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Reactive scattering of a helium seeded oxygen atom beam
Volume 49, number 1
REACTIVE SCATTERING
1 luly 1977
CHEMICAL PHYSICS LETTERS
OF A HELIUM SEEDED OXYGEN ATOM BEAM
P.A. GORRY, C.V. NOWIKOW and R. GRICE CJ~e~rùscryDepartmenr, LXversiry of Manckejter. Manchester, Mi3 9 PL. UK Received 13 April 1977
A hígh pressure microwavc discharge source operating tvith a dilute mixture of 02 in He kas been uscd to produce a supcrsonic nozzle bcam of 0 atoms seeded in He. Ttiis source has been used to sfrrdy the reactive scattering of Cl atoms with c/z and CS2 malccules at an initial translatìonal energy E = 38 kJ mDI-l. Yekxity bistritrutions of reactive scattering were measurcd over u widc angular range by cross-corrclation time-of-flight analysjs. The 0 f Cl2 reaction proceeds via 3 shorflivcd collision complex while the 0 + CS2 regction follotvs a stripping mechanism.
1. Iutroduction The reactions of oxygen atoms with haiogen molecuIes [1,2J have been studiecl in mcJlecular beams by usìng a microwave discharge source ?hrough Oz at low pressure -1 torr. Depending on the configuration of the source employed, the source temperature couM be arranged to be = 1000 K [l ] or *350 K (2]_ The reaction of oxygen atoms with CS1 molecuies bas been studíed 131 by using the thermal dissociation at 2 100 K of Cl2 at law pressure * 1 torr in an iridium oven source. Both these types of oxygen atom source suffer
from the disadvantage of producing beams wi0z Maxwell-Boltzmann velocity distributions. In ado!ition the beams are limited to fairly modest energïes; a matimum x 17 kJ moP1 for the thetmal dissociation source. A radíofrequency discharge source using a mixt ure of 5% Uz in He at high pressure *60 torr has been constructed by Miller and Patch 143, which overcumes these disadvantages by producing a beam of oxygen atoms seeded in helium. In the present experiments a high pressure Oz/Ke discharge source has been empíoyed whìch is similar to that of Miller 2nd I%tch hut which uses mícrowave excitation of the discharge and a stainiess steel skimmer to extract the core of the nozzle expansion. This source has been used to study the reactive scattering of oxygen atoms with Cl2 and CSz, using cross-correlation time-of-flight analysis [51 116
to determïne butions 0 i- Cl, -+ oc o+cs,
the OCI and OS product
+ Cl ,
+0s+cs.
velocity distri0) 01
The molecular beam scattering apparatus empIoy_ ing mass spectrometric detection, is similar to that used in previous work [6]. However differentid pumping of of the detector employs triode ion pumps whích are capabie of pumping helium. Increased dìfferential pumping of the beam sources permits the use of nozzle sources of non-condensible gases. The helium seeded
oxygen aCom beam is produced from a microwave discharge through a mixture of 15% Oz in He at r=8Q torr in a quartz tube with a 0.3 mm diameter nozzle oriflce. This yields an oxygen atom beam with a velocity distribution peaking at 2200 m s-1 and a full width --1000 m ri at half maximum intensity correspondkg to a Mach number &f * 5. The source can be oper-
ated at higher pressures 5200 torr giving a narrower velocicy distribution and higher Mach number hut $h& @es a lower oxygen atom flux. Conditions were chosen to yield maximum oxygen atom flux in these expeti-
Volume 49, number 1
1 July 1977
CHEMICAL PHYSICS LETTERS
ments. The microwave cavity is of conventional
design but is water cooled and bas a tuning electrode which can be adjusted from outside the vacuum. Tuning is monitored by a reflected microwave power meter. During typical operation at = 100 W of incident microwave power, 0, dissociation =35% is attained in the beam. The crossed beam issuing from a nozzle source has a velocity distribution peaking at ~5 10 m s-l with full width = 140 m s-l at half maximum intensity corresponding to Mach number M= 6.7 for both
Cl, and CS2. The velocity distributions of the beams were determined by the “conventional” time-of-flight method used [2] in our previous work. However, velocity distributions of reactive scattering were determined by cross-correlation time-of-flight analysis [S] utilising a pseudo-random chopper disc. The efficiency of signal recovery is improved by a factor ~20 for the cross-correlation method compared with the conventional method, due primarily, to the high fractional open time (~50%) of the pseudo-random chopper compared with that (~5%) of the conventional single slot chopper. The time-of-fligbt distribution measured by the detector using either chopper disc is determined by a minicomputer interface which employs a random access memory to achieve rapid initial data acquisition followed by periodic transfer of accumulated data to the core of the controlling minicomputer. In the cross-correlation method the rotation of the chopper disc and the advance of the interface address register are both synchronised to a crystal oscillator in the interface. As in previous work [2] the minicomputer is also used to apply a wrap-around correction to the time-of-flight data, to deconvolute the data for the chopper disc shutter function and to transform the data to a flux clistribution as a function of laboratory veiocity.
