H. €3. DUNFORD
258
(hyd.). The net enthalpy change would be lowered to 19.5 kcal. Results given by Fig. 11 show that the initial differential heat of chemisorption is obviously higher than 30 kcal. and therefore suggest that the chemisorbed water is dissociated. This conclusion agrees with n.m.r. results recalled previously.10311 I n summary, from 33 to 13 kcal., that is to say for 0 < n < no,the formation of ammonium from exchangeable protons or dissociated chemisorbed water molecules and/or ammination of exchangeable cations would occur; from 13 to 5 kcal., for n > no,physical adsorption would take place. IV. Conclusions H-montmorillonite and base saturated montmorillonite which have reacted with ammonia show mainly the same infrared spectral features as NH4-montmorillonite. This proves that chemisorbed NH3 is present as NH4+ ions rather than in molecular form. This can be understood easily for H-montmorillonite. In the case of base saturated montmorillonite, a reaction with the residual water molecules provides the mechanism necessary for S H 4 +ion formation. The calorimetric data suggest that _theresidual water in the clay exists in a dissociated state and when reacted with NH3 donates a proton, forming NH4+ ions. Residual water is suggested to be that left in interlayer spaces after outgassing in the temperature range from 20' to the temperature of dehydroxylation. The transformation of NH3 into NI&+ by reaction with HaO is supported by the relationship between the absorbances of H20 and NH4+ deformation bands. The interaction between KH4+ and water is substantiated by fast isotopic exchange with DzO and the existence in
1'01. 67
"4-montmorillonite of two bands that have been attributed to N H . . . O interactions. Constitution hydroxyls are not believed to take direct part in the transformation of chemisorbed NH3 into NHII+ since there is a relatively slow isotopic exchange between ND3 and lattice OH. On the other hand, the deuteration of lattice hydroxyls proceeds rapidly in the presence of DzO. The coiinection between the chemisorbed phase and protons belonging to the solid phase is probably provided by intermediation of HzOas follows : ND4+ reacts with HzO to form DzO and NH4+; D,O then reacts with lattice hydroxyls. This process assumes delocalization of the proton belonging either to chemisorbed HzO or the lattice. This possibility is also indicated by results obtained by others with nuclear magnetic resonance. That water is a very important intermediate in the movement of protons from lattice hydroxyls may be explained by the intermediate position of the bond energy of OH in between ?JH(NH4+)and ?JH(XH3). For NH,, EN-H= 83 kcal., EOH= 110 kcal., and for NH4+ EN-H = 114 kcal. The idea of the dissociated state of chemically adsorbed water should be taken into serious consideration when explaining catalytic processes provided by acid as well as calcium montmorillonite. These views could also have a bearing on the spontaneous aluminization of H-montmorillonite. Physical adsorption of 3" produces expansion of dehydrated montmorillonite but not vermiculite. The difference in swelling between the two expanding lattices may be explained on the basis of a difference in the charge density of the two minerals.
THE HALF-LIFE OF A3Cuf NITROGEN'p2 BY H. R . DUNFORD Department of Chemistry, University of Alberta, Edmonton, Canada Receiued June 26, 2962 The decomposition of ammonia by active nitrogen has been demonstrated to occur by two kinetically distinguishable mechanisms. When a condensed discharge is used t o activate the nitrogen, the predominant mode of decomposition of ammonia is by a metastable state of nitrogen believed to be ASZ,", which is formed mainly in the discharge tube. The half-life of this metastable species is 0.08 f 0.03:see. The less important mode of ammonia decomposition is attributed to reaction with a molecular afterglow precursor.
Introduction Although 4S nitrogen atoms are now generally accepted as the major chemically active component present in active nitrogen, there still appears to be controversy over the possibility of other reactive species. The lowest energy triplet state of nitrogen, A3&+, with 6 e.v. of energy in its lowest vibrational level, would appear to be a possibility as the next h o s t abundant species present. This triplet A state of Nzcan be formed by electron bombardment,3 by direct recombination of nitrogen atoms, and as the end product of the LewisRayleigh e m i ~ s i o n . ~It can radiate to the ground state by emission of the forbidden Vegard-Kaplan bands. (1) Supported financially by the National Reaearoh Council of Canada. (2) Presented in part a t the 141st National Meeting of the Amerioan Chemical Society, Washington, D. C., March, 1962. (3) W. Liohten, J . Chem. Phys.. 26, 306 (1957). (4) X . D. Bayas s n d G . B. Kistiakowsky, ibid., 88, 992 (1960).
