Crossed Molecular Beam Reactions of Tritium BromideL. Robbin Martin and James L. Kinsey Citation: The Journal of Chemical Physics 46, 4834 (1967); doi: 10.1063/1.1840642 View online: http://dx.doi.org/10.1063/1.1840642 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/46/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crossed molecular beam studies of the reactions of methyl radicals with iodoalkanes J. Chem. Phys. 89, 6744 (1988); 10.1063/1.455348 Crossed molecular beam study of the reactions of methyl bromide with potassium and rubidium J. Chem. Phys. 69, 5267 (1978); 10.1063/1.436580 Molecular charge transfer reactions of tritium J. Chem. Phys. 65, 3172 (1976); 10.1063/1.433488 Study of the Reaction of K with HBr in Crossed Molecular Beams J. Chem. Phys. 37, 2895 (1962); 10.1063/1.1733115 Infrared Spectra and Molecular Constants of Gaseous Tritium Bromide and Tritium Chloride J. Chem. Phys. 24, 1246 (1956); 10.1063/1.1742749

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4834 R. GORDEN AND P. AUSLOOS

lower than in the investigation of Aquilanti. It would thus seem that Process (26) is not as important as sug­gested by Aquilanti. This is consistent with the low ethylene and ethane yields formed in the xenon­sensitized radiolysis of CIL-CDcNO mixtures (Table IV) which as noted earlier in the Discussion indicate a low yield of methyne and methylene radicals.

We also irradiated a Ar-CH4-CD4 (100:4.96: 2.59) mixtures in the presence of NO and obtained a value of 0.365 for HD/D2 independent of conversion from 0.01 to 0.2%. The latter value is again considerably higher than the extrapolated value of 0.1 reported by Aquilanti for an identical mixture. The actual mechanism of the formation of hydrogen in the xenon- and argon-sensi­tized radiolysis remains to be explored. As noted before35

ion-molecule reactions as well as the decomposition of neutral excited molecules contribute to the formation

THE JOURNAL OF CHEMICAL PHYSICS

of nonscavengable hydrogen III the radiolysis of methane.

We suggest that the decrease of HD/D2 with dimin­ishing conversion, reported by Aquilanti may be an experimental artifact. In the present study, the hydro­gen was distilled from the bulk methane and analyzed separately. In the study of Aquilanti the hydrogen was analyzed in the presence of the methane. It seems likely that in the latter study the contribution to mass 4 from fragmentation of CD4+ was not correctly taken into account in the calculation of the D2 yield from the mass spectrum of the irradiated mixture. If this correction is underestimated the ratio HD /D 2 would seemingly go through the origin at sufficiently low conversion. By the same token it is believed that the extrapolated values of HD/D2 reported for the argon­and krypton-sensitized radiolyses are too low.

VOLUME 46, NUMBER 12 15 JUNE 1967

Crossed Molecular Beam Reactions of Tritium Bromide*

L. ROBBIN MARTlNt AND JAMES L. KINSEyt

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts

(Received 18 January 1967)

A radioisotope detection method was successfully employed to determine the product distribution in crossed molecular beam experiments. Angular distributions of the atomic tritium product were measured for the reactions K+TBr->KBr+T and Cs+TBr--->CsBr+T. The measured distributions correspond to "backward" scattering (X~1I') of the potassium bromide relative to incoming potassium in the center-of­mass coordinate system for both reactions, i.e., the reactions are rebound reactions.

INTRODUCTION

THE interpretation of crossed molecular beam experi­ments in terms of differential cross sections in the

center-of-mass coordinate system is well understood. l

However, unfavorable kinematic factors and limita­tions on experimental resolution frequently make it difficult to resolve clearly even the qualitative behavior of some parameters in chemical reactions. One of the reactions most thoroughly studied, K + HBr~ KBr+ H, is particularly intractable to detailed examination of the angular and velocity distribution of products. The easily observed product, KBr, is so much heavier than its partner that it is kinematically restricted to very slight motion relative to the center of mass. Resolution

* This work was supported in part by the National Science Foundation.

t Present address: Department of Chemistry, California Institute of Technology, Pasadena, Calif.

t Alfred P. Sloan Research Fellow. 1 S. Datz, D. R. Herschbach, and E. H. Taylor, J. Chern. Phys.

