J. Phys. Chem. 1986, 90, 731-738
73 1
FEATURE ARTICLE Vibrational Quenching of Small Molecular Ions in Neutral Collisions Eldon E. Ferguson Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 (Received: November 8, 1985)
Selected ion flow drift tube techniques have recently been developed for the measurement of molecular ion vibrational relaxation rate constants. The first systematic studies of ion vibrational relaxation show that the collisional quenching rate constants are generally large and generally decrease with relative energy in the thermal range. This is a clear indication that the quenching is dominated by the long-range attractive forces. A model is proposed in which transient complex formation is followed by vibrational predissociation in the complex. If one utilizes ion-neutral vibrational relaxation and three-body association of the same ion-neutral reactants, a vibrational predissociation rate constant in the range 109-10'0 S-I is found applicable to a number of cases. The magnitudes of vibrational relaxation rate constants in some cases yield new insights into ion-molecule interaction potentials.
Introduction Vibrational excitation and relaxation of small neutral molecules have been active areas of research for over 60 years, since the discovery of Pierce' of the dispersion of sound, i.e., the variation of sound speed with frequency, which allowed the experimental detection of vibrational relaxation from measurements of sound speed vs. frequency. Theory and experiment have gone hand in hand since the earliest days. Zener2 gave a wave mechanical solution for vibrational energy transfer which is the basis for modern theory. The well-known Landau-Teller3 classical theory interpreted the relaxation rate dependence on vibrational frequency, temperature, and reduced mass. The field of vibrational relaxation has made an important contribution to the understanding of molecular interactions that we have today. The vibrational excitation and relaxation of molecular ions, on the other hand, have been very little studied. The first systematic quantitative measurements providing a large enough data base to draw some generalities have only been reported recently.e7 The data base is still very sparse; nevertheless, some generalities and some mechanistic understanding have resulted from these initial studies, and they will be discussed here. Some of the previous results on vibrational relaxation of molecular ions have been reviewed in a recent paper.' Most of these have been semiquantitative measurements on large molecular ions. With the flow tube techniques that will be described in this paper and the rapidly developing laser-induced fluorescence technology, a steady flow of data on molecular ion vibrational excitation and deexcitation can be anticipated in the coming years. It was discovered very earlys that neutral vibrational deexcitation by the interconversion of vibrational to translational energy can be very inefficient; e.g., 1Olo collisions of CO(u=l) with
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(1) G. W. Pierce, Proc. A m . Acad. Arts Sci., 60,271 (1925). (2) C . Zener, Phys. Reu., 37,556 (1931). (3) L. D. Landau and E. Teller, Phys. Z . Sowjetunion, 10, 34 (1936). (4) H. Bohringer, M. Durup-Ferguson, E. E. Ferguson, and D. W. Fahey, Planet. Space Sci., 31,483 (1983). ( 5 ) H. mhringer, M. DurupFerguson, D. W. Fahey, F. C. Fehsenfeld. and E. E. Ferguson, J . Chem. Phys., 79,4201 (1983). (6) W. Dobler, W. Federer, F. Howorka, W. Lindinger, M. Durup-Ferguson, and E. E. Ferguson, J . Chem. Phys., 79, 1543 (1983). (7) W. Federer, W. Dobler, F. Howorka, W. Lindinger, M. Durup-Ferguson, and E. E. Ferguson, J . Chem. Phys., 83, 1032 (1985). (8) 0. Oldenberg, Phys. Reu., 37, 556 (1931).