Carleton and Oldenbergs have observed the First Positive spectrum in absorption in active nitrogen, so the presence of A321,+ Nz is beyond doubt. The problem is whether they are present in sufficient Tm.0 concentration to be chemically important. key questions which must be answered are (1) what is the lifetime of A3&+ N z molecules, and (2) what is the primary mechanism of their formation? With regard to the lifetime of triplet A state KZ,two answers, differing by a factor of 33, are presently available. Wilkinson and Mullikene determined the relative lifetimes of three metastable electronic states of ;"\'z by measuring the pressures a t which the absorption spectra from the lZg+ (ground) state of Nz could be detected. ( 5 ) N. P. Carleton and 0. Oldenberg, reaults presented a t the Fourth European Conference on Molecular Mpeotroscopy, Bologna, 1959. (0) P. G . Wilkinson and R. $. Mulliken, J . Chem. Phya., 81, e74 (11)51))*
HALF-LIFE OF A3Zu+NITROGEN
Feb., 1963
These were converted to absolute lifetimes by using Lichten’s molecular beam result for the a i r g state of N2. Their result is 0.026 sec. for the lifetime of the N3Xu+ state of Nt. On the other hand, Carleton and Oldenberg, from their Lewis-Rayleigh absorption measurements, have calculated a lifetime of 0.85 see. N ~ x o nin , ~a study of active nitrogen a t high pressures, deduced a lower limit of 0.1 sec. for the lifetime of the triplet A state of N2; he therefore favored the longer lifetime of Carleton and Oldenberg. With regard t o the primary mechanism of formation of h3&+ n - 2 in active nitrogen, Berkowitz, Chupka, and Kistiakowsky8showed that only a small fraction of recombining atoms enter into the afterglow mechanism which finally results in formation of A3&+ Nz. Wentink, Sullivan, and Wrayg obtained a value of 1/40 for this fraction. However, t o our knowledge, no direct experimental measurements either of the fraction of N atoms which form A3&+ Nz directly upon recombination,1° or of the fraction of N2 molecules converted to the A3& + st>ateby electron bombardment in a diischarge, have been reported. We report here on the use of an ammonia titration the nitric oxide visible endt e c h n i q ~ l2 e coupled ~ ~ ~ ~ with ~ point techniquel31~~ of titration for nitrogen atoms to study the kinetics of formation and decay of metastable molecular species in active nitrogen.
Experimental The reaction vessel was a 19-mm. i.d. Pyrex tube into which reactant could be admitted a t five different positions.15 Distances of the four reactant inlet tubes downstream from the reactant inlet (designated no. 1) nearest the discharge are: no. 2, 44 cm.; no. 3 , 8 7 cm.; no. 4, 130 cm.; and no. 5 , I73 cm. I n the case of NHa experiments, unreacted KH, was retained in a trap, surrounded by liquid nitrogen, located 40 em. below the reactant inlet furthest from the discharge. Total reaction time a t 2.45 mm. pressure could be varied from 0.086 to 0.33 sec. by choice of the appropriate reactant inlet. A 300-ml. flask was inserted between the discharge and reactant inlet no. 1 in order to allow short lived species ample time to decay and to dampen out any effects due to the intermittency of the discharge. A 1500ml. ballast volume was inserted in the flow system near the vacuum pump. Rates of NHs and NO input and of NHarecovery were determined by pressure-volume-time measurements. Pi11 experiments were conducted a t room temperature. Five different pressures of molecular nitrogen were used: 0.88, 2.45, 4.8!5, 6.95, and 11.2 mm. as measured near reactant inlet no. 1. The corresponding pressures near the product trap were 0.75, 2.30, 4.60, 6.95, and 11.5 mm. and corresponding rates of 4.65 X input of molecular nitrogen were 3.7 X lov5,1.84 X 8.0 X 10-4, and 1.6 X 10-8 mole/sec. Reaction time calculations are based on mean pressures and the assumption of linear flow rates. At all pressures a 4 yf. condensed discharge