37,2895 (1962).

of the KBr motion from that of the center of mass becomes very difficult in this case. The masses of the reactants, however, are more nearly equal, so that there is no such difficulty in measuring reactant angular distributions. Indeed, the nonreactive scattering of K by HBr has been one of the most thoroughly investi­gated systems in molecular beam experiments.2

Because of the detailed information available from the elastic-scattering work about the reactants, our study was undertaken to provide complementary in­formation about the products by measurement of their center-of-mass angular distribution. In order to operate on the favorable end of the disproportionate velocity distribution a new method of detection was developed for observing the lighter product. This was accomplished by choosing to study a slightly different reaction, K + TBr~ KBr+ T, in which a radioactive product was formed. The radioactive decay of the atomic tritium product selectively absorbed on molybdenum trioxide

2 (a) D. Beck, J. Chern. Phys. 37, 2884 (1962); (b) D. Beck, E. F. Greene, and J. Ross, J. Chern. Phys. 37, 2895 (1962).

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MOLECULAR BEAM REACTIONS OF TRITIUM BROMIDE 4835

films served as the detection basis. The reaction of Cs with TBr was also studied.

KINEMATICS

Figure 1 shows average velocity vector diagrams for collisions between K atoms at 400° C and either HBr or TBr molecules at 25°C. The average relative kinetic energy of the reactants E is about 1.4 kcal mole--1 for these temperatures. The difference, Q= E' - E, between the relative kinetic energy of the products and that of the reactants can be at most about 4.8 kcal mole--1 for K+HBrj 4.2 kcal coming from I1Doo, the difference in dissociation energies of the lowest vibrational states, and 0.6 kcal from the average rotational energy of HBr at 25°C. The larger reduced mass of TBr puts its ground-vibrational state 1.56 kcal deeper in the potential well so that the ground-state exothermicity I1Doo is reduced to 2.6 kcal. The maximum value of Q for K+TBr is therefore only about 3.2 kcal mole--1•

Since the KBr molecule comprises 99% of the total mass in the HBr reaction it will recoil from the center of mass with less than 1 % of the final relative velocity. Thus, even for the maximum allowed final relative velocity, the mass factor will restrict the velocity of KBr relative to the center of mass, V'KBr, to be small compared to the velocity of the center of mass itself:

V'KBr= [2E'mH/mKBr(mH+mKBr) J1/2•

For example, if Q is zero v'KBr/vc.m.=0.102 for the average velocities at the temperatures given above. If Q is assumed to have its maximum value of 4.8 kcal, this ratio increases to 0.216. These limits are indicated in Fig. 1. The Q= 0 recoil velocity could at most produce only about a 5° shift in the laboratory distribution of KBr with respect to a calculated center-of-mass distribution. On the other hand, in the reaction K + TBr--tKBr+T, the tritium atom is light enough that V'T/Vc.m . is 7.1 for Q=O or 13.0 for Q=3.2 kcal.

fL-_______ ---=;_ Nt --- ---

FIG. 1. Large figure, left-hand corner: velocity vector diagram for K (400°C) +HBr (25°C). Small figure, upper right-hand corner: 1/10 scale velocity vector diagram for K (400°C) +TBr (25°C). In both diagrams possible product velocities are shown for Q=O and for the largest possible Q. Average initial velocities at the temperatures given are used in both figures.

FIG. 2. Vacuum system. A-ionization pump, B-sublimation pump, C-titanium fila~ents,. D-6-in. gate. valve, E-linear motion feed through for msertmg and removmg detectors, F­roughing pump lines, G-liquid-nitrogen reservoir, H-detector assembly, I-TBr oven cold shield, J-zeolite filled stop for TBr beam K-surface ionization detector for alkali metal beam, L-b~am stop for metal beam, M-r~tating support. The.alkali­metal oven cold shield has been omitted from the drawmg. It would be partially obscured by the detector assembly in the view given.

Clearly the laboratory distribution of tritium atoms is a sensitive function of the center-of-mass distribution at all angles even for quite small recoil kinetic energy. In the limit of very large recoil velocity, laboratory and center-of-mass angles differ by a constant value.

EXPERIMENTAL

Laboratory angular distributions of tritium atoms were determined by surrounding the intersection zone of the reacting beams with small plates uniformly coated with MoOa- a substance which has previously been found to be a highly efficient "getter" for atomic hydrogen.3 After exposure to the crossed beams the plates were removed from the vacuum system and counted for weak tritium {3 activity in a windowless proportional counter well shielded from background.

