COMMUNICATIONS TO THE EDITOR
2071
of CO increases the yield t h r e e f ~ l d . ~Such an increased efficiency in the use of the 1849-A radiation suggests a long-lived intermediate such as the lowest Nztriplet. The observation that the 2537-A emission is seen a t lower pressures of CO than of N2 is consistent with the higher quenching cross section exhibited by CO for 6(3P1)H g ? 0.192 A2for N2and 4.07 A2for CO. A more thorough study of pressure effects is underway to try to establish the importance of N2 and CO triplets in the observed emission a t 2537 A and in the enhanced decomposition of ethylene. (5) T . A. Gover and H. G. Bryant, Jr., unpublished results. (6) 35.W. Zemansky, Phys. Rev.,36, 919 (1930). (7) National Science Foundation Predoctoral Fellow.
DEPARTMENT O F CHEMISTRY UNIVERSITY OF VIRGINIA CHARLOTTESVILLE, VIRGINIA
-
‘0
i 0
-5
10’0
I2 W
0 z 0
u z 9
w IO’ 2_
t In
g
T. A. GOVER H. G. BRYANT, JR.’ IO’
RECEIVED APRIL18, 1966 0
Catalyzed Enhancement of Chemi-Ionization in Atomic Nitrogen and Oxygen Mixtures’
Sir: The production of chemi-ions in N-0 afterglows
has been reported.2~~We have now found that chemiion concentrations in such systems are increased by several orders of magnitude when certain additives (C2F4, C2H4, C2H2)are present. As will be seen below, this indicates that appreciable quantities of excited molecules of sufficient energy to produce chemi-ions can result from homogeneous catalysis of atom recombination reactions. The flow apparatus and conditions (T = 300”K, P = 1 to 4 torr) were similar to those of previous N atoms, produced by passing an He-N2 mixture containing 0.2 to 3 vol. % ’ N2 through a microwave discharge, could in part or total be replaced by 0 atoms via the reaction N NO = N2 0 before being mixed with the additive. Pyrex wool plugs were placed upstream and downstream of the NO inlet nozzle for removal of excited species produced in the discharge and titration. The H e N 2 mixture was purified6 by (i) passage over Ti turnings and Zr sponge at = 1150°K and (ii) a liquid N2trap. The Langmuir probe results obtained with C2F4 are shown in Figure 1. It is striking that in the absence and presence of C2F4,the maximum ion concentration occurs a t an 0-atom fraction of about 0.25. Only a t the highest C2F4concentration does this maximum occur a t a slightly higher 0-atom fraction. These ob-
+
+
0.2
0.4
0.6
0.8
1.0
Figure 1. Effect of C2F4 on positive ion concentration. P = 2.0 torr, average gas velocity = 10 m sec-1; initial concentrations: [Helo = 6.3 X [Nz]o= 1.9 X [NIo [O]O= 1.0 X lo1*, all in molecules cc-1. The numbers 25, 2.5, and 0.25 give the ratios { [N]o [0]0)/[CzF410. ~ the Langmuir For this sequence of C Z Fconcentrations, probe was located 2.0, 1.3, and 0.3 cm, respectively, below the CZF4 nozzle, Le., the axial positions giving the maximum ion concentrations. The nozzleprobe distance used for the data obtained in the absence of C2F4 was 1.3 cm.
+
+
servations suggest a t once that the mechanism of the enhanced ionization is similar to that of the ‘‘normal” N-0 ionization in the absence of C2F4. The slight increase in fractional 0-atom content needed to obtain maximum ionization a t the highest C2F4 flow rate can be attributed to increased scavenging of atoms by C2F4 and reaction intermediates. The same fractional 0atom concentration was also observed to give the maximum ion concentration (i) for various reaction (1) This work was sponsored by Project SQUID, which is supported by the U. S. Office of Naval Research, Department of the Navy, under Contract Nonr 3623(00), NR-098-038. (2) C. R. Gate, F. T. Smith, and H. Wise, J . Chem. Phys., 40, 3743 (1964). (3) C. R. Gate, R. A. Young, and R. L. Sharpless, ibid., 39, 1234 (1963). (4) A. Fontijn, W. J. Miller, and J. M. Hogan, “Tenth Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1965,p 545. (5) A. Fontijn, J . Chem. Phys., 44, 1702 (1966). (6) R. A. Young, R. L. Sharpless, and R. Stringham, ibid., 41, 1497 (1964).
Volume 70,Number 6 June 1966
COMMUNICATIONS TO THE EDITOR
2072
times up to 10 msec, (ii) for the individual positive ion species (vide infra), and (iii) in previous work2 on the “pure” N-0 afterglow in which Nz-instead of Heh’z-was used as the discharge gas. The mechanism of ionization in the absence of the additive is probably2s3
N2*
+ NO* -+ N2 + NO+ + e-
(1)
and/or
N
+ N + NO* +N 2 + NO+ + e-
(2)
in which N2* and NO* are excited molecules formed in the afterglow a t concentrations proportional to [NI2and [N][O], respectively. This mechanism leads2 to an ionization rate proportional to [N13[0]for which function the maximum ion concentration occurs a t a fractional 0-atom concentration of 0.25. Indications that the same mechanism operates in the presence of C2F4are also obtained from the observations that (i) little or no chemi-ionization occurs in the reaction of N or 0 atoms alone with C2F4, and (ii) the primary chemi-ion, with or without3 C2F4, is NO+. Our Bennett radiofrequency mass spectrometer experiments show that this ion peaks earlier in the N-O-CZF, reaction than any other ion species and that during the first 0.5 msec of reaction its concentration is one order of magnitude higher than that of all other ions together. Many other chemi-ions are formed; their production can be attributed to charge transfer from NO + and subsequent ion-molecule reactions. The lighter of these together with their most probable
The Journal of Physical Chemistry
identification are 31 (CF+), 43 (CZF+), 47 (CFO+) 50 (CF2+), 59 (C20F+),and 69 (CF3+). Results similar to those of Figure 1 were obtained when CzH4 or C ~ H was Z used, instead of CzF4,as the additive; the enhanced N-0-type chemi-ionization then coexists with the 0-hydrocarbon-type chemiionization. It is known that molecules with unsaturated C-C and reaction intermediates such as9 CN can enhance atom recombination. The present work gives strong indications that such homogeneously catalyzed atom recombination reactions can lead to N2* and/or NO* molecules that have sufficient energy to produce ions via reactions 1 and 2. Such molecules are not necessarily in the same excited states responsible for the ionization in the absence of additives. Reaction 1 is likely to be more important to the enhanced ionization process than reaction 2, since, even though [NO*] may increase due to the presence of additive, [K] decreases. Aclcnowledgment. We thank Dr. S. C. Kurzius for helpful discussion and A. D. Freda for assistance with some of these measurements. (7) R. Kelly and C. A. Winkler, Can. J. Chem., 38, 2514 (1960). (8) E. W.R. Steacie, “Atomic and Free Radical Reactions,” Reinhold Publishing Corp., New York, N. Y., 1954,p 423. (9) I. M. Campbell and B. A. Thrush, Proc. C h m . Soc., 410 (1964).
AEROCHEMRESEARCH LABORATORIES, INC. ARTHURFONTIJN PRINCETON, NEW JERSEY 08540 PIETER H. VREE RECEIVEDAPRIL28, 1966