C O are required at 300 K to convert the vibrational energy to translational energy. It is this situation of inefficient vibrational excitation that has been treated in the greatest detail, both experimentally and theoretically. This is the most common case for neutral molecules, especially for nonpolar diatomic molecules with relatively high frequency vibrations. The vibrational deexcitation is induced by the short-range repulsive interaction of the colliding molecules, and considerable insight into the process has been gained by classical mechanics. Vibrational deactivation efficiency is promoted by increasing the violence of the collision (e.g. increasing the temperature), and the efficiency is greater for lower frequency vibrations, in which a lesser amount of vibrational energy need be converted to translational energy. However, there are also many neutral systems for which vibrational deexcitation is determined by the long-range attractive forces. This occurs for neutrals when the interaction potential is large due to chemical bonding interaction. Such interactions lead to much larger probabilities of deactivation, and they lead to decreases of deactivation probabilities with increasing relative kinetic energy (or temperature), just the contrary of the dependence arising from repulsive forces. It might have been anticipated that vibrational relaxation of molecular ions by neutrals (or of molecular neutrals by ions) would be generally more efficient because of the added electrostatic potential between ions and neutrals, and this is indeed the case, although there are still a few cases of very inefficient vibrational relaxation of ions by neutrals, either cases in which the electrostatic interaction is very weak or cases in which chemical interaction dominates the electrostatic interaction and is repulsive. Flow Drift Tube Experiments An experimental technique which allows systematic measurements of vibrational excitation and deexcitation is the selected ion flow drift tube (SIFDT) shown schematically in Figure 1. This is an outgrowth of the original flowing aft erg lo^,^ to which an electric drift field has been addedI0 to control the ion-neutral relative energy from thermal to -1 eV and to which a massselected ion source" has been added. In vibrational relaxation (9) E. E. Ferguson, F. C. Fehsenfeld, and A. L. Schmeltekopf, Adc. A t . Mol. Phys., 5 , 1 (1969). (10) M. McFarland, D. L. Albritton, F. C. Fehsenfeld, E. E. Ferguson, and A. L. Schmeltekopf, J . Chem. Phys., 59,6610 (1973).
This article not subject to U S . Copyright. Published 1986 by the American Chemical Society
732 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
Ferguson
TABLE I: Vibrational Quenching Rate Constants (cm3 s-') at 300 K ion 0*+
k, 1
quencher He Ne
O) is fast. The addition of SO2 probes Oz+(u>l), for which charge transfer to produce SO2+is fast, and O2+(u=O,l), for which charge transfer is endothermic and slow. In the same way H 2 0 detects O,+(u>2). The decrease of the monitor ion signal with addition of a quenching gas is then a direct measure of the rate constant for vibrational quenching of those ions to which the monitor responds. This technique was earlier used to study reactions of the metastable ions 02+(a4nu),12 NO+(3A),'3and O+(2D)14in the flow drift system. An electric field applied to the drift section allows a measure of the energy dependence of the quenching rate constants. Energy dependences of rate constants for molecular ions in drift tubes cannot be quantitatively converted to temperature dependences, since the velocity distribution of the ions is not Maxwellian (and is not known), but this is not a very serious problem since the kinetic energy dependences are not great and hence the departures from true "kinetic" temperature dependences cannot be large. There can of course be great departures between "kinetic" and "vibrational temperatures" of ions in drift tubes, depending on the ion and the buffer gas.
Vibrational Relaxation Data Table I gives measured quenching rate constants for O2+(u>O), 02+(u>1). NO+(u>O), and N2+(u>O). The rate constants are typically large, < k , < 1 0 - ~cm3 s-I; s-l corresponds (12) W. Lindinger, D. L. Albritton, M. McFarland, F. C. Fehsenfeld, A L. Schmeltekopf, and E. E. Ferguson, J Chem. Phys., 62, 4101 (1975) (13) I. Dotan, D. L. Albritton, and F. C. Fehsenfeld, J Chem. Phys., 71, 3280 (1979). (14) B. R. Rowe. D. W. Fahey, F. C. Fehsenfeld, and D. L. Albritton, J. Chem.'Phys., 73, 194 (1980). .
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 733 requires -40 collisions to quench C12.22 This latter case is presumably not quenching by charge transfer, which is endothermic by 310 cm-I (>kT), but rather by V T, which would be expected to be quite efficient due to the large attractive potential between C12- and C1,. The polarizability of C12 is 4.6 A', and there should be additional resonance stabilization of the cluster (as there is in 04-).Values of k, were obtained for O2+(y>l), which is approximately equal to k,(u=2), since the population of u = 2 substantially exceeded that of higher levels, and they were about twice the values for 02'(u>O), which are approximately equal to k,(u=l).
-
.
H2O
02
lo-'
-
10-2
-
.
co,
r
1
N + .-
Kr H2
E m
8 ct
e
10-3-
?
.
Ar
c.
