- 3 x cm3/s. (This rate constant has been

Transcription

1 Negative ion - molecule reactions E. E. FERGUSON Aeronomy Laboratory, Envirotzrne~tfal Sciences Services Arltnit~i.stratiot~ Research Laboratories, Bo~lrler, Colorado Laboratory reaction rate constant measurements for negative ion-atom interchangereactions, negative ion charge transfer reactions, and negative ion three-body association reactions of aeronomic interest are reviewed and the available data tabulated. The present experimental tecllniques in use are briefly summarized: Most of the rate constants have been measured only at 300 OK; in a few cases data is available at energles 2 1 ev, as well as at 300 OK, so that an indication of the energy dependence of the rate constants is available. Canadian Journal occhernistry, 47, 1815 (1969) Introduction Almost the entire available knowledge concerning negative ion - molecule reactions of atmospheric interest has been acquired since the I.A.G.A. Aeronomy Symposium held in Berkeley in The subject of positive and negative ion low energy charge exchange and ion - molecule reactions was reviewed then by Paulson (I), and at the time the only negative ion reaction rate of atmospheric interest known was the reaction which had been measured by Henglein and Muccini (2) and found to have a rate constant - 3 x cm3/s. (This rate constant has been subsequently measured by three groups and found to be 1.2 x lo-' cm3/s at 300 OK.) By way of contrast, a recent review (3) tabulated 15 negative ion neutral reactions of atmospheric interest and fairly detailed negative ion reaction schemes for the atmosphere have now been proposed (4, 5). At least 4 different laboratory groups, utilizing 4 different experimental techniques, have now obtained rate constant data on negative ion - neutral molecule reaction processes. The negative ion composition of the ionosphere has as yet not been determined experimentally so that even the qualitative predictions based on laboratory measurements have not been subjected to critical test. This is in contrast to the situation for the positive ion chemistry in which direct rocket borne mass spectrometer ion measurements have allowed detailed, quantitative checks of the most important positive ion reaction rate constants, for at least the E and F1 ionospheric layers. The present review is concerned only with negative ion reactions with neutral molecules, and specifically reactions in which a different heavy negative ion is a product of the reaction. This excludes processes of electron attachment and detachment from neutrals. One of the more important negative ion - neutral molecule reaction processes occurring in the D region is the associative-detachment process in which an electron is released from a negative ion. This process has long been expected theoretically, but was discovered experimentally only in This class of reactions is included in the electron attachment and detachment processes reviewed by Phelps (6) in the current symposium. The binary negative ion reactions known which appear likely to be important in the earth's atmosphere are listed in Table I. Brief discussions of some aspects of the role of these reactions in the quiet D region have bee^ given by Fehsenfeld et al. (4), Fehsenfeld and Ferguson (7), and Ferguson (3). Recently, LeLevier and Branscomb (5) have presented a discussion of the role of negative ion chemistry in certain disturbed atmospheric conditions. In many respects, more useful observations exist for comparison of the broad atmospheric negative ion behavior with theoretical prediction for disturbed atmospheric conditions. The limited available data for negative ion three-body association reactions are given in Table 11. Discussion of Measurements Different measuring techniques are often complementary in their capabilities and the energy ranges over which they can be operated. It is very desirable (and of course generally recognized) that the important reactions should be

2 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 TABLE I Binary ionospheric negative ion reactions Rate constant Reaction (cm3/s) Method? Energy Reference OH- + NO2 -> NO2- +OH O3-+COz + COa-+02 CO3-+O 402-+COz 03-+NO +NO3-+O NOz-+O3 + NOa-+02 NO NO C03-+NO2 + NOS-+C02 NzO N N2 -> products 03- +SiO -> SO3-+O O3 - + CO + products 02-+SO2 -> S > Sos So2 -> Soh (-11) 8.0(-10) 1.5(- 09) 7.0(- 10) 1.9(- 10) 2.9(-09) 5.0(- 10) 1.O(-09) 1.4(- 09) 1.O(-09) 9.0(-12) <2.5(-11) -1.5(-11) <6.0(-12) slow 4.0(-11) to 4.0(-10) 8.0(-iij fast < 1.O(- 15) fast slow 5.4(-10) 5.1(-10) f.a m.s. m.s. m.s. -1 ev -1 ev 3 ev (-10) = 10-lo. 7m.s. refers to mass spectrometer ion source measurements, refers to flowing afterglow experiments, and refers to crossed studies. $Extrap. means that studies above 2 ev ion energy were extrapolated to thermal energy assuming a "smooth" energy dependence of [he cross section. $Product ion uncertain, either NOn- or NO2- or both. measured in several different ways when this is possible. A comparison of measurements made by different techniques greatly aids in the evaluation of the precision and reliability of the different methods. It also acts as a safeguard against errors which may arise in experiments due to such factors as lack of knowledge of reactant states, and various subtle experimental problems which may be somewhat hidden in any particular experiment. A. Moss Spectrometer Experiments Historically, most positive ion reaction measurements have been carried out in mass spectrometer ion sources. This has not been true, however, of ionospheric positive ion reactions and is not true for negative ion reactions in general. The role of mass spectrometer ion source measurements in the elucidation of ionospheric negative ion chemistry is expected to continue to be limited because so many of the most critical re-

