J . Phys. Chem. 1988, 92, 4340-4348
4340
Photolysis of Chlorine Nitrate at 254 nm J. P. Burrows,* G. S. TyndallJ and G. K. Moortgat Max-Planck-Institut fur Chemie, Postfach 3060, Mainz, F.R.G. (Received: May 21, 1987; In Final Form: February 16, 1988)
The photolysis of C1ONO2has been studied at 254 nm. The production of NO,, NO2, and ClONO and the removal of ClON0, have been observed directly, and the yield of O(,P) has been inferred from the production of 0,.The production of Cl and NO, was found to be the dominant photolysis pathway. For ClONO, concentrations of 55 X lo1, molecules cm-,, the values determined for the quantum yields for the removal of ClONO,, -@cIoNo2, and the production of NO,, avo,,were 0.99 f 0.1 and 1.04 0.04, respectively. However, these yields appear to be dependent on the ClON0, concentration and decrease as ClONO, concentration increases. For CIONO, concentrations in the range (3-8) X 1014molecules cm-,, the yields, obtained for the formation of 0, NO2, and ClONO, are = 0.24, aNO2 = 0.25, and @cloNoS 0.2. No evidence was found for a significant production of NO. The observed behavior may be explained by the production of a long-lived excited state of C10N02. The results are discussed in relation to the stratospheric behavior of C10N02.
*
Introduction C1ONO2 is thought to act as a temporary reservoir in the stratosphere for both NO, (Le., N O and NOz) and C10, (Le., C1 and C10) species.'*z The reactions of such compounds play an important role in determining the stratospheric 0, abundance. Photolysis by ultraviolet radiation is an important loss process for C10N02 in the stratosphere. However, the precise effect of this process on the stratospheric O3budget depends on the nature of the products formed. Several photolysis pathways are available to CION02: CIONOz + hv- C10 NO2 Xl 51100 nm (1)
-- + + + - + ++ CI
C1
NO3 X2 I 700 nm NO O2 X3 I 652 nm
ClONO
+
C1
+
NOz
O(,P) O(,P)
X4 5 391 nm X5 I318 nm
(2) (3) (4) (5)
ClONO O(lD) 5 241 nm (6) The gas-phase ultraviolet photolysis of C 1 0 N 0 2 has been studied by a variety of techniques, of which two studies indicated evidence for channel 4. Smith et aL3 photolyzed C1ONOZmixtures at 302.5 nm and used end product analysis to determine the dominant photochemical pathways. Adler-Golden and Wiesenfeld4 observed the production of 0 atoms in the flash photolysis of C 1 0 N 0 2 mixtures but failed to observe C1 atoms and estimated an upper limit for the quantum yield of C1 to be 4%. In contrast, four groups have found evidence for channel 2. Chang et aL5 used the technique of very low pressure photolysis to study the photochemical pathways 1 to 5 , the light source used being a high-pressure xenon lamp. Detection of both products and reactants was made by mass spectrometry, and NO, was shown to be the major product of the reaction by the appearance of the m / e 62 peak. C1 atoms were identified by the observation of either C12 in C1ONOZ-N2 mixtures or HCl in mixtures which contained added CzH6. In addition, O(3P) yields were inferred by an NOz titration technique. Quantum yields for the production of C1 atoms, NO,, and O(,P) of 1.0 f 0.2, 0.5 f 0.3, and 10.1, respectively, were determined, and the authors concluded that channel 2 is the dominant photolysis channel. Marinelli and Johnston6 studied reactions 1-5 at 249 nm; the primary photolysis product NO, was detected by tunable dye-laser absorption spectroscopy at 662 nm. The primary quantum yield for NO, production was found to be (0.5(-0.1 + 0.3)). Margitan' also studied the photolysis of C 1 0 N 0 2 by laser flash photolysis, using a Nd:YAG laser at 266 and 355 nm. He observed the production of CI and 0 atoms by resonance fluorescence and determined the quantum yields for C1 and 0 atom production to be 0.9 and 0.1. ---*
'Present address: NOAA R/E/AL-2, Boulder, CO 80303
0022-365418812092-4340$01 SO10
Finally, Knauth and Schindler* photolyzed C 1 0 N 0 2 in the presence of NO2 at 263 and 313 nm using a high-pressure Hg lamp and appropriate optical filters. Relatively high concentrations of ClON0, were used, and end product analysis was made by mass spectroscopic, ultraviolet, and infrared measurements. These authors found quantum yields for the removal of C 1 0 N 0 2 and production of N 2 0 Sof 1.12 f 0.2 and 0.96 f 0.15, respectively. The objective of this study was to investigate the primary products formed in the photolysis of C 1 0 N 0 2 a t 254 nm. A secondary aim was to understand the apparent discrepancy in the literature between the quantum yields for the production of NO3 and CI atoms.
