Internal and translational energy effects on the charge- transfer reaction of CO+2 with O2

International Journal of Mass Spectrometry and Zon Processes, 117 (1992) 261-282

261

Elsevier Science Publishers B.V., Amsterdam

Internal and translational energy effects on the chargetransfer reaction of COT with 02* E.E. Fergusona, Jane M. Van Dorenb, A.A. Viggianob, Robert A. Morrisb, John F. Paulsonb, J.D. Stewart’, L.S. Sunderlin”’ and P.B. Armentrout’ “Climate Monitoring and Diagnostics Laboratory, NOAA, Boulder CO 80303 (USA) bPhillips Laboratory’, Geophysics Directorate, Ionospheric Effects Division (GPID), MA 01731-5000

Hanscom AFB

(USA)

“Department of Chemistry, Universiiy of Utah, Salt Lake City UT 84112 (USA)

(First received 14 January 1992; in final form 30 March 1992)

ABSTRACT We have investigated the reaction of CO: with 0, over a range of kinetic energies with a guided ion beam apparatus and over a range of temperatures and kinetic energies with a variable-temperature selected-ion flow drift tube. The rate constants decrease with increasing kinetic energy at low energy and increase at higher energy. Below about IOeV, reaction proceeds by charge transfer only. Above lOeV, O+ and CO+ product ions are observed in addition to the charge-transfer channel. At low energy, i.e. below 0.1 eV, the rate constants at a particular center-of-mass kinetic energy, (KE,), do not depend on the temperature of the buffer gas. This indicates that energy in rotations and in the bending vibrational modes does not play a major role in determining reactivity. Above O.leV, the rate constants at a particular (KE,,) do depend on temperature, such that the higher the buffer gas temperature the larger the rate constants. Analysis of the data suggests that the enhancement observed with temperature is primarily due to excitation of the CO: stretching vibrational modes. The analysis indicates that excitation of the CO: stretching modes increases the rate constant by approximately an order of magnitude. Excitation of bending vibrations may also enhance the efficiency of charge transfer above 0.2eV. Translational energy causes a small increase in the charge-transfer rate constant above 0.3 eV. Keywordr: charge transfer; CO:

; energy effects; guided ion beam; variable-temperature selected-ion flow

drift tube.

INTRODUCTION

The charge-transfer co:

+opo:

reaction

+co,

of CO: with O2 AH = - 1.70eV

(1)

Correspondence to: A.A. Viggiano, Phillips Laboratory, Geophysics Directorate, Ionospheric Effects Division (GPID), Hanscom AFB MA 01731-5000, USA.

* Dedicated to Professor Charles H. DePuy on the occasion of his 65th birthday. ’ Formerly the Air Force Geophysics Laboratory. 2Present address: Department of Chemistry, Purdue University, West Lafayette IN 47907, USA. 0168-l 176/92/$05.00

0 1992 Elsevier Science Publishers B.V. All rights reserved.

262

E.E. Ferguson et al./Znt. J. Mass Spectrom. Ion Processes II7 (1992) 261-282

has been extensively studied [l-8]. Lindinger et al. [2] inferred a large enhancement in the charge-transfer efficiency for vibrationally excited CO; ions from the kinetic energy dependences of the rate constant in a flow drift tube using He, N2 and Ar buffers. The rate constant was found to decrease initially with added kinetic energy at 300 K gas temperature. At higher kinetic energies, the charge-transfer rate constant increased. In an He buffer, the minimum value was 2 x IO-” cm3s-’ near 0.3 eV relative center-of-mass kinetic energy ((KE,)). In N, and Ar buffer gases, the rate constant began to increase at a (KE,,) near 0.1 eV. The rate constants above 0.1 eV were larger in N, than in He buffer gas and were even larger in Ar buffer gas. At 0.3 eV, the rate constant was found to be enhanced by almost an order of magnitude in the Ar buffer over the He buffer. The enhancement was attributed to differing fractions of vibrationally excited CO: ions in the reactant ion distribution in different buffer gases, where excitation increases with the mass of the buffer gas. The question of whether the enhancement observed in the rate constant is mode specific has not previously been addressed. The major goal of the present study is to quantify the enhancement by deriving mode specific charge-transfer rate constants for vibrationally excited CO: ions. This goal is achieved by measuring rate constants in a drift tube at different temperatures (91-471 K) and by measuring cross-sections in a guided beam apparatus over a large kinetic energy range (0.02-25 eV). Since the measurements have overlapping kinetic energy ranges, the results can be directly compared by converting the cross-sections to rate constants. This comparison coupled with the temperature data allows the effects of translational and internal energy to be identified independently and mode-specific charge-transfer rate constants to be assessed. This is an extension of our previous work on internal energy dependences of ion/molecule reactions [9-151 and is the first case studied in the drift tube where the ion can be vibrationally excited. Further, this is the first study of this reaction that uses ion beam methods and thus extends the study of this process to elevated energies where additional product channels are observed. EXPERIMENTAL

Variable-temperature selected-ion jlow drift tube The measurements were made using the Phillips Laboratory variabletemperature selected-ion flow drift tube apparatus (VT-SIFDT) [9]. Instruments of this type have been the subject of review [16], and only those aspects important to the present study will be discussed in detail. CO: ions were created by electron impact on CO, in a moderate pressure ion source

E.E. Ferguson et al./lnt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

263

(about 0.1 Torr). The ions were extracted from the source and mass selected in a quadrupole mass filter. CO; ions were injected into a flow tube 1 m in length through an orifice of 2 mm diameter. Helium buffer gas transported the ions along the length of the flow tube. The pressure in the flow tube was about OSTorr. The buffer gas was added through a Venturi inlet surrounding the ion injection orifice. This type of inlet aids in injecting the ions at low energy. A drift tube, consisting of 60 electrically insulated stainless steel rings connected by resistors, is positioned inside the flow tube. A voltage could be applied across the resistance chain in order to produce a uniform electric field along the axis of the flow tube for drift studies of (KE,, ) dependences. The bulk of the gas in the flow tube was pumped by a Roots type blower. The flow tube was terminated by a truncated nose-cone. A small fraction of the ions in the flow tube was sampled through a 0.2mm hole in the nose-cone, mass analyzed in a second quadrupole mass spectrometer, and detected by a channel electron multiplier. Neutral reactant gas was added through one of two inlets. The inlets are rings with a series of holes pointing upstream [9]. The area inside the ring is equal to the area outside the ring (between the ring and the flow tube wall) to aid in quickly distributing the reactant gas throughout the cross-sectional area of the flow tube. Rate constants were determined from the decay of the primary ion signal as a function of added neutral flow. By combining the slope of a plot of the logarithm of the reactant ion signal versus neutral reactant flow with the values of pressure, temperature, flow rates of the reactant and buffer, and ion velocity, a rate constant was calculated. A rate constant was determined at each of the two neutral inlets, and an end correction was determined from those data. The reported rate constant incorporates the end correction. Ion flight times were measured by applying an electrical retarding pulse to two of the drift tube rings separated by a known distance and measuring the difference in the two arrival time spectra of the ions. The ion velocity and therefore the reaction time were determined from these data and from a knowledge of the relevant distances. Pressure was monitored by a capacitance manometer. Flow rates of the buffer and of the reactant gas were controlled and measured by MKS flow controllers. The entire flow tube could be heated or cooled over the range 91-471 K. All parameters including ion velocity and the end correction were measured for each rate constant determination. The absolute accuracy of the reported rate constants is f 25% while the relative accuracy is f 15%. Guided ion beam

