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Reactive scattering of a neon seeded oxygen atom beam
Volume 55; number 1
REACTIVE
SCATTERING
CHEMIC_AL PHYSICS LETTERS
OF
1 April 1978
A NEON SEEDED OXYGEN ATOM BEAM
P-A. GORRY, C-V. NOWIKOW and R. GRICE Chemistry Deparrmenr;CTni~e~-ty of Mancksrer. Manchester Ml3 9PL. UK Received 3 January 1978 A high pressure microwave discharge source operating with a dilute mixture of Oa in Ne has been used to produce a supersonic nozzle beam of 0 atoms seeded in Ne. This low energy supersonic 0 atom beam has been used to study the reactive scattering of 0 atoms with Cla and CSa molecules at an initial translational energy E = 13 kJ mol-r. The results are compared with reactive scattering from the same reactions using a high energy 0 atom beam formed by seeding 0 atoms in He. The 0 i- Cl2 reaction proceeds via a short-lived collision complex where the lifetime of the collision complex depends upon the initial translational ener,T. However the 0 + CSa reaction follows a stripping mechanism which is unaffected by initial translational energy_ 1.
2. Experimental
Introduction
Recently a helium seeded oxygen atom beam has been used [l] in reactive scattering studies at high translational energy. The oxygen atom beam was produced [I ,2] from a high pressure microwave discharge through a dilute mixture of oxygen in helium. A principle advantage [3] of the seeded nozzle beam technique is that the translational energy of the reactive species may be varied by changing the molecular weight of the carrier gas. However, in the case of a microwave discharge source, it is also necessary to establish that the use of a carrier gas other than helium will still permit the production of an intense oxygen atom flux. in the present experiments, neon carrier gas has been used to produce a supersonic oxygen atom beam at thermal energy with an intensity comparable to the previous helium seeded source. This beam has been used to measure reactive scattering at thermal energy from the reactions Q+cl,-+ocl+cl, o+cs~-+os
(1) tcs,
(2)
which were previously studied [1] at higher translational energy. Measurements on reaction (1) with a helium seeded oxygen atom beam have also been extended in range and accuracy so that the dependence of the reaction dynamics on initial translational energy could be accurately determined for both reactions (1) and (2).
The molecular beams apparatus employed in the present work was the same as that used in our previous study [ 1] with a helium seeded oxygen atom beam. The use of neon carrier gas requires an increased nozzle diameter (0.04 cm) for optimum pzrformance at a source pressure Tz:80 torr with 15% oxygen in neon. Under these conditions the mtcrowave discharge source yields = 35%dissociatioc of oxygen; very similar to that obtained [I] with helium carrier gas. The supersonic oxygen atom beam has a velocity distribution peaking at 1300 m s-l with full width 500 m s-l at half maximum intensity, corresponding to a Mach number M = 4.1. The velocity distribution of the supersonic oxygen atom beam was determined by conventional time-of-flight measurements with the detector aperture restricted to 0.008 cm diameter_ Correspondingly narrow slots (0.05 cm width) in the time-of-flight chopper disc reduce the shutter width to 7 m, which permits accurate estimation of the fast oxygen atom velocities. The velocity distribution of the helium seeded oxygen atom beam was redetermined by this method to yield an improved estimate of the velocity distribution peaking at 2050 m s-l with a full width 800 m s-l at half maximum intensity corresponding to a Mach number M=4.3. The velocity distributions of the Cl2 and CSZ nozzle beams were the same as those employed in our previous work [ ll_ Velocity distributions of reactive scattering were measured by cross-correlation time-of19
Volume 55, number I
CHEMICAL PHYSICS LETTERS
1 April 1978.
flight analysis 1441which gives more efficient, signal recovery than the conventional time-of-flight method by a factor x20.