Experimental Section The photolysis of ClONO, at 254 nm has been investigated in the same apparatus in three ways. The first method employed continuous photolysis of flowing gas mixtures with reactant and product detection by matrix isolation Fourier transform infrared (FTIR) spectroscopy. In the second method the continuous photolysis of static gas mixtures was observed by UV-visible absorption measurements. The third approach employed the modulated photolysis of flowing gas mixtures (known as molecular modulation sptroscopy (MMS)), with detection of reactants and products by UV-visible absorption spectrometry. The apparatus used in this study has been described in detail elsewhere9J0 and is shown schematically in Figure 1. The photolysis cell consisted of a jacketed quartz vessel, 148 cm long, whose end windows were inset yielding an optical path length of 114 cm. The photolysis cell was internally coated with a thin film of Teflon to minimize wall reactions. The photolysis cell was surrounded by six low-pressure Hg lamps (Philips TUV lamps). More than 90% of the output intensity of these lamps is in the 253.7-nm emission; the rest of the power is distributed over the other Hg emission lines and a weak visible continuum. The photolysis lamps were powered by a (1) Rowland, F. S . ; Spencer, J. E.; Molina, M. J. J . Phys. Chem. 1976,
80, 2711.
(2) Eggleton, A. E. J.; Cox,R. A.; Derwent, R. G. New Sci. 1976 (May 20), 402. (3) Smith, W. S.;Chou, C. C.; Rowland, F. S . Geophys. Res. Lett. 1977, 4, 517. (4) Adler-Golden, S. M.; Wiesenfeld, J. R. Chem. Phys. Lett. 1981, 82, 281. ( 5 ) Chang, J. S.; Barker, J. R.; Davenport, J. E.; Golden, D. M. Chem. Phys. Lett. 1979, 60, 385. (6) Marinelli, W. J.; Johnston, H. S. Chem. Phys. Lett. 1982, 93, 127. (7) Margitan, J. J. J . Phys. Chem. 1983, 87, 674. (8) Knauth, H.-D.; Schindler, R. N. Z. Naturforsch., A: Phys., Phys. Chem., Kosmophys. 1983, 38A, 893. (9) Burrows, J. P.; Griffith, D. W. T.; Moortgat, G. K.; Tyndall, G. S . J . Phys. Chem. 1985, 89, 266. (10) Burrows, J. P.; Tyndall, G. S.; Moortgat, G. K. J . Phys. Chem. 1985, 89, 4848.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4341
Photolysis of Chlorine Nitrate at 254 nm
1-2-
MATRIX CRYOSTAT
I
m
n
n
WAVE FORM GENER ATGR
DIGI'IZER AMPLIFIER
Figure 1. Schematic diagram of the matrix isolation FTIR, UV-visible absorption, and modulated photolysis apparatus. TABLE I: Ultraviolet Absorption Cross Sections Used in This Study'Vb gas 220 nm 235 nm 254 nm 313 nm -C 1.9 x 10-19 c12 CION02 3.4 X 1.5 X 4.8 X 1.3 X CION0 ?d 2.1 X 7.5 X 1.0 X ClNO 9.0 x 10-18 1.5 x 10-18 2.4 x 10-19 1.2 x 10-19 4.1 x 10-19 1.5 x 1049 1.9 x 10-20 2.0 x 1 0 4 9 NO2 NO3 2.2 x 10-18 7.7 x 10-19 3.4x 10-19 1.9 x 10-20 NZOJ
350 nm 1.8 x 10-19
-
2.7 X 1.4x 10-19 3.9 x 10-19
-
2.3 x
10-21
400 nm
6X
6.1 x -
623 nm -
10-19
1.9 x 10-20 1.2 x 10-17 -
ref 13 13 22 14 15 io, 23 13
'The units are cmz molecule-'. bThe resolution is 5 nm except for NO3 where the resolution is 1 nm. c - denotes u undetermined or anticipated cm2 molecule-'). d ? denotes that u is undetermined but its value estimated by extrapolation is large (Le., u = 3 X small value (Le.. u 5 1 X 10-18 cm2 mkeche-1).
