172
A. X, WRIGHTAND C. A. WINKLER
Vol. 67
THE ACTIVE NITROGEN AFTERGLOW AND THE QUENCHING EFFECT OF ADDED AMMONIA1t2 BY A. N. WRIGHT$ AND C. A. WINKLER Upper Atmosphere Chemistry Research Group, Physical Chemistry Laboratoiy, McGill liniversit y, Montreal, Canada Received July IS, 196d The intensity of light emission in the visible region, and for einission from the 11th and 6th vibrational levels of N2(B3&),varies linearly with the square of the N atom concentration in such a way as to suggest that collision with an unpoisoned Pyrex wall may induce transition from the 6&+ state to the B state a t vibrational levels less than eleven. The results further suggest that as many as 106 collisions with N2(X1Z,+) may be necessary to induce the conversion of the quintet state of nitrogen to a triplet state capable of causing light emission. When NHa is added to active nitrogen in an unpoisoned system, its effect on light einission in the first positive system is complicated by the role of NH8 as an effective wall poison against surface recombination of N atoms, as well as its quenching action in the gas phase. However, in a system well poisoned against surface recombination, the quenching effect of NHa on light emission over the visible region, and particularly in the 5820 and 6630-h;. regions, suggests that NH, deactivates the quintet state of nitrogen with a collision efficiency of about 10-5. The quenching efficiency of NH3 for N2(6&+) is reduced in the presence of significant concentrations of N2(A.32u+), and it appears that most chemical decomposition of NH3 is induced by a collision of the Second Kind with the A state molecule.
Introduction It has been established conclusively4~5that the intensity of the Lewis-Rayleigh nitrogen afterglow, for total emission and also for emission from the 11th or 6th vibrational levels of the B3& state, depends on the square of the concentration of N( "). The characteristic vibrational distribution of the Nz(B3E,3 AS&+) 1st positive bands appears to remain unchanged during 140 see. of decay,0and the infrared, vacuum ultraviolet, and visible afterglows all decay7 identically with time. Although nitrogen atom concentrations were not determined in the recent studies in static they were estimated a t a point downstream in some of the studies4 that have been made in flow systems, in which the afterglow intensities were measured by viewing l ~ n g i t u d i n a l l ya~ ~decay ~ ~ ~ ~tube of a t least 25-cm. length. Recent worklo supports the postulate4t1I that 4s nitrogen atoms first recombine to form weakly bound sZ,+ nitrogen molecules as a stable intermediate in the emission of the afterglow, and these then populate directly the v = 12, ll, 10 (group 1) levels of the B state through a radiationless collision-induced transition. On the other hand, the strong emissioiz from the lower vibrational levels of the B3Hg st,ate, i.e., v = 7, 6, 5 (group 4) and v = 4 (groups 3a and 3b), appears to arise from populat'ion of these levels of the B &ate through intermediate elect$ronicstates of lower energy content than 5&+. Bayes and Kistiakowsky have pointed outlo that further progress in elucidating the initial steps of the afterglow mechanism requires experiments in which the nitrogen atom concentration and (1) This investigation was supported by the National Research Council of Canada and the Deience Research Board of Canada. (2) Presented a t the Edmonton Conference of the Chemical Institute of Canada, May 28, 1962. (3) Postdoctoral Fellow. (4) J. Berkowita, PI. A. Chupka, and G. B. Kistiakowsky, J . Chem. P h y a . , 46, 457 (1956). (5) G. B. Kistiakowsky and P. Warneck, ibid., 27, 1417 (1957). (6) R. A. Young and K. C. Clark, ibid., 34, 604 (1960). (7) R. A. Young, ibid., 33, 1112 (1960). (8) T. Wentink, Jr., 6. 0. Sullivan, and K. 1,. Wray, ibid., 29, 231 (1958). (9) U. H. Kurzweg and H. P. Broida, J . Mol. Spectroscopy, 3,388 (1969). (10) K. D. Bayes and G. B. Kistiakowsky, J . Chem. Phus., 32, 992 (1960). (11) A. G. Gaydon, "Dissociation Energies and Spectra of Diatomio Molecules," Chapman and Hall, Ltd., London, 1953, 2nd Ed., p. 157; Nafure, 163, 407 (1944).