A description of the guided ion beam apparatus and experimental procedure has been given elsewhere [17]. Briefly, ions were produced as

264

E.E. Ferguson et aLlInt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

described below and focused into a magnetic sector momentum analyzer for mass analysis. The mass-selected CO: ions were decelerated to a desired kinetic energy and focused into an octopole ion guide that radially traps ions over the mass range of interest. The octopole passes through a static gas cell into which O2 was introduced. The O2 pressure was maintained at a sufficiently low level (less than 0.1 mTorr) that multiple ion/molecule collisions were improbable. Studies of the effect of the neutral gas pressure on the measured cross-section verify that the beam results shown below are due to single ion/molecule encounters. After leaving the octopole, product and unreacted primary beam ions were focused into a quadrupole mass filter for mass analysis. Ions were detected with a secondary electron scintillation ion detector and the signal was processed by pulse-counting techniques. Raw ion intensities were converted to absolute cross-sections as described previously 1171. For charge-transfer reactions such as reaction (l), products may be formed through a long-range electron jump such that little or no forward momentum is transferred to the ionic products. In such instances, it is possible that up to 50% of these ions may have no forward velocity in the laboratory and will not drift out of the octopole to the detector. Such slow product ions which do traverse the octopole may be inefficiently transmitted through the quadrupole mass filter [ 171. Results reported here were reproduced on several occasions and the cross-section magnitudes shown are averaged results from many individual data sets. Based on reproducibility, the uncertainty in the absolute cross-sections is estimated as + 30% with relative errors of f 10%. The errors should be independent of energy except at the highest energies. Previous guided ion beam results of charge-transfer reactions have proven to be accurate to within these errors [21,22,48,51]. The data shown and analyzed in this paper involve the reaction of CO,t produced in a 1 m long flow tube [ 181operated at a total pressure of 0.45 Torr, with a flow rate of 6000 standard cm3min-‘ . He+ and metastable He(23S) were formed in this source by using a microwave discharge, and these species ionized CO,, which was introduced into the flow tube further downstream. He+ is known to react with CO* to form mostly CO+ (79%) and CO: (11 Oh) [19] but CO+ reacts further with CO2 to form CO: efficiently [20]. In the flow tube source, there are approximately lo5 collisions between ions and the buffer gas. These collisions effectively cool the vibrational modes of the ions formed [21,22]. The charge-transfer reaction CO: + Kr -+ CO, + Kr+

(2)

was investigated to quantify the extent of vibrational excitation remaining in the CO,+ ions emerging from the source. Reaction (2) is endothermic by 0.23 eV at 298 K [23] and ions which contain over 0.23 eV of internal energy

E.E. Ferguson et aLlInt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

265

are expected to charge-transfer with Kr with reasonable efficiency at low energies [24]. The cross-section for reaction (2) was measured at low energies and shows no apparently exothermic feature, suggesting that the ions are thermalized. Also, the cross-section for reaction (2) shows no dependence on the voltages of the lenses that extract the ions from the flow tube. This suggests that the ions are not collisionally heated during extraction from the flow tube. Finally, the measured reaction cross-sections can be converted to kinetic energy dependent rate constants for direct comparison with drift tube results. This is achieved by use of R((KE,, )) = TN(E), where a(E) is the measured reaction cross-section, E is the kinetic energy of the ion in the center-of-mass frame, v = (2E/p)“‘, p = mimn/(mi + m,), and (KE,) = E + (3/2)yk,T, where y = mi/(mi + m,) = 0.579, mi and m, are the masses of CO: and O2 respectively, and k, is the Boltzmann constant. ENERGY

DISTRIBUTIONS

In non-thermal experiments such as drift tubes and beams, internal and translational energies have different effective temperatures. A knowledge of the distributions for both internal and translational energy is therefore critical to interpreting the data. Variable-temperature selected-ion flow drift tube Several different energy distributions or effective temperatures are needed to describe the VT-SIFDT experiments. The average kinetic energy in the ion/neutral center-of-mass system, (KE,,), in the drift tube is derived from the Wannier formula [25] as

where mi, m,, and m, are the masses of the reactant ion, buffer gas and reactant neutral respectively, vd is the measured ion drift velocity, and T is the temperature. The first term in the formula is the energy supplied by the drift field, and the second term is the thermal energy. This formula is an excellent approximation of the ion energy at low ion energies [26,27]. At energies approaching 1 eV, the formula is still good to within + 10% [26,27]. The distribution of kinetic energy in a drift tube with helium buffer gas is close to Maxwellian [26-291 and therefore can be approximated by an effective temperature given by (KE,,) = (3/2)kT,,. The neutral reactant temperature in a drift tube is the same as that of the buffer gas. Therefore the vibrational and rotational distributions of 0, are

266

E.E. Ferguson et aLlInt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

determined by the He temperature and are Maxwellian. For all temperatures studied in the present experiments, O2 can be assumed to be in its ground vibrational state. The internal temperature of the CO: is more complicated. The ion internal temperature in an infinitely long drift tube is derived from the average ion-buffer center-of-mass kinetic energy [30], derived from expression (3) where the neutral mass and buffer mass are the same. The internal temperature is then given by (KE,,), = (3/2)kTn,. In a flow drift tube such as ours, the infinite length limit, i.e. temperature equilibration, for rotations is established well before the reaction region [31]. The distance necessary to establish equilibrium for vibrations is less well known and may vary with the vibrational frequency. For several reasons, we believe that the CO: vibrational distribution is quickly established and maintained along the length of the drift tube in the present experiments. The data, as shown below, are clearly different at different temperatures for a given (KE,, ). This indicates that the vibrational distribution is at least partially determined by temperature and drift field. Moreover, if the vibrational states were not coupled in the reaction region, i.e. drift region, we would expect, but do not observe, curvature in the decay plots since the different vibrational states react with substantially different rate constants. Taken together, these two experimental details indicate that the vibrational distribution of CO: is at least close to that which would be found for an infinitely long drift tube. In addition, recent studies show significant vibrational excitation in N: in an He-buffered drift tube [32], and a rate constant greater than lo-l3 cm3 s-’ at a (KE,,) of 0.38 eV was measured [32,33] for vibrational excitation of Nl by He. While equilibration of the N: vibrational distribution in the Hebuffered drift tube was not complete until about 40 cm at 0.60 eV (KE,, ), we expect the efficiency of excitation to be substantially greater for the lower frequency CO,+ ion vibrations. In particular, the Landau-Teller relation [34], which defines the vibrational quenching rate constant and by detailed balance is related to the excitation rate constant, depends on the vibrational frequency as k, = A exp( - 4z21v/u)