-0’
V. Fig. 1. Contour map of laboratory flux of the OCI product from the reaction 0 f Cl* as a function of laboratory angle 61 and velocity U. Thc velocities of the reactant beams and the centre of rnass are ïndicated by the Newton diagram.
ll0' i
rob \
96
1
y-
”
7. /
0
+
CS,-o>+cs
515
rnr
60.
so’ /
3. Results The results of the angular and tirne-of-flight measurements for the 0 + Cl, and 0 + CS2 reactions are shown in figs. 1 and 2 ascontour maps of laboratory flux for the OCl and OS reaction products. The velocities of the incident beams and of the centre of mass are shown by the Newton diagrams in figs. 1 and 2. These contour maps were constructed from velocity stans at 14 laboratory scattering angles for 0 + Cl,
Fig. 2. Contour map of laboratory flux of the OS product from the reaction 0 + CS2 as a function of laboratory angle 0 and velocity u.
117
CHEMICAL PHYSICS LETTERS
Volume 49, number 1
1 July 1977
and 27 angles for 0 + CS2. It is apparent that both contour maps at smal1 laboratory scattering angles 0 = 20” close to the oxygen atoms beam but that substantial intensity persists out to wide Iaboratory scattering angles 0 Z 90°. A prefiminary kinematic analysis of these data has been carried out using both the numerical inversion procedure of Siska [7 J and the stochastic method of Entemann [SJ to determíne the centre of mass angular distributions T(3) and product transIationai energy distributions P(E’) shown in figs. 3 and 4. The aclivation energies of these reactions Ea = 13 kJ moF1 for CI? [9], 4 kJ mol -* for CS2 [lO] lie suffïciently below the ïnitial translational energy that they should not influence the kinematic transformations for the
narrow beam velocity distributions employed in these experiments. The reactant translational energies E, reaction exoergicities ODo and total energies available to products Eto, are given in tabje 1. The angular distribution T(0) for the OCf products from 0 + Cf, shown in fig. 3 exhibits a backward peak which is a factor =2/3 lower than the forward peak. This indicates [ 111 that the reaction proceeds via a short-Lïved collision complex which persists with a mean lifetïme equal to the rotational period of the complex. This compares with the fhrdings of Parrïsh and Herschbach [ 11 that the reaction complex is long-lived compared with the rotationd period for collisions at lower initial transiational energy E = 12 kJ mol-l _ The angular distribution T(B) for the OS proGuct from 0 f CS2, shown by a solid line in fig. 4, peaks in the forward direction and is typical of a stripping mechanism [12 J . 7’his distribution is compared with that obtaïned by Geddes et al. from only angular measurements, shown by a dotted curve in fig. 4. The angular distribution from the present experiments shows a rather broader forward peak and substantially greater intensity in the backward hemisphere. Table 1 Reaction energetics: reactarzt translntional energy E; peak of the product translztional energy distribution Eik; average product translationa! energy E&; reaction exoergicity DQ; total product energy Etot; units kJ mol-’
E cl2
CS2 118
E&
37.9
3.6
38.5
26.1
Eav *
oo
&ot
6.5 39
25.5 86.5
63.4 125.0
1
I
0.
30’
60’
90’
126’
lS0’
r”
--:
0’
-.
:
kot
. . -.
0
0
10
I 20
l
I
TRANSLATIONAL
1
-_
l
20
180’
,8.
ANGLE
CM
1
4.0
50
60
ENERGY. E’ (kJ mol’).