3_ Resu!ts Measurements using a helium seeded oxygen atom beam were repeated for the 0 + C1, reaction to gain a contour map of laboratory reactive scattering with improved range and accuracy to that shown in ref. [ 13, particularly in the region close to the Cl, beam. Kinematic effects render the centre of mass differential cross section particularly sensitive to laboratory data in this region. Transformation of the laboratory data to centre-of-mass coordinates using the direct numerical inversion procedure due to Siska 151 yields a ‘contour map of the differential reaction cross section which cannot be adequately represented by the product of an angular function T(6) and a product transIational energy distribution P(E’j_Accordingly the stochastic kinematic analysis method of Enteman [6] was modified to allow modest variation of the product velocity distribution with scattering angle; the product velocity being higher in the forward direction than at wide angles_ Fig. 1 shows the angular function Ii’-(@)thus obtained for 0 + Cl, with a helium seeded oxygen atom beam and the translational energy distributions P(E’) appropriate to forward scattering (0 = 0”) and to wfde angle scattering (0 > 90”) of the OCl product_ This analysis yields an estimate of the backward peak 030 + 0.1 relative to the forward peak which is lower than the value = 0.6 obtained in our preliminary kinematic analysis [l ] but the results are otherwise substantially unaltered_ Reactive scattering from the 0 f Ci2 reaction using a neon seeded oxygen atom beam is of very much lower intensity(= 5%) than that from a helium seeded oxygen atom beam. This is as expected since the translational energy E = 13 _CW mol-l from a neon seeded beam barely equals the activation energy E, = 14 & 1 k3 mol-’ for the 0 f Cl, reaction [7]. The intensity of reactive scattering is insufficient for the determination of a contour map. The laboratory angular distribution and two velocity scans (one near the peak laboratory intensity and the other along the direction of the centroid vector) were measured for the OCl product_ Kinematic analysis of these rest&s using the stochastic 20
o-
36
so-
64
120’
lso-
180'
CM ANGLE, 0.
\\ I
I
0
10
20
Etot -.
-_ -- -- -- __ 1 I 30
TRANSLATIONAL
40 ENERGY.
50 E’
60 CkJ mOl-‘~_
Fig. 1_ Centre of mag angular distribution T(0) and product translational energy distributions P(E’) at 0 = 0” (solid curve) and 0 = 90” (broken curve) for the 0 +-Cl2 reaction at E =30.4 kJ mol-‘. The CM3product is detected and 8 = 0’ denotes the incident 0 atom direction.
method 163 again required that the product velocity distribution vary with scattering angle in centre-ofmass coordinates. The same form of dependence was adopted for the analysis of the neon seeded beam case as that determined for the helium seeded beam data. Fig. 2 shows the angular function T(0) obtained for 0 + C1, with a neon seeded oxygen atom beam and the translational energy disttibutions P(E’) appropriate to forward scattering (0 3 0”) and to wide angle
Volume 55, number 1
1,
0’
CHEMICAL. PHYSICS LETTERS
II
30’
II
60’
0,
90’
I1
120’
t1
150’
180’
CM ANGLE, 9.
TRANSLATIONAL
ENERGY, E’CkJ
mot-‘3.
Fig. 2. Centre of mass angular distribution T (0) and product translational energy disttiutions P(E')ar8= 0" (solid curve) and B = 90” (broken curve) far the 0 +-Cl? reaction at E = 13.0 kJ mol+_
scattering @I 2 9G”) of the OCI product.
It is apparent that the backward peak has now increased to 0.55 F 0.1 relative to the forward peak in the angular func-
tion T(0) for the reaction with a neon seeded oxygen atom beam. This increase in backward peak height with decreasing reactant translational energy indicates [S] a concomitant increase in the lifetime of the OCI, collision complex. The oscuIating complex model [S] estimates a mean lifetime comparable to the rotational period for the reaction of neon seeded oxygen atoms
1 April 1978
where E = 13.0 kJ mol-I. Hot&ever the lifetime decreases to approximately half of the rotational period of the complex for the reaction of helium seeded oxygen atoms with higher reactant translational energy E = 30.4 kJ mol-l. The product translational energy distributions, shown in figs. 1 and 2 for the 0 + Cl, reaction, indicate that the translational energy is higher for OCl product scattered in the forward direction (solid curves) than for that scattered at wide angles (broken curves)_ The average fraction of the total available energy disposed into product translationai energy is in the range fiv = 0.2 --O-3 where fiv = E&/E,,,. The energies at the peak Eik 3nd the average energies 15:~ of the mean product translational energy distributions determined by the Siska numerical inversion method [S] are listed in table 1, together with the reactant translational energies E, reaction exoergicities 4Do and total available energies for reaction products I!?,,. Reactive scattering from the 0 t CSz reaction with a neon seeded oxygen atom beam is of rather lower intensity (~30%) than that from a helium seeded oxygen atom beam but is still adequate to measure velocity distributions of OS reactive scattering over the full accessible range of laboratory scattering angles_ The laboratory data was transformed using the direct numerical inversion procedure of Siska [S] to obtain a contour map of &hedifferential reaction cross section which can be represented by the angular function T(0) and product translational energy distribution P(E' j shown in fig. 3. The angular function T(B) peaks strongly in the forward direction and is typical of a stripping reaction. Indeed, it is identical, to within experimentai uncertainty, to the angular function obtained [ I] for the 0 + CS2 reaction using a helium seeded oxygen atom beam. The product translational energy distribution is also identical to that obtained Table L Reaction energetics; reactant translational enero_yE; peak of the product translational energy distribution E&avenge product translational energy E& reaction esoergmry ADO; total product energy Et,,; units kJ mol-’
E Cl2 Cl2 cs2
30.4 13.0 13.2
E;k
4.0 5.4 10.4
E SW
ADo
Etor
15.0 7.0 30.8
25.5 25.5 86.5
55.9 38.5 99.7
21
Volume
55, number
0
20
CHEMICAL PHYSICS LETTERS
1
40
60
TRANSLATIONAL
80
100
ENERGY, E’ (kJ ml-‘).