was used to produce the active nitrogen. For the first two pressures the electrodes were 21 in. apart; at higher pressures, the distanice between the eleckodes was shortened to 6.75 in. in order to facilitate maintenanco of the dischargr. In all cases the flash rate was adjusted to about 20 per sec. by use of a Variac on the primary side of the high voltage transformer. T‘ariac settings in order of increasing pressure were 60,97, 80, 90, and 100. A Model 501-;lf photometer (Photovolt Corporation, h’ew (7) J. F. Noxon, J. Chem Phys., 36, 926 (1962). (8) J. Berkowitz, W. A. Chupka, and G. B. Kistiakowsky, ibzd., as, 457 (1956). (9) T. Wentink, Jr., J. 0. Sullivan, a n d K. L Wray, zbid., 29,231 (1958). (10) See, however, A. N. Wright and C. A. Winkler, Can. J . Chem.. 40, 5 (1962). (11) G.It. Freeman and C. A. Winkler, J. Phys. Chem., 159, 371 (1955). (12) G.B. Kistiakowsky and G. G. Volpi, J. Chent. Phys., as, 665 (1958). (13) Ga 13. Kistiakowsky and G. G. Volpi, w i d . , 27, 1141 (1957). (14) F. Kaufman and J. R. Kelso, ibid., 27, 1209 (1957). (15) M. A. A. Clyne a n d B. A. Thrush, Pror. Roy. SOC.(London), A261, 259 (1961).
259
York) was employed for some afterglow intensity measurements. A Type E visible region photo search head was strapped onto the flow tube with electrician’s tape. For these measurements, three right angle bends were placed in the reaction tube between the search head and the discharge. The NHs inlet was located 45 cm. upstream from the search head, and in approximately the same position relative t o the discharge in terms of flow time as reactant inlet no. 1, described above. The flow system was poisoned with metaphosphoric acid for all experiments except the photometry measurements, when the prepurified nitrogen (Canadian Liquid Air Co., Hamilton, Ont.) \+as bubbled through ire water a t atmospheric pressure before admission t o the apparatus. Except for the experiments using water vapor as a wall poison, the nitrogen gas WRS passed through a liquid nitrogen trap 1)riort o admission to thr flow system.
Results The nitrogen afterglow appears normal a t all pressures but 11.5 mm. At this pressure when the dlischarge is turned on a pink glow results which fills Ihe entire flow system. It subsides in a few minutes to be replaced by the usual Lewis-Rayleigh afterglow. The minimum voltage required to maintain the discharge drops during this interval. The peculiar pink afterglow may be that observed by Broidai6ti7and co-workers at an optimum pressure of about 7 mm. when a microwa,ve discharge is used. Results of all of the X O titration experiments are summarized in Tables I and 11. The rate of recombination of nitrogen atoms was calculated on the basis of the equation 1/[N] - l / [ i Y O ] = k[M]t, where [No] is the concentration of atomic nitrogen at the reactant inlet no. 1, [N] the concentration at any other reactant inlet, 2, [MI is the concentration of molecular nitrogen, and t is the flow time from inlets 1 to x. The value of k obtained at 0.88 mm. pressure, which could be discarded solely on statistical grounds, could perhaps be indicative of a detectable amount of heterogeneous recombination also occurring. It could also be due to a considerable contribution from diffusive, as contrasted to viscous, flow. It can be seen that our mean value for k agrees fairly well with the published values based on the NO titration method.l8v19 TABLE I COYCENTRATION OF ATOMICNITROGEN,MOLE/CC.x 108, DETERMISED BY K O TITRATION Reaotant inlet no.
----0 88
4.23 2.80 2.32 1.79 1.55
Total pressure, mm. 2.45 4 85
4.60 3.41 1.99 1.57 1.07
4.11 2.93 2.16 1.71 1.35
AS
--_---6.96
11 2
4.32 2.93 2.24 1.67 1.23
5.47 4.43 2.93 1.67 1.13
TABLE I1 RATEOF RECOMBINATION OF KITROGEN ATOMS~ Pressure, mm.
k, (mole/cc.)
0.88 (1.8 x 2.45 1.1 x 4.85 7.2 x 6.95 6.2 X 11.2 1.1 x Mean of four pressures 8.8 X li Values of Harteck, Herron, and eo-workers, 5.7 X respectively.
