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1763
+ N4+ +Nz + Nz+* + Nz N2+ + C +Nz + C + N4f + C +2Nz + C+
Ion-Catalyzed Recombination of Atomic Nitrogen and the Pink Glow
Nz*
by Robert A. Young, Carole R. Gatz, and Robert L. Sharpless-
Minute quantities of impurities in nitrogen greatly increase the amount of active nitrogen (ie., atomic nitrogen) leaving a strong discharge.2,3 However, regardless of impurities, there exists an optimum discharge length (or residence time) for producing active nitrogen in fast-flow systems. Both these observations imply that active nitrogen is rapidly destroyed as well as generated in strong dissociative discharges. Recently, an energetic short-duration afterglow (the pink glow) has been discovered a short distance downstream from the dissociative di~charge.~The emission characteristics of this glow resemble in many ways those of the discharge just preceding it. The pink glow is quenched by some catalytic impurities, and, if oxygen-containing catalysts are added directly to this glow, the nitrogen atom density leaving the region is greatly in~reased.~It is the purpose of this note to indicate a connection between these phenomena. Almost half a century ago Lord Rayleigh6 discovered that active nitrogen was destroyed by a weak discharge. This observation was confirmed in the following experiment. Nitrogen gas (at = 2 mm.) was partially dissociated by a microwave discharge approximately 1 m. upstream from a 30-cm. weak discharge region. The quenching discharge was pulsed on for short time idervals, and the intensityo of the first positive bands between 5000 and 6500 A. (which are proportional to [NI2)l an! the intensity of the NO p bands from 3200 to 3900 A. (which are proportional to [O] [N]) were detected by photomultipliers downstream from the quenching discharge. By varying the weak discharge pulse length, the decay of atoms can be followed in the oscilloscope traces shown in Figure 1. Figure 2 plots the “trough” intensities of Figure 1 on a semilog scale. The nitrogen atoms decayed exponentially during the discharge, while the small residual atomic oxygen in the stream was unaffected. These effects, and other observations published previously,3 can be explained by the mechanism
+ N2 + Nz%N4+ + Nz N4+ + N N3+ + N2* NS+ + N +N2+ + Nz* ---)
(5)
C represents an “impurity catalyst,” and the asterisk denotes species possessing excess energy. Some reactions are symbolic-(4) and (5), for example, with many possible variations.
Stanford Research Institute, Menlo Park, California (Receiued December 1.4, 1964)
Nz+
(4)
(1) (2)
(3)
Figure 1. Photomultiplier current traces for short pulses. The fmt positive band intensity is recorded on top; the @-band intensity a t the bottom. The oscilloscope gains are 0.2 mv./div. (top) and 2 mv./div. (bottom); the sweep speed is 20 msec./div. Traces are shown for discharge pulse durations of 2, 4, 7, 10, 20, 30, and 40 msec.
The reasons for proposing this mechanism and its consequences will be presented inductively. That ions are essential for the rapid nitrogen atom removal observed in discharges simply reflects the most obvious change produced by a discharge in the gas containing atomic nitrogen. However, a specific ion is required since fast atom removal does not occur despite the presence of ions created by chemionization processes.6 Altering the electron temperature (or even ion temperature) does not have a drastic effect on the atom recombination rate. Certainly, the most likely new ion formed in predischarged nitrogen by a secondary weak discharge is (1) See R. A. Young and R. L. Sharpless, J . Chem. Phys., 39, 1071 (1963), for an extensive list of references on active nitrogen, (2) J. W. Strutt, Proc. Roy. SOC.(London), A88, 539 (1913); A91, 303 (1915). (3) R. A. Young, R. L. Sharpless, and R. S. Stringham, J. Chem. Phus., 40, 117 (1964). (4) G. E. Beale, Jr., and H. P. Broida, ibid., 31, 1030 (1959); R . A . Young, ibid., 36,2854 (1962) ; C. E. Fairchild, A. B. Prag, and K . C. Clark, ibid., 39, 794 (1963); A. B. Prag andK. C. Clark, ibid., 37, 2427 (1962) ; H. P. Broida and I. Tanaka, ibid., 36, 236 (1962). (5) J. W. Strutt, Proc. Roy. SOC.(London), A92,438 (1916) ; E. J. B. Willey, J. Chem. SOC.,2831 (1927). (6) C. R. Gatz, R. A. Young, and R. L. Sharpless, J. Chem. Phys., 39, 1234 (1963).