3 FERGUSON: NEGATIVE ION - MOLECULE REACTIONS TABLE 11 Three-body negative ion reactions Rate constant Reaction (cnlg/s) Method* Reference 0- +N2+He + N20-+He -1.o(-30) (80 OK) N2 -> products < 1.5(- 31) (300 OK) 16 *d.t. rerers to drift tube, and refers to flowing afterglow experiments. actions involve relatively unstable neutral reactants, such as atomic oxygen and ozone, which have not been successfully used in positive or negative ion studies in mass spectrometer ion sources. Also, the most important atmospheric negative ions (e.g. 0,-, 0,-, C03-, etc.) are not produced by direct electron impact on a stable gas and accordingly are not suitable for the typical low pressure mass spectrometer ion source studies. Several mass spectrometer ion source measurements are tabulated in Table I. The early observation by Henglein and Muccini (2) that 0- charge transfers rapidly with NO, has subsequently been borne out by more precise measurements. The recent measurement bv Paulson (8) of the same reaction has been well supported by the flowing afterglow measurement. The report by Curran (9) that C1- charge transfers rapidly with NO, has not been supported, however, by either the thermal energy flowing afterglow results (4) or by a reasonable extrapolation to low energies of the higher energy crossed measurements (10). dependence in the cases studied. The same conclusion was obtained for many positive ion charge transfer reactions studied as a function of energy using the crossed technique. Negative ion - atom interchange reactions have not been studied in crossed experiments. Probably these reactions will often not be fast at the several volts and greater energies at which the reactions are studied. One complication in the extrapolation of charge transfer rate constants from energies to the ionospheric energy range arises if a fast ion -atom interchange process becomes significant at energies below the lowest energy studied. One example of such a situation may occur for the reaction which Rutherford and Turner (lo) measured for kinetic energies from ev. They extrapolated their data to obtain a 300 OK rate constant of 7 x lo-'' cm3/s. However, it is not unlikely that at thermal energies, the reaction goes as B. Crossed Beam Measurements rather than by charge transfer! The flowing The crossed studies carried Out by afterglow results suggested that this might be Rutherford and Turner (lo) at Atomic the case, but were inconclllsive and must be have been in the charge trans- repeated with additional measurements (the 0, fers of various negative ions to NO,. The meaconcentration) simu~taneolls]y carried out in surements were actually carried out at energies order to resolve this point. above 2 ev, up to several hundred ev in most cases, and Rutherford and Turner extrapolated C. Afterglow Meas~lrernerzts their values to thermal (ionospheric) energies. Fite and Rutherford (1 1) made some interesting A comparison of their results with thermal en- qualitative observations on negative ions in air ergy measurements shows that the charge trans- and oxygen afterglows using a stationary afterfer rate constants do not have a markedenergy glow method in which ions were sampled as a