-2
see. -1
ia16) 10’6 1015
10’6 1015
1016
12.4
x
1015 and
116) G.E. Beale and H. P. Broida, J. Chem. Phys., S 1 , 1030 (1859). (17) H.P.Broida and I, Trtnaka, ibid., 36, 236 (1962). (18) P.Harteok, R. R. Reeves, and a.Mannella, ibzd., 29, BO8 (1958). (19) J. T. Herron, J. L. Franklin, P, Bradt, and V. H , Dibeler, zbzd., 30, 819 (1959).
260
H. B. DUNFORD
Vol. 67 TABLE I11
RATESOF INPUT AND DESTRUCTION OF AMMONIABY ACTIVE KITROGEN NI rcsults expressed as mole/sec. X 108 Inlet no.
0.8
#2
0-
B 0
'2 0.4
5
,,,LAA 0.2
---
1
#a
J
*owJ#4
0
A
#5
2
0.26
.
0.22
1.51 3.47 5.60 8.14 10.47 1.81 4.34 4 47 7.43 9.83
destroyed
Inlet
no.
0.88 mm. pressurc 0.18 5 .16
.17 .15 2.45 mm. pressure 3 0.96 1.20 1.24 1.38 4 1.40 5 0.54 -62 .65 a43 .63
1
c y
3
0.20 2
4
6
Rate of NHs addition.
5
of reciprocd of nitrogen afterglow intensity us. ammonia flow rate (molc/sec. x loR).
When XH3 was added to active nitrogcn, a bright blue trap reaction occurred a t -19Go, as rcportcd by Zabolotny and A photograph of the spectrum of this trap reaction has bcen obtained on a Type 103aF plate in this Laboratory by Mr. E. R. V. Milton, who used a quartz medium Hilgcr instrument. No band structure could be detected, but rather the flame appeared as a continuum, extending from 3200 to 6800 A. It had maximum intensity in the 3800-5500-A. region. Results of the dcterminations of dccomposition are rcportcd in Table 111. The data obtained a t 2.45 mm. pressurc are shown in Fig. 1. Blank experiments to test the efficiency of NHa trapping were made, and indicated that losses of up to 10% of the input ammonia occurred. All of the data shown in Table 111have been corrected for the blanks, which were highly reproducible for all rates of NI& input up to 1.1 X 10-6 mole/sec. Results of all experiments to determine the extent of KH3 decomposition are reported in Table I11 except some a t 2.45 mm. pressure for ammonia input rates in excess of 1.2 x 10-6 mole/sec., where results of all experiments, including blanks, were not reproducible. Results of photometry experiments are shown in Fig. 2. Discussion Possible Major Sources of Error.-Possible major sources of error in our measurements which require critical discussion are: (1) Occurrence of the NH3 trap reaction: This is considered to be of negligible importance. I n our system the flow time from discharp to trap (about 0.6 sec. a t 2.45 mm.) is such that considerE. R. Zabolotny and E. Gesser, J . Phye. Chsm., 66, 408
(1962).
1
2.30 5.78 9.82 5.60 6.98 9.75 9.98 4.17 9.90 2.14 2.16 5.24 5.36 7.37 9.25
"I
NIII i n p u t
destroyed
0 . !I4 0 98 2.30 7.53
0.00 .03 .08 . 00
1.70 4.00 7.63 7.63 0.87 2.05 4.51 7.21 11.2
0.31 .26 * 33 .16 .09 .IO .17 .ll OR
.
6.95 mm.
