Volume
74, number
OBSERVED
CHEMICAL
3
TRANSLATIONAL
PHYSICS
15 September
LETI’ERS
1980
ENERGY DEPENDENCE OF THE K + C2H5 i -+ Kl + C2 A,
REACTION CROSS SECTIOhJ FROM 0.17 TO 0.55 eV (c.m.) * F.J.L.
AOIZ, V.J. HEXRERO
* and A. GONZALEZ
Departartrerrto de Qmtnlca Fmca, Umrerx~dad Complutense. Received
UREti’A Madrid 3. Spain
25 July 1980
Relatrve values of the total reactron cross section cR for the crossed molecular beam reactron K + C2H51 -* KI + CaHs have been measured over the translational energy (ET) range 0 17-0.55 eV It IS found that OR decreases monotonically rrlth ET over thts range, any maximum tn oR(.!?T) is presumed to tie below 0 17 eV.
The translational energy dependence of the molecular beam reactron cross sectron [ l] has been a subJect of increasmg Interest [2] smce the K + CH,I + KI -t CH, study by Cersh and Bernstem [3] which reveaied a sharp maulmum in oR at 0. I8 eV followed by a monotonic declme out to I eV. The present communtcation reports relative values * The Work recerved financral support from the Commtsnon Asesora. and the Juan March Foundation of Spain. * Fellow of Juan March Foundatron. bfadnd. Spatn. 1979-80
I
of the total reaction cross section for the K + C,H,I + Kl + C2H5 system measured as a function of ET, the mean translational energy in the center-of-mass (cm.) system, usmg our crossed molecular beam machine [3]. Our measurements have been carried out usmg a K thermal effusive (603 K) beam and a C2HSI supersonic He seeded, beam (nozzle temperature 375 K). Over the collisron energy range studied (0.17-0.55 ev), oR decreases with energy and any maxImum m the reaction cross section must be presumed to he below ET = 0.17 eV.
I
I
I
I
I
I
I
I
I
02
03
OL
OS
06
E, (eV1
Frg. 1. OR(&+ for the 6 + CaHsI arbrtrary umts.
398
-c KI + CaHs
reaction
The solid line IS a best-lit
of Eu’s equation
(see text).
Ordinate
scale
Volume 74, number 3
15
CHEMICAL PHYSICS LETTERS
The reactively scattered KI was detected by means of a conventional surface ionization (3) detector mounted on a synchronous motor mount which allowed measurement of the product scattered in different laboratory angles. A network circuit, coupled to the synchronous motor, allows us to program automatrcally the detector movement by setting (externally) its stepping rate, making it possrble to produce “continuous scans” of the scattering flux over the laboratory angle range as in refs [S-7]. The K and C2H51 densities n, and n2 were monitored with a Re SI detector and a quadrupole mass spectrometer (with an electron-bombardment ionizer), respectively. The C,H,I beam velocity was measured usmg the time of fhght technique. Complete laboratory differential reaction cross secnon measurements were carried out at twelve different velocities, then integrated and normalized to yield the total flux, FKI, at each velocity- The reactron cross section was calculated [2] vraok = FKr/nl n2u,A V , where ur 1s the average relative velocity and AV 1s the beam intersection volume (futed for all the experiments). Frg. 1 shows the result of the present study. The solid curve drawn through the present results is a modified Eu equatron [8] wrth n = 2.9; i.e., a different value than found for the analogous K + CH, I post-maxrmum cross sectron, where n was found to have a value of 2.5. Since the only previous estrmation of the cross sec-
September 1980
tion for the present reaction is that of Kwei et al- [9] at ET = 0.06 eV, work is now in progress in our Iabora-
tory to estimate the low energy reaction cross iection. A fuU account of the present work is in preparation, including the differential cross sections over the collision energy range studied_
References [I 1 R-B. Bernstein, Advan. At. MoL Phys. 15 (1979) 167. [2] A. Gonzilez Ureiia. V-J. Herrero and F.J. Aok, Chem. Phys.
44
< 1979)
81:
[31 M-E. Gersh and RB. Bernstein, J. Chem. Phys. 55 (1971) 4661; 56 (1972) 6131. 141 A. Gonkilez Ureiia, F.J. AOIZ. V.J. Herrero, F_L_ Tab& and V. Saez, 75 Aniversario de la R5E.F.Q. 27 (1978) 57;
is1 161 171 PI
191
FJ. Aoiz, VJ. Herrero and k Gor&kz Ureiia, to be published. S.A. Pace, H.F. Pang and RB. Bernstein, J. Chem. Phys_ 66 (1977) 3635. K.T. Wu, H.F. Pang and RS. B-stein, J. Chere. Phys. 68 (1978) 1064. H-F. Pang, K-T. Wu and R-B. Bernstein. J. Chem. Pays_ 69 (1978) 5267. B.C. Eu, J. Chem. Phys. 60 (1974) 1178; Chem. Phys. 5 (1974) 95; H.E. Litvak, A. Gonz5ler Ureiia and RX Bernstein, I_ Chem. Phyr 61(1974) 4091. G-H. Kwei, J-A. Norris and D.R. Herschbach. I. Cbem_ PhYs. 52 (1970) 1317.
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