Fig. 3. Centre of mass angular distribution T(9) and product translational energy distribution P(E’) for the 0 +CSz rcaction at E = 13.2 W mol-I. The OS product is detected and 0 = 0” denotes the incident 0 atom direction.
with a helium seeded oxygen atom beam whefi both are expressed in the form P( f ‘) where f ‘= E’/Etot. The 0 + CS, reaction disposes a similar fraction f ‘= 0.3 1 of the total available energy into product trans-
lation to that for the 0 + Cl2 forward scattering.
3. Discussion The present results confirm the conclusion drawn 22
1 April 1978
in our previous paper [l] , that the 0 + CJ2 reaction proceeds via a short-lived collision complex [S]. The mean lifetime of the collision complex is comparable to its rotational period when the total energy available to products Etot = 38.5 kJ mol-l but is only half of its rotational period when Etot = 55.9 kJ mol-I_ Thus the reaction potential energy surface must involve a hollow corresponding to the collinear O-Cl-Cl configuration, which has been discussed [9,10] for the 0 f Br2, I2 reactions. However, the hollow for 0 + Cl, must be shallower with respect to products than those for 0 + Br,, I, since these reactions proceed via longlived collision complexes with lifetimes longer than their rotational periods for Etot = 54 kJ mol-I and 76 kJ mol-l The osculating complex model [g] for reactions proceeding via a short-lived complex assumes that all reactive collisions involve many vibrational periods of the complex and that hence the differential reaction cross section is separable into an angular function T(0) and a velocity function U(U) = uP(E’). However this is clearly inappropriate to the 0 + Cl2 reaction, where our kinematic analysis indicates that the differential reaction cross section is not separable even when the mean collision lifetime is comparable to the rotational period of the complex. This indicates that not all reactive collisions for the 0 + Cl, reaction sample the O-Cl-Cl hollow on the reaction potential energy surface to the same extent. Scattering in the forward direction, which corresponds to the shortest collision lifetimes, disposes rather more energy into product translation than scattering at wide angles which corresponds to longer collision lifetimes. Thus scattering in the forward direction may be considered a stripping component of the reactive scattering which arises from collisions at the largest impact parameters. Sampiing of the potential energy well at small O-Cl-Cl internudear distances would be restricted [ 1 I ] by the higher orbital angular momentum of collisions at the largest impact parameters and this would result in shorter collision lifetimes_ The sharply forward and backward peaked angular distribution observed for 0 + Cl1 at an initial translational energy E = 13 kJ mol-‘, indicates that the maximum collision orbital angular momentum remains appreciable L, =40 h even for reaction in the threshold region. The 0 f CS2 reaction exhibits stripping dynamics which are independent of initial translational energy
Volume_SS,number f
1 April 1978
CHEMICALPHYSICSLETTERS
over the range E = 13 -38 kJ mol-l_ fn part the difference in reaction dynamics compared with the 0 + Cl2 reaction arises from the greater exoergicity of the 0 + CS2 reaction_ However, the precise invariance of 0 + CS2 angular distribution with initial translational energy indicates the absence of any hollow on the 0 +CS, potential energy surface in contrast to that for the 0 + Cl2 reaction. The study of the 0 + Cl2 and 0 + CS2 reactions using both helium seeded and neon seeded oxygen atom beams ihustrates the increased insight into reaction dynamics which can be gained by measuring reactive scattering as a function of reactant translational energy. In future experiments, the use of helium/neon mixture as a carrier gas should provide a means of varying the translational energy of the oxygen atom beam continuously_
References [ 11 P.A. Cony, C.V. Nowikowand R. Grice, Chem. Phys. Letters 49 (1977) 116. [2] D.R. MiBer and D.F. Patch, Rev. Sci. Instr. 40 (1969) 1.566. [3] R.A. Larsen, S.K. Neoh and D.R. Herschbach,Rev. Sci. instr.45 (1974) 1.511; A. Liibbert, G. Rotzoll, R. Viard and K. Schii~erl,Rev. Sci. I?str. 46 (1975) 1656; U. norkenhagen,H. Mdthan and J.P. Toennies, J. Chem. Phys. 63 (1975) 3173; J.J. Valentini,b1.J. Co_ggiola and Y-T. Lee, Rev. Sci Instr- 48 (1977)
181 Acknowledgement Support of this work by the Science Research Council is gratefully acknowledged_
58.
