Charge Transfer Products Research Articles

Charge-transfer production of predissociated N 2 states. I. The n = 3 Rydberg state region (12.8–14 eV)

An experimental study is presented on the predissociation of perturbed n = 3 Rydberg systems in N 2 converging to the X 2Σ g + and A 2Π u ion states. The Rydberg states are formed in collisions of N 2 + with cesium at energies between 4.8 and 7 keV. The dissociation fragments are recorded using translational spectroscopy. Assignment of the Rydberg states is realized with a selective laser excitation technique. The laser excitation also reveals that ion core excitation concurrent with electron transfer is a significant mechnism in the charge-transfer formation of the (3sσ gA + Rydberg states. The (3pπ u)X + G 3Π u(υ=0), (3pσ u)X + D 3Σ u + (υ=0), )3sσ gA +F 3Π u(υ=0–3), and (3sσ gA + o 1Π u(υ=0–4) states are observed with all levels predissociating to N( 2D) + N( 4S) atoms. The vibrational numbering of the F 3Π u state is reassigned with respect to earlier work.

Guided-ion beam measurements of X++NO (X=Ar, N2) reactions

Cross section and product ion time-of-flight measurements are presented for the X++NO (X=Ar, N2) collision systems over a collision energy range of 0.1–20 eV (c.m.). The experiments are carried out in an octopole guided-ion beam apparatus. Charge transfer is the main channel observed and dissociative charge-transfer products are observed for collision energies equal to or greater than their respective thermodynamic thresholds. A weak channel leading to ArN+ and/or ArO+ is observed in the X=Ar system. The charge-transfer cross sections of the two collision systems are similar in magnitude and have approximately an E−1/2 dependence at near-thermal collision energies. The near-thermal cross sections are significantly less than the Langevin–Gioumousis–Stevens predictions for a reaction mediated by ion–induced dipole interactions and complex formation. The time-of-flight distributions of the Ar+, N+2+NO charge-transfer products are characteristic of near-resonant charge-transfer processes. At all collision energies studied, the charge-transfer reaction is dominated by a direct mechanism, but below 1 eV, the formation of a long-lived intermediate complex is also significant. The results are discussed in relation to statistical models and to constraints imposed by requirements of energy resonance and Franck–Condon overlap in charge-transfer reaction dynamics.

The reactions of Zn +, Cd +, Hg + and Pb + with benzene, substituted benzenes and thiols

The reactions of Zn +, Cd +, and Pb + have been studied in the gas phase with benzene, substituted benzenes, aryl thiols and alkyl thiols. Most reactions of Zn +, Cd + and Hg + involved charge transfer as the major reaction pathway, wheres Pb + reacted to give addition produces. Zn +, Cd + and Hg + reacted with alkyl substituted benzenes, benzenethiol and p-fluorobezenethiol by the charge transfer reaction. Pb + was much less reactive and gave the addition reaction. The reactions of the metal ions with the alkythiols gave numerous products including the thiol cations, the protonated thiol ions and addition complex ions. Whereas the reactions of Zn + with the butanethiols gave addition products involving up to three thiols or parts thereof, the reaction with Cd + gave only monoaddition products. Hg + gave the charge transfer product with n- and t-butanethiols and Pb + gave addition products with the alkylthiols. The reactivity of the thiols varied with t-butanethiol being the most reactive and n-butanethiol being the least reactive.