20-kHz signal of up to 1 kV and were switched on and off at frequencies between 0.01 Hz and 5 kHz or run continuously. The ratio of light-on to light-off time could be varied between 0.1 and 9. Collimated light from either a Dz lamp or a quartz halogen lamp passed once through the cell before being focused onto the entrance slit of a 0.3-m monochromator. The wavelength calibration of the monochromator was checked at 254,313,404, and 632.8 nm with the emission from a low-pressure H g lamp and a HeNe laser. The current from the photomultiplier was converted to a voltage, V. In the absence of photolysis this voltage was then offset by an equal and constant voltage, V,. The residual signal, AU = V - V,, was sent to a 1024-channel 9-bit averager. In this manner small modulated absorptions produced in the cell can be measured with increased sensitivity. A photodiode which observed the photolysis lamps provided the trigger for the averager. The optical density, OD, for small absorptions is given by O D = Au/Vo After an experiment, the signal from the averager and the voltage V ,were sent to a microcomputer for mathematical manipulation. Modulated absorptions of both products and reactants were recorded at different wavelengths (e.g., NO3 at 623 nm, NOz a t 400 nm), and the absorption cross sections used are listed in Table I. The initial gas-phase concentration of ClONO, was determined from optical density measurements at 220 nm on its addition to and removal from the cell. Matrices were grown in a microsampling cryostat which was connected to the photolysis cell through a Teflon solenoid valve. Typically, matrices were grown at 10 K by opening the solenoid valve in five 0.1 5-s pulses. The cryostat chamber was mounted
permanently to the FTIR spectrometer, Model BOMEM DA03.01. Matrix isolation FTIR spectra were usually recorded from 400 to 3900 cm-' at 0.5-cm-' resolution. ClONO, was synthesized by mixing freshly prepared ClzO with N 2 0 5at 195 K and allowing the contents to warm slowly to 273 K." The resulting mixture was purified by trap-to-trap distillation at 195, 155, and 77 K. The fraction which collects at 155 K contains C10N02. The distillation was repeated several times to ensure high purity. Experiments were performed on both static and flowing mixtures of CION02 in N2 or Oz. Flows of C10N02 were obtained by passing dry N, through C 1 0 N 0 2 held at 195 K. As shown in Figure 1, the gas mixtures are flowed transversely through the photolysis cell to ensure thorough mixing. At 195 K the vapor pressures of the solids N 2 0 5and NzO, are much lower than that of the liquid C10N02. Consequently, any impurities in the C1ONOZdo not enter the flow. The purity of C 1 0 N 0 2 in the photolysis cell was investigated by both its UV absorption and matrix isolation FTIR spectra. Small amounts of NOz and H N 0 3 were observed. The source of the H N 0 3 is probably heterogeneous reaction of C10N02 and HzO at the wall of the cell. The source of NOz may well b e decomposition of CIONOz and H N 0 3 on the walls of the cell. The decay of static mixtures of CIONOz in the cell was monitored both by matrix isolation FTIR and UV absorption. It was essential to condition the photolysis cell to minimize wall losses: C10N02
-
products
(7)
(1 1) Schmeisser, M.; Fink, W.; Brandle, K. Angew. Chem. 1957,69,2085.