Figure 2 is a vertical cross-sectional view of the apparatus used. The vacuum envelope is a tempered glass "X" whose arms are 23 cm in diameter. The top of the "X" is a large rotary motion feed through to which the ovens, detectors, and a liquid-nitrogen reservoir are attached. Figure 3 shows a horizontal cross section of the system geometry. The two beam collimating slits were 1 mm wideX 10 rum tall. The TBr beam source slit was a i-X lO-rum array of sintered glass capillaries 5 X 1O-a mm in diameter and 0.75 rum thick.4 At the low pressures used in the source chamber of this oven ( typically about 8 X 10-3 torr) this produced a highly directional beam which greatly diminished the amount of TBr that had to be collimated out of the beam. Simple kinetic-theory calculations indicate that the fraction of gas in the beam is improved

3 R. E. Brennan and F. Fletcher, Proc. Roy. Soc. (London) A250, 389 (1959). . . .

4 Obtained from Permeomcs CorporatIOn, Southbndge, Mass.

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4836 L. R. MARTIN AND J. L. KINSEY

~BEAM i!EOLITE ~ STOP FOR TBr

2 CM. >------1

'-OQ,~ DETECTORS

"'- " " 'I ,

BEAM STOP FOR K

GLASS CAPILLARY ARRAY

FIG. 3. Geometry of ovens and detectors. The detector circle is 2.5 em in diameter. The alkali-metal beam stop and the monitor detector are shown displaced to the right of their actual positions by, respectively, about 27 em and 20 em.

by about a factor of 30 over that expected for Knudson slits. Even with the capillaries, however, only about 6% of the effusing TBr ends up in the collimated beam.

The potassium source was a two-chamber oven also equipped with a special slit arrangement. In this case a lO-mm-tallline of holes 0.15 mm in diameter in a plate approximately 3 mm thick was used. Typical operating temperatures for the alkali-metal oven were 400°C in the upper chamber and 330°C in the lower chamber when it contained potassium. The TBr source oven was operated near room temperature. A surface ionization detector provided to monitor the intensities of the alkali beam showed typically a flux of about 1.5 X 1015

atoms/sec in that beam. The TBr beam flux, estimated as ten percent of the total flow out of the oven, was about 5 X 1013 molecules/sec.s The angular width of both beams was about 5°.

Both ovens were surrounded by copper cold shields attached to a copper tank above the ovens which was filled with liquid nitrogen. The oven mounts passed through holes in the tank. Beam stops were provided for both beams to reduce the background. For the TBr beam this was a heavy-walled copper tube filled with granular zeolite and attached to the same liquid­nitrogen tank as the oven cold shields. The TBr beam entered this tube through a grid window. The stop for the potassium beam was a similar copper tube attached to a flange on one of the arms of the glass "X" and provided with an external source of liquid-nitrogen cooling.

High vacuum was maintained during the run by a titanium sublimation pump (1000 liters/sec) and a smaller ion pump (nominally 25 liters/sec) to remove residual rare gases. This pumping arrangement provided a convenient solution to the problem of avoiding con­tamination of the laboratory with radioactive material since all the tritium was buried on the walls of the

6 J. A. Giordmaine and T. C. Wang, J. App!. Phys. 31, 463 (1960).

pump. Prior to each run the system was roughed to about 5X 10-4 torr by liquid-nitrogen-trapped me­chanical pumps which were then valved off as the other pumps were started. Typical apparatus pressure during a run was 8X 10-7 torr.

The TBr was prepared by combination of excess bromine vapor with pure T2 gas (obtained from New England Nuclear Corporation) on a red-hot platinum filament in a baked-glass system. The T2 was quantita­tively converted to TBr in this process. After removal of the excess bromine the TBr was frozen into a gold­plated stainless steel bomb which was then transferred to the beam source. The preparation and handling procedures are described in detail elsewhere.6

The detectors were aluminum plates with a coated area of 3 mmX20 mm. These were arranged cylindri­cally with their long dimension vertical about the line of intersection of the two vertical ribbon beams. The detector plates were held in fixed, numbered positions on two aluminum support rings which formed the edges of a cylindrical cage 25 mm in diameter and 40 mm tall. No part of this structure could be struck by either of the direct beams. Each detector plate sub tended 14° of cylindrical angle.