Fig. 3. Centre of mass angular distribution T( 1) and product translational energy distribution BE’) for th.* 0 + Cl2 reaction; B = 0’ denotes the incident 0 atom direction. Zolid curve for P(E’) denotes result of numerical inversion methcd, dashed curve stochastic method of kinematic analysis. Dottti curve shows prediction of RRKM-AM model for a long-liverì collision complex.
The product translational energy distribution for 0 + Cl2 derived by the numerical inversion and the stochastic methods are shown by the solid and the dashed curves in fig. 3. Both methods ïndicate that con-
siderably less energy is disposed into product translation than is predicted by the RRKM-AM model for a long-lived collision complex [dotted curve in fig. 3), which accounts wel1 [1,2] for the 0 + Brzr 12 reactions at lower energy- In the present experiments the 0 + Clz reaction
disposes a fraction fav = 0.10 of the total avaíl-
Volume 49, number 1
CHEMICALPHYSICS LETTERS
1 July 1977
4. Discumion
The lifetime of the short-lived complex observed in the present experiments for the 0 f Cl, reaction is less than that of the long-lived complex observed by Parrish and Herschbach [I J due to the increased reactant translational energy of the helium seeded oxygen atom beam. The RRK theory [12] relates the lifetime of a long-lived complex r = U-I [EC/@‘, - Eo)] 3
0
20
40
00
TRANSLATIONAL
00
loo
l2tl
EIOERGV.E’ CkJ md?.
F&. 4. Centre of mass anguiardistriiution T(0) and product uanslntional energy distr:bution P(E’) for the 0 + f.& reaction. Solid curve for T(6) shows result of present experiments, dotted curve of ref. (3 J.
able energy into product translation, where & = E~/f&,,,. Roth methods of kinematic analysis yield the same distribution of product translationaI energy for the 0 + Cs, data in these experiments, as shown by the solid curve in fig. 4. This distriiution indicates that a higher fraction& = 0.32 of the total available energy is disposed into product translation by the 0 + C!$ reaction than the 0 + Cl* reaction. The energies at the peak E& and the average energies E& of the product translatic -al energy distriiutions are given in table 1.
(3)
to the total energy of the complex EC and the dissociation energy ITo of the complex with respect to products, where Y denotes the mean vibrational frequency of the complex. Application of this formula yields a rough estimate of the OCl, complex dissociation energy E. = 80 * 40 kJ molB1 with respect to products OCI + Cl. The angular distribution of the 0 + CS2 reaction determined in the present experiments is typical [ 1l] of a stripping reaction. Such a direct mt chanism would attribute the substantial intensity observed in the backward hemisphere, which was not detected by Geddes et al. [3], to collisions at small impact parameters. The flash photolysis study by Smith [ 13] of the 0 + CS, reaction estimates the fraction of the total energy available to prpducts which is disposed into product vibration f& = 0.2. Thus our determination of the fraction disposed into product translation fiv = 0.3 I in the present experiments implies that a remarkably high fraction must be disposed into product rotation f;,==0.5; in agreement with the conclusitins of Geddes et al. [3]. As pointed out by Geddes et al., the disposal of energy into product rotation in the reaction of an atom with a triatomic molecule is not restricted by angular momentum conservation as is the case for the reaction of an atom with a diatomic molecule. Indeed the molecular orbital theory of Walsh [ 141 suggests a mechanism for such high products rotational excitation. Extending* the theory presented by Waish [ 141 for tetra-atomic molecules of the form HAAH to moiecules of the form BAA& indicates that the O-S-C-S transition state with 22 valence electrons will have a bent structure like that of the N, F, molecule. This implies that the collinear CS2 molecule becomes strongly bent during reaction with the 0 atom, * Con-y et al. [ 1.5) will give a full acmunt of this work. 1J9
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
resulting in high rotatíonal excitation of the OS and CS product molecuies rotating in opposed senses. Indeed, high product rotational excitation has been attributed 1163 to just such an induced bending of the SC32 molecule in its reactions w-ith alkali atoms, which proceed by an electron jump mechanism.