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
4342 020,
P I
,
1,
0161
MATRIX ISLATION FTIR SPECTRUM ff CION0 AND CINOi Cl2+NO,’N,* hvl310rhi100nmi
I
HNO,
ClONO
Burrows et al. strengths obtained are listed in Table 11. The intensity of the photolysis lamps at 254 nm was measured by using the rate of photolysis of ClNO as an actinometer. ClNO was prepared by mixing pure C12 with an excess of NO and allowing the equilibrium to establish itself. Subsequently, the ClNO was frozen at 195 K and the excess NO pumped away. This procedure was repeated several times to ensure high purity. The photolysis rate of ClNO at 254 nm, klo, was determined from measurement of the decay of pure CINO: ClNO
OOL
I
-
+ hv (A = 254 nm) C1 + N O C12 + NO C1 + ClNO
-
(10) (11)
The decay of ClNO concentration is related to k l o by the expression 2klo = -d In (ClNO)/dt
3001 1500
1550
1650
1600
1700
1750
18bOCm-1
Figure 2. FTIR matrix isolation spectrum of a photolyzed (310 5 X 5 400 nm) gas-phase mixture of C12 (1 .O X 10l6molecules cm-’) and NO2 (7.0 X lOI4 molecules cm-)) in N2 (90 Torr) at 298 K. TABLE 11: Matrix Isolation F H R Band Strengths
molecule CION02
ClONO ClN02 N205
absorption, cm-l
assignt
778 809 1292 1730 1717 1269 1674 1245 1704
v4
V3
u2 VI1
HNO,
U6)
VI VI *4
VI2
1.o 1.5 0.59 z0.2
VI I
1745 NO2
(v2 +
VI
1616 902
SdS”, 0.41 0.30 0.73 1.09 20.25 -0.13 ~0.3 1.o
”3
v5
After this conditioning of the cell, the rate of loss of C 1 0 N 0 2 at s-I. the wall, k,, was determined to be k7 11.0 X In the experiments using matrix isolation, the bulk flow of N2 contained 10 vppm of N 2 0 which was used as a reference gas of known mixing ratio in the N2 matrix. The absorption due to the v3 band of N 2 0 was used to calculate the relative band strength Sf/SFOof absorption features due to other species. Calibration was achieved by flowing mixtures of the species, X, in the N20-N2 bulk gas through the photolysis cell and depositing a matrix. The gas-phase concentration of the species was determined from optical density changes in the UV or visible spectrum. It is assumed that mixing ratios in the matrix are the same as those in the gas phase. The relative band strengths, S y ” / S y ,were calculated and the values obtained are listed in Table 11. A C I O N 0 matrix isolation FTIR reference spectrum was obtained by deposition of gas-phase mixtures of CI2and NO2in N2, photolyzed by black lamps (310 IX I400 nm). An example of the spectrum obtained is shown in Figure 2. The photolysis of such mixtures proceeds via the reactions Cl2 hv CI CI (8)
+ CI + NO2 + M +
-+
-+
ClNO2
+
CION0
+M
+M
(9a) (9b)
Niki et a1.I2 have estimated the branching ratio k9,/ks to be 0.8 at 700-Torr total pressure. The infrared band strengths of the absorption features of C1N02 and ClONO relative to the band strength of the v3 band of N 2 0 were calculated from an estimate of the C I O N 0 and C I N 0 2 gas-phase concentrations and the matrix isolation FTIR absorptions. The concentrations of CION0 and C l N 0 2 were estimated from o tical densit measurements around 220 nm, assuming that z u$:$2 and using the recommended branching ratio k9Jkgb.I3 The relative band
&ENo
(12) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1978, 59, 78.
The value of klo was determined to be (8.3 f 0.5) X IO4 s-I per lamp. The rate of absorption of photons by C10N02 at 254 nm, kphot, was calculated from a knowledge of klo and the absorption cross sections of ClNO and C 1 0 N 0 2 at 254 nm, &lYo and uCIoN02: 254 5
kphot = C k j = k ~ o ~ ~ ~ $ “ 0 2 / d J ~ o i=1
The value of &N:o2 is taken from an evaluation of photochemical data for use in stratospheric m0de1ing.I~ At 253.7 nm uflyo was measured to be 2.6 X cm2 molecule-’ in this work and is in agreement with a recent study of the UV spectrum of CIN0.I4 A value of (1.5 f 0.09) X 10” s-I per lamp for kphotwas obtained. NO2 has a minimum in its UV absorption around 250 nm.I5 Due to the small emission at 3 13 nm from the photolysis lamps and subsequent photolysis of NO2, it was necessary to measure the photolysis rate of NO2. This was readily achieved by observation of the photolysis of pure NO2, which proceeds via the reactions NO2
+ hv
+0 NO + 0 2
+
0 + NO2 ---*
NO
(12) (13)
The NO2 decay is given by -
d l n [NO,] dt
d[NO] 1 =--- 2kl2 dt [NO210