(4)

where k, is the vibrational quenching rate constant, I is the range parameter (e-folding distance) for the repulsive part of the potential, v is the vibrational frequency, and v is the relative collision velocity. Vibrational quenching in N:-He collisions at energies greater than 0.3eV was found to be well described by this relation [32,33]. At lower energies, where the attractive potential is significant, this expression is expected to provide a lower limit on the quenching efficiency. The exponential dependence of the rate constant on

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes II 7 (1992) 261-282

267

vibrational frequency indicates that the low frequency CO: vibrations (500, 1283 and 1469 cm-‘.) [35] will be much more readily excited than those of NT (2207 cm-‘) [36]. Finally, the ability to predict results obtained in drift tubes with different buffer gases using the data obtained in the He-buffered drift tube, discussed below, further supports the assertion that the CO: ion vibrational distribution is near equilibrium. Ar and N, buffer gases are far more efficient vibrational quenchers, and by inference vibrational exciters, than He [37]. Therefore the equilibrium vibrational distribution, as described by the Wannier equation (eqn. (3)), is expected to be established more quickly in collisions with these buffer gases. Application of excited state rate constants derived from data obtained in an He-buffered drift tube, where equilibration had not occurred, would lead to an underestimation of the rate constants measured in Ar and N, buffer gases. The predicted values were slightly smaller than the observed values for the Ar buffer gas but were larger than for the Nz buffer gas. Guided ion beam

For the ion beam experiments, the absolute kinetic energy as well as the kinetic energy distribution of the ions in the interaction region is measured by using the octopole as a retarding field analyzer. The full width at half-maximum (FWHM) of the energy distribution is 0.35 + 0.07eV in the laboratory frame. Uncertainties in the absolute energy scale are f 0.05eV in the laboratory frame. Translational energies in the laboratory frame of reference are related to energies in the center-of-mass frame by E = Erabmn/(mi + m,). The data obtained in this experiment are broadened by the ion energy spread and Doppler broadening (the thermal translational motion of the O2 reactant). The FWHM of the Doppler broadening in the center-of-mass frame is 0.41E”‘(eV) at about 300 K [38]. The CO: internal temperature is given by the temperature of the flow tube ion source, i.e. 300K. As explained above, care was taken not to excite the ions upon extraction. The 0, internal temperature is 300 K, the temperature of the target cell. These internal temperatures do not vary as the kinetic energy of the ions in the beam is varied. RESULTS

The charge-transfer rate constants measured in the VT-SIFDT are plotted in Fig. 1. As mentioned above, the absolute error in these rate constants is estimated to be f 25% while the relative error is estimated to be f 15%. Also plotted in Fig. 1 are rate constants for the beam data (with errors estimated

268

E.E. Ferguson et al./int. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

co**+ 0,

‘* 10”~ -

+ 0,’ +

co,

l 0

I’

l

+ 0.0

a

l

0

Ii a .

4 l

r’

.91K n .

298K 471 K

0

GIB

m

lm

zrn

@ g

p

*oa@[ 0

lo-”

-

0.01

0.1 (KEcJW)

Fig. 1. Measured rate constants for the charge-transfer reaction of CO: with O2 as a function of (KE,,). VT-SIFDT data at 91 K, 298 K and 471 K are shown as solid circles, squares and diamonds, respectively. A portion of the data obtained with the guided ion beam technique is shown as open squares. The measured cross-sections have been converted to rate constants as described in the text.

to be 30%) as obtained from the cross-sections as described above. Both VT-SIFDT and guided ion beam data indicate that the rate constant for charge transfer decreases with increasing kinetic energy as approximately (KE, )-’ up to 0.1 eV and then levels off in value or increases, depending on experimental conditions. The rate constants measured at room temperature with different ion kinetic energies in the drift tube are in excellent agreement with previous drift tube results obtained in an He buffer [2,3]. The collision rate constant for this reaction is 6.8 x lo-“cm3 s-’ at 300 K. At low energy (less than 0.1 eV) the data taken at different temperatures and in the different apparatuses fall approximately on a single curve. Differences between the guided ion beam and VT-SIFDT data in this region are probably due to small systematic differences between the two apparatuses, although differing kinetic energy distributions may also contribute. At higher energies, the data spread into distinct curves for each temperature and experiment. In this higher energy range, the flow tube data indicate that the efficiency for charge-transfer increases with temperature at a fixed average center-of-mass kinetic energy. The guided ion beam data fall close to the values obtained at 91 K in the VT-SIFDT at a given (KE,,) in this same energy range. In this energy region, the difference between the guided ion beam and VT-SIFDT data at room temperature is largely a result of differing vibrational energy distributions, as will be seen from the more detailed analysis below.

E.E. Ferguson et a/lint. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

269

CO,* + 0, -3 products

:

Product Channel l

:

n

.

. .

0 .

*CO+ -Total

lo2

.

:

0;

.....I 0.1

.

. . '....' 1

.

. . **...I 10

.

.

. .

KEc,,,W)

Fig. 2. Cross-sectionsfor the reactionof CO: with O2as a functionof KE,, measuredwith the guided ion beam apparatus. Solid circles, squares and diamonds refer to 0;) O+ and CO+ product formation. The total reaction cross-section is given by the solid line.

