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Recombination of N2H+ ions with electrons was studied using a stationary afterglow with a cavity ring-down spectrometer. We probed in situ the time evolutions of number densities of different rotational and vibrational states of recombining N2H+ ions and determined the thermal recombination rate coefficients for N2H+ in the temperature range of 80–350 K. The newly calculated vibrational transition moments of N2H+ are used to explain the different values of recombination rate coefficients obtained in some of the previous studies. No statistically significant dependence of the measured recombination rate coefficient on the buffer gas number density was observed. [ABSTRACT FROM AUTHOR]

The Gram-Charlier method for solving the Boltzmann equation is used to compute velocity distribution functions for O+(4S3/2) ions drifting under the influence of an electric field through helium or argon gas containing small amounts of N2. This allows us to reassess the accuracy of the commonly used reaction cross section for the O+(4S3/2) +N2 reaction, perhaps the most important reaction in the upper ionosphere. It is found that the cross sections that were derived from flow-drift measurements are in considerable error for relative kinetic energies of 0.3-3 eV between the reacting species. Using the best available transport theory, flow-drift tube data of the reaction rate coefficient are inverted to obtain a better cross section. [ABSTRACT FROM AUTHOR]

We describe a quantitative and inexpensive method of demonstrating frustrated total internal reflection based on the attenuation of a laser beam through a wedge-shaped air gap between two glass prisms. An advantage of this method is that the thickness of the air gap can be determined by observing interference effects with only a slight modification of the setup. The experiment avoids the need to employ high-precision translation stages as is often the case in standard demonstrations of frustrated total internal reflection.

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.

This paper demonstrates that the transport coefficients of 4He+ in 4He can be calculated over wide ranges of E/N, the ratio of the electrostatic field strength to the gas number density, with the same level of precision as can be obtained experimentally if sufficiently accurate potential energy curves are available for the X²Σu+ and A²Σg+ states and one takes into account resonant charge transfer. We start by computing new potential energy curves for these states and testing their accuracy by calculating spectroscopic values for the separate states. It is established that the potentials obtained by extrapolation of results from d-aug-cc-pVXZ (X = 6, 7) basis sets using the CASSCF+MRCISD approach are each in exceptionally close agreement with the best potentials available and with experiment. The potentials are then used in a new computer program to determine the semi-classical phase shifts and the transport cross sections, and from these the gaseous ion transport coefficients are determined. In addition, new experimental values are reported for the mobilities of 4He+ in 4He at 298.7 K, as a function of E/N, where careful consideration is given to minimizing various sources of uncertainty. Comparison with previously measured values establishes that only one set of previous data is reliable. Finally, the experimental and theoretical ion transport coefficients are shown to be in very good to excellent agreement, once corrections are applied to account for quantum-mechanical effects. [ABSTRACT FROM AUTHOR]

Detailed measurements employing a combination of a cryogenic flowing afterglow with Langmuir probe (Cryo-FALP II) and a stationary afterglow with near-infrared absorption spectroscopy (SACRDS) show that binary electron recombination of para-H+3 and ortho-H+3 ions occurs with significantly different rate coefficients, pabin and oabin, especially at very lowtemperatures. The measurements cover temperatures from 60Kto 300 K. At the lowest temperature of 60 K, recombination of para-H+3 is at least three times faster than that of ortho-H+3 (pabin = (1.8 ± 0.4) × 10-7 cm³ s-1 vs. oabin = (0+5-0) × 10-8 cm³ s-1). [ABSTRACT FROM AUTHOR]

We report thermal rate coefficients for 12 reactions of rare gas cations (Ne+, Ar+, Kr+, Xe+) with halide anions (Cl-, Br-, I-), comprising both mutual neutralization (MN) and transfer ionization. No rate coefficients have been previously reported for these reactions; however, the development of the Variable Electron and Neutral Density Attachment Mass Spectrometry technique makes it possible to measure the difference of the rate coefficients for pairs of parallel reactions in a Flowing Afterglow-Langmuir Probe apparatus. Measurements of 18 such combinations of competing reaction pairs yield an over-determined data set from which a consistent set of rate coefficients of the 12 MN reactions can be deduced. Unlike rate coefficients of MN reactions involving at least one polyatomic ion, which vary by at most a factor of ∼3, those of the atom-atom reactions vary by at least a factor 60 depending on the species. It is found that the rate coefficients involving light raregas ions are larger than those for the heavier rare-gas ions, but the opposite trend is observed in the progression from Cl- to I-. The largest rate coefficient is 6.5 × 10-8 cm³ s-1 for Ne+ with I-. Rate coefficients for Ar+, Kr+, and Xe+ reacting with Br2- are also reported. [ABSTRACT FROM AUTHOR]

We have studied the dependence of several ion-ion mutual neutralization (MN) reactions on helium density in the range from 1.6 × 1016 to 1.5 × 1017 cm-3 at 300 K, using the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The rate coefficients of the reactions Ar+ + Br2-, Ar+ + SF6-, and Ar+ + C7F14- were found to be independent of gas density over the range studied, in disagreement with earlier observations that similar MN reactions are strongly enhanced at the same gas densities. The cause of the previous enhancement with density is traced to the use of 'orbital-motion-limit' theory to infer ion densities from the currents collected by ion-attracting Langmuir probes in a region where it is not applicable. [ABSTRACT FROM AUTHOR]

