E. E. Ferguson

Flowing Afterglow Measurements of Ion-Neutral Reactions

Publisher Summary This chapter describes the experimental and analytical techniques that have been developed for flowing afterglow applications to the quantitative study of ion–neutral reaction processes. Most other techniques for the measurement of ion–molecule reaction rate constants are inherently unsuited for the examination of an ion reacting with a neutral where the neutral has the lower ionization potential, and sufficient data for this generalization did not exist prior to the flowing afterglow results. Charge transfer reactions of negative ions have sometimes been useful in establishing relative electron affinities of molecules, which are often difficult to measure. Positive ion charge-transfer reactions have, on occasion, been useful in establishing relative ionization potentials of molecules, generally known or better measured in more direct ways. The dc discharge had other advantages over the microwave discharge as well. Its geometrical configuration was more compatible with the detailed flow analysis, and it was more easily incorporated in metal flow tubes, which were soon found to be advantageous.

Ion–Molecule Reaction Studies from 300° to 600°K in a Temperature‐Controlled Flowing Afterglow System

The flowing afterglow technique for ion‐neutral reaction rate constant studies has been modified to cover a range of gas temperature from 82° to 600°K. The experimental apparatus is described and rate constants for six reactions between 300° and 600°K are reported. The reaction of He+ with N2 is found to have a rate constant 1.2 × 10−9 cm/3sec, independent of temperature in this range. The reaction of O+ with CO2 has a rate constant 1.1 × 10−9 cm3/sec at 300°K, decreasing slightly to 8 × 10−10 cm3/sec at 600°K. The reaction of N+ with O2 appears to have a rate constant 6 × 10−10 cm3/sec throughout this temperature range. The rate constant for charge transfer of N2+ with O2 is measured to be 4.7 × 10−11 cm3/sec at 300°K, decreasing to about 2 × 10−11 cm3/sec at 500°K. The rate constant for the reaction of O+ with O2 decreases from 2 to 1.3 × 10−11 cm3/sec, and the rate constant for O+ + N2 decreases from 1.2 × 10−12 cm3/sec to about 6 × 10−13 cm3/sec between 300° and 600°K.

Laboratory measurements of negative ion reactions of atmospheric interest

Abstract Rate constants for the loss processes of negative oxygen ions in the D -region of the ionosphere have been measured in the laboratory for the first time. Associative detachment reactions for O − and O 2 − ions with atomic oxygen dominate the loss of these ions when the atomic oxygen concentration exceeds the ozone concentration. When the ozone concentration exceeds the atomic oxygen concentrations, the O − and O 2 − ions will largely charge-transfer to the ozone producing O 3 − . The O 3 − ions will rapidly react with CO 2 to form CO 3 − . The CO 3 − ions will react with atomic oxygen to regenerate the O 2 − ions and will also react with NO to produce NO 2 − ions. O 3 − ions also react with NO to produce NO 3 − ions which may be very stable in the atmosphere. The most important atmospheric negative ions may then be NO 2 − , NO 3 − , and CO 3 − , with O 2 − and O − having lower concentrations than these and O 3 − having a still lower concentration. A key consideration in the production of stable (long lived) atmospheric negative ions is the ozone to atomic oxygen ratio. A sufficient condition for large λ (ratio of negative ion density to electron density) is that the ozone to atomic oxygen ratio exceeds unity.

Thermal Energy Ion—Neutral Reaction Rates. I. Some Reactions of Helium Ions

A flowing, steady‐state afterglow system has been utilized to measure room‐temperature ion—neutral reaction rates. A description of the apparatus and technique is given. Measured rate constants for He+ reactions with O2, N2, CO, NO, and CO2 are reported, as well as upper limits for the reactions of He+ with H2, Ne, and Ar, and an estimate for the reaction of He2+ with Ne and N2.The reactions of He+ with O2, N2, CO, NO, and CO2 are all rapid, the rate constants all being ≈10−9 cm3 sec−1, implying essentially a reaction per collision.

Thermal‐Energy Ion—Neutral Reaction Rates. VII. Some Hydrogen‐Atom Abstraction Reactions

Rate constants for reactions in which the positive ions O+, N+, Ar+, N2+, CO+, CO2+, and N2O+ abstract a hydrogen atom from H2 have been measured at 300°K in a pulsed, flowing‐afterglow reaction tube. Several additional reactions are observed which occur as secondary reactions in these experiments.

Thermal Energy Ion—Neutral Reaction Rates. II. Some Reactions of Ionospheric Interest

A pulsed, flowing afterglow has been utilized to measure rate constants at room temperature for the following reactions involving NO: N2++NO→NO++N2,k1≈5(+1,−3)×10−10cm3/sec,N++NO→NO++N,k2=8(+1,−4)×10−10cm3/sec,O2++NO→NO++O2,k3≈8(+2,−5)×10−10cm3/sec,O++NO→NO++O,k4≤2.4×10−11cm3/sec. In addition the following related reactions have been measured: N++O2↗NO++O↘N+O2+,k5≈1(+0.1,−0.6)×10−9cm3/sec,N2++O2→O2++N2,k6≈1(+0.2,−0.6)×10−10cm3/sec,O2++N→NO++O,k7≈1.8(+0.2,−0.9)×10−10cm3/sec. Some additional information concerning the end products of N++O2 is offered.

Reaction of atomic oxygen ions with vibrationally excited nitrogen molecules

Abstract Laboratory experiments show that the ion-atom interchange reaction (1) O + + N 2 (T v ) → NO + + N has an increased rate constant when the N2 is vibrationally excited. For a Boltzmann distribution of N2 vibrational states such that Tv = 4000°K, the rate constant for (1) is increased by a factor of about twenty over the rate constant for ground vibrational state N2. This has important implications for electron loss rates in the ionosphere. The vibrational effect can account for previous laboratory measurements which yielded larger rate constants for reaction (1) than have been measured in this laboratory.

Additional flowing afterglow measurements of negative ion reactions of D-region interest

Abstract Some additional negative ion reactions of possible D-region importance have been measured in the ESSA flowing afterglow system at 300°K. It has been found that NO3− does not react with atomic oxygen or atomic nitrogen with a rate constant as large as 10−11 cm3/sec. The electron affinity of NO3 is found to be greater than that of NO2 by more than 0.9 eV. It is pointed out that conversion of O2− to O4− followed by subsequent reactions of the O4− may play an important role in D-region negative ion chemistry. Rate constants for the reaction of O4− with NO, CO2 and O are reported. The reaction of O4− with CO2 produces CO4−. Rate constants for the reactions of CO4− with NO and O are reported.

Thermal‐Energy Ion—Neutral Reaction Rates. III. The Measured Rate Constant for the Reaction O+(4S)+CO2(1Σ)→O2+( 2Π)+CO(1Σ)

The rate constant for the reaction O+(4S)+CO2(1Σ)→O2+(2Π)+CO(1Σ)+1.5eV is measured to be 1.2±0.4×10−9cm3/sec in a pulsed flowing afterglow system. This reaction violates the Wigner spin‐conservation rule and is probably the fastest such reaction to have been observed.

Thermal‐Energy Ion—Neutral Reaction Rates. V. Measured Rate Constants for C+ and CO+ Reactions with O2 and CO2

The rate constants for the charge‐transfer reactions of CO+ with O2 and CO2, and the ion—atom interchange reactions of C+ with O2 and CO2 have been measured at 300°K in a pulsed‐discharge flowing‐afterglow system. These reactions are all efficient, having rate constants ∼10−9 cm3/sec except for CO+ charge transfer to O2 which has a rate constant 2×10−10 cm3/sec.