3110
J . Phys. Chem. 1986, 90, 3 1 10-3 1 16
Primary Energy Distribution in the Products of the Reaction F 4- H8r
-
HF(v’)
+ Brt
P. M. Aker, D. J. Donaldson, and J. J. Sloan* National Research Council Canada, Ottawa, Canada, K1 A OR6,and Department of Chemistry, Carleton University, Ottawa, Canada, K1S 5B6 (Received: November 22, 1985; In Final Form: February 25, 1986)
It is shown that the title reaction creates HF with approximately the maximum vibrational and rotational excitation permitted by the exoergicity. Thus it appears to exhibit the same dynamics as the other members of the X HY reaction family (X, Y halogen atoms). This result was obtained from a low-pressure infrared chemiluminescence experiment in which special procedures were introduced to show the absence of all gas-phase and surface energy transfer processes which might distort the measured distribution.
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Introduction The isolation of the elementary steps of certain complex or extremely fast reactions, for the purpose of detailed energy partitioning measurements, presents a severe experimental problem. We have previously reported an example of a complex (two-step) reaction for which the primary (atom plus molecule) and the secondary (atom plus radical) steps give different energy distributions.’ Under certain experimental conditions, both steps of the reaction occur, with the result that the observed energy distribution is a composite which varies in an unusual way when the experimental conditions are changed. Extraordinarily fast reaction rates can also cause difficulties with energy partitioning measurements on “simple” reactions consisting of only a single step. In the following, we report an energy distribution measurement on the reaction F HBr HF(v’,J’) + Br, for which several previous have obtained widely varying results. We shall give evidence suggesting that when the mean-free path is extremely long (the usual condition for measurements of very fast reactions) the reactive process being observed may not be isolated in the desired part of the apparatus. This has implications for the observed result which will be discussed later. The previous results on the F HBr system fall into two categories: In one case, the observed HF vibrational distribution is inverted, having its maximum in the third* or f o ~ r t h ~vi-. ~ , ~ the distribution is monotonically brational level. In the ~ther:,~.~ decreasing; the most-populated level which is observed is HF(v’=l). We shall refer to the former result as “hot” and to the latter as “cold”. These results were obtained in several laboratories by two different experimental techniques. The first, the lowpressure infared chemiluminescence te~hnique,’~produces either the hot or cold result, depending upon the conditions used in the experiment. A cold distribution is observed for low reagent flows and the distribution recomes more vibrationally inverted (“hotter”) as the reagent flows are increased. These changes are just the opposite of those caused by gas-phase deactivation processes, for which an initially inverted distribution becomes progressively colder with increasing reagent flows. The other technique, a fast-flowing afterglow experiment,s obtains the hot distribution exclusively. The initial energy distributions from other exothermic reactions of the form X + H Y (X, Y are halogen atoms) all produce vibrationally inverted product energy distributions. This has been extensively documented for the F/HCl r e a c t i ~ n ~and . ~ ,in~ the * ~ ~ ~ ~one ~*~ majority of the work on the F / H I r e a c t i ~ n . ~However, study of the latter showed that it could produce either hot or cold energy distributions under conditions in which the F/HBr reaction produces the same conflicting results. This dual behavior has never been observed for the F/HCl reaction. In this context, it should be noted that the rates of F/HBr and F / H I are both approximately equal to 4.5 X lo-” cm3 molecule-’ s-l, while that of the F/HCl reaction is about a factor of five ~ m a l l e r . ~ J ~ In view of the wealth of information on the family of X / H Y reactions and of the conflicting results on the F/HBr member of
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‘Issued as NRCC No. 25672.
0022-3654/86/2090-3 110$01.50/0
this family, we have examined this system using a low-pressure apparatus which was orginally designed for the measurement of energy partioning in multistep (atom/molecule/radical) reactions. This apparatus is well-suited to the present problem. It provides several internal checks on the energy distributions being measured and the design of the reagent mixer permits a large range of pressure and flow geometfies while ensuring that the reaction zone is localized in a known part of the apparatus. With this apparatus, both the ‘hot” and “cold” F/HBr distributions could be obtained under experimental conditions which suggest an interpretation of these results which is consistent with all of the available data on the system.