[4] V.L. .Iirschy and J.P. AIdridge, Rev. Sci. Instr. 42 (1971) 381. rql P-E. S&a. J. Chem. Phvs. 59 (1973) 6052. [L6iE.A. Entemann, Ph.D. Thesis.Har&d University (1967). 171 M.A.A. Clyne md J.A. Coxon, Trans. Faraday Sot. 62
191 [JOI [Ill
(1966) 2175; J-N. Bradley, D.A. Whytock and T-A. Zaleski, J. Sot. Faraday Trans. I69 (1973) 1251. G.A. Fisk, J.D. McDonald and D.R. Herschbach, sions Faraday Sot. 44 (1967) 228. D-D_ Parrish and D-R. Herschbach, J. Am. Chem. (197;) 6133. DSt_A_G_ Radlein, J-C. Whitehead and R. Grice, Phys. 29 (1975) 1813. D.R. Rcrschbach, Faraday Discussions 55 (1973)
Chem. DiscusSot_ 95 hloi_ 113.
23
REACTIVE
SCATTERING
CHEMIC_AL PHYSICS LETTERS
OF
1 April 1978
A NEON SEEDED OXYGEN ATOM BEAM
P-A. GORRY, C-V. NOWIKOW and R. GRICE Chemistry Deparrmenr;CTni~e~-ty of Mancksrer. Manchester Ml3 9PL. UK Received 3 January 1978 A high pressure microwave discharge source operating with a dilute mixture of Oa in Ne has been used to produce a supersonic nozzle beam of 0 atoms seeded in Ne. This low energy supersonic 0 atom beam has been used to study the reactive scattering of 0 atoms with Cla and CSa molecules at an initial translational energy E = 13 kJ mol-r. The results are compared with reactive scattering from the same reactions using a high energy 0 atom beam formed by seeding 0 atoms in He. The 0 i- Cl2 reaction proceeds via a short-lived collision complex where the lifetime of the collision complex depends upon the initial translational ener,T. However the 0 + CSa reaction follows a stripping mechanism which is unaffected by initial translational energy_ 1.
2. Experimental
Introduction
Recently a helium seeded oxygen atom beam has been used [l] in reactive scattering studies at high translational energy. The oxygen atom beam was produced [I ,2] from a high pressure microwave discharge through a dilute mixture of oxygen in helium. A principle advantage [3] of the seeded nozzle beam technique is that the translational energy of the reactive species may be varied by changing the molecular weight of the carrier gas. However, in the case of a microwave discharge source, it is also necessary to establish that the use of a carrier gas other than helium will still permit the production of an intense oxygen atom flux. in the present experiments, neon carrier gas has been used to produce a supersonic oxygen atom beam at thermal energy with an intensity comparable to the previous helium seeded source. This beam has been used to measure reactive scattering at thermal energy from the reactions Q+cl,-+ocl+cl, o+cs~-+os
(1) tcs,
(2)
which were previously studied [1] at higher translational energy. Measurements on reaction (1) with a helium seeded oxygen atom beam have also been extended in range and accuracy so that the dependence of the reaction dynamics on initial translational energy could be accurately determined for both reactions (1) and (2).