Ne++C60 collisions: The dynamics of charge and energy transfer, fragmentation, and endohedral complex formation

Interaction of Ne+ with C60 has been studied for collision energies ranging up to 100 eV. The dominant process is charge transfer, with C60+ accounting for 85% to 100% of the total product signal in this energy range. At 25 eV collision energy, C60-2n+ dissociative charge transfer products appear, accounting for ∼13% of the total product signal at high energies. At ∼25 eV, NeC60+, NeC58+, and NeC56+ all appear together, followed by a series of NeC60-2n+ products where n increases with increasing collision energy. These products are believed to be endohedral complexes, Ne@C60-2n+, where neon is physically trapped inside the fullerene cage. The production efficiency of the endocomplexes reaches a maximum of ∼1.2% at 60 eV, and decreases to 0.5% at 100 eV. Modeling the results leads to the following picture of Ne++C60 collision dynamics: Charge transfer occurs in nearly all collisions, and is the only significant process for high impact parameters. At collision energies below ∼25 eV, translational-to-internal energy transfer is inefficient, even for low impact parameter collisions. At higher energies, the inelasticity jumps to near 100%, leading to fragmentation of the nascent products. Both endohedral penetration and efficient energy transfer appear to require bond rupture in the C60.

Ion-molecule reaction chemistry of Fe+ with NO: excited-versus ground-state reactions

Ion-molecule reactions of excited- and ground-state Fe[sup +] with NO are reported. The charge-exchange and clustering ion-molecule reactions of Fe[sup +] with Fe(CO)[sub 5] are used to study the reaction dynamics of metastable versus ground electronic state Fe[sup +] ions. The ground-state [sup 6]D(4s[sup 1]3d[sup 6]) Fe[sup +] ions are unreactive with NO. Conversely, the metastable electronic states react with NO to form charge-transfer product ions (NO[sup +]) and radiative association adduct ions [Fe[sup +](NO)] and by the collisional relaxation process. The collisional relaxation process gives rise to the ground-state [sup 6]D(4s[sup 1]3d[sup 6]) and excited-state [sup 6]S(4s[sup 2]3d[sup 5]) Fe[sup +] ions. The [sup 6]S(4s[sup 2]3d[sup 5]) Fe[sup +] ion is unreactive with NO and constitutes approximately 6% of the Fe[sup +] ion population. Molecular orbital concepts are used to explain the lack of reactivity for the [sup 6]S(4s[sup 2]3d[sup 5]) Fe[sup +] ions and also the collisional relaxation process by the high-lying 4s[sup 1]3d[sup 6] states to form the [sup 6]S(4s[sup 2]3d[sup 5]) Fe[sup +] ions. 31 refs., 5 figs.

Guided-ion beam measurements of the X++H2O(D2O) (X=Ar,N2) collision systems

Guided-ion beam cross section and product kinetic energy measurements of charge-transfer and atom-abstraction reactions of the Ar++H2O(D2O) and N2++H2O(D2O) collision systems are presented for collision energies ranging between 0.2 and 20 eV c.m. Both charge-transfer systems exhibit large hyperthermal cross sections exceeding 10 Å2 and are characterized by a long-range interaction, evidenced by the small amount of angular scattering observed in product-ion time-of-flight (TOF) measurements. Weak forward-scattered signals due to orbiting collisions are detected and are interpreted to stem from a dipole orientation that maximizes the long-range attractive forces. The charge-transfer product ion velocity distributions are well described by an osculating complex model. The charge-transfer systems exhibit weak isotope effects that are related to competition with the atom-abstraction channels. The atom-abstraction cross sections represent ∼10% of the total cross section and the observed distinct isotope effects and TOF distributions are consistent with a pairwise-energy model.

Collision-induced dissociation of Ag (C 6H 6) +

Collision-induced dissociation (CID) of Ag (C 6H 6) + is studied by using guided ion beam mass spectrometry. A flow tube source is used to produce thermalized silver benzene ions. Ag + is the only observed ionic product; no charge transfer product C 6H + 6 is found, in contrast with previous photodissociation results. Careful analysis of the CID threshold yields a 0 K bond dissociation energy of D(Ag +−C 6H 6) = 1.62 ± 0.07 eV (37.4 ± 1.7 kcal/mol). This value is compared with several literature values.