Q2
Vibrational Relaxation Model The following model has been d e ~ e l o p e din ~ ,an ~ ~attempt to extract mechanistic data from the measured quenching rate constants. It is assumed that colliding ions and neutrals form a transient complex, an orbiting motion under the influence of the electrostatic attractive potential (augmented perhaps in some cases by chemical forces) with lifetimes enhanced by the rotational (and possibly vibrational) excitation of the neutral and/or ion
0
c
10-4-
0
c Q)
6 10-5-
I lHe 0'1 012 013 014 0)s O(6 017 018 019 02+
'
- X Complex Bond Energy (eV)
AB+ + C
Figure 2. Vibrational quenching probability for O,'(u=l) ions in collision with various neutrals as a function of the 02*-neutral bond disso-
+ 02(u=0)
+
02(u=1)+ O2+(u=O) 325 cm-' (1) the quenching rate equals the charge-transfer rate, within experimental uncertainty. This is also true for the more precisely measured quenching for 02+(u>2) (mostly u = 2), and the kinetic energy dependence is precisely the same (very little dependen~e).~ The same finding occurs for the exothermic (468 cm-I) NO+(u) quenching by N O and for the well-measured N2+ quenching by N2, which in this case is endothermic by 155 cm-I ( kvp,which is the case when k, > k,. Both situations are reported in Table I. In the latter case, of course, vibrational relaxation is rate limited by the collision frequency. This occurs for deep wells such as 02+ H 2 0 or NO+ + NH, and is a somewhat predictable situation from the electrostatic parameters of the quencher. Neither T nor k,, is a priori known. In order to make some coherence out of this scheme, we have introduced additional experimental evidence on 7. A situation much more studied than vibrational relaxation of molecular ions is their three-body association with neutrals
+
AB'
+C +M
k3 +
AB+C
+M
(5)
where the transient complex [AB'C] * is stabilized by collision with a third body M before unimolecular decay [AB+.C]*
+M
ks
AB+*C
+M
(6) The stabilization rate constant k, is approximately equal to the collision rate constant. The rate constant k3 is given by k3 = kck,7 in this simple energy-transfer model so that k, and k3 are related by k, = k v p k 3 / k , (when k, O). It seems likely that an anisotropy of the interaction potential is essential in order to have efficient vibrational relaxation. This can be important in two ways. First, the formation of a long-lived complex is facilitated by an anisotropic potential, which allows conversion of relative translational energy into internal rotational energy. This has been shown in trajectory calculations by Schelling and Castleman.40 In the case of nonpolar diatomic and linear neutrals, the anisotropy of the polarizabii :y provides the anisotropy of the potential. The polarizabilitics are typically 50% greater along the molecule axis then perpendicular to it. In additif !I, the quadrupole moment is directional and may make a small c "tribution,which could either add to or subtract from that of thi polarizability, depending on the sign of the quadrupole moment. The largest anisotropies are due to the dipole moments of polar molecules. In every case the vibrational quenching of 0," or NO+ by a polar molecule is efficient. The magnitude of the interaction potential and its anisotropy are not of course independent; the existence of a strong directional force requires a deep interaction well, so it is not very easy to separate the effects of well depth and anisotropy. The inefficiency of complex formation should also be manifest in the magnitude of three-body association rate constants. In the case of NO' with Kr, the value of k3 is very low so that the failure cm3 s-l, does not necessarily imply to observe quenching, kq < a low value of kvp,only that k,, < 7 X IO9 s-' (Table 11). The second possible role of anisotropy in the potential is that it would facilitate conversion of vibrational energy to rotational energy of the products of a quenching reaction. It may be that rotatic a1 excitation is important, Le., that the favored process T, R, not simply V T, and that the R component is is V significant. A conclusion reached by Osborn and Smith4I based on quasi-classical trajectory studies of neutral molecule v'hrational energy transfer in collisions is that quite modest interniolecular interaction can appreciably enhance the extent of vibrational energy transfer, but this requires an angular anisotropy in the potential energy hypersurface in order to couple the vibrational to the translational-rotational motions. It appears that similar considerations are applicable to the ion vibrational relaxation by neutral collision processes.