The molecular beams apparatus employed in the present work was the same as that used in our previous study [ 1] with a helium seeded oxygen atom beam. The use of neon carrier gas requires an increased nozzle diameter (0.04 cm) for optimum pzrformance at a source pressure Tz:80 torr with 15% oxygen in neon. Under these conditions the mtcrowave discharge source yields = 35%dissociatioc of oxygen; very similar to that obtained [I] with helium carrier gas. The supersonic oxygen atom beam has a velocity distribution peaking at 1300 m s-l with full width 500 m s-l at half maximum intensity, corresponding to a Mach number M = 4.1. The velocity distribution of the supersonic oxygen atom beam was determined by conventional time-of-flight measurements with the detector aperture restricted to 0.008 cm diameter_ Correspondingly narrow slots (0.05 cm width) in the time-of-flight chopper disc reduce the shutter width to 7 m, which permits accurate estimation of the fast oxygen atom velocities. The velocity distribution of the helium seeded oxygen atom beam was redetermined by this method to yield an improved estimate of the velocity distribution peaking at 2050 m s-l with a full width 800 m s-l at half maximum intensity corresponding to a Mach number M=4.3. The velocity distributions of the Cl2 and CSZ nozzle beams were the same as those employed in our previous work [ ll_ Velocity distributions of reactive scattering were measured by cross-correlation time-of19
Volume 55, number I
CHEMICAL PHYSICS LETTERS
1 April 1978.
flight analysis 1441which gives more efficient, signal recovery than the conventional time-of-flight method by a factor x20.
3_ Resu!ts Measurements using a helium seeded oxygen atom beam were repeated for the 0 + C1, reaction to gain a contour map of laboratory reactive scattering with improved range and accuracy to that shown in ref. [ 13, particularly in the region close to the Cl, beam. Kinematic effects render the centre of mass differential cross section particularly sensitive to laboratory data in this region. Transformation of the laboratory data to centre-of-mass coordinates using the direct numerical inversion procedure due to Siska 151 yields a ‘contour map of the differential reaction cross section which cannot be adequately represented by the product of an angular function T(6) and a product transIational energy distribution P(E’j_Accordingly the stochastic kinematic analysis method of Enteman [6] was modified to allow modest variation of the product velocity distribution with scattering angle; the product velocity being higher in the forward direction than at wide angles_ Fig. 1 shows the angular function Ii’-(@)thus obtained for 0 + Cl, with a helium seeded oxygen atom beam and the translational energy distributions P(E’) appropriate to forward scattering (0 = 0”) and to wfde angle scattering (0 > 90”) of the OCl product_ This analysis yields an estimate of the backward peak 030 + 0.1 relative to the forward peak which is lower than the value = 0.6 obtained in our preliminary kinematic analysis [l ] but the results are otherwise substantially unaltered_ Reactive scattering from the 0 f Ci2 reaction using a neon seeded oxygen atom beam is of very much lower intensity(= 5%) than that from a helium seeded oxygen atom beam. This is as expected since the translational energy E = 13 _CW mol-l from a neon seeded beam barely equals the activation energy E, = 14 & 1 k3 mol-’ for the 0 f Cl, reaction [7]. The intensity of reactive scattering is insufficient for the determination of a contour map. The laboratory angular distribution and two velocity scans (one near the peak laboratory intensity and the other along the direction of the centroid vector) were measured for the OCl product_ Kinematic analysis of these rest&s using the stochastic 20
o-
36
so-
64
120’
lso-
180'
CM ANGLE, 0.
\\ I
I
0
10
20
Etot -.
-_ -- -- -- __ 1 I 30
TRANSLATIONAL
40 ENERGY.
50 E’
60 CkJ mOl-‘~_
Fig. 1_ Centre of mag angular distribution T(0) and product translational energy distributions P(E’) at 0 = 0” (solid curve) and 0 = 90” (broken curve) for the 0 +-Cl2 reaction at E =30.4 kJ mol-‘. The CM3product is detected and 8 = 0’ denotes the incident 0 atom direction.
method 163 again required that the product velocity distribution vary with scattering angle in centre-ofmass coordinates. The same form of dependence was adopted for the analysis of the neon seeded beam case as that determined for the helium seeded beam data. Fig. 2 shows the angular function T(0) obtained for 0 + C1, with a neon seeded oxygen atom beam and the translational energy disttibutions P(E’) appropriate to forward scattering (0 3 0”) and to wide angle
Volume 55, number 1
1,
0’
CHEMICAL. PHYSICS LETTERS
II
30’
II
60’
0,
90’
I1
120’
t1
150’
180’
CM ANGLE, 9.