Experimental Section A series of experiments were carried out over a broad range of conditions with an apparatus developed for the study of fast multistep atom/radical reactions. (In such processes, careful control of the reagent mixing is of paramount importance.’,’ Two different kinds of experiments were carried out: one explored the effect of the reagent mixing and flow characteristics; the other was designed to ensure that the influences of gas-phase energy transfer and of any possible processes occurring on the surface of the reagent inlets themselves were removed. Each experiment used a reagent-inlet configuration consisting of three concentric quartz tubes mounted in the center of the lid of a cylindrical stainless steel vacuum chamber (25 cm i.d. by 25 cm high). An observation zone, defined by a multipass optical cell, was directly below the inlet tubes (at the mid-plane of the chamber). The location of these components is shown schematically in Figure 1. The region surrounding the optical cell (indicated by dashed lines in the figure) contained a series of liquid-nitrogen-cooled copper vanes, in a (vertical) radial chevron configuration. All three tubes were Teflon-coated to reduce the loss of atomic reagent on their walls. The innermost tube, the injector, introduces (1) (a) Donaldson, D. J.; Parsons, J.; Sloan, J. J.; Stolow, A. Chem. Phys. 1984,85, 47. (b) Donaldson, D. J.; Goddard, J.; Sloan, J. J. J . Chem. Phys. 1985,82, 4524. ( 2 ) (a) Sung, J. P.; Setser, D. W. Chem. Phys. Lett. 1977, 48, 413. (b)
Tamagake, K.; Setser, D. W. In State to S f a t e Chemistry, Brooks, P. R., Hayes, E. F., Eds.; ACS Symposium Series Vol. 56; American Chemical Society: Washington, DC, 1977. (3) Beadle, P.; Dunn, M. R.; Jonathan, N. B. H.; Liddy, J. P.; Naylor, J. C. J . Chem. SOC..Faradav Trans. 2 1978. 74. 2170. (4) Brandt, D.’; Dick&, L. W.; Kwan; L. ‘N. Y.; Polanyi, J. C. Chem. Phys. 1979, 39, 189. (5) Tamagake, K.; Setser, D. W.; Sung, J. P. J . Chem. Phys. 1980, 73, 2203. ( 6 ) Jonathan, N. B. H.; Sellers, P. V.; Stace, A. J. Mol. Phys. 1981, 43, 215. -..
(7) Dill, B.; Heydtmann, H . Chem. Phys. 1983, 81, 419. (8) Jonathan, N.; Melliar-Smith, C. M.; Okuda, S.; Slater, D. H.; Timlin, D. Mol. Phys. 1971, 22, 561. (9) (a) Kirsch, L. J.; Polanyi, J. C. J . Chem. Phys. 1971, 57, 4498. (b) Ding, A. M. G.; Kirsch, L. J.; Perry, D. S.; Polanyi, J. C.; Schreiber, J. L. Faraday Discuss. Chem. Soc. 1973, 55, 252. (10) Wurzberg, E.; Houston, P. L. J . Chem. Phys. 1980, 72, 5915.
0 1986 American Chemical Society
F + HBr Reaction Energy Distribution
The Journal of Physical Chemistry, Vola90, No. 14, 1986 3111
E
$ 3 . 3.
/--
CONFIGURATION A
oan.
Figure 1. Cross-sectionalsketch of reaction chamber lid showing location of reagent inlets with respect to optical cell mirror.