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(37) F. C. Fehsenfeld, E. E. Ferguson, and C. J. Howard, J.Geophys. Res., 78, 327 (1973). (38) F. C. Fehsenfeld, J . Chem. Phys., 61, 1588 (1974). (39) E. E. Ferguson, F. C. Fehsenfeld, P. D. Goldan, and A. L. Schmeltekopf, J. Geophys. Res., 70, 4323 (1965). (40) F. J. Schelling and A. W. Castleman, Chem. Phys. Lett., 111, 47 (1984). (41) M . K. Osborn and I. W. M. Smith, Chem. Phys., 91, 13 (1984).
Ferguson Accepting the importance of neutral rotational excitation in complex formation or the importance of interaction potential anisotropy in general, it is the relatively efficient vibrational relaxation of Oz'(u) by Ar and Kr that is the anomaly to be explained. The observation of the large ratio of k, for 0 2 + ( u ) and NO+(u) by Kr (> 11) led to the speculation that 0,' chemically bonds to Kr, whereas NO+ does not.,' This was supported by the finding that 0,' clusters to Kr in a three-body reaction an order of magnitude more rapidly than does NO+. This has been beautifully substantiated by Jarrold et al.43in photodissociation studies of the Kr-02+cluster ion, in which a bond strength of -0.33 eV and a Kr-O-0 bond angle of 139O are found. Thus, it is clear that the 02'-Kr potential is both fairly strong and anisotropic due to specific chemical bonding forces augmenting the electrostatic forces. Presumably there is also a similar, but with Ar. The open-shell 211s weaker, specific interaction of 02+ structure of 02+ leads to greater reactivity than for the closed-shell NO+('B+). It would be of considerable interest to measure the rotational excitation imparted to the ions and neutrals in vibrational quenching experiments, but that is certainly a formidable task. One can anticipate generally that vibrationally excited ions will be quenched effectively by species which have strong directional forces with the ion. An example might be 0 atoms quenching 0 2 + ( u ) , since 03+ is stable and bent. On the other hand, O(3P) atoms probably do not quench vibrationally excited NO+('Z) ions efficiently since the state correlation is to a repulsive NO2+triplet state.34 If borne out, this will be another example of repulsive chemical forces overriding attractive electrostatic forces. Another case of interest is CH4, which is electrostatically almost spherically symmetrical, having no quadrupole moment or polarizability anisotropy. NO+(u) is quenched rather efficiently by CHI, Z = 38, and 02'(u) is quenched extremely efficiently, 2 = 1.9. The rotational constant is large ( B = 5.2 cm-I), and the polarizability is substantial, 2.56 A,3both of which favor complex formation. However, it is not so easy to understand how the ions exert a torque on the CH4 to rotationally excite it. Perhaps this is a case which demonstrates the role of rotational excitation of the ion by the shorter range re3ulsive forces or vibrational excitation of CH4 modes, which a 2 accessible within the expected electrostatic well depth. The extremely efficient quenching of 0 2 " ( u ) by CH, may be explicable as a consequence of long-lived complex formation associated with "incipient" chemical reaction between 02+ and CH,. Hydride ion (H-) transfer to produce CH3++ HO, is 0.24 eV endothermic, and electron transfer to produce CH4+ + 0, is 0.6 eV endothermic. The existence of these slightly endothermic reaction channels could give rise to chemical forces which enhance the lifetime of the 02+.CH4complex. For example, it is wellknown that charge-transfer complexes (e.g. Bz-I,) are stabilized due to the mixing of an endothermic charge-transfer (Bz+.12-) state in the wave function of the ground state of the species.44 It does seem clear from the relative 0 2 + ( u ) and NO+(u) quenching efficiencies by CH, that the 0,' interaction is stronger.