4 1818 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969 function of time following cessation of a discharge in the gas. They found the ion sampling efficiency to vary with time in the afterglow and this effect precluded any quantitative rate measurements in the stationary afterglows. The flowing afterglow system developed in the Environmental Sciences Services Administration Laboratory has proved to be very useful for negative ion reaction studies. Most of the reactions tabulated in Table I have been measured by this method. For the ion - atom interchange reactions, few other measurements exist. The flowing afterglow measurements of negative ions have so far been carried out at 300" K. Recently, a new temperature controlled flowing afterglow system has been constructed which operates in the temperature range OK. The rate constants of Table I will in time be measured over this temperature range, which includes the temperature range of most concern for atmospheric negative ion reactions. D. Drift Tube Measurements So far the application of drift tubes to negative ion studies of~tmospheric interest has been concentrated on three-body rate constant measurements. Most of the three-body rate constants of Table I1 have been measured in drift tubes. The recent upsurge in binary positive ion reaction measurements in drift tubes will perhaps be followed by similar negative ion measurements. A particular advantage of the drift tube technique is in the capability of varying the reactant ion kinetic energy over a range of values. Variations in ion kinetic energy cannot be assumed in all cases to be equivalent to similar variations in gas temperature in which rotational and vibrational degrees of freedom are equilibrated as well as translational, but will in acy case provide very useful data. The temperature or energy variation feature is not so important for ionospheric negative ion charge-transfer and ion - atom interchange reactions as it is in the case of ionospheric positive ion reactions, because the range of temperatures in the D region where negative ions can be important is so much more limited. However, three-body reaction rate constants appear to be much more dependent on temperature than binary reaction rate constants. One difficulty with drift tube measurements, as well as mass spectrometer ion source and crossed experiments, will be to achieve ion energies in the sub-300 OK temperature range characteristic of the D region. The afterglow technique, which has been successfully operated at 82 OK with all degrees of freedom equilibrated, has a distinct advantage in this respect. On the other hand, the drift tube can be used to measure reactions which are endothermic at thermal energy. Slightly endothermic collisional dissociation of negative ion clusters may prove to be important in controlling their concentration in the D region, as one possible application where the drift tube could provide perhaps unique data. Moruzzi and Phelps have successfully determined the stability of the negative ion C0,- in drift tube studies, for example (17). The drift tube technique is, of course, quite restricted in the variety of reactants which can be studied. General Discussion The C1- charge transfer with NO, has been included in this review, not because that particular reaction is expected to be of direct ionospheric interest, but because Curran's report that this process was fast has led to the widely accepted conclusion that the process was exothermic, and consequently that the NO, electron affinity exceeded that of C1. Since the recent studies do not find this charge transfer to occur at thermal energies, this conclusion should be reexamined. The value of the NO, electron affinity is of some importance in the physics and chemistry of the D region. The observation that F- does not charge transfer rapidly with NO, is also reported in Table I for the same reason, F having about the same electron affinity as C1. Observation of charge transfer reactions at thermal energy is often a useful way to obtain limits on electron affinities. For example, the observation of fast NO- charge transfer with 0, establishes an upper limit of 0.46 ev on the NO electron affinity. Since there are no serious discrepancies between negative ion rate constants measured by different experimental techniques and since no quantitative atmospheric negative ion chemistry models or direct ionospheric negative ion measurements exist, the question of the precision of measurement is not at this time one of compelling urgency so far as aeronomy is concerned. In favorable cases, the flowing afterglow and the mass spectrometer ion source techniques are probably reliable to within 30%. This is supported by the intercomparison of the techniques