Energy partitioning in hyperthermal O+ + H2O charge-transfer collisions

The energy partitioning in hyperthermal O+ + H2O charge-transfer collisions is determined from the analysis of guided-ion beam time-of-flight measurements. The experimentally determined H2O+ laboratory velocity distributions exhibit a main band due to near-thermal ions and a weak channel due to center-of-mass backscattered ions. The velocity distributions are fit to simulated distributions based on an osculating complex model. The charge-transfer products formed via the shortest lived complexes exhibit little translational-to-internal energy transfer, resulting in internal energies of 1.0 ± 0.2 eV. Contributions due to longer-lived complexes are observed and result in increased internal excitation. The internal energy distribution is, however, non-statistical because the lifetime of these complexes is considerably shorter than their rotational period.

Rovibrational product distributions of O2+ from the reaction of O+(4S) with CO2

The nascent rovibrational population of O2+ produced from the reaction of near-thermal O+(4S)+CO2 is measured by observing the translational energy spectrum of its dissociative charge transfer products following neutralization in Cs vapor at keV energies. It is found that of the 1.19 eV exothermicity in the formation reaction, the product O2+ is formed on the average with 0.48 eV of internal energy partitioned nearly equally between vibrational excitation (0.27±0.03 eV) and rotational excitation (0.22±0.03 eV). Populations measured for O2+(v=0−5) are: 0.38, 0.30, 0.18, 0.09, 0.05, 0.01, respectively. The O2+ rotational temperature decreases with increasing vibrational excitation, from Trot=3100 K in v=0 to Trot=450 K in v=5. The present populations show some disagreement with recent flow tube measurements. Details of the technique and a determination of the O2+ rovibrational population following electron-impact ionization of O2 are also presented. The O+(4S)+CO2 reaction and its products are important in the ionospheres of Mars and Venus and in CO2 releases in the Earth’s ionosphere.

Nitrogen ion (N+) + C60 fullerene reactive scattering: substitution, charge transfer, and fragmentation

N + +C 60 reactive scattering has been studied over the collision energy range from 1 to 80 eV. Charge transfer, producing C 60 + is the dominant low-energy channel. At high collision energies a significant fraction of the charge-transfer products also fragment producing C 60-2n + (n=1-8). Chemical reactivity of C 60 with N + is low compared to that observed for B + , C + , and O + and shows evidence of a barrier for N addition

Collisions of H+, H2+, H3+, ArH+, H−, H, and H2 with Ar and of Ar+ and ArH+ with H2 for Energies from 0.1 eV to 10 keV

Graphical and tabulated data and the associated bibliography are presented for cross sections for elastic, excitation, and ionization collisions of H+, H+2, H+3, ArH+, H−, H, and H2 with Ar and of Ar+ and ArH+ with H2 for laboratory energies from 0.1 eV to 10 keV. Where appropriate, drift velocities and reaction or excitation coefficients are calculated from the cross sections and are recommended for use in analyses of swarm experiments and electrical discharges. In the case of H+ in Ar, cross sections for momentum transfer, charge transfer, electronic excitation, and electron production are recommended. Drift velocity calculations predict runaway for H+ in Ar for electric field to gas density ratios E/n greater than 4.3×10−18 V m2. For H+2 in Ar, the cross sections include those for ArH+ formation, charge transfer, electronic excitation, and electron production. Drift velocities and average cross sections are calculated versus E/n. In the case of ArH+ collisions with Ar, only cross sections for ion molecule reactions are recommended. For H− collisions with Ar only collisional detachment data is available. Momentum transfer, electronic excitation, and electron production cross section data are available for collisions of H with Ar. Collisions of H with Ar are of special interest because of the very large cross sections for excitation of the H atoms at low energies. For H2 in Ar cross sections for momentum transfer and excitation of the Hα and Hβ lines are recommended. For Ar+ collisions with H2, cross sections for charge transfer and ion molecule reactions are recommended. Cross sections for proton transfer are available for ArH+ in H2.

