COMMUN~CATIONS TO THE EDITOR ments (in our treatment, CH2 and CH3 groups) form a continuum about any arbitrary plane described in the bulk phase of the liquid. Fowkes in his comments disagreed with this assumption on the basis that summation gives more precise results than integration. We agree that summation could give better values of potentials than integration but only providing that the position and number of nearest neighbors is precisely known. This knowledge can come from the radial or pair distribution function of the liquid. In Figure 3 of our previous paper‘ we showed that our assumed distribution function of water was slightly different from that derived experimentally. However, any revision of the calculated surface energy to take account of the most probable distance of closest approach, rll, obtained from the experimental distribution function must also include a revision of N o , the number of volume elements per unit volume. (See eq 5, ref 1). To alter rll without altering No would, of course, lead to an erroneous bulk density. Fowkes2 has summed the total energy of n-octane using an assumed lattice model for the distribution of n-octane molecules at a surface. He then equated the lattice separation of n-octane molecules, obtained from X-ray diffraction, to the interparticle distance between two CH2 groups on adjacent molecules. I n the absence of the distribution function of C atoms or CH2 and CH3 groups in a liquid n-alkane, we prefer to assume a continuum rather than the further assumptions made by Fowkes-namely, that nearest neighbors are arranged in a regular quasi-crystalline lattice. References 6-8 cited by Fowkes in his comments refer to criticism against calculating the potential of an adsorbed atom or molecule at a solid surface. Although these criticisms may apply in the present case, they refer specifically to the adsorption of a vapor at a solid surface. When such adsorption takes place, the condensed vapor is supposed to form a closely packed monolayer. For such conditions the potential of an atom in the adsorbed layer should be obtained by summation and not integration. Reference 13 quoted by Fowkes seems to us somewhat irrelevant. Elmas3 criticized our calculation of the adsorption energy of a monolayer of surfactant at a solid-liquid interface which was calculated partly by integration, and partly by summation where lattice arrangement was known. Elmas3 then compared our model with his own for the adsorption of a rare gas on graphite. Comparison of forces of an adsorbed monolayer of a solute a t a solid-liquid interface with interfacial forces of a pure liquid with its vapor is not justified. We feel the case between summation and integration of potentials in a liquid system is not settled, mainly because the distribution of nearest neighbors is not known with any certainty. I n the absence of dis-
3701 tribution functions, we feel that integrat’ion is no more of a dangerous trap than summation. An assumption common to both the treatments of Fowkes3 and our own,’ namely, that somehow the surface entropy is included or compensated for in the calculation; seems to us more fundamental and inore difficult to justify. (2) F. M. Fowkes in “Surfaces and Interfaces. I. Chemical and Physical Characterisations,” J. J. Burke, et al., Ed., Syracuse University Press, Syracuse, N. Y., 1967, p 203. (3) H. M. Elmas, “Wetting,” Monograph No. 25, R. H. Ottewill, Ed., Society of the Chemical Industry, London, 1967, p 246.
RESEARCH LABORATORY J. F. PADDAY KODAKLIMITED N. D. UFFINDELL WEALDTONE, HARROWS, MIDDLESEX, ENGLAXD RECEIVED JULY 11, 1968
Formation of Electronically Excited Species in Nitrogen Atom-Oxygen Atom Recombination Reactions Catalyzed by Carbon Compounds: NO(A2ZI, B2H) and O(lS)l
Sir: It is known that the rate of chemiion formation in N-0 mixtures may increase by factors of lo2 to 103 upon addition of carbon compounds (C2F4, CzH2, C2H4, and C2N2).2,3Formation of ions can be attributed to N2* NO* N2 NO+ e-, in which [Nz*] and/or [NO*] are increased via catalyzed atom-recombination reactions (apparently involving CN) which can proceed considerably faster than normal three-body recombination.2 In an effort to find further evidence for formation of excited species in catalyzed N-0 recombination, weoarenow studying chemiluminescence in the 2000-6000-A region. Tubular flow reactors and reaction conditions (temperature, = 300’K; pressure, 0.5-10 torr) similar to those of the ion studies2s3are being used. N atoms produced in a microwave discharge are partly or totally replaced by 0 atoms via NO titration upstream from the nozzle through which the carbon compound is added. Such addition is found to enhance NOy(A28-X211) and O(’S-lD) emission intensities as well as excitation of some vibrational levels of the NO(B211) state. The catalyzed atomrecombination reactions involved are not necessarily identical for all these phenomena. Figure 1 illustrates the effect of C2F4 addition on NO emission. The y (v’ = 0) bands are increased in
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(1) This work was sponsored by Project SQUID, which is supported by the Office of Naval Research, Department of the Navy, under Contract N00014-67-A-0226-0005, NR-098-038. (2) A. Fontijn and P. H. Vree, “Eleventh Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1967, p 343; J . Phys. Chem., 70, 2071 (1966). (3) A. Fontijn and R. Ellison, unpublished data. Volume 72, Number 10
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enhancement (see above), the y bands again become the dominant emission feature below ~ 3 0 0 0A, The intensity of O(lS-lD) emission can be enhanced by a factor of 20 when C2F4 is added. Maximum intensity, in both the presence and the absence of CLF4, occurs for [N]/[O] = 2. O(%) formation can probably be attributede to N(“) NO(a411) --t Nz(XI2) O(lS) and/or N@) O(3P) 4 N,(X‘B) O(lS), in which NZ($)represents an unspecified triplet state of Nz the concentration of which is a [N]2. The emission enhancement can again be accounted for by catalyzed atom-recombination reactions resulting in increased [NO(a411)] and/or [N#) I. The previously reportedla strong enhancement of CN emission (in the presence of C2H2) occurring when N atoms are partially replaced by 0 atoms is also observed in the present work for the N-O-CzNz and X-O-CZF4 reaction systems. Investigation of the latter also reveals NCN(A3II-X32),l1 NCO(AzZX211),12and Pu’F(b1Z-X3Z)13emission. A corollary of the present results is that caution should be exercised in the use of N-0 afterglow intensities for atom concentration measurements in the presence of a third reactant.