TRANSLATIONAL
ENERGY, E’CkJ
mot-‘3.
Fig. 2. Centre of mass angular distribution T (0) and product translational energy disttiutions P(E')ar8= 0" (solid curve) and B = 90” (broken curve) far the 0 +-Cl? reaction at E = 13.0 kJ mol+_
scattering @I 2 9G”) of the OCI product.
It is apparent that the backward peak has now increased to 0.55 F 0.1 relative to the forward peak in the angular func-
tion T(0) for the reaction with a neon seeded oxygen atom beam. This increase in backward peak height with decreasing reactant translational energy indicates [S] a concomitant increase in the lifetime of the OCI, collision complex. The oscuIating complex model [S] estimates a mean lifetime comparable to the rotational period for the reaction of neon seeded oxygen atoms
1 April 1978
where E = 13.0 kJ mol-I. Hot&ever the lifetime decreases to approximately half of the rotational period of the complex for the reaction of helium seeded oxygen atoms with higher reactant translational energy E = 30.4 kJ mol-l. The product translational energy distributions, shown in figs. 1 and 2 for the 0 + Cl, reaction, indicate that the translational energy is higher for OCl product scattered in the forward direction (solid curves) than for that scattered at wide angles (broken curves)_ The average fraction of the total available energy disposed into product translationai energy is in the range fiv = 0.2 --O-3 where fiv = E&/E,,,. The energies at the peak Eik 3nd the average energies 15:~ of the mean product translational energy distributions determined by the Siska numerical inversion method [S] are listed in table 1, together with the reactant translational energies E, reaction exoergicities 4Do and total available energies for reaction products I!?,,. Reactive scattering from the 0 t CSz reaction with a neon seeded oxygen atom beam is of rather lower intensity (~30%) than that from a helium seeded oxygen atom beam but is still adequate to measure velocity distributions of OS reactive scattering over the full accessible range of laboratory scattering angles_ The laboratory data was transformed using the direct numerical inversion procedure of Siska [S] to obtain a contour map of &hedifferential reaction cross section which can be represented by the angular function T(0) and product translational energy distribution P(E' j shown in fig. 3. The angular function T(B) peaks strongly in the forward direction and is typical of a stripping reaction. Indeed, it is identical, to within experimentai uncertainty, to the angular function obtained [ I] for the 0 + CS2 reaction using a helium seeded oxygen atom beam. The product translational energy distribution is also identical to that obtained Table L Reaction energetics; reactant translational enero_yE; peak of the product translational energy distribution E&avenge product translational energy E& reaction esoergmry ADO; total product energy Et,,; units kJ mol-’
E Cl2 Cl2 cs2
30.4 13.0 13.2
E;k
4.0 5.4 10.4
E SW
ADo
Etor
15.0 7.0 30.8
25.5 25.5 86.5
55.9 38.5 99.7
21
Volume
55, number
0
20
CHEMICAL PHYSICS LETTERS
1
40
60
TRANSLATIONAL
80
100
ENERGY, E’ (kJ ml-‘).
Fig. 3. Centre of mass angular distribution T(9) and product translational energy distribution P(E’) for the 0 +CSz rcaction at E = 13.2 W mol-I. The OS product is detected and 0 = 0” denotes the incident 0 atom direction.
with a helium seeded oxygen atom beam whefi both are expressed in the form P( f ‘) where f ‘= E’/Etot. The 0 + CS, reaction disposes a similar fraction f ‘= 0.3 1 of the total available energy into product trans-
lation to that for the 0 + Cl2 forward scattering.
