8 Reactions of N (A Σ+u) 3
2
ROBERT A. YOUNG and GILBERT A. ST. JOHN 1
Stanford Research Institute, Menlo Park, Calif. 94025 The interaction of N (A Σ+u) with atomic nitrogen and NO was studied by observing the steady state and transient behavior of Vegard-Kaplan band emission from N (A Σ+u) excited in a Tesla-type discharge in flowing nitrogen. N (A Σ+u) was deactivated by Ν 3
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2
2
2
3
3
N (A Σ+u) + Ν-->N * + Ν 3
2
2
with a rate coefficient of 5
× 10
-11
cc./sec. and by NO
N (A Σ+u) + NO-->Ν (Χ Σ) + NO* with a rate coefficient of 7 × 10 cc./sec. NO was excited to the A Σ level by N (A Σ+u) with a rate coeffi cient of 3 × 10 cc./sec. and the A Σ+ level of NO was excited by N (A Σ+u) with a rate coefficient of 1.3 × 10 cc./sec. 2
3
1
2
-11
2
v'
=
0
3
2
-11
2
v' =
0,1
2
3
v'
=
v'
=
3
1
-12
*Tphe chemical effects occurring in an electrical discharge are the consequence of the injection of energy into a neutral gas from an applied electrical field by way of electron impact processes. Collisions of energetic electrons with neutral species produce ionization, fragmentation of molecules, and electronic, vibrational, and rotational excitation of the neutral gas. Reactions of the positive ions, free radicals, and excited species with the unexcited gas are, of course, favored over reactions between the species generated by electron impact because of the much larger density of the original neutral, unexcited gas. In many cases, however, the neutral gas is inert even to energy rich species and then reaction between minor constituents, either those produced by electron impact or others purposely added to the gas, predominates and can be studied. Discharges in molecular nitrogen have been studied for almost a century. This is partially because it is so readily available, and partially 1
Present address: Physics Department, York Univ., Toronto, Ontario, Canada.
105 Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
106
CHEMICAL REACTIONS IN ELECTRICAL DISCHARGES
because N is so inert that reaction between electron impact produced species predominates. Despite this long period of study, the phenomena occurring in discharged nitrogen are poorly understood. The ionic phenomena occurring in a pure N discharge are evidently relatively simple. Since N does not form a stable negative ion, electrons are removed by N dissociative recombination. However, N and N ions are also observed and are probably produced by reactions of Ν and N with N . The recombination of nitrogen atoms, originally produced by the discharge, gives rise to phenomena of remarkable complexity. Perhaps the best known is the Lewis-Rayleigh afterglow. This afterglow consists entirely of the 1st positive bands of N and reflects a peculiar vibrational population distribution in the emitting state. Since this state cannot be directly formed from unexcited atoms, it is necessary to postulate subse quent events causing the originally formed molecules to make nonradiative transitions to the radiating state. 2
2
2
2
2
2
+
3
+
4
+
+
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2
The processes involved in the afterglow, following the initial atom recombination, necessarily involve excited N . Hence, to understand this phenomenon, it is essential to study the behavior of these excited N molecules and, in particular, those processes which alter its quantum state while removing little energy. Although the excitation of the LewisRayleigh afterglow requires a quantum state change with little energy removal, the complete dissipation of the energy of nitrogen atom recom bination requires either large energy removal from N in the A S meta stable state (radiative lifetime 10-12 sec.) directly to produce unexcited N , or a large internuclear-change-small-energy-removal transition hori zontally on the potential diagram of N so as to place the molecule on the vibrational ladder leading to unexcited N . The A 2 state is singled out in this consideration of recombination energy degradation because (1) almost all the emission during atom recombination terminates on this level, and (2) direct recombination into the A 2 state is, by statisti cal weight, 1/3 of the total recombination flux. Other considerations make it likely that a much larger fraction of the successful recombination events pass through the A 2 state. If the N ( A 2 ) molecules formed by recombination are removed only by radiation, this emission should be intense in the Lewis-Rayleigh afterglow. However, this emission has never been observed as a conse quence of atom recombination. To account for this, collisional deactiva tion must occur rapidly. However, emissions from N ( A 2 ) have been observed in appropriate nitrogen discharges and in very high pressure, short lifetime afterglow. The major difference between those situations where emission is observed and where emission is not observed appears to be in the concentration of atomic nitrogen. Hence, a working hypothe2
2
3
2
2
2
3
2
3
3
2
3
U
u
u
u
+
+
+
+
2
3
U
+
Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
U
+
8.
