J . Phys. Chem. 1984, 88, 2339-2343 other forms of nitrogen oxides which may inhibit the further oxidation or be inert and not affect the continued oxidation. It seems apparent that NO, has relatively little effect upon the aqueous oxidation of SOz. With the major constituent in a flue gas being the relatively insoluble NO, the effect upon the aqueous-phase oxidation of S(1V) is minimized. Even if the NOz were the major constituent, the effect would not be given unless NOz
2339
were in concentrations comparable to or greater than that of SOz. Nitrogen dioxide would react directly with the S(IV) species; however, once the NOz were depleted, no catalytic effect would be noted. Acknowledgment. The financial support of the National Science Foundation is gratefully acknowledged.
-
The K 4- C,H,Br BrK 4- C2H, Reaction up to 0.9 eV of Collision Energy. Maximum in the Excitation Function and Comparison with Related Systems V. J. Herrero, V. Saez Rabanos, and A. Gonziilez Ureiia* Departamento de Quimica Fhica, Facultad de Quimica, Universidad Complutense de Madrid, Madrid- 3, Spain (Received: June 23, 1983)
-
An extension of the collision energy dependence of the K + CzHSBr BrK + CzH5reaction cross section studied in our previous work is presented. Excitation function measurements are reported until 0.9 eV of collision energy. The existence of a weak maximum in uR(ET)at about 0.35 eV is confirmed from where the reaction cross section slightly decreases until 0.9 eV. A simple dynamical model, based on angular momentum conservation,was used to account for this excitation function shape. An analysis of the differential cross section is made via a simple treatment based on the (Franck-Condon type) requirements for the dissociative capture of an electron in the assumed electron-jump process, M + RX M+ + RX-,which provides a general frame for the understanding of the products' translational energy and angular distributions.
-
1. Introduction
-
In a former work' the collision energy dependence of the K + C2HSBr KBr C2Hsreaction cross section was studied in the energy range between 0.10 and 0.50 eV. Special attention was paid to the determination of the threshold energy for reaction which was found to be Eo = 0.12 eV. The excitation function showed a sharp increase after that threshold, and then it leveled off between 0.2 and 0.5 eV. The existence of a possible maximum between 0.3 and 0.5 eV was also suggested. In the present work we have extended the excitation function measurements until 0.9 eV of collision energy and we have confirmed the existence of a weak maximum which seems to lie at about 0.35 eV. After that maximum the excitation function goes slowly down in contrast with the abrupt post maximum decline observed for the alkali metal + alkyl iodide reaction^.^-^ Section 2 reports the scattering measurements including experimental conditions. An analysis of the data is presented in section 3. We have also extended the differential reaction cross-section study carried out in previous work'" by performing a LAB to CM inversion of the new data (section 3.1). The present investigation confirms the conclusions concerning the products' (CM) angular and velocity distributions and extends their validity up to 0.9 eV. In section 3.2 we present a DIPR-DIP analysis7-13 of the dif-
+
(1) Herrero, V. J.; Siez Ribanos, V.; GonzAlez Urefia, A. Mol. Phys. 1982, 47, 725. See also: Herrero, V. J. Ph.D. Thesis, Universidad Complutense de Madrid, Madrid, 1982. (2) Litvak, H. E.; Gonzllez Ureiia, A,; Bernstein, R. B J . Chem. Phys. 1974, 61, 738. Ibid. 1974, 61, 4091. (3) Pace, S. A.; Pang, H. F.; Bernstein, R. B. J . Chem. Phys. 1974, 66, 3635. (4) Gersh, M. E.; Bernstein, R. B. J. Chem. Phys. 1971, 55,4601. Ibid. 1972, 56, 6 13 1. ( 5 ) Aoiz, F. J.; Herrero, V. J.; Gonzilez Urefia, A. Chem. Phys. 1981, 59, 61. Aoiz, F. J.; Herrero, V. J.; Gonzilez Ureiia, A. Chem. Phys. Lett. 1980, 74, 398. See also: Aoiz, F. J. Ph.D. Thesis, Universdad Complutense de Madrid, Madrid, 1980. (6) Herrero, V J.; Tabarbs, F. L.; Siez Rabanos, V.; Aoiz, F. J.; Gonzllez Ureiia, A. Mol. Phys. 1981, 44, 1239. (7) Herschbach, D. R. Faraday Discuss. Chem. SOC.1973, 55, 233. (8) Truhlar, D. G.; Dixon, D. A. In "Atom-Molecule Collision Theory"; Bernstein, R. B., Ed.; Plenum Press: New York, 1979; Chapter 18 and references cited therein.