A laboratory study of charge transfer and hydrogen atom pickup reactions in N +H 2O(D 2O) collisions at suprathermal energies

Charge transfer and hydrogen atom pickup cross sections are reported for N +H 2O(D 2O) collisions at center-of-mass energies between 1 and 25 eV. Time-of-flight measurements were performed to determine the charge transfer product ion kinetic energies. The exothermic charge transfer proceeds rapidly at large impact parameters with little momentum transfer, and is independent of the water isotope. Above 2 eV, this reaction is governed by a direct mechanism without a long-lived complex. The endothermic hydrogen atom pickup cross section increases with the collision energy, and is approximately two orders of magnitude smaller than the charge transfer cross section.

Cross Sections and Swarm Coefficients for Nitrogen Ions and Neutrals in N2 and Argon Ions and Neutrals in Ar for Energies from 0.1 eV to 10 keV

Graphical and tabulated data and the associated bibliography are presented for cross sections for elastic, excitation, and ionization collisions of N+, N+2, N, and N2 with N2 and for Ar+ and Ar with Ar for laboratory energies from 0.1 eV to 10 keV. Where appropriate, drift velocities and reaction or excitation coefficients are calculated from the cross sections and recommended for use in analyses of swarm experiments and electrical discharges. In the case of N+ in N2, cross sections for momentum transfer, charge transfer, electronic excitation, and electron production are recommended. Drift velocity calculations predict runaway for N+ in N2 for electric field to gas density ratios E/n greater than 4.3×103 Td, where 1 Td (townsend)=10−21 V m2. For N+2 in N2, the cross sections include those for N+ and N+3 formation, electronic excitation, and electron production. Drift velocities and average cross sections are calculated for E/n≥500 Td. In the case of N in N2, only cross sections for momentum transfer are recommended. For N2 in N2, cross sections for momentum transfer, electronic excitation, and electron production are recommended. Collisions of electronically excited states with N2 are not included. For Ar+ in Ar, cross sections for charge transfer, electronic excitation, and electron production are recommended. For Ar in Ar, cross sections for momentum transfer, electronic excitation, and electron production are recommended.

Experimental and theoretical total state-selected and state-to-state absolute cross sections. I. The H+2(X,v′)+Ar reaction

Total state-selected and state-to-state absolute cross sections for the reactions, H+2(X̃,v′=0–4)+Ar→H2(X,v) +Ar+(2P3/2,1/2) [reaction (I)], ArH++H [reaction (II)], and H++H+Ar [reaction (III)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.48–100 eV. Experimental state-selected cross sections for reactions (I) and (II) measured at Ec.m.=0.48–0.95 eV are in agreement with those reported previously by Tanaka, Kato, and Koyano [J. Chem. Phys. 75, 4941 (1981)]. The experiment shows that prominent features of the cross sections for reactions (I) and (II) are governed by the close resonance of the H+2(X̃,v′=2)+Ar and H2(X,v=0)+Ar+(2P1/2) vibronic states. At Ec.m.≤3 eV, the vibrational state-selected cross section for the charge transfer reaction (I) is peaked at v′=2. The enhancement of the charge transfer cross section for v′=2 as compared to other v′ states of reactant H+2 increases as Ec.m. is decreased. The state-to-state cross sections for reaction (I),measured at Ec.m.≤3 eV, show that the enhancement for the charge transfer cross section for v′=2 is due to the preferential population of Ar+(2P1/2). At Ec.m.=0.48–0.95 eV and v′=2, nearly 80% of the charge transfer product Ar+ ions are formed in the 2P1/2 state. However, at Ec.m.>5 eV, the intensity for charge transfer product Ar+(2P3/2) is greater than that for Ar+(2P1/2). Contrary to the strong vibrational dependence of the cross section for reaction (I), the cross section for reaction (II) is only weakly dependent on the vibrational state of H+2. At Ec.m.≤3 eV, the cross section for the formation of ArH+ is the lowest for v′=2 compared to other v′ states, an observation attributed to the competition of the nearly resonant Ar+(2P1/2)+H2(X,v=0) charge transfer channel. The cross section for reaction (II) decreases with increasing Ec.m.. At Ec.m.≥20 eV, the cross sections for the formation of ArH+ become negligible compared to those for Ar+. The appearance energies for the collision-induced dissociation H+2(X̃,v′=0–4) are consistent with the thermochemical threshold for reaction (III). The cross sections the formation of H+ are ≤20% of those for H+2. Theoretical state-to-state cross sections for reaction (I) at Ec.m.=19.3 and 47.6 eV calculated using the nonreactive infinite-order sudden approximation are found to be in fair agreement with experimental results.