the molecular reagent, which is deflected horizontally outward through an annular opening 1 mm high between the lower opening of the injector and a small Teflon deflector plate suspended just below the injector tip. This lateral injection of the molecular reagent ensures its rapid and efficient ‘mixing with the atomic reagent flowing downward in tube 2, thus localizing the reagent mixing zone in the region immediately surrounding the injector tip. The injector is mounted in a threaded fitting which allows it to be raised or lowered with respect to the two outer tubes. Raising the injector tip causes the reagent mixing zone be moved to a region of higher atomic-reagent density. In this way, the effective densities of the reagents can be changed without the necessity of changing their flows. If the injector tip is raised until it is inside tube 2, the probability that the newly formed product will collide with the inner wall of tube 2 is greatly enhanced, permitting exploration of the effects of such wall collisions. If the injector tip is extended well below the ends of the other tubes, the Teflon deflector plate causes the molecular reagent to move laterally across the downward stream of reagent atoms and into the cryobaffle. This has an open structure, which effectively traps and cryopumps condensible molecular reagents such as HBr. As a consequence, the spatial extent of the reaction zone is limited to a disk-shaped region below the injector tubes and the total interaction time between the two reagents is very short. By moving the injector over a sufficiently long distance, therefore, a complete range of conditions can be explored-from negligible reaction under isolated conditions to nearly complete reagent consumption and strong interference due to the reagent inlet walls. The arrangement of the two outer tubes (labeled tubes 1 and 2 in Figure 1) was different for each of the two kinds of experiment which were carried out. The two configurations and the reagent locations are shown in Figure 2. Configuration A, Figure 2a, is used to explore (and ultimately remove) the effects of gas-phase and surface energy transfer processes. In this case tubes 1 and 2 were the same length and the injector tip, a t its lowest point, was 3 cm below both other tubes. Figure 2b shows configuration B, which was used to explore reagent mixing effects. For this arrangement, tube 2 was terminated 4.5 cm inside tube 1. At its lowest position, the injector tip was even with the bottom of tube 1. For the present studies, the injector was kept 2.5 cm below the end of tube 2. The results to be reported and discussed in later sections were largely obtained from “switching experiments” in which one reagent is substituted for another in the injector without changing any other experimental parameters. Two different kinds of switching experiments were done, each of which used one of the
CONFIGURATION B
Figure 2. The configurationsof the three concentric reagent inlet tubes (approximately to scale) and reagents in each for the two types of experiments described in the text. The inner tube is the injector, the intermediate inlet is tube 2, the outer one is tube 1.
inlet configurations just described. The initial distribution from the F/HBr reaction was determined by performing a switching experiment using reagent configuration A, with the injector tip at its lowest position. HBr and H2were alternately switched into the injector, with a constant flow of F atoms in tube 2 a t all times. One series of experiments was performed with D2 flowing in tube 1, followed by a second series with the D2 absent. For the switching experiment which determined the initial F/HBr distribution, appropriate flows of F atoms and D2 were first established in tubes 2 and 1, respectively. The experiment consisted of accumulating 25 interferograms with HBr flowing in the injector, switching H2into the injector in place of the HBr, accumulating 25 more interferograms (which are stored separately), and then repeating the cycle until a total of 100 interferograms have been accumulated for each reagent. Following the experiment, the two sets of interferograms are added separately, yielding two spectra: one of the F/HBr/D2 system, and the other of the F/H2/D2 system. This entire procedure was carried out for various values of the important experimental parameters such as F atom flow, injector location, etc. The F/H2 spectra taken before and after each F/HBr run served two purposes: they demonstrated the effects of changing the reagent flows, injector position, etc. on the HF(0’) distribution produced by the (well-understood) F H2 reaction and their invariance with time showed that no changes in the apparatus occurred during the experiment. The F/D2 distributions were measured in order to show that the behavior of the apparatus did not change on switching from H2 to HBr. The D2 flowed constantly during the experiment and the D F distributions obtained during the F/H2 and F/HBr measurements were always identical. From this we conclude that the presence of HBr caused no change in the performance of the apparatus. The second kind of switching experiment used inlet configuration B, Figure 2b. In this case, F-atoms flowed in tube 2 while H2and D2 were alternately switched into the injector. HBr flowed in tube 1 for one H2/D2 switch cycle; it was absent for the next. First, a switching experiment (as described above) on H2/D2was carried out with no HBr flowing, then one with HBr flowing in the outer tube, followed by one with no HBr, then one with a different HBr flow, etc. These data provided a measurement of the extent of which HF(0’) is collisionally deactivated by HBr. This information was obtained by subtracting the absolute HF vibrational populations (the normalized distribution multiplied by the total emission intensity, both of which are measured) obtained in the F/HBr/D2 cycle from those obtained in the F/HBr/H2 cycle. The subtraction eliminates those parts of the HF/(u’) populations which are
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3112 The Journal of Physical Chemistry, Vol. 90, No. 14, 1986
Aker et al.
TABLE I
HF populations
flows“
run 19 20 21 22 25 26 28 30 31 34 35 36 38 39
H,/D, H2 D2 H2 D2 H2 D2 H2 H2 D2 H2 D2 H2 H2 D2
SF,
HBr 0.0 0.0
