chemical physics of discharges - Argonne National Laboratory

33
velocity of the reacting partn rs is sufficie tl y great. Giese and Maier (60) have
f shown that for the reaction Ar + CO 4 Ar + C + 0, which is endothermic by 6.62
+
and 6.44 e\’ for the 2p state of Ar , the threshold for the
“co
Their measured values were in agreement with this relation.
(60) C. F. Giese and W. B. Plaier, J. X m . a. 39, 197 (1963)
(61)
(62)
(63)
React ion
1
112
Table 11
Some Second-Order Rate Constants
10 10 k, cc/molecule sec Reference
D2 -3 D3 + D 14.5 4,11
+
CH4 + CH4 + CH
5+ + m3
10 61,62,63
H20 + H20 --f H30 + OH 12.7 1,2,13
+ +
C H + C H + C H +CH4 14.5 9
2 6 3+5
2+3
+ C H + ~ CH o + n 0.126 22
O2 + 33
CH4 + O2 + CH30 + OH
0.257 22
V. L. Tal’roze and E. L. Frankevich, J. Phys. Chem. USSR 34, 1275 (1960)
C. W. Hand and H. von Weyssenhoff, Can. J. Chem., 42, 195, 2385 (1964)
J. L. Franklin, Y.
2353 (1966)
Wada, P. Natalis and P. M. Hierr J. Phys. Chem. 70, -
All reaction rate studies have not been limited to extremely low pressures.
When the pressure in the ion source becomes sufficiently great the reactions follow
the psuedo first order rate laws over rather wide ranges of pressure, unless subse-
quent reactions interfere. Thus, Field et al. (51) found the disappearance of CH4
in methane to obey first law kinetics over a pressure range of about 0.1 to 400 Torr.
However, as the pressure is raised, in certain systems, reactions of a higher order
do occur. Field, (37) studying ethylene, has reported some reactions having apparent
orders as high as about 6. These, of course, are not truly 6th order reactions, but
simply represent a succession of perhaps five secondary reactions which show dependence
upon the 6th power of the pressure. Rates of reactions of such high order do
not yield satisfactory rate constants, but it has been possible to determine rate
constants for reactions of 3rd order, and Field and several others have made approximate
measurements of the rate constants for such third order processes. As is the
case with the second order processes, these reaction rates are relatively high. For
example, in ethylape eld deter ined several third order rate constants to be
F3 2 -1
in the order of molecule- sec .
Although most of the measurements of rate constants in the literature were
obtained with the source operating in a continuous mode employing a variation in
pressure to establish the rate, some studies have been made in which retention time
in the source was varied.
Such measurements were originally made by Tal’roze and
Frankevitch (61), but subsequent measurements have been made by Hand and von Weissenhoff
(62) and Franklin, Natalis, Wada, and Hierl (63). In order to obtain a satisfactory
variation of time, a pulsed mode is employed. In such an operation a pulse
of electrons is fired through the gas. After it is stopped, the resulting ions
can be retained in the source for a controlled period of time and then rapidly
extracted by a pulse of high energy. By varying the delay time, the time of retention
in the source is varied, and rate constants determined in the manner more usually
employed by chemists in rate studies. Results obtained in this way generally agree
-
I
+