E. W. McDaniel

Ion identity and transport properties in CO2 over a wide pressure range

We have investigated in drift tube mass spectrometers the identity and the transport properties of ions formed in CO2 gas at pressures ranging from 10−4 to 762 torr. Under bombardment by low energy (20–100 eV) electrons in the ion source, the primary positive ion is predominantly CO+2, with traces of C+, O+, and CO+. The predominant ion becomes O+2 at pressures above 100 μ (0.1 torr), and clustering of CO2 molecules to the O2+ occurs even at pressures below 1 torr. Break‐up of the clusters also occurs, the ion identity changing many times in the drift region. The zero‐field reduced mobility of the O+2⋅ (CO2)n charge carrier is a function of pressure, and varies from (1.30±0.03) cm2/V⋅sec at 0.2 torr to (1.18±0.03) cm2/V⋅sec at 1 torr. The sole negative ion produced directly by the electron bombardment is O−, which clusters to form the stable ion CO−3, whose reduced mobility is (1.27±0.06) cm2/V⋅sec for E/N ?60 Td at all pressures below 1 torr. At much higher pressures and under somewhat different conditio...

The Li+–He interaction potential

New measurements of the mobility of Li+ ions in He gas at 300°K are reported for a wide range of E/N, the ratio of the electric field strength to the gas number density. These data are used in conjunction with kinetic theory to test various Li+–He interaction potentials over a wide range of separation distance. It is shown that the ab initio potential of Hariharan and Staemmler gives mobility values in excellent agreement with experiment at low and moderate E/N, but that significant discrepancies exist at high E/N. The mobility data are also directly inverted to give the Li+–He interaction potential. This directly determined potential is in excellent agreement with the ab initio at intermediate and long range, but differs significantly in the short‐range region. In the latter region, however, it is in agreement with the potential obtained by analysis of beam‐scattering experiments.

Mobility, diffusion, and clustering of K+ ions in gases

We have measured, with a drift tube mass spectrometer, the mobilities and longitudinal diffusion coefficients of K+ ions in nitrogen and carbon monoxide at 300°K. The measurements were made over a range of E/N extending from thermal values up to 636 × 10−17 V·cm2. Here E is the drift field intensity and N is the gas number density. The zero‐field reduced mobilities of K+ ions in N2 and CO were determined to be (2.54 ± 0.05) and (2.30 ± 0.04) cm2/V · sec, respectively. The low‐field diffusion coefficients are in excellent agreement with the values calculated by the Einstein equation from the experimental zero‐field mobilities. The experimental diffusion coefficients are compared with the predictions of an equation developed by Wannier on the assumption that the ion‐molecule interaction consists of only the attractive polarization force, of which a constant mean free time between collisions is a consequence. Comparison is also made with a modified version of this equation which contains the ionic drift velo...

Mobilities and longitudinal diffusion coefficients of K+ ions in argon gas

We have used a drift tube mass spectrometer to measure the mobilities and longitudinal diffusion coefficients of mass‐identified K+ ions in argon at 300°K. The range of E / N extended from 1 to 610 × 10−17V · cm2, where E is the electric drift field intensity and N is the number density of the neutral gas. The zero‐field reduced mobility of K+ ions in argon was found to be 2.66 ± 0.05 cm2/V · sec, in close agreement with the value previously reported by Tyndall for ions in argon that were assumed to be K+ but not mass identified. The low‐field diffusion coefficients agree well with the value predicted from the Einstein equation using the experimental zero‐field mobility. Our diffusion data are compared with theoretical predictions of Wannier and of Whealton and Mason, and with experimental data obtained by Skullerud using a drift tube lacking a mass spectrometer.

Mobility of Cl− ions in Xe gas and the Cl−–Xe interaction potential

The mobility of Cl− ions in Xe gas at 300 °K has been measured in a drift tube mass spectrometer for a wide range of values of the ionic energy parameter E/N (the ratio of the electric field strength to the neutral gas number density). A Cl−–Xe interaction potential is assumed and a kinetic theory appropriate for the ion motion is used to derive the mobility from the potential. Then an iterative technique is used to modify the potential so as to fit the predicted mobility to the experimental data. This interaction potential is directly determined by the experimental data for a range of internuclear separation distances from about 4 to about 10 a.u. At distances greater than 10 a.u., the potential is the induced dipole polarization potential.