Acknowledgement We gratefully acknowledge the assistance of Dr. D.J. Malcolme-Lawes during the aOse_mbly of the apparatus uséd in these experiments and the support of thïs work by the Scíence Research Council. The experi-
mental work reported here was carried out at the University Chemical Laboratory, Cambridge University.
References [J 1 D.D. Parrish and D.R. Herschbach, J. Am. Chem. Sec. 9.5 (1973) 6133.
120
1 July 1977
[Z] D.St.A.G. Radlein, J.C. Whitehead and R. Grice, Mol. Phys. 29 (1975) 1813. [3] J. Geddes, P.N. Clough and P.L. Moore, J. Chem. Phys. 61(1974) 2145. 14) D.R. Mrller and D.F. Patch, Rev. Sci. Instr. 40 (1969) 1566. [S] V.L. Húschy and J.P. Afdridge, Rev. Sci. Instr. 42 (1971) 381. i61 C.F. Carter, M.R. Levy and R. Grice, Faraday Discussions 55 (1973) 3.57. (71 P.E. Siska, J. Chem. Phys. 59 (1973) 6052. 181 E.A. Entemann, Ph.D. Thesis, Harvard University (1967). r91 M.A.A. Clyne and J.A. Coxon, Trans. Faraday Sec. 62 (1966) 2175; J.N. Bradley, D.A. Whytock and T.A. Zaleski. J. Chem. Sec. Faraday Trans. 69 (1973) 1251. I.W.M. Smith, Trans. Faraday Sec. 64 (1968) 378; A.A. Westenberg and N. de Haas, J. Chem. Phys. 50 (1969) 707. 1111 G.A. Fisk, J.D. hfcDonald and D.R. Herschbach, Discussion! Faraday Sec. 44 (1967) 228. 1121 D.R. f-ferschbach, Advan. Chem. Phys. 10 (1966) 319. 1131 I.W.M. Smith, Discussions Faraday Sec. 44 (1967’1 194. r141 A.D. Walsh, J. Chem. Sec. (1953) 2288. 1151 P.A. Gorry, C.V. Nowikow and R. Grfce, to be published. 1161 R. Grice, M.R. Cosandey and D.R. Herschbach, Ber. Bunseages. Physik. Chem. 72 (1968) 975.
REACTIVE SCATTERING
1 luly 1977
CHEMICAL PHYSICS LETTERS
OF A HELIUM SEEDED OXYGEN ATOM BEAM
P.A. GORRY, C.V. NOWIKOW and R. GRICE CJ~e~rùscryDepartmenr, LXversiry of Manckejter. Manchester, Mi3 9 PL. UK Received 13 April 1977
A hígh pressure microwavc discharge source operating tvith a dilute mixture of 02 in He kas been uscd to produce a supcrsonic nozzle bcam of 0 atoms seeded in He. Ttiis source has been used to sfrrdy the reactive scattering of Cl atoms with c/z and CS2 malccules at an initial translatìonal energy E = 38 kJ mDI-l. Yekxity bistritrutions of reactive scattering were measurcd over u widc angular range by cross-corrclation time-of-flight analysjs. The 0 f Cl2 reaction proceeds via 3 shorflivcd collision complex while the 0 + CS2 regction follotvs a stripping mechanism.
1. Iutroduction The reactions of oxygen atoms with haiogen molecuIes [1,2J have been studiecl in mcJlecular beams by usìng a microwave discharge source ?hrough Oz at low pressure -1 torr. Depending on the configuration of the source employed, the source temperature couM be arranged to be = 1000 K [l ] or *350 K (2]_ The reaction of oxygen atoms with CS1 molecuies bas been studíed 131 by using the thermal dissociation at 2 100 K of Cl2 at law pressure * 1 torr in an iridium oven source. Both these types of oxygen atom source suffer
from the disadvantage of producing beams wi0z Maxwell-Boltzmann velocity distributions. In ado!ition the beams are limited to fairly modest energïes; a matimum x 17 kJ moP1 for the thetmal dissociation source. A radíofrequency discharge source using a mixt ure of 5% Uz in He at high pressure *60 torr has been constructed by Miller and Patch 143, which overcumes these disadvantages by producing a beam of oxygen atoms seeded in helium. In the present experiments a high pressure Oz/Ke discharge source has been empíoyed whìch is similar to that of Miller 2nd I%tch hut which uses mícrowave excitation of the discharge and a stainiess steel skimmer to extract the core of the nozzle expansion. This source has been used to study the reactive scattering of oxygen atoms with Cl2 and CSz, using cross-correlation time-of-flight analysis [51 116
to determïne butions 0 i- Cl, -+ oc o+cs,
the OCI and OS product
+ Cl ,
+0s+cs.
