Chemical Reactions in Electrical Discharges

6 Ion-Molecule Reaction Rates Measured in a Discharge Afterglow

Downloaded by UNIV OF MINNESOTA on April 24, 2013 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch006

Ε. E. FERGUSON, F. C. FEHSENFELD, and A. L. SCHMELTEKOPF Aeronomy Laboratory, Environmental Science Services Administration, Boulder, Colo.

The application of a flowing afterglow technique to the measurement of thermal energy ion-molecule reactions is briefly discussed. The flowing afterglow system has the ad vantage of producing reactant species in their ground elec tronic and vibrational states in many cases and, additionally, allows the measurement of reactions of ions with unstable neutrals such as Ο, H, N, and O . Reactions observed in CO and Ar-H systems are shown to be consistent with ion composition studies in discharges in these gases. Several associative-detachment reactions, including O + Ο-->O + e, Η + Η --> H + e, and O + H --> H O + e are found to have large rate constants and such reactions may be important in determining negative ion concentrations in certain discharges. 3

2

2

-

-

-

2

2

2

2

A flowing afterglow system has been utilized for the past several years in the ESSA Laboratories in Boulder, Colorado, for the measurement of reaction rate constants at 300 °K. for both positive and negative ions reacting with both stable and unstable neutral species (5, 6, 7, 8, 9,10,11). Figure 1 illustrates one version of the flowing afterglow tube which has been utilized. A tube of about 1 meter in length and 8 cm. in diameter serves as the reaction vessel. A gas, usually helium, is introduced at one end of the tube and exhausted at the other end at a rate of around 200 atm. cc./sec, the helium pressure being typically — 0.3 torr. The helium is ionized by a pulsed d.c. discharge producing about 10 He ions and He(2 S) metastable atoms per cc. Positive ions are produced in most cases by adding a relatively small concentration of neutral gas into the helium afterglow by means of a small nozzle. The positive ions are prod ucts of He and He(2 S) reactions with the added neutral. The reactions 10

+

3

+

3

83 In Chemical Reactions in Electrical Discharges; Blaustein, B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

84

CHEMICAL REACTIONS IN ELECTRICAL DISCHARGES

of these ions with a second neutral added at a second downstream nozzle are then measured. The ion composition of the afterglow is monitored by means of a frequency scanned quadrupole mass spectrometer covering the mass range 1-200 a.m.u. The rate of disappearance of a reactant ion with neutral reactant addition leads directly to a reaction rate constant. Our estimate of the reliability of the rate constants so determined is ± 3 0 % in favorable cases. Our experience in comparing our rate constants with other measured rate constants generally supports this estimate.


ROOTS

GAS Figure 1.

MASS SPECTROMETER

Flowing afterglow reaction system

Experimental This experimental scheme mentioned above has many variations. We have, for example, successfully used borosilicate glass and quartz reaction tubes as well as stainless steel, and microwave and electron beam ionization as well as the d.c. discharge. We sometimes find it desirable to produce reactant ions by adding a suitable gas through the discharge with the helium rather than downstream in the afterglow, particularly in the case of certain negative ions such as O" and H", which are readily created by dissociative-attachment by fast electrons in the discharge. We sometimes use carrier gases other than helium, particularly argon. Very recently a modified flowing afterglow system has been operated in the temperature range, 80° to 600 °K. Some of the important features of the flowing afterglow experimental technique are the following: (1) The reactant ions in many cases are known to be in their ground states, either because of the reaction which produces them or by virtue of superelastic electron collisions in the plasma prior to neutral reactant addition. Stable neutral reactant species are added without being subjected to discharge or excitation conditions so that they can be assumed to be neither vibrationally nor electronically excited. On the other hand, some selective excitation is possible. For example, the reaction 0 ( S) + NsOS) -> NO (i2) + N( S) + 1.1 e.v. +

4

+

4

In Chemical Reactions in Electrical Discharges; Blaustein, B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

(1)

6.

FERGUSON ET AL.

Ion-Molecule Reaction Rates

85

has been measured (27, 28) as a function of vibrational temperature from 300° to 6000°K. (2) It is possible to add chemically unstable neutral reactants into the afterglow so that reactions such as N 0

2

2

+

+ Ο -> NO + N,

(2)

+ Ν -> NO + O,

(3)

+

+

+

Ο" + Ο -> 0 and

(4)

Ο" + 0 -> 0 - + Ο 3


+ e,

2

(5)

3

have been studied in this system. The importance of this fairly unusual capability of the flowing afterglow system for discharge reaction studies is clear, particularly in view of Kaufman's discussion on the production of atoms and unstable neutrals (18). (3) The difficulty of resolving concurrent reactions does not arise, as it does in mass spectrometer ion sources, and we have measured the reaction C0