The Mo03 coating was achieved by sublimation of the oxide from a hot molybdenum filament in about 4 torr of oxygen, followed by baking in air to 450°C. Extremely careful baking of the detectors was found to be necessary to obtain stable films. The rate of heating and the final temperature reached were precisely controlled. The process found to be most successful and followed in this work is also described in detail in Ref. 6. The relationship between a given detector's position during the preparation procedure and its location on the detector circle was random from run to run to average out possible nonuniformities.

The detector assembly was introduced into the scattering chamber through a vacuum lock after a high vacuum had been obtained. This avoided a burst of radioactive material occurring when the sublimation pump was first fired. After a run was completed, the exposed detectors were removed through the same vacuum lock while the system remained at high vacuum, before the cold surfaces were allowed to warm up. This substantially reduced exposure of the molybdenum oxide film to stray TBr and other tritium-containing compounds, the principal source of noise in counting. The rotating mount for the ovens and detectors allowed them to be placed in a position allowing easy access to the detectors for introduction and removal. During exposure, this assembly was reoriented so that the alkali metal beam was aimed directly at the surface ionization monitor. Typical exposure times were about 15 min.

6 L. Robbin Martin, Ph.D. thesis, "Chemical Reactions in Crossed Molecular Beams," MIT 1966 (unpublished).

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MOLECULAR BEAM REACTIONS OF TRITIUM BROMIDE 4837

The samples were counted in Matheson P-IO count­ing gas flowing through a modified Amperex 400 PC windowless proportional counter. The counter anode was 0.02S-mm-diam tungsten wire operated at 1850 V. Pulses of 1-14 mV across the 10-MQ input load were counted in a pulse-height analyzer. The over-all efficiency for counting tritium on films in this fashion was estimated by dissolving some of the films in water, converting the water to H2 gas, and counting the gas in a batch-type proportional counter. The estimated efficiency obtained was 25 %.

RESULTS

The data are presented in the form of bar graphs in which the angular widths and positions of the detectors are indicated by the widths and positions of the bars. The exit regions for the two beams are indicated by cross hatch. The angular coordinate system is that of Fig. 1. The vertical coordinate is in measured counts per second corrected for (1) cosmic-ray background (approximately 0.7 cps) and (2) tritium background due to residual TBr pressure in the system (0.2-0.6 cps, depending on the run). The latter background was determined by counting blank detectors placed in the system in a location where they were shadowed from the reaction center. The activity on the blank detectors was found to be independent of their location in the vacuum system. Figure 4 represents the tritium atom distribution for a potassium beam at 400°C and TBr at 2SoC. Figure 5(a) is a similar graph for cesium at 300°C and TBr at 25°C. Figure 5(b) is an average of data on four separate cesium runs. Reasonable counting errors figures are ±O.OS cps for Figs. 4 and Sea) and ±0.02 for Fig. 5 (b) .

Since the peak in the T distribution fell in the vicinity of the expected peak intensity of elastically scattered potassium, an experiment was performed to rule out the possibility of interaction of adsorbed potassium on the film with background TBr to produce false signals. A group of twelve detector plates was introduced into

CPS K

5

4.5

4 TBr

3.5

3

2.5

2

1.5

0.5 oL,~~J+~~~~~~~~~~~J,~~~

180 140 100 60 20 0 340 300 260 220 180

DEGREES FROM K BEAM

FIG. 4. Tritium atom distribution for K+TBr. Beam exits are indicated by cross hatch.

CPS r--------,~------mr----------------_, 1.2

1.0

0.8

0.6

0.4

0.2

1.2

1.0

0.8

0.6

0.4

0.2

{aJ

180 140 100 60 20 0 340 300 260 220 180

DEGREES FROM C s BEAM

FIG. 5. (a) Tritium atom distribution for Cs+TBr, (b) is an average of four experiments.

the system. Four of them were exposed to a potassium beam to produce varying amounts of discoloration. The others were kept free from potassium contamina­tion. All of the detectors were then moved out of a line of sight with either beam and exposed to a transient of TBr. The data, shown in Fig. 6, show no significant correlation between counting rate and exposure to potassium. In the figure detectors numbered 5, 6, 7, 8, are the ones exposed to the potassium beam.

DISCUSSION

Reference to the velocity detector diagram of Fig. 1 will show that the observed laboratory distributions of tritium atoms can only correspond to "backward"7 scattering of the products in the center-of-mass systems.

Before reaction: M-TBr

After reaction: MBr~T.