velocity distri0) 01
The molecular beam scattering apparatus empIoy_ ing mass spectrometric detection, is similar to that used in previous work [6]. However differentid pumping of of the detector employs triode ion pumps whích are capabie of pumping helium. Increased dìfferential pumping of the beam sources permits the use of nozzle sources of non-condensible gases. The helium seeded
oxygen aCom beam is produced from a microwave discharge through a mixture of 15% Oz in He at r=8Q torr in a quartz tube with a 0.3 mm diameter nozzle oriflce. This yields an oxygen atom beam with a velocity distribution peaking at 2200 m s-1 and a full width --1000 m ri at half maximum intensity correspondkg to a Mach number &f * 5. The source can be oper-
ated at higher pressures 5200 torr giving a narrower velocicy distribution and higher Mach number hut $h& @es a lower oxygen atom flux. Conditions were chosen to yield maximum oxygen atom flux in these expeti-
Volume 49, number 1
1 July 1977
CHEMICAL PHYSICS LETTERS
ments. The microwave cavity is of conventional
design but is water cooled and bas a tuning electrode which can be adjusted from outside the vacuum. Tuning is monitored by a reflected microwave power meter. During typical operation at = 100 W of incident microwave power, 0, dissociation =35% is attained in the beam. The crossed beam issuing from a nozzle source has a velocity distribution peaking at ~5 10 m s-l with full width = 140 m s-l at half maximum intensity corresponding to Mach number M= 6.7 for both
Cl, and CS2. The velocity distributions of the beams were determined by the “conventional” time-of-flight method used [2] in our previous work. However, velocity distributions of reactive scattering were determined by cross-correlation time-of-flight analysis [S] utilising a pseudo-random chopper disc. The efficiency of signal recovery is improved by a factor ~20 for the cross-correlation method compared with the conventional method, due primarily, to the high fractional open time (~50%) of the pseudo-random chopper compared with that (~5%) of the conventional single slot chopper. The time-of-fligbt distribution measured by the detector using either chopper disc is determined by a minicomputer interface which employs a random access memory to achieve rapid initial data acquisition followed by periodic transfer of accumulated data to the core of the controlling minicomputer. In the cross-correlation method the rotation of the chopper disc and the advance of the interface address register are both synchronised to a crystal oscillator in the interface. As in previous work [2] the minicomputer is also used to apply a wrap-around correction to the time-of-flight data, to deconvolute the data for the chopper disc shutter function and to transform the data to a flux clistribution as a function of laboratory veiocity.
-0’
V. Fig. 1. Contour map of laboratory flux of the OCI product from the reaction 0 f Cl* as a function of laboratory angle 61 and velocity U. Thc velocities of the reactant beams and the centre of rnass are ïndicated by the Newton diagram.
ll0' i
rob \
96
1
y-
”
7. /
0
+
CS,-o>+cs
515
rnr
60.
so’ /
3. Results The results of the angular and tirne-of-flight measurements for the 0 + Cl, and 0 + CS2 reactions are shown in figs. 1 and 2 ascontour maps of laboratory flux for the OCl and OS reaction products. The velocities of the incident beams and of the centre of mass are shown by the Newton diagrams in figs. 1 and 2. These contour maps were constructed from velocity stans at 14 laboratory scattering angles for 0 + Cl,
Fig. 2. Contour map of laboratory flux of the OS product from the reaction 0 + CS2 as a function of laboratory angle 0 and velocity u.