Both the decay of NO2 and the production of NO were monitored. The NO was measured by resonance absorption of the y(0,O) band of NO at 226 nm emitted from a microwave lamp, through which a flow of air at low pressure passed. The value of k12was determined to be ( I .22 f 0. l ) X per lamp. Results ( a ) Matrix Isolation FTIR Studies. CIONO, in N2was flowed through the cell, and matrix isolation FTIR spectra of both the initial mixture and the photolyzed mixture were recorded. Two concentrations of CION02, 3 X lOI4 and 7 X I O l 4 molecules ~ m - ~ , were photolyzed, and the residence times, tlS, in the cell were 45 and 3 s, respectively. In both cases the total pressure in the cell was maintained at 20 Torr and the temperature was 298 K. An example of the FTIR spectrum obtained is shown in Figure 3. As well as the precursor CIONO, and impurities H N 0 3 , NO2, and H,O, the end products N 2 0 5and CION0 are identifiable, N205 by its characteristic 1704- and 1745-cm-’ absorptions and C I O N 0 by its weak absorption at 1717 cm-I. The ratio of (13) DeMore, W. B.; Margitan, J. J.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. JPL Publication 85-37, July 1985. (14) Tyndall, G. S.; Stedman, K. M.; Schneider, W.; Burrows, J. P.; Moortgat, G. K. J . Photochem. 1987, 36, 133. (15) Schneider, W.; Moortgat, G. K.; Tyndall, G. S . ; Burrows, J. P. J . Photochem. Photobiol., A 1987, 40, 195.
Photolysis of Chlorine Nitrate at 254 nm
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4343
0 20 MATRIX ISOLATION F T I R SPECTRUM
OF CIONO,
m
PHOTOLYSIS
A
0 16
CIONO2 + hv Ih: 25Lnml lo1
CIONO,
Ibl
CIONO,+N,+
+
N,
hvl?,:25lnml
10-
.
090 12
08 -
w
0
c z
---I\\ \
m,'
E
2
0
2 tamps 3 tamps L Lamps but l110 light 1 tamp
\
008
0 OL
L 1.10'2
Figure 3. FTIR matrix isolation spectra of gas-phase mixtures of CIONO2 (3.0 X lOI4 molecules ~ m - in ~ )N2 (20 Torr) at 298 K: (a) unphotolyzed and (b) photolyzed (A = 254 nm).
i11013
Figure 4. Plot of
-ICIONOJ
1.10'' molecule cm-'
-
I
1 1015
1.10'6
aOD versus C10N02 concentration.
[CIONO]/[NZO5] changed from 0.5 to 1.2 as the C 1 0 N 0 2 concentration was increased from 3 X lOI4 to 7 X lOI4 molecules cm-,. The formation of ClONO may be interpreted as evidence for the Occurrence of pathway 4. The appearance of N2Os is explained by reaction of NO, with impurity NO2: NO2 + NO, + M = NzOS + M (14)
NO3 is generated directly by channel 2 and also by reaction of the C1 or 0 atom products with excess CIONOz via reactions 15 and 16: C1 C 1 0 N 0 2 C1, NO3 (15)
+
0 + ClONO,
-
+ C10 + NO3
(16)
As the ratio k16[C10N0zJ/k,3[NOz]was about 0.4 in these experiments (no added O,), the majority of 0 atoms reacted with NO2 via reaction 13. In Figure 3 the amount of NOz also increases on photolysis of C10N02. This could possibly indicate that a photolysis pathway such as reaction 5 is present. However, it may also result from secondary chemical reactions. Provided pathway 2 dominates, the amount of NzOSobserved should be approximately the same as the amount of CION02 photolyzed, namely, 2t,,kph,[C10NOz]o (assuming that the NO3 formed reacts with the NOz impurity). However, the amount of NZOsobserved was less than that expected. An upper limit for the yield of ClONO can be estimated assuming the C I O N 0 to be in a stationary state determined by production via C10N02 photolysis (Le., kph,[CION02]o) and flow out of the cell. @cloNowas found to be 10.2 at the concentrations of ClONO, used. ( b ) UV-Visible Studies. In order to quantify the somewhat qualitative observations from the matrix isolation FTIR studies, experiments were performed on static and flowing mixtures with detection of products and reactants by UV-visible absorption spectrometry. The bulk gas was either Nzor Oz at a pressure of 20 Torr, and the cell was normally held at 298 K. (i) Photolysis of Static Mixtures of CIONOz in N 2 . In one set of experiments static mixtures of C1ONOZin N2 were photolyzed. The C I O N 0 2 concentration was varied from 8 X lo', to 1.0 X 10l6 molecules cm-,, and the initial decay of optical density at 220 nm for each concentration was recorded. The rate of change of optical density, d(OD,,,)/dt, is related to the path length, 1, absorption cross sections at 220 nm, ( T : ~ ~ , and concentrations of absorbing species, ci (e.g., i are C1ONOz, NO,, NO2, NzOs, and CIONO), by the equation n
d(OD2,o)/dt = l ( E4 2 0 dci/dt) i=l
Since C10N02 has the largest absorption cross section at 220 nm
0
2.5
-t ( s )
-
5
7.5
10
Figure 5. Plot of the NO, concentration versus time in the modulated photolysis (A = 254 nm) of CIONOz (8.0 X l O I 4 molecules ~ m - in ~ )N, (23 Torr) at 313 K.