Figure 2 shows the complete cross-section data set measured in the beam apparatus. At energies less than approximately 10 eV, only charge transfer is observed. Above 10 eV, O+ and CO+ are formed. The cross-section for charge transfer is found to decrease from 10 A2 at low energy to about 0.5A2 at approximately 1 eV. A broad minimum is found between 1 and 1OeV. Above approximately 10 eV, the cross-section for charge transfer decreases while the cross-sections for formation of O+ and CO+ increase. The total reactive cross-section increases above about 10 eV as a result of the rapidly increasing cross-sections for the new dissociative reactive channels. Above 30 eV the very rapid fall-off in the Of and 0; cross-sections may be due to inefficient collection efficiencies at those energies. As a consequence, the uncertainties in the cross-sections in this energy region may be much greater than f 30%. DISCUSSION

The charge-transfer

reaction

The general shapes of the curves describing the dependence of the rate constants on center-of-mass kinetic energy (Fig. 1) have been discussed in detail previously [l-4,7]. The decrease in the efficiency of this reaction with increasing kinetic energy at low kinetic energies is typical for slow ion/molecule reactions [39]. We will show that most of the increase in the rate constant with kinetic energy at higher energies can be attributed to the presence of

270

E.E. Ferguson et aLlInt. J. Mass Spectrom. Ton Processes 117 (1992) 261-282

vibrationally excited CO: reactants. At the time of the earlier flow drift tube experiments it was not recognized that ions can be vibrationally excited in an He buffer. Therefore these workers did not recognize that the enhancement in He at 300 K and high kinetic energies could arise from vibrational excitation of the CO: ion. However, the larger rate constants measured in Ar and N, buffers did lead them to conclude that the charge-transfer rate constant is increased by the addition of vibrational excitation in the CO: ion [l-4]. A slight increase in rate constant with energy is seen in the beam data, indicating that even in the absence of increasing vibrational excitation, the chargetransfer rate constant increases slightly. Here, we will concentrate on the new aspects of the data, namely the temperature dependence of the rate constants at a particular (KE,,) and the observation of new reaction channels at high energy. We have recently shown that dependences of rate constants or branching ratios on the internal temperature of the reactant neutrals can be derived from measurements of rate constants or branching ratios as a function of (KE,,) at several temperatures in a VT-SIFDT [9-121. For reactions of monatomic ions with neutrals, comparison of these rate constants or branching ratios at a particular (KE,, ) but different temperatures gives information on the dependence of the measured reaction parameter on the internal energy/ temperature of the reactant neutral. In particular, if the value of the kinetic parameter measured at a given (KE,,) varies with temperature, one concludes that the kinetic parameter depends on the extent of excitation of internal degrees of freedom in the molecule. All our previous studies with the VT-SIFDT have involved monatomic ions or diatomic ions in which no vibrational excitation of the ion by either the drift field or the temperature was possible. In this study, however, higher vibrational states of the ion can be excited in the drift field. Therefore, when analyzing and interpreting the VT-SIFDT data together with the guided ion beam data, one must consider excitation of internal degrees of freedom of both the ion and neutral species. At (KE,,) up to about 0.1 eV, the value of the rate constant to a good approximation depends only on the average center-of-mass kinetic energy. Above 0.1 eV, the observed rate constant for the charge-transfer reaction of CO: with O2 depends on the temperature of the He buffer as well as the kinetic energy. These dependences are the result of differing internal energy distributions of the reactants, since the distribution of kinetic energies at a particular (KE,, ) is independent of the temperature of the buffer gas [26-291. In addition, the rate constants obtained in the VT-SIFDT and beam apparatuses at 300 K differ principally because the internal energy distributions in the reactants differ and, to a lesser extent, because the center-of-mass kinetic energy distributions differ in the two apparatuses. More specifically, in the beam experiment the reactant ion has an internal temperature of 300 K,

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

271

while in the VT-SIFDT the internal “temperature” of the ion depends on the buffer gas temperature and the drift field, i.e. internal modes of the ion can be excited by collisions with the buffer gas in the presence of a drift field, as mentioned above. Previous work in our laboratory has generally found that rotations play a minor role in determining reactivity [ 12,40-441. In the present system, we find that the rate constants do not depend on temperature at a particular (KE,,) at low kinetic energies, a region where rotational energy is a significant fraction of the total energy available. Since increasing temperature increases the average rotational energy in both 0, and CO:, this is evidence that rotations do not play a major role in the present system. Furthermore, contributions to the rate constants from reactions of vibrationally excited O2 reactant are not possible in these experiments because the 0, vibration is not significantly excited over the range of temperatures investigated. The preceding arguments suggest that the differences in the rate constants at a particular (KE,,) measured in the VT-SIFDT are not the result of differences in rotational temperature of the reactants, extent of vibrational excitation of the O2 reactant, or kinetic energy distributions. By a process of elimination, only differences in the extent of vibrational excitation of CO,+ can be responsible for the differences in the rate constants observed. Previous drift tube measurements have also shown the importance of the CO: vibrations in affecting the efficiency of this reaction [2,3]. Those measurements found differences in the rate constants at a particular (KE,) when different buffer gases were used. In what follows in this paper, we examine the question of whether these differences are mode specific. Figure 3 shows the fraction of CO: ions with excited vibrational levels for the differing experimental conditions. The fraction was calculated from the Boltzmann distribution, assuming that the internal temperature of the reactant CO: in the VT-SIFDT is given by the center-of-mass kinetic energy with respect to the buffer (eqn. (3)) as (KE,,), = (3/2)kTinl. The internal temperature of the CO: ions in the beam experiment is constant and equal to 300K, as determined by the temperature of the buffer gas in the flow tube source. At 300 K in the beam experiment, only bending vibrations are significantly excited, as indicated in Fig. 3. Vibrational frequencies were treated as harmonic and taken from Gauyacq et al. [35]. The asymmetric stretch (1469cm-‘) and symmetric stretch (1283cm-‘) are combined into one parameter since the frequencies are similar, i.e. the indicated fraction of CO: ions with ZJ> 0 in the stretch includes ions with excitation i.n the asymmetric stretch or symmetric stretch. The degenerate bends (500cm-‘) were treated similarly. The rate constants shown in Fig. 1 show almost no dependence on temperature at energies less than 0.1 eV. From Fig. 3, we see that at these energies

272

c

.e

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

0.4 ---91K

. -29SK ’ -471

K

1

0.01 Wo,,,)@W

Fig. 3. Fraction of CO: with w > 0 for different vibrational modes. Excitation in either or both of the stretches is given by broken lines while solid lines refer to excitation of the bend(s). Thin, medium, and thick lines refer to 91 K, 298K and 471 K respectively. The fraction of the excitation of bending vibration(s) in the guided ion beam experiment is given by the solid horizontal line.