117
CHEMICAL PHYSICS LETTERS
Volume 49, number 1
1 July 1977
and 27 angles for 0 + CS2. It is apparent that both contour maps at smal1 laboratory scattering angles 0 = 20” close to the oxygen atoms beam but that substantial intensity persists out to wide Iaboratory scattering angles 0 Z 90°. A prefiminary kinematic analysis of these data has been carried out using both the numerical inversion procedure of Siska [7 J and the stochastic method of Entemann [SJ to determíne the centre of mass angular distributions T(3) and product transIationai energy distributions P(E’) shown in figs. 3 and 4. The aclivation energies of these reactions Ea = 13 kJ moF1 for CI? [9], 4 kJ mol -* for CS2 [lO] lie suffïciently below the ïnitial translational energy that they should not influence the kinematic transformations for the
narrow beam velocity distributions employed in these experiments. The reactant translational energies E, reaction exoergicities ODo and total energies available to products Eto, are given in tabje 1. The angular distribution T(0) for the OCf products from 0 + Cf, shown in fig. 3 exhibits a backward peak which is a factor =2/3 lower than the forward peak. This indicates [ 111 that the reaction proceeds via a short-Lïved collision complex which persists with a mean lifetïme equal to the rotational period of the complex. This compares with the fhrdings of Parrïsh and Herschbach [ 11 that the reaction complex is long-lived compared with the rotationd period for collisions at lower initial transiational energy E = 12 kJ mol-l _ The angular distribution T(B) for the OS proGuct from 0 f CS2, shown by a solid line in fig. 4, peaks in the forward direction and is typical of a stripping mechanism [12 J . 7’his distribution is compared with that obtaïned by Geddes et al. from only angular measurements, shown by a dotted curve in fig. 4. The angular distribution from the present experiments shows a rather broader forward peak and substantially greater intensity in the backward hemisphere. Table 1 Reaction energetics: reactarzt translntional energy E; peak of the product translztional energy distribution Eik; average product translationa! energy E&; reaction exoergicity DQ; total product energy Etot; units kJ mol-’
E cl2
CS2 118
E&
37.9
3.6
38.5
26.1
Eav *
oo
&ot
6.5 39
25.5 86.5
63.4 125.0
1
I
0.
30’
60’
90’
126’
lS0’
r”
--:
0’
-.
:
kot
. . -.
0
0
10
I 20
l
I
TRANSLATIONAL
1
-_
l
20
180’
,8.
ANGLE
CM
1
4.0
50
60
ENERGY. E’ (kJ mol’).
Fig. 3. Centre of mass angular distribution T( 1) and product translational energy distribution BE’) for th.* 0 + Cl2 reaction; B = 0’ denotes the incident 0 atom direction. Zolid curve for P(E’) denotes result of numerical inversion methcd, dashed curve stochastic method of kinematic analysis. Dottti curve shows prediction of RRKM-AM model for a long-liverì collision complex.
The product translational energy distribution for 0 + Cl2 derived by the numerical inversion and the stochastic methods are shown by the solid and the dashed curves in fig. 3. Both methods ïndicate that con-
siderably less energy is disposed into product translation than is predicted by the RRKM-AM model for a long-lived collision complex [dotted curve in fig. 3), which accounts wel1 [1,2] for the 0 + Brzr 12 reactions at lower energy- In the present experiments the 0 + Clz reaction
disposes a fraction fav = 0.10 of the total avaíl-
Volume 49, number 1
CHEMICALPHYSICS LETTERS
1 July 1977
4. Discumion
The lifetime of the short-lived complex observed in the present experiments for the 0 f Cl, reaction is less than that of the long-lived complex observed by Parrish and Herschbach [I J due to the increased reactant translational energy of the helium seeded oxygen atom beam. The RRK theory [12] relates the lifetime of a long-lived complex r = U-I [EC/@‘, - Eo)] 3
0
20
40
00
TRANSLATIONAL
00
loo
l2tl
EIOERGV.E’ CkJ md?.
F&. 4. Centre of mass anguiardistriiution T(0) and product uanslntional energy distr:bution P(E’) for the 0 + f.& reaction. Solid curve for T(6) shows result of present experiments, dotted curve of ref. (3 J.
able energy into product translation, where & = E~/f&,,,. Roth methods of kinematic analysis yield the same distribution of product translationaI energy for the 0 + Cs, data in these experiments, as shown by the solid curve in fig. 4. This distriiution indicates that a higher fraction& = 0.32 of the total available energy is disposed into product translation by the 0 + C!$ reaction than the 0 + Cl* reaction. The energies at the peak E& and the average energies E& of the product translatic -al energy distriiutions are given in table 1.