4.85 mm. 1
(20)
0 80 2.24 3.41 7.93
;O
1
Pig. 2.--Plot
"8
NHa input
5
2.25 4.20 6.98 8.05 11.05 11.35 2. '15
0.22 .25 .14 .18 .20 .22 .00
11.5 mm. 2 3 . l l 4 1.02 5 0.70
2.38 2.34 2.24 2.35
0.29 ,I6 .I2 .16
0.23 .52 .60 .44 -48 .a2 .:32 .IO .19 0.48 .46 *.,
9.154
.65 -88
9.83
.37
I
able decay of active nitrogen has occurred even in the absence of NH3 bcforc the reaction at - 196' can comis located mencc. Furthermore, when the frozen "3 near the discharge,'* the rate of its decomposition is ncgligible in comparison to the rate of the gas phase reaction for NH3 admitted through inlet no. 1 in our system. (2) Inefficiency of the U-trap used to retain unreacted NI.18: This introduces an error into the calculation of reaction time. It was observed that recovered XH3 was deposited in a 6-cm. region beginning about 0.5 cm. above the level of liquid nitrogen, so that reaction distance from inlet no. 5 to the trap was estimated to be 43 cm., rather than the 40 cm. from inlet to liquid nitrogen level. The extent of the remaining error in the estimate of reaction time is difficult to estimate, but even if it is appreciable, its magnitude decreases in a linear manner with increasing distance from ammonia inlet tube to the trap. It may therefore affect our quantitative results, but not the observed trend in our results. (3) Lack of reproducibility in the reaction system: This can be shown statistically by making a plot of the data in Table 111, which shows that extent of scatter of
Feb., 1063
HALF-LIFEOF A32,+ NITROGEN
results is much larger a t 11.5 mm. than at the two lower pressures, where it is quite small. This may be due to the formation of the pink active nitrogen a t higher pressures, which occurs in a highly non-reproducible fashi0n.l' Previous Results on NH3 Addition to Active Nitrogen.-Freeman and Winklerl' have demonstrated that there are a t least two chemically reactive species present in active nitrogen. They showed that under their conditions, the extent of NH3 decomposition rises with NH3 concentration to a limiting value which is only onesixth the concentration of atomic nitrogen. The latter was found to be about 8 mole % by titration with ethylene. It is now known that the yield of HCN from ethylena indicates an atomic nitrogen concentration which is considerably lower than that indicated by the NO so that the discrepancy between the optimum extent of NH3decomposition and the available supply of atomic nitrogen may be considerhbly greater than a factor of six.22 The reaction of active nitrogen with NH, has been investigated mass spectrometrically a t nitrogen atom concentrations of 1 mole yo or less. No decrease in atomic nitrogen or ammonia concentration could be detectecllz but a very slight extent of NH3decomposition was observed by scanning the Hs peak.l9 The effect of NH, addition on the nitrogen afterglow intensity, also studied a t atomic nitrogen concentrations of 1 mole or less, is much more striking.4112 A plot of the reciprocal of afterglow intensity us. NH3 concentration is linear, which is consistent with reaction of NH3 with a molecular afterglow precursor. Bayes and Kistiakowsky suggest this may be the 5Zg+ state of Nz. Interpretation of Present Results.--The results shown in Fig. 1 confirm the rapid and appreciable rate of NH3 destruction when it is added to active nitrogen produced in a condensed discharge. For higher rates of NHa addition a t inlet no. 5 the extent of reaction becomes independent of NH3 concentration. For higher rates of addition of NH3 a t inlets no. 1, 2, and 3, a further slight but perceptible increase in rate of NH3 destruction occurs, which appears linear in rate of NH3 addition, and is more pronounced the nearer to the discharge the position of NH3 addition. The latter effect was not observed under the conditions used by Fresman and Wink1er.l' The results, summarized in Fig. I , are not compatible with reaction with only one excited molecular species, whether the single species is generated by one or more mechanisms. By extrapolation of the linear portions obtained a t higher flow rates of XH3 shown in Fig. 1, back to zero flow rate of NH3, values for the extent of NH3 decomposition by the more abundant excited molecular species are obtained. These values shown in Table IV represent complete reaction of the more abundant excited molecular species, which we shall designate as A, with NHI. For reasons already partially outlined, we believe the species A is predominantly, if not entirely, A3&+ NP. That it is not an afterglow precursor is supported by the data shown in Fig. 2. The plot of the reciprocal of afterglow intensity vs. concentration of NH3 added is convex toward the NH3 concentration axis. It can be
grated first-order equation is obeyed quite well, and indicates that the half-life of species A is 0.094 ~ e c . 2 ~ The deviations from the first-order plot, while scarcely outside experimental error, might be attributed to a small extent of A3Zu+Nz formation by direct atom recombination or, if species A is a composite entity, to the more rapid decay of one of the species present. With regard to the former possibility, our data, with generous estimates of errors, become incompatible with the assumption of more than 10% of the recombining atoms forming A3&+ Nz molecules. It therefore supplements the data of Wentink, Sullivan, and Wray, who
(211 G . J. Verbeke and C . A. Winkler, J Phys. Chem., 64, 319 (1960). (22) For work supporting the HCN method of measuring N atoms Bee ( a ) A. N. Wright and C. A. Winkler, Can. J. Chen., 40, 5 (1962); (b) A. N. Wright, R. I,. Nelson, and C. A. Winkler, abtd., 40, 1082 (1962).