the fraction of reactant ions with excitation in the bending modes varies considerably with the temperature of the buffer gas. For instance at O.O6eV, only 4% of CO,’ molecules are excited at 91 K, while approximately 40% are excited at 471 K. We can therefore conclude that bending mode excitation does not strongly influence the rate constant at low kinetic energy. Appreciable differences in the rate constants are observed only at energies where appreciable stretching mode excitation is possible. This suggests that excitation of one or both of the stretching vibrational modes strongly influences the rate constant. It should be pointed out, however, that the first overtone of the bending vibration becomes excited at approximately the same energy as the symmetric stretch. While not an unequivocal conclusion, we assume that the overtone will not substantially affect the reactivity since a single quantum in the bend does not appear to affect the rate constant substantially, or at least that the effect of the stretching modes will be larger than that for the overtones. With several assumptions, we can derive mode-specific rate constants from the VT-SIFDT data. First, we assume that rotations play little role in the reactivity, as suggested by the data. Second, we do not distinguish between the two stretching modes, and we calculate the fraction of ions with excitation in the stretching and bending modes as described above. Finally, we note that in this treatment excitation of overtones is not distinguished from that of fundamentals. With these assumptions, the rate constant at a given (KE,,) and

E.E. Ferguson et al./lnt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

temperature W,

W%

213

is given by >) = k, pop(O) + kb pop(b) + k, pop(s)

(5)

where k,, k, and k, are the rate constants for the charge-transfer reaction of CO: ions with O2 when the CO: ions are in their ground vibrational level, excited vibrational levels of the degenerate bend, and excited vibrational levels of either one of the stretches, respectively. The parameters pop(O), pop(b), and pop(s) are the fractional populations of the various modes calculated as described above, with one exception, as follows. In this formulation, we must choose which term will incorporate ions with excitation of both bending and stretching vibrations. Because we believe that the data indicate that stretching vibrations play the greatest role in the efficiency of the charge-transfer reaction, we designate any ions with excitation of stretching vibrations in the stretching mode term. For purposes of normalization, the relative population of reactant ions with excitation of the bending vibrations is given by the difference between unity and the sum of the fraction of ions with no vibrational excitation and of those with excitation of the stretching vibrations. Since the VT-SIFDT experiments were made at three temperatures, we have three equations and three unknown rate constants. The beam data are neglected in this analysis and will be used to test the model we have developed. The calculations gave meaningful answers only over the energy range 0.1-0.28 eV. Below 0.1 eV, the differences between the data points were not large enough to allow making the analysis, and at higher energies the results had no physical meaning, i.e. the rate constants derived for the bending excitation were negative. The model is unable to yield meaningful rate constants at higher energies probably because the three-state model inadequately accounts for overtone excitation present at these energies and because the contribution to the total rate constant by the term k,pop(s) is dominant at these higher energies and makes it impossible to distinguish other smaller terms. The results of the calculations are shown in Fig. 4. As was deduced from examining Figs. 1 and 3, excitation of the bending vibration of CO: has little effect on the rate constant at low energy, i.e. k, and kb are comparable. In contrast, excitation in one (or possibly both) of the stretches enhances the efficiency of charge transfer by up to an order of magnitude, as described by k,. From 0.1 to 0.2 eV, k, is found to decrease, and k, and k, are found to remain approximately constant. Above 0.2eV, k, is found to decrease substantially and k,, is found to increase. As mentioned above, the decrease in k, may be a consequence of the model breaking down. At 0.3 eV and above, k,, is found to be negative, a clear indication that the model is no longer valid. The validity of the model is best tested by how well it predicts other data. Three types of data can be predicted: ion beam data, pure temperature data

274

E.E. Ferguson et aI./Int. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

co,++ 0,

8 .

-3

0,’

+

co,

_ _ _k . . .kb

s Two State Model -ko-kb=k 0,s -. k

1 Wcm)WI

Fig. 4. Derived rate constants for the charge-transfer reaction of CO: with O2 in different vibrational levels as a function of (KE,,). Thick solid, thick long broken, and thick short broken lines refer to ions in their ground vibrational level, excited levels of the bending vibration, and excited levels of the stretching vibrations respectively. The thin broken line is derived from a two-state model described in the text and refers to ions which have vibrational excitation in one or both of the stretches. In this model, ions in the ground vibrational level are represented by the guided ion beam data, shown by the thin solid line.

taken at temperatures up to 900 K [I], and drift tube data taken in Ar and N, buffers [2]. These data sets have considerably different internal temperatures of the CO: ions and considerably different fractions of CO: ions in excited vibrational levels. Using the appropriate experimental conditions to evaluate the relative populations of the ground and excited vibrational levels and our derived mode-specific rate constants, we can calculate the charge-transfer rate constant expected for each energy and experiment. The average internal energy of the CO: ions in the flow drift experiments involving non-helium buffer gases was calculated as with He buffer gas, using the Wannier formula (eqn. (3)) where the reactant neutral and buffer masses are identical. Vibrational quenching and therefore excitation are known to be more efficient in collisions with Ar and N2 than in collisions with He [37]. Therefore equilibration of ion vibrational distributions is expected to be established more quickly in Ar- and N,-buffered flow drift tubes than in He buffer. Comparison of the calculated rate constants with the observed values as a function of center-of-mass kinetic energy is shown in Fig. 5, where the predicted values are shown as lines and the data as points. The rate constants measured at high temperatures [l] and the rate constants measured in argon and nitrogen buffers [2] at room temperature are predicted well over the entire energy range, the agreement being on the order of lo-20%. The present

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

1

-

0'11

t

Ar pred

.

kAr

-

-N2pred

.

kN2

-

-GIB

0

k(GIB)

0

k(T) meas

k(T)pred

275

--

0.1

0.2 WE,,)

0.3

WY

Fig. 5. Comparison between predicted and measured rate constants for the charge-transfer reaction of CO: with 0, as a function of (KE,,). Triangles, diamonds, squares and circles refer to data taken in a drift tube using an Ar buffer [2], a drift tube using an Nz buffer [2], guided ion beam (present results), and pure temperature data using an He buffer gas [l] respectively. Solid, dashed-dotted, infrequently broken, and frequently broken lines refer to predictions for data taken in a drift tube using an Ar buffer, a drift tube using an N, buffer, guided ion beam, and pure temperature data, respectively.

guided ion beam data are predicted well at low energy. At higher energies, the rate constants obtained in the guided ion beam experiment are larger than the predicted values. While this disagreement is still within the combined error bars of the beam and predicted rate constants over this energy range, the disagreement appears to be systematically increasing with energy. The disagreement occurs at energies where the precipitous drop in the calculated value of ki, is found and where the bending vibration term in the equation for the total rate constant (eqn. (5)) makes a large contribution to the predicted beam data (because no appreciable excitation of the stretch occurs under the experimental conditions). Therefore we expect that at least some of the disagreement arises from the breakdown of the model as described above. The other data sets are not strongly affected by the decline in k, since the rates are largely controlled by k,. Since the three-state model fails at higher (KE,), an estimate of the effect of excitation of the stretching vibration at high energy can be obtained by using a two-state model. In this model, the rate constants for CO: in the vibrational ground state and those for CO: with excitation in the bending mode are assumed to be equal and are set equal to the guided ion beam data, where very little excitation of the stretching vibration is present. The second “state” represents all ions with vibrational excitation in one or both of the