The reaction of K with TBr is somewhat less exother­mic than that of K with HBr because of the large differences in zero-point vibrational energies of HBr and TBr. Nevertheless, it seems likely that the behavior of HBr would be similar to that observed here for TBr since the potential surface is the same in the two cases. Therefore, we expect that K+HBr--tKBr+H should also be a reaction in which the products are scattered predominately backward. This is consistent with some of the general features of "rebound reactions"s: The size of the total-reaction cross section is in the neighbor­hood of that usually found for rebound reactions,S and clear cut "rainbow angles" are observed in the elastic

7 We shall follow the convention of calling the scattering "for­ward" if the alkali halide velocity is parallel to the initial alkali­metal velocity in the center-of-mass coordinate system, and "backward" for the opposite situation.

8 D. Herschbach, in Molecular Beams, J. Ross, Ed. (John Wiley & Sons, Inc., New York, 1966), pp. 319-393.

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4838 L R. MARTIN AND J. L. KINSEY

CPS 5

4

3

2

t-

I-

l-r-

l-

t-

.--r-

e- r-r- e-.--r- 0-

0-r-

2 3 4 5 6 7 8 9 10 II 12

DETECTOR NUMBER

FIG. 6. Test of background interaction.

scattering of the reactants.9 It should be pointed out, however, that our conclusion for K + HBr is in dis­agreement with that reached by Grosser, Blythe, and BemsteinlO in their extremely careful study of this system. The reasons for this difference are not under­stood at present.

The crudeness of our laboratory resolution prohibits the precise determination of either the center-of-mass differential cross section for the products O"p(X) or of the collision exothermicity Q. However, it is possible to place qualitative limits on these quantities by computer simulation of the experiments using a wide variety of initial assumptions and observing which are consistent with the data. We have carried out a large number of computations for both K + TBr and Cs+ TBr using a program whose input includes an assumed O"p(x) and a distribution in values of Q. The program averages over the Maxwellian velocity distributions in both beams and transforms the results into the laboratory coordi­nate system taking into account both the in-plane and out-of-plane resolution.

In all our calculations the O"p(X) were taken to be velocity independent, and the distribution in Q con­tained a single value each time. The angular distribu­tions used all peaked at x= 1800 and decreased to zero at a variable angle in the "backwards hemisphere." For none of the angular distributions tried was it possible to reproduce the data with Q's that were appreciably positive. The cesium data appear to require a definitely

9 E. F. Greene, A. L. Moursund, and J. Ross, in Ref. 8, pp. 135-170.

10 A. E. Grosser, A. R. Blythe, and R. B. Bernstein, J. Chern. Phys. 42,1268 (1965).

negative Q. It was further found that extremely broad angular distributions were inconsistent with both sets of data. In the case of the potassium reaction, a full width no greater than about 300 is indicated, although the data for this case are poorer than for the cesium reaction. The Cs+TBr results are compatible with a slightly broader O"p(x). Most of the increased breadth of the laboratory distribution in the cesium experiments, however, can be attributed to kinematic effects rather than wider center-of-mass distributions.

Possibly of greater significance than the specific results on the two reactions studied in this work is the demonstration of the applicability of radioisotope counting techniques to crossed molecular beam experi­ments. The collection and detection efficiency is quite high. If our best estimates of beam intensities are combined with the measured counting efficiency and the known cross section for K+HBr~KBr+H, the total activity observed in any of the runs was found to be consistent with a sticking probability for T atoms on the detectors between 10% and 100%.

The method used here should be applicable to almost any reaction yielding atomic hydrogen as one of the products. There is also some evidence that small organic radicals (CRa·, C2Hs' ) have a large sticking coefficient on MoOa filmsll and therefore could be studied if labeled either with T or Cl4. The small signal per captured atom per second for weakly radioactive atoms such as tritium or C14 is offset by the possibility of using long exposure times and by the fact that the entire laboratory angular distribution is collected simultaneously.

Selectivity of the detectors for products is a desirable but not always essential characteristic since consider­able "kinematic separation" of products and reactants can occur in favorable cases. In any event, the absence of an observable signal from elastically scattered TBr in our data indicates a strong preference for T over TBr by the detectors. This difference would probably be even more pronounced with reactants less polar than TBr.

ACKNOWLEDGMENTS

We thank Professor James Dubrin for his assistance in the preparation and handling of the TBr and for many valuable discussions. The computer calculations were carried out at the MIT Computation Center.

11 G. M. Burnette and H. W. Melville, Chern. Rev. 54, 225 (1954) .

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