2

+ H -> C 0 H + Η

+

2

2

(6)

+

without complication from the reaction H

2

+ C 0 -> C 0 H + H,

+

2

2

(7)

+

since H is not ionized in the flowing afterglow arrangement (12). 2

Thermal Energy Charge-Transfer Reactions The flowing afterglow system is well suited to exothermic chargetransfer reaction measurements, since the neutral reactant, necessarily of lower ionization potential, does not go through the ionization region and hence is not selectively ionized as would be the case in some experimental arrangements used for ion-molecule reaction studies. For positive ions, either atomic or molecular, charge-transfer to molecular neutrals is usually fast (barring occurrence of a fast competitive exothermic rearrangement reaction). Many examples of such fast charge-transfers have been ob served and only very few exceptions. Examples of the latter are the reactions between He + H , which is observed not to have a rate con stant as large as 10" cc./sec, and Ne + N which has a rate constant < 10" cc./sec. Ar , C O , C 0 , N , N\ and H 0 all charge-transfer with 0 to produce 0 with rate constants greater than 10" cc./sec. (or cross sections greater than 20 A. ) as a more typical finding. The situation appears to be the same for negative ion charge-transfer although much less data is available in this case. Table I gives a few examples of some very recent measurements of negative ion chargetransfers which might be important in some gas discharge applications. It is of interest to note in Table I that some recent measurements of +

2

13

12

2

+

2

+

+

2

+

+

2

+

2

2

+

10

2


86


Rutherford and Turner (25, 26) in crossed beam experiments at much higher energies yield similar rate constants in several cases. This suggests that the rate constants are not likely to change drastically with tempera ture over the range of temperatures of interest in gas discharges. Also the O" + N 0 charge-transfer measurement of Paulsons (22), which is in good agreement with the 300°K. flowing afterglow measurement, was made at an average ion energy of 2 e.v. in a mass spectrometer ion source. The fast charge-transfer of O" with N 0 agrees with earlier results of Curran (2), Henglein and Muccini (17), and Paulson (22). The slow charge-transfer of CI" with N 0 does not agree with Curran's (2) estimate that Cl~ charge-transfers with N 0 with a cross section in the range 8 to 70 A . at low energy. At present we are not sure whether the CI" (and F") charge-transfer with N 0 does not go at all or is merely exceptionally slow. This is an important point to be established in further investigations since Curran's report of C r charge-transfer with N 0 is the basis for the generally accepted 3.6 e.v. lower limit on the electron affinity of N 0 . 2

2

2


2

2

2

2

2

Table I.

Laboratory Rate Constants k (cc./sec.) X 10-

Reaction

FA(300°K.)

o - + o -» o - + ο o - + o -> o - + o Ο" + N 0 - » N 0 - + Ο 0 - + N0 - » N0 - + 0 Η" + N 0 - » N 0 - + Η OH" + N 0 -> NOo" + OH NHo" + N 0 -> N 0 - + NH F" + N 0 - » N 0 - + F Cl" + N 0 -> N 0 - + CI 3

2

3

3

3

2

2

2

2

2

2

2

2

2

2

2

2

2

a 6

2

2

2

9

0.53 0.40 1.2 0.8 2.9 1.0 1.0 < 0.025 < 0.006

Beam"

3 2 0.5 3 —0.015 Slow >

E = 2e.v.

2 e.v. 2 e.v. 25 e.v. 2 e.v. 3 e.v. 5 e.v.

J. A. Rutherford and B. R. Turner, Refs. 25, 26. J. F. Paulson, Ref. 22.

The finding of generally large charge-transfer rate constants contra dicts most theoretical predictions which had assumed that charge-transfer would be slow except in unusual cases involving fortuitous energy reso nances. In the absence of experimental data one would certainly predict that exothermic charge-transfer to a molecular neutral will more likely be fairly fast than slow. Ion-Atom Interchange Reactions The most commonly studied ion-molecule reactions have involved changes in molecular configuration. Typical examples are


6.

FERGUSON ET AL.