276

E.E. Ferguson et al./lnt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

stretching modes. The charge-transfer rate constant for CO,+ with excitation in one or both of the stretches is evaluated from the beam data and the 471 K VT-SIFDT data in a manner similar to that described for the three-state model. The results of these calculations are also shown in Fig. 4. At low (KE,,), the value of the rate constant for ions with excitation in the stretching vibrations is in reasonable agreement with that derived from the three-state model. At higher (KE,,), the charge-transfer rate constant for vibrationally excited CO: ions, with excitation in one (or both) of the stretches, stays approximately constant over the entire energy range. Because a significant fraction of the CO: ions with some excitation in the stretching modes have in fact more than one quantum of excitation at high (KE,,), the derived rate constants in this energy region must be viewed as an average for a number of states. Nonetheless, application of a two-state model to the data shows that internal excitation continues to enhance the rate constant over the entire energy range investigated. The generally excellent agreement between predicted and observed rate constants in the different experiments as well as the general agreement between the two models provides strong support for the following assertions. The data clearly indicate that vibrational excitation in the CO,+ reactant strongly increases the efficiency of charge transfer with 0,. It appears that excitation of a stretching vibration of the reactant CO: ions enhances the charge-transfer efficiency to a much greater extent than does excitation of the bending vibration, with the caveat that the first overtone of the bending vibration is approximately equivalent to the frequency of the symmetric stretch and cannot be distinguished from a stretch in our analysis. The three-state model suggests that excitation in the bending mode may also enhance the charge-transfer rate constant/efficiency to a small extent near 0.2eV, although this conclusion is equivocal. The exact values of the modespecific rate constants as well as the shape of the derived curves which describe the dependence of the mode-specific rate constants on kinetic energy are more uncertain, especially at higher energies, where the three-state model breaks down. However, application of a two-state model to the data indicates that the enhancement due to vibrational excitation continues to high (KE,). Finally, we recognize that the two- and three-state models used in the analysis above are oversimplifications, as a large fraction of the molecules are excited into overtones of both the stretch and bend, and no attempt to distinguish between the two stretches was made. However, application of such models enables us to evaluate the differing effects of translational energy and of energy in several internal degrees of freedom in the reactant ion on the efficiency of charge-transfer reactions. As mentioned earlier, several other studies have examined the effects of vibrational excitation on the reaction of CO: with OZ. The results of earlier

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes II 7 (1992) 261-282

211

drift tube studies in different buffer gases clearly indicated that vibrational excitation of the CO: reactant enhances the charge-transfer efficiency, but no analysis of the data was made to reveal the vibrational mode responsible for the enhancement [2,3]. Additionally, Derai et al. [7] have studied this reaction in an ion cyclotron resonance (ICR) mass spectrometer. In that study the CO: was produced in two ways, one which formed CO: ions with an internal temperature of 300K and one which formed CO: ions with approximately 1.4 eV of internal excitation. The rate constants for the lower internal energy ions agree with the thermal measurements reported here at low energy and are a factor of 2 higher than the beam results at energies greater than 0.2 eV. The discrepancy may indicate that the ions in the ICR had an effective internal temperature greater than 300 K. The rate constants for the highly excited CO: ions in the ICR experiment are approximately the same as the rate constants derived from the present work for ions with vibrational excitation of the stretch(es). While the exact vibrational distribution of the internally excited ions in the ICR is not known, the agreement between the rate constants for the excited species in the ICR experiments and the rate constants for CO: excited in the stretch(es) in this work suggest that overtones of the stretches, which are likely to be present in the highly internally excited CO: ions in the ICR experiment, do not have drastically larger rate constants than do those for the ZJ= 1 levels. Understanding the physical origin of the remarkable enhancement of an electronic transition, i.e. charge transfer, by relatively modest increases in ion vibrational energy is a challenge. First, it should be noted that the phenomenon appears to be rather general. Qualitatively similar behavior has been observed [3] using flow drift tubes with He and Ar buffers for the charge transfer of CO: with both 0, and NO and for the charge transfer of N,O+ and NO: with NO. No cases of charge-transfer rate constants which decrease with increasing vibrational energy of the reactants have been reported. In fact, for every Slow, i.e. k << kcollision,triatomic ion charge transfer studied, a vibrational enhancement has been observed. By contrast, at least one case is known [45] where vibrational excitation of CO: decreases a rate constant for a “chemical” reaction, namely CO: + H2 -+ COzH+ + H

(6)

Thus there appears to be a distinction between reactions involving electronic transitions (change in potential surface) and chemical reactions on a single adiabatic potential surface. We suggest the following qualitative model for these observations. The fact that the charge-transfer rate constant is small implies a low probability of crossing between the two electronic surfaces corresponding to reactants and products of reaction (1). In the present case, k is approximately equal to

278

E.E. Ferguson et al./Int. J. Mass Spectrom.