0022-3654/84/2088-2339$01.50/0
TABLE I PH2I
torr
30 56 87 110 129 146 128 150
PC2H50rI
torr
T"0ZZd
OC
29 30 33 33 33 33 29 30
Tk/K
598 598 598 598 598 598 598 598
114 114 114 114 114 114 114 114
1o - v, ~/ cm s-I 12.00 17.30 20.00 21.65 22.75 23.31 23.50 24.00
ET/
eV 0.27 0.51 0.64 0.75 0.82 0.86 0.87 0.90
ferential cross section for the present reaction; we shall show that the main features of the molecular reaction dynamics can be derived from the repulsive nature of the RX-interaction originated M+ + RX- assumed to be the in the electron jump M RX first step of the reactions of this family. These repulsive interactions determine both the shape and the collision energy dependence of the products' angular distribution function. By using the DIPR model, we shall establish a selectivity in the (reactive) angle of attack. Some energy requirements for the proposed electron attachment step are also discussed. Section 3.3 reports the application of a simple dynamical model to the present excitation function. In section 3.4 the comparison of the present and related excitation function shapes initiated in ref 1 is extended. We confirm here the differences between the alkyl iodide + alkali metal reactions and the corresponding bromide ones and extend the comments on the relationship between chemical reactions with alkali metals and dissociative electron attachment by R X molecules. Finally, section 4 is dedicated to
+
-
(9) Kuntz, P. J. Trans. Faraday SOC.1970, 66, 2980. Kuntz, P. J. Mol. Phys. 1972, 23, 1035. (10) Marron, M. T. J . Chem. Phys. 1973, 58, 153. (1 1) Harris, R. Ph.D. Thesis, Harvard University, Cambridge, MA, 1970. (12) Weinstein, N. D. Ph.D. Thesis, Harvard University, Cambridge, MA, 1970. (13) Hsu, D. S . Y. Ph.D. Thesis, Harvard University, Cambridge, MA, 1974. See also: Verdasco, E. Tesis de Licenciatura, Universidad Complutense de Madrid, Madrid, 1982.
0 1984 American Chemical Society
2340 The Journal of Physical Chemistry, Vol. 88, No. I I, I984
Herrero et al.
1
05 VI
a
f
c
2.
x
3
1
U
d
c
e 05
P,(8) = exp(-ln 2 I(8 - 6’*)/pI2)
(1b)
Pr(r) = expi-ln 2 I(r - r*)/aI2)
(IC)
r is defined by r = W ’ / W ’ ~ where ~ ~ , w’is the relative velocity of the KBr relative to the centroid and w’,,, the maximum value of w ’allowed by energetics; 8 is the C M angle of scattering of the KBr product with respect to the relative velocity vector (see section 2.2 of ref 6 for more details). In our previous work, we had performed the laboratory to center of mass inversion for reactants’ collision energies up to 0.5 eV. Here we have extended the same analysis to the rest of the collision energy values; the present calculations showed that the set of parameters for P,(8) and P,(r) already used in our previous (i.e., r* = 0.75, 8* = 180) could equally well fit the present LAB angular distributions corresponding to the higher energy range here reported. Typical results are shown in Figure 1, where laboratory and computed (dashed lines) KBr angular distributions are compared. The conclusions drawn in ref 6 for the C2H5Br+ K KBr C2H5 differential reaction cross section are confirmed here; i.e., over the whole range of collision energy studied (0.1-0.9 eV) the KBr is backward peaked and most of the products’ energy goes into translation, which is characteristic for a direct and rebound (repulsive) mechanism. 3.2. DIPR-DIP Model Analysis. In order to perform this analysis, we have used the M XR XM + R reaction scheme (where M is the alkali metal, X is the halogen, and R is the alkyl group). In this model the differential cross section per steradian is given by8
-
+
0 L A B I DEG
Figure 1. Comparison of calculated (dashed line) and measured (solid line) laboratory total BrK angular distributions. For simplicity only four cases are shown scaled to unity at the peaks.
