J . Phys. Chem. 1987, 91, 317-322 through the encounter in polar solution, some typical exciplex systems such as pyrene-p-DCNB and pyrene-DMA show the formation of plural kinds of ion pairs undergoing C R deactivation and dissociation into free i o n ~ . l ~ ,Namely, ~ ~ , ~ , some ~ ion pairs undergo easily the dissociation into free ions, while others undergo only the CR deactivation without dissociation. Presumably, former ion pairs may have considerably more loose structure, and the latter will have more compact structure. In this respect, we have examined the behavior of pyrene-PMDA systems when ion pairs are formed by electron transfer at the encounter in the strongly polar solvent, acetontrile. We have made time-resolved transient absorption spectral measurements upon concentrated acetontrile solution with a large amount of the ground-state EDA complex ([pyrene] 5 X lo-’ M, [PMDA] -0.2 M) by exciting the solution with the third harmonic (355 nm) of the picosecond YAG laser. In this case, we excite mainly the ground-state EDA complex, and most of the produced ion pairs disappear within ca. 50 ps due to the rapid C R deactivation. However, we can recognize a small amount of absorbance due to ion radicals even in the few nanosecond to 100 ns regime, which means that a small amount of dissociated ions are produced in competition with CR deactivation of the loose ion pair. This loose ion pair may be formed by encounter between excited pyrene and ground-state PMDA, since a small amount of free pyrene is excited in this experiment. The rise time of the loose ion pair estimated by the SternVolmer equation 7 = so/(l kq70 [PMDA) is ca. 250 ps for the concentrated solution with [PMDA] 0.2 M (saturated). Since we have not detected any decay of the ion pairs in the 100-ps regime, the lifetime of the geminate ion pairs produced by encounter should be shorter than 250 ps. Therefore, it is difficult to detect directly the decay process of the loose geminate ion pairs in the concentrated solution. In order to obtain the approximate value of the lifetime of the of loose ion pairs, we have examined the quantum yield (aion) dissociated ion formation by using dilute solution ([PMDA] 0.02 M) where we excite almost exclusively the free pyrene by a 355-nm laser pulse. The concentration of the initially produced
-
-
+
-
-
317
pyrene SI state has been determined from the absorbance observed immediately after excitation, and that of the pyrene cation has been determined from the absorbance obtained at 5 4 s delay time, by using the molar extinction coefficients of the respective species.I2 From these procedures, approximate values of ai,,,,have been estimated to be 0.1 -0.2. Assuming competition between the CR deactivation (ICCR) and dissociation (kd)in the geminate ion pair, is given by kd/(kd 4- kCR). On the other hand, we can obtain both aiOn and qp= (kd kCR)-lseparately in the case of pyrene cation-tetracyanoethylene anion geminate ion pairs produced by encounter in acetontrile, from which the rate constants have been estimated to be kd = 2.6 x IO9 s-l and kCR= 2.5 x IO9 s-l. In the case of the TCNB anion-toluene cation ion pair in acetonitrile, we have obtained kd = 1.5 X lo9 s-l.lC3*In view of these kd values in acetontrile, we assume kd 2 X lo9 s-I, which gives, with aion i= 0.1-0.2, kCR 2 x 10”-8 x lo9 S-I. The above results clearly indicate that the lifetime of the loose geminate ion pair produced by encounter between excited pyrene and ground-state PMDA in acetontrile is considerably longer than that of the more “tight” geminate ion pair produced by excitation of the ground-state EDA complex due to the small CR deactivation rate constant in the former ion pair. Therefore, not only the energy gap dependence of the kCRof the geminate ion pairs but also the structure (“loose” or “tight”) of the geminate ion pairs, which depends on the way of its production are important factors determining the ionic dissociation yield of various electron donoracceptor systems including exciplexes as well as much stronger EDA complexes.
+
-
Acknowledgment. This work was partly supported by a Grant-in-Aid (No. 58430003, 58045097) from the Japanese Ministry of Education, Science and Culture to N. M. Registry No. THF, 109-99-9; PMDA, 89-32-7; PMDA-pyreneEDA complex, 3470-21-1; pyrene, 129-00-0;benzene, 71-43-2;ethyl acetate, 141-78-6; acetone, 67-64-1; acetonitrile, 75-05-8. (12) Schomburg, H. Ph. D. Thesis, Gottingen, 1975. (13) Mataga, N.; Kanda, Y . ;Okada, T. J . Phys. Chem. 1986, 90, 3880.
