ionization, it was found that about 60% of the ions had internal energies greater than 0.8 eV. At collisionless pressures, the ions were found to relax below 0.8 eV at a rate of 3 s −1. At higher pressure, where the collision rate is approximately 1 s −1, the relaxation rate was measured at 4.5 s −1. The results are compared with the energy deposition function inferred from the PES spectrum, and it is suggested that electron impact at 17 eV gives a somewhat smaller fraction of ions above 0.8 eV than does photoionization at 21 eV.

A study of the symmetric charge transfer reaction H+2+H2 using the high resolution photoionization and crossed ion–neutral beam methods

A new ion–molecule reaction apparatus, which combines the crossed ion–neutral beam method, high resolution photoionization mass spectrometry, and charge transfer detection, has been developed. Using this apparatus, we have examined the relative total charge transfer cross sections of H+2+H2 as a function of the vibrational state of H+2, v′0 =0–4, at the center-of-mass collision energy (Ec.m.) range of 0.38–200 eV. The relative total charge transfer cross sections measured at Ec.m. =8, 16, 22.5, and 200 eV are in general agreement with a recent theoretical calculation based on the semiclassical energy conserving trajectory formulation. The vibrational energy effects on the cross sections for the charge transfer and the H+3+H channels at low collision energies (Ec.m. ≤1 eV) were directly observed. The rotational states, J=0, 1, and 2, of H+2(v0=0) were also selected in this experiment. Within experimental uncertainties, the rotational excitations of H+2(v′0 =0) have no effect on the relative total charge transfer cross sections at Ec.m. =2 and 4 eV. By calibrating the nominal relative total charge transfer cross sections obtained with an ionizing photon energy of 18 eV (688 Å) to absolute total charge transfer cross sections determined previously using low energy electron impact ionization, absolute total charge transfer cross sections for v0 =0 and 1 in the kinetic energy range of Ec.m. =8–200 eV were estimated. The absolute total charge transfer cross sections thus obtained for v′0=0 and 1 are lower than the theoretical values by approximately a factor of 2. However, the kinetic energy dependence of the total charge transfer cross section is in agreement with the theoretical calculation. The final vibrational state distributions of the charge transfer products H+2 from the reaction H+2(v0=0) +H2(v″0=0) → H2(v′)+H+2(v″) at Ec.m. =4, 8, and 16 eV have been probed by charge transfer reactions H+2(v″)+N2 and H+2(v″)+CO. The results are consistent with the theoretical prediction that approximately 92% and 85% of the product H+2 ions formed at Ec.m. =8 and 16 eV are in the v″=0 state, respectively.

Low energy electron impact attachment and ionization in HgBr2

Cross sections for low energy electron impact attachment and ionization in mercuric bromide (HgBr2) have been measured using an electron beam experiment. The ion products of these reactions, as identified by mass analysis, are Br− and chiefly HgBr+2. Rate coefficients computed from the measured cross sections were found to be in good agreement with values determined independently from an electron swarm experiment.