+
-
summarizing the main conclusions.
2. Experimental Section
where 8 is the center of mass scattering angle of MX relative to The molecular-beam machine used for the present experiments the initial direction of M, P({) is the differential cross section per has been extensively described e l ~ e w h e r eby ; ~means ~ ~ ~ ~of it, we unit Eulerian angle J’, and P(R)is the probability function for have carried out measurements of KBr formed in the collisions total repulsive energy release R . The relationship among 8, R , of K atoms with C2HSBr molecules. The experimental setup and { is given by8 consists of an effusive K beam intersected at 90° by a supersonic 6({,R) = arctan lsin {/@ cos {)I (3) beam of C2H5Brseeded in H2 The K beam and the KBr produced in the reactive collisions were detected by means of a Re surface where ionization detector. As in previous works, the angular range studied was -20-120O with respect to the K-beam direction (for P = (ETMMMR/RM~MMX)‘/~ (4) a more detailed description on the geometry of the experimental The distribution function P ( R ) can be obtained either from system see ref 5). The velocity distribution of the K beam was experimental velocity measurements or from simple theoretical assumed to be Maxwellian. The C2H5Brbeams’ relative densities models. and velocity distributions were measured by means of an electron bombardment mass spectrometer, as in previous ~ o r k . ’For ~ ~ ~ ~ In the present treatment we have used the same method applied in the previous work (see ref 14) to calculate the P(R) distribution the determination of the C2H5Brvelocity distributions, we have function and the products’ average translational energy E$ as a used the time of flight (TOF) technique. function of the collision energy ET. The method is based on the Relevant experimental conditions are given in Table I; the rest “quasi-diatomic reflection approximation” for the electron jump of them are the same as those of ref 1. from the (bond) RX to the (repulsive) RX- potential involved in this type of reaction. The repulsive energy distribution is given 3. Results and Discussion by P ( R ) $2(r),where +(r) is the vibrational wave function of 3.1 Reactive Scattering Data and Products’ CM Distributions. the RX bond. The products’ average translational energy can be The KBr laboratory angular distributions for the collision energies calculated from14 of the present study were obtained by the method used in ref 1, 5, and 6 (Le., by extrapolation of the nonreactive tail and substraction of the nonreactive signal from the total signal). Some E’, = of these experimental (net KBr flux) angular distributions are displayed in Figure 1. Since the total (reactive and nonreactive) rnin signal has been smoothed out by a solid line (similar procedure was used in previous work) drawn through the points, the net where qo(r)represents the ground-state wave function for the RX reactive KBr angular distributions are represented by solid lines vibration in the haromic oscillator approximation; Le., qo(r) = in Figure 1. exp(-a(r - ro)2)(used in the present work because the In order to get the (center of mass) angular and recoil velocity temperature of the RX beam source was kept low). The intedistributions of the products, we have applied the same method gration limits are given by the following conditions (see the as in ref 6 . The detailed differential reaction cross section was corresponding figure in ref 14): approximated by the uncoupled angular and velocity distributions, I/onic(r.max)= vcovaient(rmax) i.e. (6) Vonic(rmin)- vcovalent(rmin) = E* I C M ( ~ ? Pdo) ~ ) PAr) (14
+
-
J
-
The functional shapes of Pe(8)and Pr(r)were taken to be the same as in ref 1 and 6:
(14) Saez RBbanos, V.; Verdasco, E.; Segura, A.; Gonzllez Ureiia, A. Mol. Phys. 1983,50, 825.
