Reactive scattering of hydrogen and deuterium atoms from halogen molecules

15 November

CEiElllICAL PHYSICS LETTERS

REACTIVE

SCATTERING

OF

FROM

HYDROGEN

HALOGEN

Iiccciwd

AND

DEUTERIUM

1970

ATOMS

MOLECULES

25 September

1970

far the reaction prcxiucts from the rcnctions of If nnd D aloms with Cl2 ant1 Br2 B’ititia cspcrimcntnl error the nn&uinr distributions clo not v:try upon changing frtm If to D ntorns. Nest of the h+xqyzn hnlidc moiecu!eS recoil bnckwnrds with respect to the incomdistribution occurs at larger angles for chlorine thnn for ~ng hvrlrwcn be:~m. ‘The rxtk of thq sngulnr hrtkino. D~tta rcsuits are in ngreement with trjcctorg cntcutntions of Pofnnj+i et al. ;\ng~l:~r

molecules

distributions

;zre rcpt~rtcti.

The reactions of hydrogen atoms with chloand bromirte molecules have been studied some time ago by Poianyi [I 1 and coworkers. From their chenilluminescence experiments. they obtain the distribution of vibrational energy of the products. We report here angular distributions of the reaction products for the same reactions. Similar conclusions c&an be drawn from these two very different ertperiments. rme

The central part of the scattering LS shown in fig_ 1. There are four

pumped

chambers:

apparatus

separately two oven chambers, a scat-

tezing chamber and a detector chamber. One of the oven cflambers has not been used in the present experiment: the hatogen was shot directly into the scattering center pumping. The intt!nsities

Fig.

442

1. Diagr:im

without

of the central

widths

part of the apparntus.

are also given in fig. 1: the absolute zre correct only within a factor of

about five. The hydrogen atom beam is formed in a tungsten oven at 3000’K. The degree of dissociation is between 30 and nearly 100% depertding on the pressure in the oven. The halogen nlolecules are shot into the scattering volume fponl below. This “out-of-plane” geometry has been ckosen for technical reasons. It was found that hydrogen halide molecules were formed irl the cletector chamber - probably on the hot surfaces of the ion source. As the halogen beam is never shot directly into the detector chamber, the partial pressure of the hydrogen hatide molecules is kept small and the signal to noise ratio is irnproved.

differential

and angular

the beams intensities

of

An “in-plane” experiment should give nearly the same angular distributions as ours, because the center of mass velocity is small compared to the hydrogen and hydrogen halide velocities [l J. Reaction prodncts are detected and analyzecl by a space charge focussing ion source 121 and a conventiona 180° magnetic mass spectrometer. The efficiency of this system is such that one out of every thousand thermal nitrogen molecules passing through the ion source is detected. Detector and reaction chamber are separated by :I small channel: the detector can be closed off from the rest of the apparatus by a bakable high xicuum valve. The hydrogen beam is chopped at a frequency of 1500 Hz and the hydrogen halide signal is analyzed by a phase sensitive amplifier. The high chopping frequency makes it possible to estimate the mean product

velocity.

A more detailed description will be pubtishcd etsewvhere 13 1. The measured angular distributions of hycirogen halides are shown in fig, 2. Intensity is ptotted versus the LAB scattering angle, which is measured against the direction of the hydrogen beam as indicated in fig. 1. Because the hydrogen and product velocities are much larger than the velocity of the center of mass of the three particles, C.M. and LAB scattering angles are roughly equal. Inspection of fig. 2 reveals the following: 1. Most of the products recoil backwards with respect to the hydrogen beam. The pe:ti of the angular distributions occurs at larger angles for chlorine than for bromine. 2. Within the experimental error, the angular distributions do not vary when changing f ram H to D atoms. As there is a pronounced forward-backward asymmetry, the reaction is mainly direct, and only a small part of the products (if any) are formed from a long lived complex. On the othei hand, the radical Cl-H-C1 has been found in a matrix isolation study [4]. This implies that “basin” in the HCl2 potenthere is an attractive tial energy surface, where the three particles cuuid be trapped for some time, forming 3 10% lived complex. Since the angular distributions do not show the behaviour expected for a complex

(Symmetrgr with respect to 90° [5]). the lifetime of the collision complex is too short in our case to produce a noticeable effect on the angular distribUtion. An influence on the angular distribution can be expected only if such a comples lives ::t least a rotationnl period of the halogen. This seems rather improbable because of the high velocity of the hydrogen. The similarity of the If and D results permits some conclusions to be wade about the potential energy surface. The only important difference in the hamiltonians for the systems HBrZ and DBr2 is in the incominji channel. where the reduced masses differ by a factor of nearly two. The reduced masses in the exit chnnnel xre nearly equal; the potential energy surface is identical for both systems. If the interaction in the inconring channel is dominant, sljillificant differences might be expected in the deuteriutn and hydrogen mylar distributions. However since they arc xlmost equal, these results suggest, that most of the reaction esothermicity is releasi?d as product repulsion. Howcvor this conclusion is not unique. Conclusions: 1. The reactions of hydrogen atoms with chlorine and bromine molecules are mainly direct. 2. Probnbty, most of the exothermicity is liberated as repulsion between the products and not as attraction of the reactants. The same conclusions have been drawn by PoIanyi and coworkers (11 in an entirely different kind of study. They performed chcmiluminescencc espcriments anti trajectory calculations on the reactions FI - Cl2 - %I c Cl and H + Br? HBr a Br. The trajectory studies show pi-cd&iscattering, the mean scattering mnt backward angle lying at 2140’ for both reactions_ In our results the mean scattering :mgIe is Larger for &Cl than HBr. Also the comyuteci angular distributions deem to be narrower than the esperintcntal ones. Nevertheless. the agreement between our esperintents and the trajectory calcufations done for an entirely different esperiment. must be considered as very good. The reaction D -- Br2 -DBr -+ Br has also been measured by Datz et al. [6], whose reSiilt is very different from ours. Their angular distribution poxks at a LAB angle of z’iO” whiIe in this eaperirnent we obtain Z~120O. Angular distributions for the reactions deutcrium plus halogens IXWC also been measured by Le Breton et al. [Tj. Qualitative.;~reement is found with these measurements though our angular distributions are IXWFOWCX’.

We

thank

Mechanikermeister

Karl

Schmid

rbr

.

machining tus.

the more

REFERENCES

delicatt?

parts

of the appara-

131 J.Grosser nncl H. IIaberlnnd. to be publish&. [-I] P. N. XohIc :!nd G. C. PintnnLe[. J. Chcm. Phys. -13 #IJ~;R) 31 G.5.