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Figure 1. Effect of C z F 4 on NO emission spectrum (pressure, 2.0 t o r r ; average gas velocity, 15 m sec-1): t h e initial concentrations in molecules are: [Helo = 6.2 X [ N ~ I= O 2.9 X [N]o = [O]o = 4.4 X lot3, [ C ~ F ~=I O5.1 X 1013.
intensity by =lo2 and are rotationally more excited than those of the normal afterglow (C2F4 absent). Near 10 torr the degree of intensity enhancement for the indivudual y bands4 decreases with increasing v’; at the lower pressures used such a comparison cannot readily be made. The /3 band (B211-X211) intensities increase by a factor less than 3 and increase more for v‘ = 2 than for v’ = 0. The intensities of the ,8 (v’ = 3) and 6 (v’ = 0) bands (excited by a pre-association mechanismb), as well as the N2 first positive bands (v’ = ll), decrease monotonically with increasing [CzF4]. Emission from the latter two band systems indicates that the enhanced emissions occur in reaction environments in which free N and 0 atoms are still present. The maximum intensities of the NOy bands occur in both the presence and the absence of C2F4 for [N] = [O], indicative of emitter formation reactions of equal order in N and 0. The process Nz(A32) XO(X211) 4 Nz(X’2) NO(A2Z) has been studied (e.g., ref 6). The same type process may explain our observations, since both [Nz(A22)]and [NO(X211)]can be enhanced by catalyzed atom recombination and in the presence7s8of N atoms [N2(A32)] a [XI and [NO(X211)] a 101. Catalyzed N-0 recombination to form YO(a411), the potential energy curve of which crosses that of the A22: state, may also contribute significantly. I n fact the preferential enhancement of the u’ = 2 level of B211 points to just that mechanism involving N(4S) and O(1D) which can combine via the b 4 2 state. The potential energy curve of b 4 2 crosses that of B2rI near v‘ = 2. A slight increase of the y (v’ = 0) intensities is also observed with C2H4. Because of the simultaneous decrease in intensity of those NO bands not subject to
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Acknowledgment. We thank S . R. Grossman for assistance with some of this work. (4) The absolute change in intensity of the v‘ = 1 bands of the 7 and p systems could not be measured. Intensity enhancements of the y bands up to v‘ = 3 have been observed. (5) R. A. Young and R. L. Sharpless, Discussions Faraday Soc., 33, 228 (1962). (6) K. H. Welge, J . Chem. Phys., 45, 166 (1966). (7) C. H. Dugan, ibid., 47, 1512 (1967). (8) R. A. Young and R. L. Sharpless, ibid., 39, 1071 (1963). (9) R. A. Young and G. Black, ibid., 44, 3741 (1966). (10) A. Fontijn, ibid., 43, 1829 (1965). (11) G. Herzberg and D. N. Travis, Can. J . Phys., 42, 1658 (1964). (12) R. W. B. Pearse and A. G. Gaydon, “The Identification of Molecular Spectra,” 3rd ed, Chapman and Hall Ltd., London, 1963, p 221. (13) A. E. Douglas and W. E. Jones, Can. J . Phys., 44, 2251 (1966).
AEROCHEM RESEARCH LABORATORIES, INC. ARTHURFONTIJN SUBSIDIARY O F RITTERP F A U D L E R CORPOR.4TION R O Y ELLISON PRINCETON, NEWJERSEY08540 RECEIVED JULY 8, 1968
The Inefficienoy of Triplet Energy Transfer from Ketones to Trivalent Rare Earth Ions’
Sir: Possible laser applications of rare earth fluorescence have been under active study for some years now.2 (1) Triplet Energy Transfer. V. This work was supported by the Office of Naval Research under Contract No. N00014-68-A-01090001. (2) (a) N. E. Wolff and R. J. Pressley, A p p l . Phys. Lett., 2, 152 (1963); (b) A. Lempicki and H. Samelson, ibid., 2, 159 (1963); (c) E. J. Schimitschek, ibid., 3, 117 (1964).