K
+ C,H,Br
-
+ CZH5 Reaction
BrK I
I
The Journal of Physical Chemistry, Vof. 88, No. 11, 1984 2341 I
I
I
-
I2
I
-
I
I
I
I
‘
I
I
T
ICzH5 + K - l K + C z H s 0.6
rl
i
1 L
.
11 -c
x 5 VI
C
2C
/
.8-
-.
6-
-1
/.7
-
L-
/” 0.L
-
/’
t-
i
0
0 2 00
02
01
0 6
10
08
E, / e v Figure 2. Experimental and model prediction average product translational energy E!, plotted vs. average translational energy of reactants ET for the indicated reaction: open symbols, present experimental results obtained from the P ( r ) distribution (see text); solid (dashed line), calculated from eq 5 with E* = ET(ET + Q,,,), The K + C2H51 KI + C2H5data are from ref 6 . The experimental points shown cover the whole range of energy studied; for clarity we have not plotted all the existing data.
-
yon,c(r)and VclValent(i) correspond to the potential energies of RX- and RX as a function of the R-X distance r; we have used for them W e n t w ~ r t h and ’ ~ Morse functionalities, respectively (see ref 1 and 14 for numcerical values of the parameters). E* is the energy used in the electron jump which is bounded by the conditions ET < E*
< ET+ Qmax
(7)
em,,
where ET is the collision energy and the maximum exoergicity. (The ET + Q, bound represents an upper limit determined by the conservation of total energy; we can expect a priori the E* value to be much closer to ET than to ET + Q,,,, since the electron jump is believed to be the first step of the reactive collision and it must be thus a characteristic of the entrance channel.) The experimental E$ values were calculated from6
where P,(r) was defined in eq IC. Figure 2 shows the “experimental” data vs. model prediction for the E ; dependence on ET in the present K C2H5Br BrK C2H5 and analogous K C,H,I KI C,H, reactions. The model calculations were performed by using eq 5; we have represented the results corresponding to the two E* limits just mentioned. As expected, the best fit corresponds to E* = ET. Wtih this E* value, both the slope (dE’r/dET) and intercept of the E $ vs. ET function are satisfactory. In the other limit (E* = ET Qmax) the calculated E’, values are significantly greater than the experimental ones. This result confirms the a priori expectation that the electron jump occurs before the R group separates from the halide X- and supports the well-known fact that most of the energy available to the products appears as translation via the R Xrepulsion.
+
+
-
+
+
-
2-
-
+
-
20
LO
-
60
I
I
I
I
I
00
100
120
1L0
160
e
I DEG
-
Figure 3. (top) Experimental vs. computed angular distributions for the K + C2HsBr BrK + C2HSand K + C2HSI KI + C2H5 reactions: solid line, CM angular distribution P ( 0 ) of KI from ref 6 ; dashed line, CM angular distribution obtained by the present DIPR-DIP model (see text and ref 14) with a = 0.8 and E* = ET = 4.85 kcal/mol. The top and bottom dashed-pointlines are the same as before, but with a = 0.7 and 1.0, respectively. (bottom) Same presentation but for BrK from the present study reaction: solid line, from ref 1 and 6 ; dashed line, from the present model calculation (see text and ref 14) with a = 2.0; dashed-point line, same as before, but with a = 1.0. Both calculations refer to E* = ET = 12 kcal/mol. All distributions are scaled to unity at the peak.