Product Yields and Mechanism of the Excimer Laser Photolysis of Azomethane at 193 nm James E. Baggott,+ Mark Brouard,: Mary A. Coles, Andrew Davis,g Phillip D. Lightfoot: Martyn T. Macpherson,l and Michael J. Pilling* Physical Chemistry Laboratory, Oxford University, Oxford OX1 3Q2,England (Received: May 27, 1986) The photodecomposition of azomethane by single pulse excimer laser photolysis at 193 nm has been studied by using end product analysis and Lyman a resonance fluoresence. The major product (290%) was ethane, as expected, but methane, ethene, ethyne, and propane were also formed in small quantities. The methane and ethene yields were observed to increase with increasing pulse energy, while the propane yield remained constant. Lyman a resonance fluorescence shows that H atoms are produced directly by photolysis and that, at high pulse energies, they are also formed by a secondary reaction. Two minor H-producing photolysis channels are indicated, the first producing H + CH2N2CH, and the second involving secondary photolysis of vibrationally excited CH3 radicals and producing H + CH2; the fractional product yields from the second channel incrgase linearly with pulse energy. A rate constant of (3.4 A 0.2) X cm3 molecule-’ s-l is obtained for the reaction of H atoms with azomethane at 300 K. Introduction Excimer laser flash photolysis is widely used in the production of reactive s p i e s for time-resolved kinetic measurements.’ Rate ‘Present address: Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD. *Present address: Department of Chemistry, The University, Nottingham. 8 Present address: Smith, Kline and French Research Ltd., The Frythe, Welwyn, Hertfordshire. Present address: BP Research Center, Chertsey Road, Sunbury-onThames, Middlesex.
0022-3654/87/2091-0317$01.50/0
data of high precision may be obtained2 provided the photolysis sytem is well-defined, i.e. competing fragmentation pathways are shown to be unimportant or can be incorporated quantitatively in the data analysis. Azomethane photolysis at 193 nm was used as the methyl radical source in a recent study of the pressure and (1) See, for example, Baggott, J. E.; Pilling, M. J. Annu. Rep. Prog. Chem. Sect. C. 1982, 80, 199.
(2) Tulloch, J. M.; Macpherson, M. T.; Morgan, C. A.; Pilling, M. J. J . Phys. Chem. 1982, 86, 3812.
0 1987 American Chemical Society
318
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
Baggott et al.
temperature dependence of the methyl radical recombination r e a ~ t i o n . ~ This paper describes the characterization of the azomethane source and an assessment of the importance of minor photolysis channels using an A r F laser in combination with (i) end product analysis by gas chromatography and (ii) timeresolved detection of H atoms using Lyman a resonance fluorescence.
Experimental Section An Oxford Lasers KX2 excimer laser, operating at 193 nm and providing pulse energies of typically 50 m J and pulse widths of 12 ns, was employed for the end product analysis experiments. The pulse energy incident on the photolysis cell was varied by using quartz disks and the beam energy on exit from the photolysis cell was measured wifh a Gentec Joulemeter and Datalab DL920 transient digitizer. The photolysis cell was constructed from a stainless steel cylinder which was machined internally so that its cross section had the same dimensions as the central, homogeneous section of the laser beam. The cell length was 7 cm. The rings and the Spectrosil end windaws were sealed with Viton "0" cell was filled, or its contents flushed to the gas chromatograph, via a 1-mm groove machined in each end of the steel cylinder and around the central reaction volume. This design is based on that of Ledford and B r a ~ nbut, , ~ due to the large volume which was sampled in the present study, the extent of peak tailing in the gas chromatograms remained significant. It was found that such tailing could be almost completely removed by trapping the cell contents on the column as described below. The cell was evacuated and filled via a conventional vacuum line. Premixed samples of azomethane diluted in argon were photolyzed and transferred to a Varian Vista 44 gas chromatograph via a pneumatically operated six-port gas sampling valve (Valco), the cell and transfer lines forming the sample loop. The transfer lines were constructed of 1/ 16-in. 0.d. stainless steel tubing to minimize the dead volume. A 2 m X 1/8 in. stainless steel column packed with n-octane on Porasil C (100-120 mesh) was used for the separations. A section at the head of the column was coiled and maintained at 77 K for 4 min following the switching of the gas sampling valve, allowing the entire contents of the cell to be trapped on the column. The cell was then isolated and the trap removed; subsequent temperature programming to 70 OC resulted in efficient separation of C,-C4 hydrocarbons within 25 min with little or no peak tailing. The Varian Vista 44 allows storage and subtraction of the base line FID signal so that small peaks can be detected and integrated despite base line drifts due to temperature programming. Product yields of 0.05 mTorr could be analyzed with signal-to-noise ratios of ca. 50:l and a reproducibility of -5%. Thus, products arising from minor photolysis channels following single-shot excitation could be analyzed, enabling direct comparisons to be made between the present study and the time-resolved experiments which were performed under similar condition^.^ The apparatus for the laser photolysis/resonance fluorescence experiments is described in detail e l s e ~ h e r e . ~A Lumonics TE861S excimer laser was operated at 193 and 248 nm with pulse energies between 30 and 80 mJ and a repetition rate of 3 Hz. The Lyman a resonance fluorescence was excited by a microwave discharge lamp and detected by an EM1 Gencom solar blind photomultiplier (KBr, MgF2). The signal was digitized on a Biomation 8 100 waveform recorder, averaged, typically over 200 shots, and analyzed on a Research Machines 3802 microcomputer. The reaction mixture was flowed through a stainless steel cell, fitted with Spectrosil windows. Azomethane was prepared by the method of Renaud and Leitch;6 its purity was checked by mass spectrometry and gas chromatography. After several freeze-pumpthaw cycles at -80 "C, the samples were found to be better than 99.9% pure. Re(3) Macpherson, M. T.;Pilling, M. J.; Smith, M. J. C. Chem. Phys. Lett. 1983, 94, 430. (4) Ledford, A. E.; Braun, W. Rev. Sci. Instrum. 1977, 43, 5 3 1 . ( 5 ) Brouard, M.; Macpherson, M. T.;Pilling, M. J.; Tulloch, J. M.; Williamson, A. P. Chem. Phys. Letr. 1985. 113, 413. ( 6 ) Renaud, R.; Leitch, L. C . Can. J . Chem. 1954, 32, 545.