(3)
to the total energy of the complex EC and the dissociation energy ITo of the complex with respect to products, where Y denotes the mean vibrational frequency of the complex. Application of this formula yields a rough estimate of the OCl, complex dissociation energy E. = 80 * 40 kJ molB1 with respect to products OCI + Cl. The angular distribution of the 0 + CS2 reaction determined in the present experiments is typical [ 1l] of a stripping reaction. Such a direct mt chanism would attribute the substantial intensity observed in the backward hemisphere, which was not detected by Geddes et al. [3], to collisions at small impact parameters. The flash photolysis study by Smith [ 13] of the 0 + CS, reaction estimates the fraction of the total energy available to prpducts which is disposed into product vibration f& = 0.2. Thus our determination of the fraction disposed into product translation fiv = 0.3 I in the present experiments implies that a remarkably high fraction must be disposed into product rotation f;,==0.5; in agreement with the conclusitins of Geddes et al. [3]. As pointed out by Geddes et al., the disposal of energy into product rotation in the reaction of an atom with a triatomic molecule is not restricted by angular momentum conservation as is the case for the reaction of an atom with a diatomic molecule. Indeed the molecular orbital theory of Walsh [ 141 suggests a mechanism for such high products rotational excitation. Extending* the theory presented by Waish [ 141 for tetra-atomic molecules of the form HAAH to moiecules of the form BAA& indicates that the O-S-C-S transition state with 22 valence electrons will have a bent structure like that of the N, F, molecule. This implies that the collinear CS2 molecule becomes strongly bent during reaction with the 0 atom, * Con-y et al. [ 1.5) will give a full acmunt of this work. 1J9
Volume 49, number 1
CHEMICAL PHYSICS LETTERS
resulting in high rotatíonal excitation of the OS and CS product molecuies rotating in opposed senses. Indeed, high product rotational excitation has been attributed 1163 to just such an induced bending of the SC32 molecule in its reactions w-ith alkali atoms, which proceed by an electron jump mechanism.
Acknowledgement We gratefully acknowledge the assistance of Dr. D.J. Malcolme-Lawes during the aOse_mbly of the apparatus uséd in these experiments and the support of thïs work by the Scíence Research Council. The experi-
mental work reported here was carried out at the University Chemical Laboratory, Cambridge University.
References [J 1 D.D. Parrish and D.R. Herschbach, J. Am. Chem. Sec. 9.5 (1973) 6133.
120
1 July 1977
[Z] D.St.A.G. Radlein, J.C. Whitehead and R. Grice, Mol. Phys. 29 (1975) 1813. [3] J. Geddes, P.N. Clough and P.L. Moore, J. Chem. Phys. 61(1974) 2145. 14) D.R. Mrller and D.F. Patch, Rev. Sci. Instr. 40 (1969) 1566. [S] V.L. Húschy and J.P. Afdridge, Rev. Sci. Instr. 42 (1971) 381. i61 C.F. Carter, M.R. Levy and R. Grice, Faraday Discussions 55 (1973) 3.57. (71 P.E. Siska, J. Chem. Phys. 59 (1973) 6052. 181 E.A. Entemann, Ph.D. Thesis, Harvard University (1967). r91 M.A.A. Clyne and J.A. Coxon, Trans. Faraday Sec. 62 (1966) 2175; J.N. Bradley, D.A. Whytock and T.A. Zaleski. J. Chem. Sec. Faraday Trans. 69 (1973) 1251. I.W.M. Smith, Trans. Faraday Sec. 64 (1968) 378; A.A. Westenberg and N. de Haas, J. Chem. Phys. 50 (1969) 707. 1111 G.A. Fisk, J.D. hfcDonald and D.R. Herschbach, Discussion! Faraday Sec. 44 (1967) 228. 1121 D.R. f-ferschbach, Advan. Chem. Phys. 10 (1966) 319. 1131 I.W.M. Smith, Discussions Faraday Sec. 44 (1967’1 194. r141 A.D. Walsh, J. Chem. Sec. (1953) 2288. 1151 P.A. Gorry, C.V. Nowikow and R. Grfce, to be published. 1161 R. Grice, M.R. Cosandey and D.R. Herschbach, Ber. Bunseages. Physik. Chem. 72 (1968) 975.