Figure 3 shows the “experimental” vs. computed angular distributions for the above two systems. The experimental distributions are from ref 6, and the theoretical ones were calculated by using eq 2. The P({) distributions were obtained by the method described in ref 8, 9, and 14. The Eulerian angle is related to the “attack angle” 7 (Le,, to the angle between Rc,the distance from M to the center of mass of RX, and the internuclear axis of RX) via (R,Z - b2)’/2
RC
+ (sin {)-b
R,
cos 4
1
where 4 is the corresponding Eulerian angle and b the collision impact parameter. For the attack angle (7) distribution we have used the same functionality as in ref 14: P(7) = e-a’Jzl.A “steric factor” with pronounced P(7) distribution, peaking at 7 = 0, seems to be necessary to reproduce the angular differential cross section. An inspection of Figures 2 and 3 indicates that the present DIPR model can satisfactorily explain both the experimental products’ translational energy and CM angular distributions. The influence of the a parameter in Z(0) can be seen in Figure 3. Note how the functionality seems to be sufficiently flexible to represent the differential reaction cross-section dependence upon the attack angle. 3.3. Excitation Function. In order to get the new reaction cross-section values, we have used the same procedure as in our previous work,’ and for simplicity we omit here its description. The aR(Er)values for the studied reaction are shown in Figure 4. The open circles symbolize the points of the present work; the new data confirm the existence of a maximum in the excitation function at about 0.35 eV. After that maximum the cross section decreases gently in the energy range measured. The solid line
2342 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
Herrero et al.
3
10
I
1
c-I
I C
O F
,‘ii‘86 It
1 I 02
I 06
OL
I
I
08
10
VI
-
c
C
E,/eV
+
2
+
%
Figure 4. U R ( E for ~ ) the K C2HSBr KBr C2H5reaction: open squares, from ref 1; open circles, present results; solid line, calculated excitation function from the present dynamic model (eq 9 of the text) with ycrr= 2.4; dashed lines, model calculationsfor different yerfvalues
n
as shown.
P
-+
bond
5
c
. ;;j
10
is the cross section calculated by using a simple hard-spheres model based on angular momentum con~ervation’~ in which the total reaction cross section is given by 0.5
where uLc is the “line of centers” cross section. In the original model15y was defined as the quotient between the reactants’ and In order to apply the model to products’ reduced masses (PIN‘). our present reaction, we have taken y as free parameter. This is equivalent to using an effective y defined by
P
” max
where bma? and b‘,,: denote respectively the reactants’ and products’ maximum impact parameter. In the work cited above b’,,, and b,,, were assumed to be the same. In the present application of the model we allow bmaxand b’,,, to take different values. The model calculation reproduces satisfactorily the experimental behavior for yeff= 2.4. For a better illustration of the model, sensitivity calculations with different yervalues (2.0 and 2.6) are also shown (dashed lines) in Figure 4. This model reproduces the shape of the excitation function and especially its maximum by taking into account the restrictions imposed by angular momentum conservation on the number of reactive trajectories; e.g., by introducing a recrossing mechanism of some sort back to the reactants valley, they can also reproduce the shape of the excitation function. At the present moment there exists no general treatment that can account for the translational features reflected in the excitation functions. A systematic study based on reliable potential energy surfaces for these systems is still required. 3.4. Comparison with Related Systems and Electron Attachment Data. In our previous paper’ we analyzed the experimentally obtained energy threshold for the K C2H5Br KBr C2H5 reaction in terms of a model based on an electron jump;I6 we indicated also the existence of certain similarities between the excitation functions for reaction and those for dissociative electron attachment by these molecules. In the present work we want to extend the discussion on the possible close relationship between reactive and electron-capture processes. Models based on an electron transfer from the alkali atom to the RX molecule as first step of the reaction have been widely
+
+
-
(15) Gonzllez Urefta, A.; Aoiz, F. J. Chem. Phys. Lett. 1977, 51, (16) Wu, K. T.J . Phys. Chem. 1979,83, 1043.
281.