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Energy Transfer Time Scales The vibrational quenching measurements of NO+(c) and Ozt(u), in conjunction with the model proposed, suggest molecular energy transfer times from the ions to "orbiting" or "proximate" neutrals of the order of 10-9-10-10 s. How does this relate to other molecular energy transfer rates for similar situations? The well-studied and rapidly progressing field of vibrational predissociation of van der Waals molecules is the most obvious area to investigate. Raising one partner of a van der Waals (or H-bonded) complex to a vibrational level in the dissociation continuum is sort of a "half-collision" of the type with which we are concerned in ion collisional vibrational relaxation. There are (42) E. E. Ferguson, D. Smith, and N.G. Adams, In?. J . Mass Spectrom. Ion Phys., 57, 243 (1984). (43) M. F. Jarrold, L. Misev, and M. T. Bowers, J . Chem. Phys., 81, 4369 (1984). (44) R. S . Mulliken, J . Am. Chem. SOC.,74, 811 (1952)
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 737
Feature Article a number of reported vibrational predissociation lifetimes for weakly bound neutral molecules. Many of these have been obtained from spectroscopic line widths, via the uncertainty principle. However, Gentry has shown45that the line width derived lifetimes may not correspond to the vibrational predissociation rates, but to faster nondissociative vibrational relaxation processes. When this uncertainty is recognized, there is not such a large body of quantitative vibrational predissociation lifetimes. There are, to be sure, a number of vibrational predissociation lifetimes in the nanosecond range, but there are also much longer and much shorter lifetinies; Le., there appears to be a very large dynamic range for the process.46 The vibrational predissociation rate for C,H4(V7,0=1) bound to C2H446appears to be greater than lo’, s-l, and for C12(u=l) bound to Ne,47 less than lo5 SKI. To the extent that our limited data set of k.p‘s (Table 11) is representative, there appears to be a substantial difference in behavior, Le., much less variability in energy transfer time for the molecular ion vibrational energy transfer to colliding or “orbiting” neutrals. We speculate that this difference reflects the difference between the relatively fixed orientations for the interacting vibrationally excited van der Waals pairs (the half-collisions) and the dynamic trajectories of the interacting ion-molecule partners. In the latter case, all relative reactant orientations are sampled to some extent; the reactants cannot be constrained to an “unfavorable” orientation as they might be in a relatively fixed geometry van der Waals molecule. One can imagine that the most favorable orientation for momentum transfer (V T) could be orthogonal to the most favorable orientation for chemical bonding. The ion-molecule collision, in the “orbiting” trajectory, more or less “averages” the orientations, leading to a relatively invariant vibrational predissociation rate constant (in the 109-1010-s-1range), when the attractive forces (electrostatic or chemical) are dominant. Experiments on neutral vibrational relaxation have been reported recently which bear a striking correlation with the ion vibrational relaxation studies. Heilweil et al.48have measured the vibrational deactivation rates of OH(o= 1) molecules adsorbed on SiO, surfaces in different chemical environments. The deactivation rate under vacuum, 4.9 X IO9 s-I, is increased when various solvents are introduced. These solvent deactivation rates (Table I, ref 48) are in the same range as the kyp)s of Table 11, Le., a few times lo9 s-’. Specifically, the values found by Heilweil et al.,48in units of lo9 s-’, are as follows: CC14, 1.4; CF,Br2, 2.2; CH2C12, 4.9; C6H6, 6.1; H20, 13. The remarkable similarity in quenching rates suggests a similarity in the physical mechanism. This may simply be that in both cases the vibrating molecule is constrained to be in close proximity to the quencher, without being orientationally constrained by chemical bonding. In the van der Waals molecules, even with the extremely weak bonds, a more specific geometry applies, leading to a more specific dependence on molecular interaction details and therefore much greater variability in rate constant. Another intriguing correlation involves the vibrational relaxation of benzene vibrations in pure benzene crystals, measured by Velsko and H o c h ~ t r a s s e r . ~The ~ vibrational relaxation rates for five vibrations, from 606 to 1603 cm-’ in frequency, range from 3.8 X lo8 s-’ (606 cm-I) to 2.1 X 1O1O s-l (1603 cm-I); coincidentally, this is almost the same range as that given in Table 11. Other systems have also been studied, and Velsko and Hochstrasser conclude “the studies of benzene and other systems have shown that the fundamentals other than those at the lowest frequency
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(45) W. R. Gentry in “Resonances in Electron Molecule Scattering, van der Waals Complexes, and Reactive Chemical Dynamics”, D. G.Truhlar, Ed., American Chemical Society, Washington, DC, 1984, ACS Symp. Ser. No. 263, p 289. (46) M. P. Cassassa, C. M. Western, and K. C. Janda in “Resonances in Electron Molecule Scattering”, D. G. Truhlar, Ed., American Chemical Society, Washington, DC, ACS Symp. Ser. No. 263, p 305. (47) D.‘E.Brinza, C. M. Western, D. E. Evard, F. Thommen, B. A. Swartz, and K. C. Janda, J . Phys. Chem., 88, 2004 (1984). (48) E. J. Heilweil, M. P. Cassassa, R. R. Cavanaugh, and J. C. Stephenson, J . Chem. Phys., 82, 5216 (1985). (49) S . Velsko and R. M. Hochstrasser, J . Phys. Chem., 89, 2240 (1985).