5 FERGUSON: NEGATIVE ION - MOLECULE REACTIONS 1819 on the 0- + NO, charge transfer (Table I) and by very much more extensive comparisons for positive ion reactions where the measurement,precision considerations are presumably not essentially different. In addition, the flowing afterglow measurements on several negative ion associative-detachment reactions have been compared with drift tube measurements and the agreement has been consistent with such estimated error limits. The uncertainties in reactions involving unstable neutrals such as 0, O,, and even NO, are greater than for reactions involving stable neutral reactants such as NO and CO,, however, due to the greater difficulty of reactant concentration measurements in these cases. The unknown distribution of vibrational states in the molecular negative ion reaction experiments so far carried out adds an element of uncertainty to their measured reaction rate constants. In principle it seems possible to resolve this uncertainty to some extent in flowing afterglow experiments since the lifetime of the negative ion prior to reaction can be varied. Experience would suggest that exothermic charge transfer reactions will not be sensitive to the vibrational state of the molecular ion, since indeed they seem relatively insensitive even to the choice of ion. Recent refinements in data analysis along with certain improvements in experimental apparatus suggest that flowing afterglow measurements can probably be refined to f 10% type errors in future experiments for favorable cases such as reactions involving stable neutral reactants. For the near future, aeronomical D region calculations are more likely to be limited by uncertainties in the critical neutral constituent concentrations (e.g. NO, 0,, 0, NO,, and CO,) than by the uncertainties in reaction rate constants. Need for Future Work Of most concern are the possibilities of important reactions not yet considered. To date, no negative ion reactions of the kind considered in this review have been measured involving excited states of the neutral reactants as one example of interest. A possibility which has been pointed out by Pack and Phelps (12) is that the primary negative ions may rapidly cluster in the lower ionosphere. Ions such as C04-, 0,-(H,O),, NO,-(H,O),, etc., may be very important. Therefore a number of three-body association reactions of the type may be of considerable importance. The very limited available data of this type is compiled in Table 11, without regard for whether the specific processes tabulated there are likely to occur in the earth's atmosphere or not, since a knowledge of the magnitudes of the few observed rate eonstants may be helpful in attempting to estimate probable ranges of such three-body coefficients, prior to measurements of the appropriate processes in the laboratory. A considerable amount of effort will be required in three-body measurements in order to furnish the data which can be expected to be important in the ionosphere. Each cluster ion forming reaction has the new degree of variability associated with different possible third bodies. One would like to know the efficiencies of both 0, and N, in most D region negative ion association reactions. Additionally, the effect of temperature variations on three-body rate constants is generally very much greater than it is for the binary charge transfer and ion - atom interchange reactions. Finally, one of the more important clustering agents in the D region is likely to be the polar H,O molecule, which is very difficult to work with quantitatively in the laboratory. Recent developments in the technology of laboratory ion - molecule reaction rate constant measurements certainly will assure a steady increase in the quantitative data required for aeronomical applications. 1. J. F. PAULSON. Ann. Geophys. 20, 18 (1964). 2. A. HENCLEIN and G. A. MUCCINI. J. Chem. Phys. 31, 1426 (1959). 3. E. E. FERGUSON. Rev. Geophys. 5, 305 (1967). 4. F. C. FEHSENFELD, A. L. SCHMELTEKOPF, H. I. SCHIFF. and E. E. FERGUSON. Planetary. Svace - Sci. i5, jij ((1967). 5. R. E. LELEVIER and L. M. BRANSCOMB. J. Geophys. Res. 73, 27 (1968). 6. A:V. PHELPS. Can. J. Chem. This iss ue. 7. F. C. FEHSENFELD and E. E. FERGUSC )N. Planetary Space Sci. 16,701 (1968). 8. J. F. PAULSON. Advan. Chem. Ser. 58, 28 (1966). 9. R. K. CURRAN. Phys. Rev. 125, 910 (1962). 10. J. A. RUTHERFORD and B. R. Turner. J. Geophys. Res. 72, 3795 (1967). 11. W. L. FITE and J. A. RUTHERFORD. Discussions Faraday Soc. 37, 192 (1964). 12. J. L. PACK and A. V. PHELPS. J. Chem. Phys. 45, 4316 (1966). 13. E. E. FERGUSON, F. C. FEHSENFELD, and A. L. SCHMELTEKOPF. Advan. Chem. Ser. In press.

6 1820 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, B. R. TURNER, D. M. J. COMPTON, and J. W. Mc- 17. J. L. MORUZZI and A. V. PHELPS. J. Chem. Phys. GOWAN. General Atomic Report 7419, Nov. 14, 45, 4617 (1966) D. S. BURCH and R. GEBALLE. Phys. Rev. 106, 183, 15. F. C. FEHSENFELD, E. E. FERGUSON, and A. L. 188 (1957). SCHMELTEKOPF. J. Chem. Phys. 45, 1844 (1966). 19. E. C. BEATY, L. M. BRANSCOMB, and P. L. PATTERSON. 16. F. C. FEHSENFELD. Unpublished results. Bull. Am. Phys. Soc. 9, 535 (1964). COMMENTS BY F. E. NILES Comments Your statement about the possible importance in HN,O,- (mass 93) being the dominant negof clustering agents, particularly H,O, in the D ative ion. Other negative ions in decreasing order region for the negative ions is supported by re- of the mass spectrometric abundance were cent laboratory measurements at BRL. Photo- NO,-.H20(mass 64), NO2- -2H,O (mass 82), ionization of a small admixture of NO in a NO,- (mass 46), and NO2-.3H,O (mass 100). few hundred mtorr of laboratory air resulted Other negative ions were also observed.