Mobilities and longitudinal diffusion coefficients of mass‐identified hydrogen ions in H2 and deuterium ions in D2 gas

We have measured in a drift tube mass spectrometer the mobilities and longitudinal diffusion coefficients of H+, H3+, and H− ions in H2 gas and D+, D3+, and D− ions in D2 gas. These measurements were made at 300°K and over a substantial range of E/N, where E is the electric field intensity and N is the gas number density. The zero‐field reduced mobilities of the ions in H2 were determined to be 16.0 ± 0.3, 11.3 ± 0.2, and 43.0 ± 1.3 cm2/V · sec, respectively; the values for the ions in D2 were 11.7 ± 0.4, 8.07 ± 0.25, and 30.1 ± 0.9 cm2/V · sec, respectively. The fact that the mobilities of H− and D− are so much larger than those of the corresponding positive ions, H+ and D+, is unexpected and thus far has not been quantitatively explained. Measurements of the longitudinal diffusion coefficients as a function of E/N are in fair agreement with the theoretical predictions of Whealton and Mason. The low‐field diffusion coefficients are in good agreement with the values predicted from the measured mobilities ...

Longitudinal diffusion coefficients of Li+ and Na+ ions in He, Ne, and Ar: Experimental test of the generalized Einstein relation

We have measured, with a drift tube mass spectrometer, the longitudinal diffusion coefficients of Li+ and Na+ ions in He, Ne, and Ar at a gas temperature of 300 °K. The measurements were made as a function of the energy parameter E/N, where E is the electrostatic drift field strength and N is the gas number density in the drift tube. The range of the steady‐state, average ionic energy thereby covered extended from close to the thermal value up to 10.1 eV in the case of Li+ in He and a few eV less in the other ion–atom combinations. The experimental data are compared with the results of computations based on three different equations: the original Wannier equation derived for the polarization attraction model, our modification of this equation, and the generalized Einstein relation discussed in the paper immediately preceding this paper [Larry A. Viehland and E. A. Mason, J. Chem. Phys. 63, 2913 (1975)]. The latter relation is superior to the other two in the present application and gives excellent agreeme...

Test of the Li+–He interaction potential

The mobility of Li+ ions in He has been measured at a gas temperature of 300 °K over a wide range of E/N, where E is the electric field intensity and N is the gas number density. A new theory of ion mobility is used to calculate from these data the standard Ω integral ?(1,1) of kinetic theory for Li+ in He over a range of effective gas temperature extending from 300 to 28 700 °K. The ab initio quantum mechanical interaction potential for the Li+–He system computed by Catlow et al. is also used to calculate ?(1,1) as a function of Teff. A comparison of the two sets of data serves as a test of the Catlow potential. The test is extended to still higher effective temperatures (up to 60 000 °K) by comparison with values of ?(1,1) computed from the short‐range repulsive potential derived from Li+ beam scattering experiments in helium. The Catlow potential reproduces the general features of the dependence of the ’’experimental’’ Ω integral on Teff over the entire range of the test. However, significant discrepan...

Measurement of the rate coefficient of the reactions H+ + 2H2 → H3+ + H2 and D+ + 2D2 → D3+ + D2 in a drift tube mass spectrometer

The rate coefficient of the reaction H+ + 2H2 → H3+ + H2 has been measured at a gas temperature of 300 °K and over a pressure range 0.25–0.50 torr. The transport properties and average energy of a given species of ion drifting in a gas in a uniform electric field are determined by E/N, where E is the electric field intensity and N is the gas number density. E/N is expressed in units of the townsend (Td), where 1 Td = 10−17 V · cm2. Our measurements were made over a range 25–50 Td, where the lower value corresponds to an average energy for the reacting H+ ions which is very close to the thermal energy of the H2 molecules at 300 °K. No systematic variation of the rate coefficient was observed over the range of E/N which was covered. The measurements were made with a drift tube mass spectrometer and involved the detailed analysis of the arrival time spectra of the product H3+ ion. The rate coefficient was evaluated to be (3.05 ± 0.15) × 10−29 cm6/sec. The same result was also obtained for the reaction D+ + 2...

Possible Sources of Large Error in Determinations of Ion–Molecule Reaction Rates with Drift Tube–Mass Spectrometers

Drift tube–mass spectrometers are now used extensively to measure the rate coefficients of ion–molecule reactions. The accuracy which can be obtained in favorable cases is about 10%, but if careful attention is not paid to a number of factors, very large errors can result. The purpose of this paper is to call attention to this fact, which seems not to be widely known, and to enumerate the factors which can lead to these large errors.