(23) A. N. Wright and C. A. Winkler, data presented at the 46th Cooference of the Chemical Institute of Canada, Edmonton, May, 1962. (24) A half-life of 0.084 set., based on a different interpretation, has been determined independently.JZb
0.7
8 0.1 o.2
sbo
0.0
~
1
\
n
"\
-0.1
0 \
-0.2
I
I
I
I
0.1
0.2
0.3
\
Time, sec.
Fig. 3.-First-order
plot for decay of species A, believed to be A3ZU+Nz.
made essentially linear if 1/1 is plotted us. NH3 ;recovered, rather than vs. NH3 added. This has been demonstrated independently by data considerably mare extensive than 0urs.2~ As already discussed, the difference between NH3 added and NHa recovered represents mainly reaction with species A. TABLE IV EXTENTOF REACTION OF NHa WITH SPECIES A NITROGEN Inlet no.
Reaction time, sec.
1 2 3
0 0.086 .170 .254 .334
4
5
IN
ACTIVE
NHa decomposed, mole/ce. = [AI
7.60 x 3.80 X 2.00 x 1.10 x 0.69 X
10-lo lo-" 10-10
10-10 10-'0
It would appear plausible to correlate the afterglow quenching with the slight decomposition of NHa which only becomes kinetically observable for high rates of NH3 addition through inlets nearest the discharge tube. Reaction of Ammonia with the More Abundant Molecular Species A.--lf species A were generated solely in the discharge tube, then its decay would follow the simple first-order equation
A plot, shown in Fig. 3, shows that in fact the inte-
H. B. DUSFORD
262
Yo]. 67
measured the fraction of atoms which enter into the also note that in Wilkinson’s observed bands29the four afterglow mechanism. The fraction of one-tenth repbranches were blended int,o one, and suggest a collision resents an upper limit on the fraction of atoms which induced mechanism to account for the observed broadencan form A3&+ Nzby all possible mechanisms. It ing and shorter lifetime obtained at atmospheric presagrees with a conclusion reached by Benson and Fueno,26 sure. However, the broadening is due to an instrumenwho, by comparison of the agreement between theory tal effect, not pre~sure.~OFurthermore, a beautiful, and experiment for atom combination to form ground sharp photograph of the 9-0 Vegard-Kaplan band in state molecules, concluded that few atoms can form absorption a t at,mospheric pressure has since been obelectronically excited molecules. The estimate of not t ained. 3 l more than 10% of the atoms forming triplet state moleConclusions Regarding Use of Condensed Discharges, cules contradicts the quantum statistical prediction -Whatever t,he final outcome of the apparent disthat nitrogen atoms will combine in the ratio 1:3 :5 into crepancy in the lifetime of A 3 & +Nz, our result of 0.08 the lL’,+, 3ZU+,and 5Zg+ states of Nz. The latter pref 0.04 sec. indicates the half-life (or half-lives) of diction ignores third body effects. It appears that the metastable species produced in a condensed discharge importance of third bodies in determining the relative through nitrogen. Our value might include the effect accessibilities to combining atoms of “allowed” elecof 6ome collisional quenching as well as radiative detronic states of molecules has yet to be examined theocay.32 It indicates that a condensed discharge is a retically. Our result also directly contradicts a conclupoor way to activate nitrogen for a study of nitrogen sion of Winkler and co-workersZ2that the species responatom reactions, unless it is used under very mild condisible for NH3 decomposition is generated, apparently tions, with the added safety factor of considerable decay solely, by atom recombination. With regard to the postime between the discharge t,ube and reaction vessel. sibility that species A is a mixture of different electronic Our results also may explain the apparent discrepancy states, it appears most likely that only electronic states between the two mass spectrometric st,udies of active of energy less than 9.756 e.v. need be considered.26 nitrogen. Berkowitz, et aZ.,s using a gIow discharge, Of these, states which are afterglow precursors or which detected 4S nitrogen atoms only, whereas Jackson and can decay by allowed transitions need not be considered. Schiff, using a condensed discharge, detecOed a second This restricts the discussion to the following states: species, which could be metastable molecules with a%,, a’1&-, A3&+,wlAu, and 3Au. The mean lifetimes energy 1.4 =t 0.6 e.v. below that of the dissociation of the first three states are estimated as 1.7 X limit of 9.756 e.v. for lZg+N2.33 0.04, and 0.026 sec.,6 and if, as discussed above, these Reaction of NH3 with the Afterglow Precursor.