Ion Processes 117 (1992) 261-282

0*2kcollision at (KE,, ) = 0.01 eV, then declines to 0.1 kcollision at 300 K (0.04 eV), and then falls to a minimum of 0.02 kcollision at (KE,,) = 0.3 eV, where the efficiency remains relatively constant as the kinetic energy increases further. The inefficiency of this reaction can be attributed to the non-resonant character of reaction (l), i.e. the Franck-Condon factors for ionization of O2 at 13.8 eV, the ionization potential of CO,, are very small. Thus charge transfer cannot occur efficiently at long range via a vertical transition but must involve a collision complex in order to facilitate adiabatic charge transfer. The lifetime of such a complex is directly influenced by the relative kinetic energy of the reactants since this becomes the internal energy of the intermediate complex. The observed kinetic energy dependence of the rate constant shows that as the energy is raised, the lifetime of the complex decreases, thereby decreasing the probability of crossing between the electronic surfaces of the reactants and products. If the coupling of energy to the intermediate lifetime were the only effect, then it should not matter whether the energy is introduced as relative translational energy or as internal energy of the reactants. The observation that the charge-transfer rate constant is significantly enhanced by excitation in the stretching vibration(s) and not by kinetic or bending excitation shows that this system is more complex. Some insight into this mode-specific behavior for neutralization of CO: by 0, can be obtained by examining what is known about the reverse process, ionization of CO,. The photoelectron spectrum of CO, shows a strong (O,O,O)+ (O,O,O) transition and progressively weaker transitions (090,O)-+ (l,O,O) and (O,O,O)-+ (2,0,0), corresponding to excitation of the symmetric stretch [46]. The equilibrium C-O bond distances in CO: are slightly greater than those in CO, [47]. Since both CO,(X’Zl) and CO:(X*I&) are linear and symmetric molecules in their ground states, excitation of the bend or the asymmetric stretch is neither expected nor observed upon ionization. This result is in concert with our observation that it is excitation of a stretch that couples most efficiently with the charge-transfer probability in reaction (l), while excitation of the bending vibrations in CO,+ has a mild to negligible effect. Note that if excitation of the symmetric stretch but not the asymmetric stretch enhances the charge-transfer reaction probability, then the value of k, shown in Fig. 4 could be twice as large as shown. We now discuss a possible explanation for why small amounts of excitation in the symmetric stretch can facilitate the ability of the CO,+ + 0, system to reach the seam where the electronic surfaces of the reactants and products intersect, while much larger amounts of relative kinetic energy fail to do so. The suggestion is that charge transfer is enhanced by facilitating the ability of the CO, nuclei to sample bond lengths characteristic of both the ion and neutral. (Technically, this is not a Franck-Condon argument since we imagine

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes II 7 (1992) 261-282

179

that the reaction still requires a fairly intimate collision between CO: and 0,, and thus electron transfer is not a vertical process. Therefore the fact that the (O,O,O)-+ (O,O,O)transition dominates is not relevant.) This is apparently accomplished easily when the reactant energy comes directly in the form of stretching excitation, while translational excitation does not promote the reaction. This may be because in the translational energy range where the vibrational enhancement is observed (Fig. 4), the lifetime of the complex is sufficiently short for energy randomization to be incomplete. An alternative explanation (suggested by-the referee) is that near-resonant V-V transfer between the CO: stretch at 1469cm’ and the 0, vibration at 1580cm-’ could enhance the charge-transfer reaction. In this regard, we note that the Franck-Condon factors for ionization of O2 favor production of 0: (‘II,) in its first vibrational level [46]. Since only low lying levels (ZJ= 0 and 1) are involved, the potentials are close to harmonic and therefore the converse should be true, i.e. vibrationally excited 0, should have a better FranckCondon overlap with ground state 0:(2 I&, ‘u = 0) than will ground state OZ. One final question that cannot be answered by the present study is “What is the rate constant at low translational energies for ions with excitation in the stretching modes ?‘. We note that at (KE,, ) = 0.1 eV, k, is approximately the same as k,, at (KE,,) = 0.01 eV. The question is whether a combination of a long intermediate complex lifetime, which occurs at low (KE,), and stretching excitation will lead to an even greater value for the rate constants, or whether k, will remain relatively independent of (KE,,) down to 0.01 eV. The answer to this question could elucidate whether the mechanism by which a long complex lifetime enhances the charge-transfer probability is by enabling internal energy of the complex to randomize into excitation of the symmetric stretch. Further experimental work that addresses more specilically the mode-specific behavior of reaction (1) is warranted. Dissociative

reaction channels

Above about lOeV, new products are formed from the reaction of CO: with 0,. The new ionic products are O+ and CO’ , formed in reactions which are 5.0 (or 5.3) and 5.7eV endothermic respectively [23]. The drop in the charge-transfer cross-section above about 10 eV correlates well with the onset of these processes, indicating that formation of either or both of these products (reactions (7)-(9)) competes with charge transfer: co,f+02~o++o+coz

AH = 4.96eV

(7)

co:+02-,o++co+02

AH = 5.30eV

(8)

CO:+02-,CO++O+0,

AH = 5.leV

Formation

(9) of O+ via dissociative charge-transfer reaction (7) would compete

280

E.E. Ferguson et aLlInt. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

directly with charge transfer by depleting the 0: product. However, the Of product could also be formed via collision-induced dissociation (CID) of CO,+ (reaction (8)), a possibility that cannot be ruled out by these experiments. The CO+ product must come from CID of CO: (reaction 9) and therefore its formation does not compete directly with 0: formation. The increase in the overall cross-section at about 1OeV is consistent with the occurrence of such a direct CID process. The fact that reactions (7)-(9) do not occur efficiently at their thermodynamic thresholds is similar to other observations in our laboratory for dissociative charge-transfer processes [14,48-501. CONCLUSIONS

The present work extends the VT-SIFDT technique developed in the Phillips Laboratory for the study of internal energy dependences of ion/molecule reactions to polyatomic ions and compares the VT-SIFDT results directly with guided ion beam results. In the charge-transfer reaction of CO: with 0,) our results indicate that excitation of the bending vibration(s) in the reactant ion has little effect on the charge-transfer efficiency (at least at low energy) while excitation of the stretching vibration(s) greatly enhances the reaction efficiency. Rotations are also found to have little effect on the reactivity. These observations suggest that a detailed study of this reaction with vibrationally state-selected CO: ions may lead to some interesting insights into the reaction dynamics of this charge-transfer reaction. While the technique used in these experiments to study internal energy dependences of ion/molecule reactions may not yield as much detail about reaction dynamics as do studies of “state’‘-selected ions, it can identify key features of reactions which are sensitive to internal energy relatively quickly. As such, in addition to providing useful data on internal energy dependences in a variety of reactions, this technique can provide guidance toward those systems which may be of greatest interest for more detailed studies. The combined use of beam and drift tube data provides several useful functions. The energy range over which a reaction may be studied is extended. In particular, the drift tube can be operated at liquid nitrogen temperature and the beam data can be extended to tens of electronvolts. The high energy range has resulted in observation of the products CO+ and O+ for the first time. The internal energy distributions of ions are different in the two experiments, providing information that could not be obtained from either experiment alone. The good agreement between the VT-SIFDT and guided ion beam results at low energy supports the accuracies of the two techniques.