ET lev Figure 5. Experimental excitation functions for reactive and dissociative
-
-
electron attachment processes: (top) The circles correspond to CH,I + K KI + CH,, the triangles to C2HSI+ K KI + C2H$ the inverted triangles to CH31+ Rb RbI + CH3,2*3 and the squares to CH31+ Xe(’P,,J XeI* + CH3.24The solid curve corresponds to the I- ion current vs. electron collision energy reported in the work of Stockdale et a1.I8 for the process CH31+ eI- + CH3 (they give a maximum at 0.15 eV and a fwhm of 0.30 eV). In the case of the CH31+ Rb RbI + CH3reaction we have plotted uRvalues for ET lower than 1.0 eV. It was that uR increases with ET for ET > 0.9 eV. (bottom) The squares and triangles refer to CH,Br + Xe()P2,J KeBr* + CH?l with 330 and 490 K nozzle-beam (CH,Br) temperatures, respectively. The circles are the results for the C2HSBr+ K KBr + C2H5 reaction (the filled circles represent the experimental measurements of the present work). Both curves correspond to dissociative electron capture; the solid one shows the results for the process CH,Br + eBr- + CH, (values reported by Stockdale et al.:’* maximum at 0.35 eV, fwhm of 0.40 eV), the dashed one is the excitation function of Christophorou et al.” for C2HSBr+ e- Br- + C2H5, In both parts the curves have been scaled to one at the maximum; the points have been scaled for best to the solid curves.
-
-
-
-
-
-
-
-
used to explain some of the experimentally observed features of the M RX family of reactions, particularly the products’ energy and angular distributions at thermal collision e n e r g i e ~ . ~In ,~~~ section 3.2 of this paper we have applied a similar treatment to the data of the present reaction in the 0.1-0.9-eV collision energy range, taking Wentworth’s empirical function20a for the VRxpotential energy curve. From an experimental point of view, the measurement of the dependence of the cross sections for dissociative electron attachment with electron energy17J8ought to allow the determination of the VRx-(r)curve shape, since the electron jump to the RX-
+
(17) Christophorou, L. G.; Carter, J. G.; Collins, P. M.; Christodoulides, A. A. J . Chem. Phys. 1971, 54, 4691.
(18) Stockdale, J. A,; Davies, F.J.; Compton, R. N.; Klots, C . E.J . Chem. Phys. 1974, 60, 4279. (19) (a) Wentworth, W. E.; George, R.; Keith, H. J . Chem. Phys. 1969, 51, 1791. (b) Wentworth, W. E.; Becker, R. S.; Tung, R.J . Phys. Chem. 1967, 71, 1652. (20) (a) Wu,K. T.; Pang, H. F.; Bernstein, R. B. J . Chem. Phys. 1978, 68, 1064. (b) Pang, H. F.; Wu, K. T.;Bernstein, R. B. Ibid. 1978,69,5267.
K
+ CzH5Br
-
BrK
+ C2H5 Reaction
1‘he Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2343
repulsive state must be essentially a Franck-Condon process. If the electron-transfer mechanism as the first step of the R X M reactions extends to collision energies higher than thermal, one can expect the excitation functions for these processes to reflect somehow the influence of the C-X bond Franck-Condon factors on the dynamics, since electron transfer involves an electron capture by the R X molecule. The potentials VRxand VM- can be affected by the presence of the alkali atom M in the neighborhood of the R X molecule; it seems reasonable to compare directly the shapes of the excitation functions for reaction and those of dissociative electron capture; this comparison is performed in Figure 5. The points in Figure 5 correspond to reactive scattering measurements; the curves correspond to dissociative electron-capture experiments. Since the main reactivity differences (at least in the post-threshold range) in the reactions of alkali metals with alkyl halides seem to be determined by the C-X bond, we have grouped separately the C-I bond processes and those involving a C-Br one. An inspection of Figure 5 shows that the excitation functions for the iodide reactions have a shape similar to that of the electron attachment curve of Stockdale et al.18 in the low-energy range. For collision energies greater than 0.4-0.5 eV, the reaction cross sections are either stabilized or even smoothly increased (see ref 23 and 3) while the electron attachment probability decreases sharply. The bromide reactions have not been studied so extensively. In