Volume 69, Number 6 M a y 1966
NOTES
1764
I
I
I
20
30
1
4t
10
40
so
PULSE DURATION I-msCC
Figure 2. Light intensity us. pulse duration (from Figure 1).
Nz+.(Since reaction 1 establishes a rapid equilibrium between Nz+and Nqf,further comments will be stated in terms of Nz+ for simplicity.) Hence, Nz+ is likely to be the species involved in increasing the rate of atom removal during the discharge. Ion densities in these weak discharges probably do not exceed 10IO/cc. so the new ions must be extremely effective. Ion-molecule reactions such as reactions 1 to 5 can have rate coefficients lo3 times larger than similar reactions between neutral species.’ As the recombination of atomic nitrogen does not change the ion densities, ions probably play a catalytic role. In absolutely pure nitrogen, reaction 5 cannot occur because [C] = 0. As the “atom-producing” catalyst is added, [Nv:!+] decreases as the net flux through reaction 5 increases until Nz+is destroyed before reaction 1 can occur. Without N2+,atoms are lost only by other much slower processes, and [N] must increase to balance the (assumed undisturbed) production of N in the discharge. Hence, C i s not a catalyst but a destroyer of a pre-exis& ing ion catalyst. The concentration of atomic nitrogen would be least when [C] = 0 and reach a maximum (or plateau) for :some finite [C]; this maximum would be The Journal of Physical Chemistry
independent of whatever “anticatalyst” C was used, as is observed.6 The minimum concentration of several different “catalysts” which produce maximum atom concentrations will be in the ratio of their rates in reaction 5. If the listed reactions, with the addition of loss processes independent of [C] for N and N2+,are subjected to a steady-state analysis, it can be shown that [N] a [C] if Jcb[C] is greater than the rate of N2+destruction. This linear dependence of [N] on [C] has been ob~erved.~ Furthermore, if reaction 2 is the rate-controlling step, [N] will decay exponentially during the quenching discharge (Figure 2). The Lewis-Rayleigh afterglow results from the “uncatalyzed” recombination of nitrogen atoms1 (first into electronic excited metastable states which then undergo nonradiative transitions to the observed radiating states). The catalyzed recombination of atomic nitrogen by reactions 1 to 3 must also lead to energy-rich molecules. However, these molecules can be in entirely different excited states from those which ultimately lead to the Lewis-Rayleigh afterglow. In fact, the excited molecules produced by ion-catalyzed nitrogen atom recombination may well be the precursors of the pink nitrogen afterglow since this afterglow follows a strong discharge in pure nitrogen (when [Nz+]is large). Absorption spectra strongly indicates that the precursors of the pink nitrogen afterglow are vibrationally excited nitrogen molecules (Nz*). Several neutral examples of processes similar to reactions 2 and 3 produce vibrationally excited products. The theoretical reasoning9used in discussing such reactions is unchanged if one reactant is an ion. Thus, reactions 2 and 3 may well produce predominantly vibrationally excited Nz which then interacts (by an unspecified path) to excite the pink nitrogen afterglow. (7) D. Stevenson and D. Schissler, J. C h . Phys., 29, 282 (1958); G. Giomousis and D. Stevenson, ibid., 29, 294 (1958). (8) A. M. Bass, ibid., 40, 695 (1964); Y. Tanaka, private communication. (9) J. C. Polanyi, J . Chem. Phys., 31, 1338 (1959).
Surface OH Groups on Zeolite XI
by H. W. Habgood Research Council of Alberta, Edmonton, Canada (Received December 83,1964)
A recent paper by Carter, Lucchesi, and Yates2 (CLY) has presented evidence from infrared spectros-