Absolute state-selected and state-to-state total cross sections for the reaction Ar+(2P3/2,1/2)+O2

Absolute spin–orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2)+O2→O+2+Ar [reaction (1)], O++O+Ar [reaction (2)], and ArO++O [reaction (3)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.044–133.3 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+O2 reaction at Ec.m.=2.2–177.6 eV have also been examined. The appearance energies for the formation of O+ (Ec.m.=2.9±0.2 eV) and ArO+ (2.2±0.2 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The cross sections for O+2, O+, and ArO+ depend strongly on Ec.m. and the spin–orbit states of Ar+, suggesting that reactions (1)–(3) are governed predominantly by couplings between electronic potential energy surfaces arising from the interactions of Ar+(2P3/2)+O2, Ar+(2P1/2)+O2, and O+2+Ar. In the Ec.m. range of 6.7–22.2 eV, corresponding to the peak region of the O+ cross section curve, the cross sections for O+ are ≥50% of those for O+2. The production of O+ by reaction (2) is interpreted to be the result of predissociation of O+2 in excited states formed initially by reaction (1). The formation of charge transfer O+2(ã 4Πu) has been probed by the charge transfer reaction O+2(ã 4Πu)+Ar. The results indicate that in the Ec.m. range of 0.4–3.0 eV charge transfer product O+2 ions are formed mainly in the O+2(ã 4Πu) state. Experimental evidence is found supporting the conclusion that the vibrational distributions of O+2(ã 4Πu) formed in reaction (1) and by photoionization of O2 in the energy range between the O+2(ã 4Πu, v=0) and O+2(Ã 2Πu, v=0) thresholds are similar. The population of O+(4S) formed by reaction (2) has also been measured by the reaction O+(4S)+N2→NO++N. In the Ec.m. range of 3–44 eV, product O+ ions of reaction (2) are shown to be dominantly in the O+(4S) ground state. At Ec.m.≥14 eV, the retarding potential energy analysis for O+2 shows that more than 98% of the charge transfer O+2 ions are slow ions formed mostly by the long-range electron jump mechanism. Product ArO+ ions are observed only in the Ec.m. range of 2.2–26.6 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (2) and (3), the overwhelming majority of ArO+ and O+ ions are scattered backward and forward with respect to the c.m. velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArO+ and O+ via nearly collinear Ar+–O–O collision configurations at Ec.m. near the thresholds of reactions (2) and (3).

Observation of the formation of N+ and ArN+ in the collisions of Ar+(2P3/2,1/2) with N2

Absolute total cross sections for the reactions, Ar+(2P3/2,1/2)+N2→N++N+Ar [reaction (1)] and ArN++N [reaction (2)], have been measured in the center-of-mass collision energy (Ec.m.) range of 6.2–123.5 eV. The appearance energy for the formation of N+ (Ec.m.=8.65±0.21 eV) is in agreement with the thermochemical threshold for reaction (1). The comparison of the collision energy dependence of the N+ cross section with the photoionization efficiency spectrum of N+ from N2 suggests that the predissociative multielectron states of N+2, C̃ 2Σ+u, F̃ 2Σ+g, G̃ 2Σ+g, and 2Σ+g (2σg)−1, which are responsible for the dissociative photoionization of N2, also play a role in the formation of N+ via reaction (1). Product ArN+ ions of reaction (2) are only observed in the Ec.m. range of 8.2–41.2 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (1) and (2), the majority of ArN+ and N+ ions are scattered backward and forward with respect to the center-of-mass velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArN+ and N+ ions via nearly collinear Ar+-N-N collision configurations at Ec.m. near the thresholds of reactions (1) and (2). At Ec.m.≥11 eV, more than 92% of the charge transfer product N+2 ions are found to be slow ions formed mostly by the long-range electron jump mechanism.