3. Discussion The present results confirm the conclusion drawn 22
1 April 1978
in our previous paper [l] , that the 0 + CJ2 reaction proceeds via a short-lived collision complex [S]. The mean lifetime of the collision complex is comparable to its rotational period when the total energy available to products Etot = 38.5 kJ mol-l but is only half of its rotational period when Etot = 55.9 kJ mol-I_ Thus the reaction potential energy surface must involve a hollow corresponding to the collinear O-Cl-Cl configuration, which has been discussed [9,10] for the 0 f Br2, I2 reactions. However, the hollow for 0 + Cl, must be shallower with respect to products than those for 0 + Br,, I, since these reactions proceed via longlived collision complexes with lifetimes longer than their rotational periods for Etot = 54 kJ mol-I and 76 kJ mol-l The osculating complex model [g] for reactions proceeding via a short-lived complex assumes that all reactive collisions involve many vibrational periods of the complex and that hence the differential reaction cross section is separable into an angular function T(0) and a velocity function U(U) = uP(E’). However this is clearly inappropriate to the 0 + Cl2 reaction, where our kinematic analysis indicates that the differential reaction cross section is not separable even when the mean collision lifetime is comparable to the rotational period of the complex. This indicates that not all reactive collisions for the 0 + Cl, reaction sample the O-Cl-Cl hollow on the reaction potential energy surface to the same extent. Scattering in the forward direction, which corresponds to the shortest collision lifetimes, disposes rather more energy into product translation than scattering at wide angles which corresponds to longer collision lifetimes. Thus scattering in the forward direction may be considered a stripping component of the reactive scattering which arises from collisions at the largest impact parameters. Sampiing of the potential energy well at small O-Cl-Cl internudear distances would be restricted [ 1 I ] by the higher orbital angular momentum of collisions at the largest impact parameters and this would result in shorter collision lifetimes_ The sharply forward and backward peaked angular distribution observed for 0 + Cl1 at an initial translational energy E = 13 kJ mol-‘, indicates that the maximum collision orbital angular momentum remains appreciable L, =40 h even for reaction in the threshold region. The 0 f CS2 reaction exhibits stripping dynamics which are independent of initial translational energy
Volume_SS,number f
1 April 1978
CHEMICALPHYSICSLETTERS
over the range E = 13 -38 kJ mol-l_ fn part the difference in reaction dynamics compared with the 0 + Cl2 reaction arises from the greater exoergicity of the 0 + CS2 reaction_ However, the precise invariance of 0 + CS2 angular distribution with initial translational energy indicates the absence of any hollow on the 0 +CS, potential energy surface in contrast to that for the 0 + Cl2 reaction. The study of the 0 + Cl2 and 0 + CS2 reactions using both helium seeded and neon seeded oxygen atom beams ihustrates the increased insight into reaction dynamics which can be gained by measuring reactive scattering as a function of reactant translational energy. In future experiments, the use of helium/neon mixture as a carrier gas should provide a means of varying the translational energy of the oxygen atom beam continuously_
References [ 11 P.A. Cony, C.V. Nowikowand R. Grice, Chem. Phys. Letters 49 (1977) 116. [2] D.R. MiBer and D.F. Patch, Rev. Sci. Instr. 40 (1969) 1.566. [3] R.A. Larsen, S.K. Neoh and D.R. Herschbach,Rev. Sci. instr.45 (1974) 1.511; A. Liibbert, G. Rotzoll, R. Viard and K. Schii~erl,Rev. Sci. I?str. 46 (1975) 1656; U. norkenhagen,H. Mdthan and J.P. Toennies, J. Chem. Phys. 63 (1975) 3173; J.J. Valentini,b1.J. Co_ggiola and Y-T. Lee, Rev. Sci Instr- 48 (1977)
181 Acknowledgement Support of this work by the Science Research Council is gratefully acknowledged_
58.
[4] V.L. .Iirschy and J.P. AIdridge, Rev. Sci. Instr. 42 (1971) 381. rql P-E. S&a. J. Chem. Phvs. 59 (1973) 6052. [L6iE.A. Entemann, Ph.D. Thesis.Har&d University (1967). 171 M.A.A. Clyne md J.A. Coxon, Trans. Faraday Sot. 62
191 [JOI [Ill
(1966) 2175; J-N. Bradley, D.A. Whytock and T-A. Zaleski, J. Sot. Faraday Trans. I69 (1973) 1251. G.A. Fisk, J.D. McDonald and D.R. Herschbach, sions Faraday Sot. 44 (1967) 228. D-D_ Parrish and D-R. Herschbach, J. Am. Chem. (197;) 6133. DSt_A_G_ Radlein, J-C. Whitehead and R. Grice, Phys. 29 (1975) 1813. D.R. Rcrschbach, Faraday Discussions 55 (1973)
Chem. DiscusSot_ 95 hloi_ 113.
23