the inteiisities in the afterglow spectrum are simultaneously measured. The intensity of the afterglow in active nitrogen produced by a microwave discharge is quenched12 by the addition of XH,, especiallylo the bands originating from the lowest vibrational levels of the B3E, state. This interaction does not appearI2 to involve N atoms and the pressure dependence of the quenching efficiency of added NH3 has been attributedl0 to interaction between NH3 and N2 (T2,+). On the other hand, the chemical decomposition of NH3, in active nitrogen produced by a condensed di~charge,l~-~s appears to be caused mainly by collisions between Kz (A3&+) and "3, with an over-all rate constantiGof about 1O1O cc. mole-l sec.-l. The reaction apparently does not involve N atoms,:5 although the addition of NHI does quench the CY emission from the reactions of active nitrogen with many carbon-containing compounds.'7118 The negligible extent of NH3 decomposition in active nitrogen of microwave origin has been attributedlGto the low concentration of N atoms, and hence lorn concentration of excited nitrogen molecules formed by atom recombination, in such systems. The present paper reports measurements on the afterglow intensities a t different levels in a flow system, corresponding to known times of decay of the active nitrogen, for a system iii which values had previously been obtained for both the X-atom concentrations,15 and the concentrations of NH, left unreactedl6 a t these levels, for different initial flow rates of added "3.
Experimental The afterglow was monitored by a 1P21 photomultiplier tube used in conjunction with an Eldorado photometer. The phototube was mounted on guide rails inside a dark box constructed around a straight tube reaction vessel. This permitted positioning of the phototube, so as always to view the center of the reaction vessel at any level betn-een 0.2 and 45 cm. below the reactant inlet. The nitrogen was activated by a condensed discharge and the linear flow rate of the gas through the cylindrical flow system (12) (13) (14) (15) (1962). (16) (17) (18)
G. R. Kistiakonsky and G. G. Volpi, J . Ghem. Phys., 28, 665 (1958). G. R. Freeman and C. A. Winkler, J . Phya. Chem., 69, 371 (1955). R. Kelly and C. A. Winkler, Can. J . Chem., 38, 2514 (1960). A. N. Wright, R. L. Nelson, and C. A. Winkler, i b d , 40, 1032
A. E.Wright and C. A. Winkler, ibzd., 40, 5 (1962). K. D. Bayes, tbzd., 39, 1074 (1961). A. N. Wright and C. -4. Winkler, ibirl., 40, 1291 (1962).
Jan., 1963
ACTIVENITROGEN AFTERGLOW AND QUENCHING EFFECT OF ADDED AMMONIA
was 478 cm. sec.-l. All experiments were conducted a t a pressure of 3 mm. and, in all other respects, the system was identical with that described previously.161’6Js Experiments involving introduction of N H 3 were initiated after the condensed discharge had been allowed to operate for about 20 min., at which time the temperature of the active nitrogen had reached a maximum value, constant for about 5 min. With suitable collimation by a pin hole through 1/4-in. thick lucite in front of the phototube, readings were obtained for the unfiltered afterglow that approached the upper limit of the photometer, while the readings in the presence of filters were alwalys at least a factor of 10 higher than the background values. Although the afterglow emission is given by the photomultiplier in terms of “yotransmission,” and consequently offers only a measure proportional to the light intensity, the readings were all made under similar geometrical conditions and comparisons between them therefore are valid. Optical filters, constructed by Spectcolab of California for transmission peaked at 5820 and 6630 A., and with less ,than 25 A. transmiwsion widths, permitted‘ intensity measurements of the emission centered about the 11th and 6th vibrational levels, respectively, of the B3II state. Readings were made for the visible afterglow (unfiltered system) and a t the two chosen wave lengths a t selected positions for the unpoisoned Pyrex system and for the system when it was poisoned against atom recombina,tion with a minute amount of water vapor.16 The photomultiplier readings decreased with time of operation of the condensed discharge but, after approximately 22 min., reached values that were constant for the subsequent 8 min. Only these steady valueei are recorded in the results. Similar readings were made, down to the 45-cm. level below the reactant inlet, in the presence of known amounts of added NH3. The NHa flow was continued for as long as 2 min, Le., until three readings of the photomultiplier, spaced 30 sec. apart, were identical,