the lower part of Figure 5 the experimental results corresponding to two studied reactions are shown together with the curves representing measurements of electron-capture excitation functions. (There are also data available on the CH3Br + K and CH,Br Rb reactions,20 but they have not been included in the figure because they refer to the MBr product’s “in-plane yield” and not to excitation functions. The reactive behavior, however, seems to be similar to that of the analogous C-Br reactions plotted XeBr CH3 and C2H5Br here.) The CH3Br Xe(,P2,J K KBr C2H5 reactions’ data are represented by symbols; the solid lines represent the experimental measurements of the present work. Both reaction excitation functions are more similar in shape at small collision energies to the electron-capture curves of Stockdale et a1.I8 than to that of Christophorou et al.” Hennessy and Sirnonsz1had shown that the excitation function for the Xe(3P2,0)+ CH3Br reaction does not vary appreciably for collision energies between 0.4 and 0.7 eV, deviating thus from the electron attachment curve of Stockdale et a1.I8 The experimental measurements of this paper show a similar deviation for the CzH5Br+ K KBr CzH5excitation function in the 0.4-0.9-eV collision energy range from the mentioned electron-capture curve. These deviations would imply that the VRx(r)or Vk-(r) potential curves for the RX electron RX- process are signif-
+
+
-+
+
-
+
-
+
+
+
-+
(21) Hennessy, R. J.; Simons, J. P. Mol. Phys. 1981, 44, 1027.
+
icantly affected by the presence of the electron-donating atom (alkali or metastable Xe) in the repulsive zone or else that other factors different from the simple requirements for electron capture by the RX molecule begin to play an important role. In the approximation considered here we have neglected the possible influence of the “bond-forming” process (Le., the “exit channel’! features in the potential energy surface) in the molecular reaction dynamics. This process could be relevant at collision energies where the reactive and electron-capture excitation functions deviate. 4. Concluding Remarks By performing experimental measurements of the relative reaction cross section for the K C2HSBr KBr C2H5reaction in the collision energy range between 0.5 and 0.9 eV, we have completed a former study.’ The new data confirm the existence of a nonpronounced maximum in the excitation function and also confirm the difference between the shapes of the excitation functions for the alkyl iodide alkali metal reaction^^^^ and the corresponding bromide and the analogy between the reactions of alkali metals and those of metastable rare-gas atom^*'^^^ over a wider energy range. The extension of the differential reaction cross-section study to the new collision energies shows that there are not any significant changes in the reaction mechanism (i.e., KBr C M angular distribution backward peaked at 180° relative to the direction of the incoming K) or in the fraction of the total exoergicity that appears as translational energy of the products over the energy range between 0.1 and 0.9 eV. We have carried out a DIPR-DIP model analysis of the differential cross section of the present reaction, which provides a general frame for the understanding of the products’ translational energy and C M distribution. A functionality of the e 4 type seems to be adequate to recover the shape of the experimental angular distributions. The weak maximum in the excitation function which has been reported as the main experimental finding of the present work can be reproduced by using the nR(ET)functionality given by a simple dynamical model based on angular momentum conservation.
+
-
+
+
Acknowledgment. We are indebted to A. Segura and E. Verdasco for their help with the calculations and experiments. This work received financial support from the Comision Asesora of Spain. Registry No. K, 7440-09-7; C2HSBr,74-96-4. (22) Rettner, C. T.; Simons, J. P. Faraday Discuss.Chem. SOC.1979,67, 329. (23) Hennessy, R. J.; Simons, J. P. Chem. Phys. Lett. 1980, 75, 43. (24) Hennessy, R. J.; Ono, Y . ;Simons, J. P. Chem. Phys. Lett. 1980, 74, 47. Hennessy, R. J.; Ono, Y . ; Simons, J. P. Mol. Phys. 1981, 43, 181.