YOUNG AND ST. JOHN
Reactions of N (A % ) 2
3
107
¥
U
sis (8) is that N ( A 2 ) is converted to another state of N in a process involving atomic nitrogen—i.e., 3
2
U
+
2
N ( A V ) + Ν -> N * + Ν 3
2
2
To study this process it is desirable to produce a detectable (by emission) amount of N ( A 2 ) and study its behavior as the concentration of atomic nitrogen is increased. It is also advantageous to find other com pounds which essentially remove all the energy residing in N ( A 2 ) and thereby short circuit the flow of energy through N (A 2u ) to other states of N . Finally, if the molecules which abstract all the energy from N ( A 2 „ ) rapidly radiate, then this energy can have no further effect within the system. A by-product of such a phenomenon is the develop ment of a sensitive detector of the energy entering the A S state of N . 2
3
U
+
2
3
2
3
U
+
+
2
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2
3
+
3
n
+
2
Experimental Figure 1 shows the experimental setup. Nitrogen was excited by a pulsed 150-kc, 5-kv. oscillator (of the type used in high-voltage supplies) in a 5-liter quartz bulb with exterior electrodes of silver-conducting paint applied above and below the center plane of the bulb with a radius of approximately half that of the bulb. A 1/2-meter Jarrell-Ash SeyaNamioka monochrometer was used to scan the discharge spectrum or to isolate bands for decay measurements. The signal from the photomultiplier used to detect emission leaving the exit slit of the monochrometer was very low, and an Enhancetron signal integration unit (Nuclear Data Inc., Palatine, 111.) was used to abstract the decaying signal from the noise. Prepurified nitrogen was passed through a liquid nitrogen trap, then through heated titanium-zirconium and two additional liquid nitrogen N FROM PRESSURE REGULATOR 2
jr- PRESSURE GAUGE ^_^T0 LIQUID Ν2TRAP " AND FORE PUMP PHOTOMULTIPLIER AND FILTERS CHEMILUMINESCENT OBSERVATION BULB SILVER PAINT ELECTRODES
LIQUID N TRAP 2
MICROWAVE DISCHARGE LIQUID N TRAPS 2
Figure 1. System used for studying the effects of gases on N (A % ) radiation 2
Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
3
+
U
108
CHEMICAL REACTIONS IN ELECTRICAL DISCHARGES
traps, past a microwave excitation section, an N O titration inlet, and finally into the 5-liter observation bulb, from which it was exhausted by a mechanical vacuum pump at approximately 1000 cc./sec. A filtered photomultiplier downstream from the bulb observed the N first positive bands resulting from atom recombination (9). This permitted the atom concentration to be computed after calibration against an N O titration 2
(9).
Interaction Of N (A t ) s
2
With Ν
+
u
Results and Discussion. Figure 2 shows spectrometer scans of the discharge. In Figure 2(a) the N second positive and NO γ bands pre dominate below 3000 A. The β bands were the only other system of NO detected, and these were very weak. If the microwave discharge is adjusted to produce a small quanity of atomic nitrogen, the N O bands can be suppressed almost entirely. [See Figure 2(b)]. Although the Vegard-Kaplan bands are also reduced, the second positive bands remain relatively unchanged. Addition of NO causes a large increase in the intensity of the NO γ bands [See Figure 2(c)]. In Figure 3 the transient behavior of the Vegard-Kaplan band in tensity, 7, is shown. The rise of I with time t when the RF excitation is turned on is given by I = J ( l — e" ) and the decay is given by I = J e~ , where τ has the same value in both expressions. On the assump tion that N ( A 2 ) (called N * hereafter) decays more slowly than any of its sources, τ represents the actual lifetime of N * . All T'S SO far measured are much larger than the characteristic decay of electron excitation (^0.1 msec.). The value of l / τ for N * in the zero vibrational level vs. [N] is plotted in Figure 4 for a variety of conditions. The linear dependence of l / τ on [N] implies a reaction such as
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2
t/r
0
0
i/r
3
2
U
+
2
2
2
N * + N^>N ** + N 2
(1)
2
and the slope of Figure 4 is k = 5 Χ 10" cc./sec. The intercept in Figure 4, representing l / τ for zero added nitrogen atoms, gives a τ much shorter than that caused by radiation. Although atoms were not added from the upstream source, they were present because of the RF discharge. Measurements of the atom concentration when only RF excitation is occurring indicate that Reaction 1 accounts for the short lifetime of N * when atoms are not added. This value of k agrees with the value de duced by Wray from studies in shock-heated Ν—N mixtures (6). Figure 4 indicates that the lifetime of N * is independent of pressure and that fc is independent of the presence of He. However, with small amounts of N in rare gases the concentrations of Ν produced by the RF discharge is smaller and hence the lifetime of N * without adding Ν is larger. 11
x
2
x
2
2
x
2
2
Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
3600
3500
3400
3300
φ —
3200
—
θ"
C O
3000
—
2900
θ"
—
CJ
θ"
—
CJ
O* —
to O*
2400
*O l CJ
2500
ro O* — CJ
2600
*Ο Ι