I
I
I
0; ( v 51) Kr -%O;(v= k +
I
0 ) Kr +
present data prehminately o;(v=I)
0.01
03
1
5
KECm lev)
Figure 4. Vibrational relaxation of O,+(u) by Kr as a function of relative energy.
tend to relax in the time regime 5C-150 ps”, that is, have relaxation rates 2 X 10’” to 6 X lo9 s-l. Perhaps this is more than mere coincidence and reflects a predisposition of vibrating molecules to transfer energy to nearby molecules on this time scale. There can of course be orientational effects on the ion vibrational quenching, but not, apparently, sufficiently strong to lead to 6 orders of magnitude variability in kvp. A likely example of a modest orientational effect is the quenching of 02+(u) and NO+(u) by N,. The faster quenching of the higher frequency NO+(u) is an anomaly, as mentioned above. The magnitude of the anomaly is about an order of magnitude. The quenching of NO+(u) is 3.7 times faster than 0 2 + ( u ) , and we would roughly have expected the reverse ratio. Alternatively, k,, is an order of magnitude greater for NO+ + N2.This may reflect the fact that the equilibrium geometry for 02+.N2is T-shaped with the 02+ at the top,50whereas NO+.N2 is a bent configuration, O-N+-N-N, with an angle of 93’ between the ON+ and N, axes.51 It would seem intuitively much easier for vibrational and rotational energy transfer to occur in the NO+”, case. Perhaps if NO+(u).N2 and 02+(u).N2were prepared from ground-state ion-neutral complexes with those geometries, the ratio of vibrational predissociation rates would be very large indeed. The angle averaging orbiting presumably greatly attenuates the orientational effect.
New Results Lindinger and his students at Innsbruck are currently carrying out some very interesting studies of vibrational relaxation and vibrational excitation of molecular ions.52 They have managed to operate a drift tube at relatively high E / N , which makes these experiments possible. The experimental problem that they have overcome better than others is to avoid electrical breakdown in the drift tube. They find vibrational excitation and deexcitation of N2+ ions in He. These processes are related by detailed bala n ~ e .This ~ ~ opens up a new area of research; the first data are only now being analyzed. A very nice result from Innsbruck is shown in Figure 4. The vibrational quenching of O,+(u), mostly u = 1 with some u = 2, by Kr as a function of energy has been obtained.54 The Boulder results at low energy5 are well reproduced. As the kinetic energy of the 02+ is increased, the quenching rate constant reaches a minimum and then increases. This is a beautiful example of the (50) G.S. Janik and D. C. Conway, J . Phys. Chem., 71, 823 (1967). (51) M. T. Nguyen, Chem. Phys. Lett., 117,571 (1985). (52) W. Lindinger, private communication. (53) J. ROSS,J. C. Light, and K. E. Shuler in “Kinetic Processes in Gases and Plasmas”, A. R. Hochstim, Ed., Academic Press, New York, 1969, Chapter VIII. (54) M. Kriegl, R. Richter, P. Tosi, W. Lindinger, and E. E. Ferguson, Chem. Phys. Lett., in press.
738 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 transition between the low-energy regime dominated by long-range attractive forces and the higher energy regime dominated by repulsive forces. This transition is often observed for neutral quenching, e.g. for hydrogen halide self-quenching such as HCI(u) + HCI, and NO(u) quenching by NO.25This appears to be the first observation of the transition for molecular ion vibrational relaxation. The minimum in quenching rate constant, which is very broad, is in the general energy range of the 0 2 + - K rbond energy, 0.3 eV.43 Competitive Vibrational Quenching and Reaction Several studies have been carried out in which vibrational quenching of an ion competes with reaction. This is a very much more complicated situation to analyze but offers the prospect of gaining insight into molecular reaction dynamics by analyzing the competitive processes. The competitive reaction quenching situation which has been and CH45S studied in most detail involves 02+ O2+(u)+ CH4
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-
02+(v'