Ion-Molecule Reaction Rates C + 0 +

-> CO + O,

(8)

+

2

87

whose rate constant, 1.1 χ 10" cc./sec, from flowing afterglow experi ments (9) agrees with an earlier value 9 X 10" cc./sec. measured in a mass spectrometer ion source by Franklin and Munson (15); the reaction 9

10

0

+ C 0 -> 0

+

2

2

+ CO

+

(9)

with a rate constant of 1.2 χ 10" cc./sec. from both flowing afterglow (7) and mass spectrometer ion source measurements (23); and 9

C + C 0 -> CO + CO Downloaded by UNIV OF MINNESOTA on April 24, 2013 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch006

+

+

2

(10)

with a rate constant 1.9 χ 10" cc./sec. Such reactions also seem to be more often fast than slow. One of the slowest exothermic ion-molecule reactions (again barring cases where fast charge-transfer competes) is 9

0

+

+ N - » NO + N, +

2

(11)

with a rate constant 1.3 Χ 10" cc./sec. . This rate constant increases (27, 28) to about 5 Χ 10" cc./sec. for an N vibrational temperature of 6000 °K. and also increases with 0 kinetic energy (16, 29). No case of a fast ion-atom interchange reaction involving the break ing of two bonds has so far been reported. The exothermic reaction 12

19

11

2

+

0

2

+

+ N ^NO 2

has a rate constant less than 10"

15

+

+ NO

(12)

cc./sec, for example (14).

Associative Detachment Reactions A number of associative detachment reactions have been recently measured to be fast in the flowing afterglow system—i.e., k > 10" cc./sec.—including those shown in Table II. The most recent flowing afterglow rate constant determinations are somewhat lower than the earlier published values, which are also included in Table II. Moruzzi and Phelps (20, 24) have measured some of the same associative detach ment reactions in drift tube experiments at Westinghouse and their results are included in Table II. The flowing afterglow and drift tube results agree within better than a factor of two in each case. Moruzzi and Phelps are also able to obtain information on the energy dependence of these rate constants in the drift tube studies. Several exothermic associative detachment reactions do not occur at measurable rates (k < 10" cc./sec), an example being 10

12

Ο" + N - » N 0 + e + 0.2 e.v. 2

2


(13)

88


Table II.

Laboratory Rate Constants k (cc./sec.) X 10~

9

Reaction Ο + Ο -> 0 + e OH" + Ο - » H 0 + e OH" + Η - » HoO + β Η" + Η - » Η + e Cl" + Η —> HC1 + e CN" + Η - » HCN + e Ο" + H -> H 0 + β O" -f CO —» C 0 + e O + NO -> N 0 + e 9

2

2

2


a

0.14 0.20 1.0 1.3 0.9 0.8 0.6 0.44 0.16

2

2

FA(300°K.)

FA(300°K.)

2

(1.5) (-0.5) (0.5)

D7?

1.0 0.8 0.15

" F . C. Fehsenfeld, Ε. E. Ferguson, and A. L. Schmeltekopf, Réf. 11. J. L. Moruzzi and Α. V. Phelps, Refs. 20, 24. b

Ion-Molecule Reactions in Discharges One obvious application of measured rate constants to the qualitative interpretation of the ion composition of a gas discharge is the case of the C 0 discharge ion composition measured by Dawson and Tickner (3). Dawson and Tickner observed the dominant ions in a glow discharge in C 0 to be 0 and C 0 . This is quite reasonable in view of the known occurrence of Reactions 8, 9, and 10 above, together with the fast chargetransfer of C O with C 0 to produce C 0 . The results of these reactions are graphically illustrated in Figure 2, which shows that all of the ions, C , 0 , and C O , do convert to 0 and C 0 by reaction with C 0 in the —6 milliseconds reaction time in the flowing afterglow. If sufficient molecular oxygen were added, one would expect the dominant ion to become 0 alone, since C 0 is known to charge-transfer rapidly with 0 (21). As a matter of interest, these same reactions dominate the Martian ion chemistry (21), since the Martian atmosphere is largely C 0 . In a like manner Figure 3, showing ion composition in an A r - H afterglow (12), suggests that the ion-molecule chemistry is such that the dominant ion is H , and this would very likely be true for certain active discharge conditions as well. This has recently been shown to be the case for the negative glow of a discharge in an H + 8.7% Ar mixture at 0.3 torr pressure (4). An example of the possible importance of associative detachment reactions in discharges was noted many years ago by Massey (19), who speculated that the low negative ion density in oxygen discharges (rela tive to iodine discharges for example) might be attributed to electron detachment by the reaction Ο" + Ο -> 0 + e. In iodine the analogous reaction Γ -f- I -> I + e is endothermic. In view of the subsequent finding that the O" reaction is indeed fast (Table II), Massey's suggestion 2

2

2

+

2

+

+

+

+

2

2

+

2

2

+

2

+

+

2

+

2

+

2

2

2

3

+

2

2

2



6.

0

89


FERGUSON E T AL.

1

2 3 4 RELATIVE C0 CONCENTRATION

5

6

2

The Journal of Chemical Physics

Figure 2.