-If absolute values are seriously in error, a t least their relthe extent of NH3 decomposition with species A is subative values should be fairly precise. This establishes tracted from the total extent of NH3 decomposition the ala, state as too short lived t o be of concern here. shown in Fig. 1,then, as discussed above, the difference We are not aware of a measurement of the lifetime of may represent reaction with an afterglow precursor. the w state, but the 3Au state has been estimated to The extent of this reaction is too great to be accounted have a lifetime of 1-2 S ~ C . ~ ’ for by reaction of NH3 with B311gNz,if the lifetime of It would appear from Fig. 3 that if species A is a the latter species is 10-6 sec. If the scheme mixture, then either one species is predominant, or the h-1 two or more species present have nearly identical lifeI X N ic1 I -Xz** ( 5 & + ) M times. If any mechanism is available for Nz molecules k-1 to cross among the metastable states, even though they might all be formed in the discharge, they would evenkZ tually all populate the lowest lying metastable state, Y*** M . -,sz*(3&) M namely A3&+. The latter possibility is, however, sheer speculation, It should be noted, though, that considerk3 able stimulus for the present work mas provided by Nz** NH3 --+ Nz ? workers who postulated the occurrence of the reaction is applicable, then the linear plots of the extent of NH3 between NH3and As&+ N z . ~ ~ decomposit’ion by afterglow precursor us. ammonia The Lifetime of A3& + Nz.-Since the slight extent of concentration indicat,e that k z ) [MI is consideratom recombination which could form A3&+ Nz and ably larger than k3[XH3]. the possibility of an underestimation of reaction time due to inefficient trapping would both tend to shorten (29) P. G. Wilkinson, J . Chem. Phys., 30, 773 (1959). (30) P. G, Wilkinson, J. O p t . Soc. Am., SO, 1002 (1960). somewhat our observed half-life for species A, we esti(31) P. G. Wilkinson, results presented a t the First International Conmate it to be 0.08 f 0.04 sec. If this species is A3ZU+, ference on Vacuum Ultraviolet Radiation Physics, Loa Angeles, April, 1962. then our result is more in accord with the mean lifetime (32) Our limited and not too precise data a t higher pressures indicate t h a t collisional quenching is not likely t o be predominant, unless by coinoidence obtained by Wilkinson and Mulliken. Carleton and i t is compensated a t lower pressure by wall deactivation. Oldenberg point out that the discrepancy of a factor of (33) D. S. Jackson and H. I. Schiff, J. Chem. Phye., 28, 2333 (1945). 33 in the published lifetimes of A3&+ Nz is reduced to a However, referring to Fig. 3 of their paper, they extrapolated t h e upper section of what is presumably a composite curve. Actually, the inflection factor of about 8 when the difference in v3factors, for the point might be taken more correctly as the onset of the higher energy different observed bands, is taken into account. They process. This gives a n appearance potential of 16.8 e.v. rather than 16.1 B.V.
+
+ +
+
+
+
+
+
(25) (26) (27) (28) (b) K.
S. W. Benson a n d I. Fueno, J . Chem. Phys., 36, 1957 (1962). K. R. Jennings and J. W. Linnett, Quart. Rev., 12, 116 (1958).
C.Kenty, J . Chem. Phys., 35, 2267 (1961). (a) R. Kelly and C. A. Winkler, Can. J . Chem., 38, 2314 (1950’; D. Bayes, zbad., 89, 1074 (1961).
and hence corresponds t o a possible metastable mulecule 2.1 e.v. below the dissociation limit. If one uses the correct I ( N ) = 14.6 e.v. rather t h a n their experimental value of 14.7 e,v., the position of the metastable molecule is further lowered t o 2.3 e.v. below the dissociation limit for ground state nitrogen. We are indebted to Dr. John Herron for these observations.