Results Dependence of Afterglow Intensity on [N]in ,the Absence of NHz.-Photomultiplier (PM) readings a t the various levels may be compared with a measure of the N atom flow rates at the same levels, estimated from the extent of HCN production during the C:IH4 rea~ti0n.l~ For the unfiltered afterglow, plots of the PM readings against the square of the N atom flow rates yield reasonably straight lines, as expected (cf. Fig. lA, for the poisoned system).l9 These lines appear not to pass exactly through (0,0),20 and therefore have been described (Table I) by parameters a and b in the linear equation
PM readings = a(N atoms flow rate)Z
+b
For emission from u = 11 (5820 8.)and from v = 6 (6630 8.), the values of a and b for the straight lines obtained up to the 15 cm. level also are given in Table I. At levels above 15 cm., however, the PM readings fell off markedly, as shown by the typical data plottied in Fig. 1B and 1C for the poisoned system. This behavior was almost certainly due to an increase in the (19) The vertical height of the larger symbol in these figures corresponds to the limits of variation of a considerable number of experiments, while %he circles represent a single experiment. Note t h a t the origin in these plots is not a t (0,O). (20) It is of interest t h a t equally good straight lines may be obtained if the PM readings are plotted against the N atom flow rate as determined by the NO titration method.” in which case there is a considerable intercept on the ordinate, the sign of which is differentfor the poisoned and unpoisonied systems. These intercepts are in agreement with t h e suggestion16 t h a t t h e destruction of NO may be due to both NdA) and N atoms, with the concentration of NdA). relative t o that of N-atoms, increasing with decay tiine of the active nitrogen i n an unpoisoned system. If the NO method indicates a considerable value for the N-atom flow rate after decay times for which i t had. in fact, decreased to low values in t h e unpoisoned system, a negative intercept would result. On the other hand, the proportionately greater aoncentration of Nz(A) after shorter decay times15 in the poisoned system would tend t o produce a line of too small slope, and hence extrapolate to a poaitive intercept.
173
7 00
6 00
,500 W 0
z
a -
400
I cn z
a
P
70
8
I W
5 0
a W a
68
58 1.8
z a
1.4
1.C
-?’
9.5 X mole cc.-l. Discussion The Active Nitrogen Afterglow.-The linear relation between PM readings and the square of the N-atom flow rate, as illustrated in Fig. 1, demonstrates that the intensity of the visible22afterglow depends only on the extent of homogeneous recombination of N atoms. Obviously, there is no important contribution to the visible afterglow from A state molecules formed on the Pyrex walls, in contrast to the emission from the v = 8 and 6 levels of the B state following recombination on a cobalt surface.23 The ratios of the slopes, a, for the unpoisoncd system to those for the poisoned system are 1.78,2.40, and 1.32
straight lines were obtained when lo/l values a t a given level were plotted against the corresponding concentrations of unreacted NH3 in the system, as shown in Fig. 2A (unfiltered afterglow), Fig. 2B (afterglow a t 5820 A.),and Fig. 2C (afterglow a t (3630 8.)z1The slopes of these straight lines are summarized in Table 111.
(21) Values of Io were obtained for the filtered afterglows a t a n y given upper level ( L e . , clohe to the reactant inlet), where Fig. 1B and 1C show a fall-off i n the absence of "8, b y extrapolating the linear relations for the lower levels and reading the PM value for the N-atom concentration which the previous work16 showed t o correspond t o the upper level. T h e corrected value ok I then was obtained by subtracting from this value of Io the decrease in intensity observed in the presence of the added NHn a t the same level. (22) The spectral response of the phototube mas limited t o the range 3000 to 6200 h.