-
of all reactants and products, then, to a first approximation as t 0 (Le,, at times less than the induction period for secondary products), this yields (d(ODzzo)/dt)o= ~ U $ ~ ~ ~ ~ ~ ( /dt), ~ [ C ~ O N O ~ J In excess CIONOz every Cl atom produced by channels 2, 3, and 5 reacts rapidly via reaction 15 to remove a C1ONOZand generates an NO3. In this way the C1 concentration rapidly reaches a small stationary state. This implies that, under the conditions used, every photon which produces a C1 atom removes two C1ONOZmolecules. Therefore, the yield for the removal of C1ONOZ,determined from the changes in OD at 220 nm, @oD, may be defined by the relationship @OD
(d(ODZ20)/dt)O
= - 2kphot[C10N02]ola$~~No~
where [C10N02Jois the initial concentration of ClONO,. If every photon absorbed by C1ONOz produces a C1 atom, then the value of aOD is 1. If other processes take place, then the value will be less than 1. A plot of aoDversus initial ClONO, concentration is shown in Figure 4. For CIONO, concentrations of 1 5 X lOI3 molecules ~ m - @OD ~ , values lie close to 1 (mean @OD = 0.99 i O.l), but for ClONO, concentrations of 25 X IO', molecules ~ m - ~ , decreases toward a mean value of 0.31 & 0.05 above l X l O I 5 molecules ~ m - ~ . The influence of photolysis lamp intensity on GOD was investigated by varying the number of lamps and/or reducing their output intensity. No strong dependence of the quantum yield on light intensity was observed. (ii) Modulated Photolysis of Flowing Mixtures of C10N02 in Nz. A second set of experiments were performed onflowing C10N02 mixtures. The NO, produced by modulated photolysis of these mixtures was monitored by optical density measurements
Burrows et al.
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
4344
0
via reaction 13 in the absence of O2to form NO, which subsequently reacts with NO3 to produce NOz:
1 Lamp
3 Lamps A
L Lamps
NO 10-
0.9 -
oa -
1- :::-
2 *
1
- 05
0.5
- OL - 03
0.1 -
0.3-
02-
402
01 i
101
I
1
1x10'2
1.10'~ -[CICNOII
Figure 6. Plot of
1x10" 1.10'5 molecule cm3 4
1.10'6
aNO, versus CIONOz concentration.
at 623 nm. An example of the production of NO, as a function of time is shown in Figure 5. NO, is produced by both channel 2 and reactions 15 and 16. Provided that C 1 0 N 0 2 is in excess, the initial rate of increase of NO3 concentration, (d[NO,]/dt),, is given by (d[N031 /dt)o = kz[ClONO2]o + kIS[Cl] [CION021 + k1,5[0] [CION021 The production of NO3 by reaction 16 is small as the majority of 0 atoms react with impurity NO2. The parameter aNP, may be considered as the yield of NO3 and is defined by the relationship @NO?