E.E. Ferguson et al./Int. J. Mass Spectrom. Ion Processes 117 (1992) 261-282

281

ACKNOWLEDGMENTS

The authors dedicate this paper to Professor Charles H. DePuy in recognition of his major contributions to ion/molecule chemistry using the SIFDT technique. Those of us who have had the good fortune to be colleagues of Chuck DePuy in Boulder especially appreciate his contributions to our scientific careers as well as his friendship. This research was sponsored in part by the Phillips Laboratory, United States Air Force, under Contract No. F19628-86X0224 and F49620-89CO019. The United States Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation herein. The Utah work has been funded by the National Science Foundation, grant number CHE-8917980. REFERENCES

2 3 4

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

W. Lindinger, F.C. Fehsenfeld, A.L. Schmeltekopf and E.E. Ferguson, J. Geophys. Res., 79 (1974) 4753. W. Lindinger, M. McFarland, F.C. Fehsenfeld, D.L. Albritton, A.L. Schmeltekopf and E.E. Ferguson, J. Chem. Phys., 63 (1975) 2175. M. Durup-Ferguson, H. Bohringer, D.W. Fahey and E.E. Ferguson, J. Chem. Phys., 79 (1983) 265. E. Alge, H. Villinger, K. Pesca, H. Ramler, H. Stori and W. Lindinger, J. Phys., Colloq., 40 Suppl. (Vol. 1) (1979) 77. A.B. Rakshit and P. Warneck, J. Chem. Sot., Faraday Trans. 2, 76 (1980) 1084. T.M. Miller, R.E. Wetterskog and J.F. Paulson, J. Chem. Phys., 80 (1984) 4922. R. Derai, P.R. Kemper and M.T. Bowers, J. Chem. Phys., 82 (1985) 4517. N.W. Copp, M. Hamdan, J.D.C. Jones, K. Birkinshaw and N.D. Twiddy, Chem. Phys. Lett., 88 (1982) 508. A.A. Viggiano, R.A. Morris, F. Dale, J.F. Paulson, K. Giles, D. Smith and T. Su, J. Chem. Phys., 93 (1990) 1149. A.A. Viggiano, R.A. Morris and J.F. Paulson, J. Chem. Phys., 89 (1988) 4848. A.A. Viggiano, R.A. Morris and J.F. Paulson, J. Chem. Phys., 90 (1989) 6811. A.A. Viggiano, J.M. Van Doren, R.A. Morris and J.F. Paulson, J. Chem. Phys., 93 (1990) 4761. D.A. Hales and P.B. Armentrout, J. Cluster Sci., 1 (1990) 127. E.R. Fisher and P.B. Armentrout, Chem. Phys. Lett., 179 (1991) 435. E.R. Fisher, M.E. Weber and P.B. Armentrout, J. Chem. Phys., 92 (1990) 2296. D. Smith and N.G. Adams, Adv. At. Mol. Phys., 24 (1988) 1. K.M. Ervin and P.B. Armentrout, J. Chem. Phys., 83 (1985) 166. R.H. Schultz and P.B. Armentrout, Int. J. Mass Spectrom. Ion Processes, 107 (1991) 29. N.G. Adams and D. Smith, J. Phys. B., 9 (1976) 1439. F.C. Fehsenfeld, A.L. Schmeltekopf and E.E. Ferguson, J. Chem. Phys., 45 (1966) 23. E.R. Fisher and P.B. Armentrout, J. Chem. Phys., 94 (1991) 1150. R.H. Schultz and P.B. Armentrout, Chem. Phys. Lett., 179 (1991) 429. S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 17 (suppl. 1) (1988).

282 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

E.E. Ferguson et aLlInt. J. Mass Spectrom.

Ion Processes 117 (1992) 261-282

J.B. Laudenslager, W.T. Huntress and M.T. Bowers, J. Chem. Phys., 61 (1974) 4600. G.H. Wannier, Bell Syst. Tech. J., 32 (1953) 170. L.A. Viehland and R.E. Robson, Int. J. Mass Spectrom. Ion Processes, 90 (1989) 167. L.A. Viehland, A.A. Viggiano and E.A. Mason, J. Chem. Phys., 95 (1991) 7286. R.A. Dressler, J.P.M. Beijers, H. Meyer, S.M. Penn, V.M. Bierbaum and S.R. Leone, J. Chem. Phys., 89 (1988) 4707. R.A. Dressler, H. Meyer, A.O. Langford, V.M. Bierbaum and S.R. Leone, J. Chem. Phys., 87 (1987) 5578. E.A. Mason and E.W. McDaniel, Transport Properties of Ions in Gases, Wiley, New York, 1988. M.A. Duncan, V.M. Bierbaum, G.B. Ellison and S.R. Leone, J. Chem. Phys., 79 (1983) 5448. M. Kriegel, R. Richter, W. Lindinger, L. Barbier and E.E. Ferguson, J. Chem. Phys., 88 (1988) 213. M. Kriegel, R. Richter, W. Lindinger, L. Barbier and E.E. Ferguson, J. Chem. Phys., 89 (1989) 4426. L.D. Landau and E. Teller, Z. Sowjetunion, 10 (1936) 34. D. Gauyacq, C. Larcher and J. Rostas, Can. J. Phys., 57 (1979) 1634. K.P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. H. Biihringer, M. Durup-Ferguson, D.W. Fahey, F.C. Fehsenfeld and E.E. Ferguson, J. Chem. Phys., 79 (1983) 4201. P.J. Chantry, J. Chem. Phys., 55 (1971) 2746. Y. Ikezoe, S. Matsuoka, M. Takebe and A.A. Viggiano, Gas Phase Ion-Molecule Reaction Rate Constants Through 1986, Maruzen, Tokyo, 1987. R.A. Morris, A.A. Viggiano and J.F. Paulson, J. Chem. Phys., 92 (1990) 3448. R.A. Morris, A.A. Viggiano and J.F. Paulson, J. Chem. Phys., 92 (1990) 2342. R.A. Morris, A.A. Viggiano and J.F. Paulson, J. Phys. Chem., 94 (1990) 1884. T. Su, R.A. Morris, A.A. Viggiano and J.F. Paulson, J. Phys. Chem., 94 (1990) 8426. A.A. Viggiano, R.A. Morris, J.M. Van Doren and J.F. Paulson, J. Chem. Phys., 96 (1992) 270. D.L. Albritton, in P. Ausloos (Ed.), Kinetics of Ion-Molecule Reactions, Plenum, New York, 1979, p. 119. J. Berkowitz, Photoabsorption, Photoionization, and Photoelectron Spectroscopy, Academic Press, New York, 1979. S.D. Peyerimhoff, R.J. Buenker and J.L. Whitten, J. Chem. Phys., 46 (1967) 1707. E.R. Fisher, M.E. Weber and P.B. Armentrout, J. Chem. Phys., 92 (1990) 2296. E.R. Fisher and P.B. Armentrout, J. Phys. Chem., 95 (1991) 4765. E.R. Fisher and P.B. Armentrout, J. Phys. Chem., 95 (1991) 6118. E.R. Fisher and P.B. Atinentrout, J. Chem. Phys., 93 (1990) 4858.