I40
I20
I
oc
14C
I2C
I oc
/
4
I
16 24 32 40 N H 3 C O N C E N T R A T I O N - M O L E I C C X IO6
8
Jan., 1963
ACTIVENITROGEN AFTEEGLOW
AND
for the unfiltered afterglow, and the emissions from the 5820 and 6630-A. regions, respectively. The greater values for the unpoisoned system, for emission in all three spectral regions used, probably results from the greater proportion of the N atom decay (approximately 0.9 of the total)l5 that occurs without light emission by surface recombination, which is first or zero-order in N atoms.24 The greater effect of a change of wall condition on the values of a for the 5820 A. region, in particular, might be explained if the unpoisoned Pyrex walls were more effective than the poisoned walls for inducing an electronic transition from the quintet state to the 3rIg state25 (cf. ref. 27). The wall-induced change of spin would have to be accompanied by a loss of energy ~~@ greater than the negligible loss that O C C U ~ S ~during the radiationless collision-induced transition between N2(58g+)and N2(X1Zg+)in the gas phase, so as to populate the B state into vibrational levels less than v = 11, rather than directly into the v = 12, 11, 10 levels, as assumed by Bayes and Kistiakowsky.l@ The currently accepted theory of the a f t e r g l o ~ ~requires s~~ that the 5 X : g + molecule make many collisions with N2(X) during a considerable lifetime and, at a pressure of 3 m.m., with a mean free path of about lov3 cm., i t is possible that it might make a substantial number of collisions with the wall during its lifetime in the present system. With similar wall conditions, the linear relation between [N]2 and the intensity of the afterglow d.oes show some dependence upon the spectral region under investigation. If the change in PM readings, down to the 45-cm. l e d , is expressed in terms of a percen.tage decrease in light emission from that observed a t the 15-cm. level (a change-of 153 and 38.8 X mole/ sec.2 iii the abscissas for the poisoned and unpoisoned systems, respectively), the following variation is obtained Unfiltered afterglow
Emission from Emission from v = 11 v = 6 (5820 8.) (6630 1.)
QUENCHING EFFECT OF ADDEDAMMONIA
*
175
-/ 1
/
I40
/
I20
IO0
B
k /
I60
/
/ /
I O
-*
/
/
I I40
./
I20
I00
I40
I20
IO0
-
I I
0
15 30 DISTANCE BELOW NH3 INLET
-
45 CM.
Fig. 3.-Plot of l ~ / against l distance below the point of introduction of NHI a t the initial Concentrations given by: 0,9..5 X mole cc.-l; 0 , 2 3 X mole cc.-'; A, 31 X mole CC.-'; A,47 X mole cc.-l. Subdivided for the spectzal conditions: A, unfiltered afterglow; B, afterglow a t 5820 A.; C, afterglow a t 6630 A.
further indicated by the observat'ion that, for positions close to the inlet jet, the emission from particular spectral regions (5820 and 6630 A.)>but not the total Emission from v = 11 in the unpoisoned system is more visible afterglow emission,29showed a falling-off from the dependent upon [El2than is the emission in this same linear dependence of intensity on [NI2(Fig. 1). I n the syst*em from the unfiltered afterglow or from v = 6, poisoned system, this fall-off occurs, for emission from while the reverse is true in the poisoned system.28 either v = 11 or v = 6, a t a value of (N-atom flow This might again suggest that a collision of N2(52,+) rate)2 that corresponds to a decay time of about 18 with the unpoisoned walls effectively induced transition msec. I n the unpoisoned system, the corresponding to the I3 state a t v < 11. decay time is about 24 msec. That the m.anner of populating the B state is modified (28) This variation is not explicable by the temperature dependence of by wall collisions of a precursor of the afterglow is the intensity of the group 1 bands in active nitrogen,lo since the decrease Poisoned system Unpoisoned syritem
30 40
26 48
36 31
(23) (a) P. Harteck, R. R. Reeves, and G. Mannella, Can. J . Chem., 88, 1648 (1960); (b) Although NO may be destroyed b y excited nitrogen molecules produced by wall recombination,16 light emission, if initiated a t all by A state molecules in such low vibrat,ional levels, would occur in the infrared (group 3b) and not be detectable in the present study. (24) R. A. Back, W. Dutton. and C. A . Winkler, Can. J . Chem.,87, 2059
(1959). (25) Although water vapor is known*B t o be very effective for induclng vibrational relaxation in the gas phase, it is assumed that, because af the very short lifetime' of the B state, the vibrational distribution within this state will not be affected significantly, at the pressure of 3 mm., by collision of Nz(B) with the trace of water vapor present in the poisoned system. This property OF water vapor would not be expected t o operate for collisions with the much longer lived quintet state, because of the very shallow potential energy well associated10 with this nitrogen molecule. ( 2 6 ) S. J. Lukasik and J. E. Young, J . Chem. Phys., 27, 1149 (1957). (27) E. R. V. Milton, H. B. Dunford, and A. E. Douglas, {bid., SS, 1202 (19bl).
in temperature between the 0.2 and 45 om. levels was only 43 and 36O in the poisoned and unpoisoned systems,'5 respectively. It is conceivable, in view of the effect of added foreign gases,'O t h a t the introduction of even the trace of H?O vapor (