Factors controlling the etching rate and etching profile in the O2 reactive ion etching pattern transfer step in multilevel lithography

AbstractPhysical bombardment plays a dominant role in the O2 reactive ion etching (RIE) pattern transfer step in multilevel lithography. Etching rates are determined by the flux and energy distribution of the bombarding ions and energetic neutrals (charge transfer products), while anisotropy is determined by their directionality. Measurements of the sheath thickness and voltage drop may be used to estimate flux, energy distribution, average energy, and angular distribution of ions and the energetic neutral products of charge transfer collisions. The estimated flux of bombarding particles allows measured etching rates to be converted into yields. The trends for the etching rate as a function of pressure, bias voltage, and other system variables reflect a single fundamental trend for the yield as a function of bombardment energy. Etching rates of an organic novolac polymer are proportional to the energy flux from bombarding particles while the yield per bombarding particle is proportional to its energy. These kinetics are combined with angular distribution and interface evolution models to predict etching profiles in multilevel lithography.

Mass spectra of some acetals

The electron impact (El), methane chemical ionisation (CH4 Cl) and ammonia chemical ionisation (NH3 Cl) mass spectra of five acetals have been obtained. The [graphic omitted] ions dominate the total ion current under El (70 and 14 eV). In the CH4 Cl mode, the [(M+ 1)–CH3OH]+ ions form the base peaks, except for methyl 2-(dimethoxymethyl)-5-(2-hydroxyethyl)-cyclopent-3-en-1-ylacetate, compound (V), which yields [(M+ 1)–2CH3OH]+ as the base peak. The [M+ 1]+ ion is absent from the NH3 Cl mode, but the fragmentation patterns are still complicated. The main fragmentation pathways include fragmentation from the cluster adduct ions, from the charge-transfer products and from the aldehyde, a product from the reversed reaction of the acetal. The difference in energy gained by the molecular ions during the ionisation process leads to this feature.

Electrochemical characteristics of nitro-heterocyclic compounds of biological interest. II. Nitrosochloramphenicol.

The electrochemical characteristics of nitrosochloramphenicol have been studied in aqueous buffer systems (pH 7.1) using direct current (d.c.) and differential pulse polarography, cyclic voltammetry and coulometric techniques. Up to 4 charge-transfer steps can be identified. The first reduction step is reversible both chemically and electrochemically, the charge-transfer product showing no tendency to undergo further reaction on the electrochemical time-scale. In contrast, the second reduction step is irreversible, with the product undergoing a fast following reaction to yield a redox-active species which was detected by cyclic voltammetry. From the data and by comparison with related systems, two reduction mechanisms are possible and are discussed.

Direct observation of charge-exchange collisions between mass-selected Nan+ clusters and Cs atoms.

We have measured total charge-exchange cross sections in collisions between mass-selected $\mathrm{Na}_{n}^{+}$ cluster ions ($1\ensuremath{\le}n\ensuremath{\le}21$) and atomic cesium. Cross sections for clusters ranging from the monomer to the 21-mer lie between 40 and 10 ${\mathrm{\AA{}}}^{2}$, decreasing slowly with increasing mass. Fast neutral products of charge exchange are photoionized and mass analyzed in a secondary time-of-flight spectrometer. Charge-transfer products present a narrow distribution of at most three masses ${\mathrm{Na}}_{n}$, ${\mathrm{Na}}_{n\ensuremath{-}1}$, and ${\mathrm{Na}}_{n\ensuremath{-}2}$, with the most probable product being ${\mathrm{Na}}_{n}$ or ${\mathrm{Na}}_{n\ensuremath{-}1}$.