=
(d[N031 /
WO
2kphot[C10NoZ10
(d[NO3]/dt>, is the initial rate of increase of [NO,], and [ClONOz], is the initial concentration of C1ONOZ. If every photon absorbed by CIONOz produces a C1 atom and an NO, via channel 2, then the value of is 1. If other photolysis pathways are is less than 1. present, then aNO, aN03 was determined as a function of [CIONOz]o,and a plot is shown in Figure 6. This curve is similar in appearance to that obtained for aOD.For CIONOz concentrations of 1 5 X lo', molecules ~ m - ~ , is close to 1 (mean @NO, = 1.04 f 0.05). At higher concentrations ONO3decreases. At the highest flowing concentrations of CIONOz used, 8 X l O I 4 molecules cm-,, the is 0.44 f 0.09. mean value of was invesThe influence of photolysis light intensity on tigated by varying the number of photolysis lamps. No significant on photolysis light intensity was noted. To dependence of aNO, reduce the yield of NzOs, several experiments were undertaken at 3 13 K. N20, dissociates approximately 10 times faster at 3 13 K than at 298 K, regenerating NOz and NO,. No significant changes in @NO, were observed a t this temperature. To obtain values of @OD in the modulated photolysis of ClON02 mixtures, the absorption at 220 nm was monitored. In this case the photolysis of relatively high concentrations of ClONOz (4.8 X lOI4 and 8.5 X l O I 4 molecules cm-,) were studied at 313 K. The values of CpoD were in agreement with those obtained in the investigations of static mixtures. In a further set of measurements the modulation of optical density at both 350 and 400 nm was monitored during the photolysis of flowing CIONOz mixtures in N 2 at 3 13 K. The modulated absorption at 400 nm is attributed to NOz. For concentrations of ClONO, around 8 X 1014molecules cm-, in 20 Torr of N2 the yield of NOz, determined from the relationship *NO2
=
(d[NOzI /do, kphot[C10N0210
was found to be approximately 0.25. When O2replaced N 2 as the bulk gas, a decrease in absorption at 400 nm was observed. These observations may be qualitatively explained as follows. Any 0 atoms generated react with NO2
+ NO,
-
NO2
+ NO2
(17)
When Ozis present in excess, the majority of 0 atoms generated react with O2to form 0, and the amount of N O produced is diminished. As a result, no net production of NO2 is observed but rather an overall removal of NO2 via its reaction with NO3. Conversely, this implies that in the N2 carrier gas experiment @N% is approximately the same as O0. The modulated absorptions observed at 350 nm in the photolysis of C10N02 mixtures may be due to NOz, C12, or ClONO. After subtraction of the NO2 contribution at 350 nm, a residual modulated signal was present. As C12 and ClONO have similar absorption cross sections at 350 nm, it is not possible to deconvolute their individual contributions from this modulated absorption signal. However, the absorption at 350 nm is consistent with an initial rate of increase of C12 concentration, (d[Cl,]/dt),, which is approximately half of (d[NO,] /dt),. In order to investigate the production of 0 atoms in the photolysis of C1ONOZ,experiments were carried out using CIONOz and O2 mixtures. 0 atoms were converted to 03,and this was monitored at 270 nm. The concentrations of reactants in this set [NO,] of experiments were as follows: [C10N02] = 3.0 X = 1.7 X lOI3, and [O,] = 7.5 X lo'* (the units are molecules cm-,). Any 0 atoms generated on photolysis of these mixtures will either react with NOz or form 0, via 0
+ 0 2 + M-03 + M
(18)
The absorption at 270 nm was observed to increase and that at 400 nm to decrease on photolysis of these mixtures. This is consistent with the production of 0, via reaction 17 and the removal of NO2 by reaction 14. The rate of production of O,, d[O,]/dt, is much larger than that expected from photolysis of NOz in the system and is approximately 18% of the observed (d[NO,]/dt),. Assuming that the 0 atoms are produced in a primary step, then the quantum yield for production of 0 atoms, a0,is given by
a0 = -d[031 dr
1 k18[0Z1[Ml + k13[N021 k p h ~ t [ C ~ ~ ~ ~ Z l k18[021 O [MI
and was estimated to be approximately 0.24. Finally, any production of N O in the system was investigated by using resonance absorption as the detection technique. Due to the presence of a 50-Hz modulation in the resonance lamp, the minimum detectable absorption (3 X was higher than for the D2and quartz halogen lamps. Such an absorption corresponds to an N O concentration of 3.3 X 1Olomolecules ~ 3 1 3 ~No ~ . signal attributable to N O was detected in a modulated photolysis experiment using a CIONOz concentration of 1.5 X lOI4 molecules cm-,, which implies that the initial rate of increase of N O was less than 2