Ion reactions with C0

2

takes on renewed interest. The same argument could be applied to H discharges in view of the rapidity of the reaction H " + H —> H + e, and in the case of other gases as suggested by Table II. 2

2

It has been observed that the electron density decay in the afterglow of microwave discharges in oxygen is anomalously slow in view of the known electron attachment cross sections for oxygen. Biondi (I) has suggested that this might be explained if a species capable of causing electron detachment is produced in substantial numbers in the discharge. We have recently (13) found the reaction 0 - + Ο -> 0 2

3

+ e + 0.5 e.v.

(14)

to be very efficient (k = 3.3 Χ 10" cc./ sec.) and it may well be that this is the detachment process involved. It is not unlikely that the reaction 10

i4

O- + 0

2

(a!A ) -> 0 g

3

+ e + 0.5 e.v.

(15)

is also fast, in view of the fairly general occurrence of fast associative



90


detachment reactions, and this process might be important in oxygen discharges in certain cases.

FLOW H

2

Figure 3.

Ion reactions in an Ar-H system 2

Conclusions The growing body of quantitative ion-molecule reaction rate data now available should allow in favorable cases prediction and in many cases correlation of observed ion compositions of gas discharges with known ion-molecule chemistry. Literature Cited (1) Biondi, Μ. Α., Advan. Electron. Electron Phys. 18, 132 (1963). (2)

Curran, R. K., Phys. Rev. 125, 910 (1962).



6.

FERGUSON E T A L .


91

(3) Dawson, P. H., Tickner, A. W., Proc. Intern. Conf. Ionization Phenomena Gases, 6th, Paris, France 2, 79 (1963). (4) Dawson, P. H., Tickner, A. W., J. Chem. Phys. 45, 4330 (1966). (5) Fehsenfeld, F. C., Goldan, P. D., Schmeltekopf, A. L., Schiff, H. I., Ferguson, Ε. E., ]. Chem. Phys. 44, 4087 (1966). (6) Ibid., 44, 4095 (1966). (7) Fehsenfeld, F. C., Schmeltekopf, A. L., Ferguson, Ε. E., J. Chem. Phys. 44, 3022 (1966). (8) Ibid., 44, 4537 (1966); erratum 46, 2019 (1967). (9) Ibid., 45, 23 (1966). (10) Ibid., 45, 404 (1966). (11) Ibid., 45, 1844 (1966). (12) Ibid., 46, 2802 (1967). (13) Fehsenfeld, F. C., Schmeltekopf, A. L., Schiff, H. I., Ferguson, Ε. E., Planet. Space Sci. 15, 373 (1967). (14) Ferguson, Ε. E., Fehsenfeld, F. C., Goldan, P. D., Schmeltekopf, A. L., J. Geophys. Res. 70, 4323 (1965) (15) Franklin, J. L., Munson, M. S. B., "Tenth International Symposium on Combustion," p. 561, Combustion Institute, Pittsburgh, Pa., 1965. ;

(16) Giese, C. F., ADVAN.

CHEM.

SER. 58, 20 (1966).

(17) Henglein, Α., Muccini, G. Α., J. Chem. Phys. 31, 1426 (1959). (18) Kaufman, F., ADVAN.

CHEM.

SER. 80, 29 (1969).

(19) Massey, H. S. W., "Negative Ions," 2nd ed., Cambridge University Press, Cambridge, 1950. (20) Moruzzi, J. L., Phelps, Α. V., J. Chem. Phys. 45, 4617 (1966). (21) Norton, R. B., Ferguson, Ε. E., Fehsenfeld, F. C., Schmeltekopf, A. L., Planet. Space Sci. 14, 969 (1966). (22) Paulson, J. F., ADVAN.

CHEM.

SER. 58, 28 (1966).

(23) (24) (25) (26) (27)

Paulson, J. L., Mosher, R. L., Dale, F., J. Chem. Phys. 44, 3025 (1966). Phelps, Α. V. (private communication). Rutherford, J. Α., Turner, B. R., G. A. Rept. 7649 (Feb. 23, 1967). Rutherford, J. Α., Turner, B. R., J. Geophys. Res. 72, 3795 (1967). Schmeltekopf, A. L., Fehsenfeld, F. C., Gilman, G. I., Ferguson, Ε. E., Planet. Space Sci. 15, 401 (1967). (28) Schmeltekopf, A. L., Fehsenfeld, F. C., Gilman, G. I., Ferguson, Ε. E., J. Chem. Phys. (April 1, 1968). (29) Stebbings, R. F., Turner, B. R., Rutherford, J. Α., J. Geophys. Res. 71, 771 (1966).

RECEIVED April 24, 1967. This work was supported in part by the Defense

Atomic Support Agency.


Chemical Reactions in Electrical Discharges

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