Ion‐molecule reaction cross sections between 0.03 and 3 eV can best be inferred from data taken in drift tube mass spectrometers. For the title reactions, the cross sections inferred from drift‐tube measurements in the late 1970s have been accepted as accurate, even though the methods used to infer them made substantial assumptions about the ion velocity distribution functions. Here the Gram‐Charlier method for solving the Boltzmann equation is used to obtain ion distribution functions that are anisotropic, having different average energies parallel and perpendicular to the uniform electrostatic field. The Gram‐Charlier method incorporates skewness, kurtosis, and correlation between the motions parallel and perpendicular to the field. This leads to more accurate ion distribution functions from which more accurate reaction cross sections have been inferred. The results differ by as much as 50% from the generally accepted cross sections that are currently used to model ion‐neutral reactions in the upper atmosphere. Key Points: Drift‐tube measurements of the rate coefficients for the title reactions are reanalyzedThe results indicate that the reaction cross sections previously extracted from the data are in significant errorNew, improved cross sections are inferred from the data for energies between 0.03 and 3 eV [ABSTRACT FROM AUTHOR]

Measurements in H3+ afterglow plasmas with spectroscopically determined relative abundances of H3+ ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H3+ ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H3+ recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution. [ABSTRACT FROM AUTHOR]

High-level ab initio potential energy curves are calculated for the lowest few states of the [He–Ne]+ complex. RCCSD(T) calculations are employed with large basis sets (up to sextuple-ζ), including extrapolation to the basis set limit, taking account of spin-orbit coupling. In addition, core-valence correlation and multireference effects are investigated. We calculate spectroscopic parameters and compare these to experimentally determined values, to other high-level ab initio results, and to results from potentials that are fitted to experimental data. We present the results of new measurements of the mobility of Ne+ in He. We also calculate mobilities for Ne+ in He, and He+ in Ne, from our potentials and from recent fitted potentials; and compare the calculated and experimental mobilities graphically and statistically. [ABSTRACT FROM AUTHOR]

New experimental and theoretical results are presented that address the movement of O+ ions through argon gas. On the experimental front, improved ion mobility results are presented. These results confirm the presence of the oft-cited mobility minimum as a function of electrostatic field strength at room temperature. On the theoretical side, high-level ab initio potential energy curves are calculated for the Ar–O+ system and, from these, transport properties are calculated and compared to experiment. A crossing between the lowest 2Π curve and the ground state 4Σ- curve near the minimum of each potential becomes an avoided crossing on the inclusion of spin-orbit coupling. It is shown that the more appropriate potential for the description of the motion of O+(4S°) through Ar at the energies of interest is the diabatic potential, neglecting fine structure. By using an improved 4Σ- potential, agreement with the mobility measurements is obtained for low and intermediate electrostatic field strengths, although small discrepancies remain for high field strengths. The appropriate choice of diabatic or adiabatic potentials is also considered for related systems of interest: He–O+, Ne–O+, and Rg–O- (Rg=He,Ne,Ar). [ABSTRACT FROM AUTHOR]

The results of both experimental and theoretical studies of the mobility of O+ in Ne are reported. Errors in the experiments have been carefully assessed, allowing the obtained data to serve as stringent tests of the ab initio potentials. These potentials were calculated using the RCCSD(T) method, employing basis sets of quintuple-ζ quality. Curves were calculated for the lowest 4Σ- state [arising from O+(4S) interacting with Ne] and for the 2Π state [arising from O+(2D) interacting with Ne]. Then, the effects of spin-orbit coupling were incorporated by using the Breit-Pauli operator. The resulting ground state (Ω=3/2) of NeO+ gives mobility values in good agreement with experiment at all field strengths. Values of spectroscopic quantities for the ground electronic state of NeO+ are also presented. [ABSTRACT FROM AUTHOR]

The authors have investigated CO band emissions arising from the dissociative recombination of HCO+ and HOC+ ions with thermal electrons in a flowing afterglow plasma. The quantitative analysis of the band intensities showed that HCO+ recombination forms the long-lived CO(a 3Π) state with a yield of 0.23±0.12, while HOC+ recombination favors formation of CO(a′ 3Σ+) and CO(d 3Δ) with a combined yield of greater than 0.4. The observed vibrational distribution for the CO(a) state reproduces theoretical predictions quite well. The vibrational distributions for CO(a′) and CO(d) are, in part, inverted, presumably as a consequence of a change in CO equilibrium bond length during recombination. The observations are compatible with current knowledge of the potential surfaces of states of HCO and HCO+. [ABSTRACT FROM AUTHOR]

The authors have studied CN(B-X) and CN(A-X) emissions produced by the dissociative recombination of HNC+ ions with thermal electrons in a flowing afterglow experiment. A separate drift tube study showed that the reaction Ar++HCN, the precursor reaction used in the flow-tube experiment, produces predominantly HNC+ rather than the more energetic HCN+ isomer. Models simulating the ion-chemical processes, diffusion, and gas mixing in the afterglow plasma were fitted to observed position dependent CN(A-X) and CN(B-X) band intensities. Absolute yields of CN(B) and CN(A) were then obtained by comparing the CN band intensities to those of CO bands produced by recombination of CO2+ ions. It was concluded that the 300 K recombination coefficient of HNC+ is close to 2×10-7 cm3 s-1, that CN(B) is formed with a yield of 0.22±0.08 and CN(A) with a yield of 0.14±0.05. By comparison to synthetic spectra, the rotational temperature of CN(B) was estimated to be approximately 2500 K. It was also found that recombination produces CN(B) and CN(A) with far greater vibrational excitation than would be expected from the “impulse model” of Bates [Mon. Not. R. Astron. Soc. 263, 369 (1993)]. [ABSTRACT FROM AUTHOR]

New experimental measurements are reported for the mobility of O+ ions in He gas at 300 K. The accuracy of these new values is estimated as ±2.5%, which allows them to serve as a stringent test of a new ab initio potential that we have calculated using the RCCSD(T) method. We employed the aug-cc-pV5Z basis set with counterpoise corrections and took spin-orbit coupling into account. The present experimental values lie below the calculated ones, but the difference becomes statistically significant only at moderate and high values of the ratio of the electric field strength to the gas number density; even there they are only marginally significant. [ABSTRACT FROM AUTHOR]

Quantitative spectroscopic observations of the N2 first positive band system (N2(B 3Πg-A 3Σu+))/electron in a recombining N2H+ flowing-afterglow plasma indicate that a substantial fraction of the product N2 molecules are formed in one or more of the low-lying triplet states, B 3Πg, A 3Σu+, and W 3Δu. The total measured N2(B-A) emission intensity from N2(B,v′>=1) is equivalent to a yield of (19±8)%. The effect of rapid collision-induced transitions between states of the triplet manifold is discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quantitative spectroscopic observations of the N2 first positive band system (N2(B 3Πg-A 3Σu+))/electron in a recombining N2H+ flowing-afterglow plasma indicate that a substantial fraction of the product N2 molecules are formed in one or more of the low-lying triplet states, B 3Πg, A 3Σu+, and W 3Δu. The total measured N2(B-A) emission intensity from N2(B,v′>=1) is equivalent to a yield of (19±8)%. The effect of rapid collision-induced transitions between states of the triplet manifold is discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The flowing afterglow technique has been applied to the dissociative recombination CO[sub 2][sup +]+e[sup -]→CO[sup *]+O, in order to determine the radiative cascade contribution from high-energy triplet states of CO, (a[sup ′] [sup 3]Σ[sup +]), (d [sup 3]Δ[sub i]), and (e [sup 3]Σ[sup -]), to the total yield of the long-lived (a [sup 3]Π) state. The yields of the a[sup ′], d, e states were found to be approximately 18%, 20%, and 0.3%, respectively, of the total CO(a) yield, which leads us to conclude that (38±8)% of the total CO(a) yield is due to radiative cascading, rather than direct formation. When combined with our previously reported total CO(a) yield of 0.29±0.10, the absolute yields of the triplet states become f(a[sup ′])=0.053±0.024, f(d)=0.057±0.022, and f(e)∼9×10[sup -4]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The temperature dependence of experimental rate coefficients for association of diatomic ions in diatomic gases is compared to model calculations in which the stabilization of the initially formed unstable association complex is assumed to occur by formation of a ‘‘super complex.’’ It is shown that acceptable agreement between the model and experimental data on four sample reactions can be obtained with a minimum of plausible assumptions. The agreement indicates that earlier theoretical models may provide an accurate description of the formation and destruction of the association complex, but failed to reproduce the experimental data, because the temperature dependence of the stabilization step was inadequately treated. [ABSTRACT FROM AUTHOR]

Determines the yield of OH radicals produced by the electron-ion recombination of protonated molecules. Yield for v=0 radicals determined relative to the known OH yield from the recombination of H[sub 3]O[sup +] ions.

Determines the yield of OH radicals produced by the electron-ion recombination of H[sub 3]O[sup +] ions. Use of a flowing afterglow apparatus in conjunction with laser induced fluorescence diagnostics in the study; Results' corroboration of previous experimental results.

We present experimental evidence that reactions of argon cations Ar+ with the halogen anions Br- and I- do not occur exclusively by mutual neutralization, but also produce the cations Br+ or I+ ions by transfer ionization (TI). The experiments were carried out in flowing-afterglow plasmas at gas temperatures between and 300 and 500 K, and employed a variant of the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The measured TI rate coefficients are 1.9 ± 0.6 × 10-9 cm3 s-1 and 1.1 ± 0.30.8 × 10-9 cm3 s-1 for the Br- and I- reactions, respectively. We find that the TI rate coefficients decline with temperature as T-0.5 to T-1. No indication of TI was found in the reaction with Cl-, where it is endoergic. [ABSTRACT FROM AUTHOR]