Pulse energylrelative units Figure 1. Plot of the fractional photolysis F (see text) vs. laser pulse energy (10 Torr total pressure, 300 K).
0~10lY(X) 008 -
A
M-Ao-Aw+
F Figure 2. Plot of the fractional product yield Y(X) (see text) vs. the
fractional photolysis: (0)Y(CH,); ( 0 ) Y(C2H4);(A) Y(C,H,). Simulations performed using FACSIMILE,'^ based on the reaction mechanism described in Table 11, are also shown: -, least-squares fits to data; ---, simulations (566 Torr total pressure, 300 K).
search grade argon (99.997%) was supplied by B.O.C. Ltd. Results and Discussion Azomethane was photolysed by using single 193-nm pulses at total pressures in the range 10-760 Torr and at temperatures of 300 and 400 K. Argon was used as the diluent gas, and the partial pressures of azomethane were limited to a maximum of 1% of the total gas pressure with an upper limit of 100 mTorr so that the optical density never exceeded 0.1, The major product under all conditions was ethane, with very small quantities of methane, ethene, and propane and even smaller quantities of ethyne being produced. W e define a fractional photolysis, F , [CH3N&H31photolyzed/ [CH3N2CH3] initial which can be calculated from the expression F=
w 4 1 / 2 + [C,H,I + [C,H,I + [C,H,I + 3 [ ~ , ~ , 1 / 2 [CH3N2CH31initial
where the concentrations in the numerator refer to product yields. Figure 1 shows a plot of F vs. laser pulse energy, which was found to be linear, within experimental error, under all conditions, up to the highest values of F studied experimentally (0.13), thus demonstrating that F may be used as a relative internal measure of the pulse energy. Figure 2 shows a plot of the product yields from the minor photolysis channels, Y(X), defined by Y(X) = [XI/([CH41/2 + [C2H21 + [C2H4I + [C2Hd + 3[C3Hs1/2)
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 319
Excimer Laser Photolysis of Azomethane TABLE I: Dependence of the Minor Product Yields, Y(X), on the Fractional Photolysis of Azomethane (Y(X)= M(X)F Y0(X))O
+
conditions 566 Torr, 300 K
CH4
C2H4
M(X)
0.56 f 0.10 102p(X) 2.3 f 0.8 all conditions M(X) 0.56 f 0.15 (10-566 Torr, lO*r”(X) 1.9 f 0.8 300,400 K) a
C3H8
0.23 f 0.04 0.56 f 0.27 0.35 f 0.05 0.21 f 0.03 0.72 f 0.18 0.39 f 0.03
Error limits represent f2 standard deviations.
vs. F for a total pressure of 566 Torr at 300 K. Both Y(CH4) and Y(C2H4)exhibit an approximately linear variation with F (and hence with pulse energy) and a positive intercept at zero pulse energy. By contrast, the yield of propane is independent of F. Similar results were found over the pressure range 10-566 Torr a t 300 and 400 K (see Table I). The minor products formed from the conventional (white light) flash photolysis of azomethane have previously been studied by Bass and Laufer7 and by Pilling et a1.8 A mechanism involving photolysis of CH3 radicals (see Table I1 for notes on the rate constants of each elementary reaction considered in this work) was proposed:
A 2CH3 + N 2
CH3N2CH3 CH3
hU
3CH2
+H
-
+ CH3 k, C2Hs k2 CH3 + H CH4 k3 T H 2 + CH3 C2H4 + H 3CH2+ 3CH2-% C2H2+ H2 CH3
4
+
C1H4
+H
k5 +
C2H5
+ H +M+H2 + M
(PI) (P2)
A CH3 + T H 2 + H + N 2
CH3
hu
+ N2
3CH2+ H
CH3N2CH3% CH3
+ 3CH2+ H + N2
-
+ CH3 ki C2H6 k2 CH3 + H CH4 k3 3CH2+ CH3 C2H4 + H 3CH2+ 3CH2 C2H2+ H 2 ks H + C2H4 CzH5 CH3 + CzH5 C3H8 CH3
k4
k6
022)
H
023)
(R5) (R8)
(P3)
The second channel could either involve direct two-photon photolysis of azomethane, or secondary photolysis of CH3 radicals. The absence of a structured transition from the ground vibrational state of CH3 at 193 nm suggests that such secondary photolysis might proceed via absorption by vibrationally excited CH3. Kinetic spectroscopy at 216.36 nm, following photolysis at 193 nm, shows that [CH3(u=O)] increases on a time scale of 10 ps at 10 Torr total pressure, demonstrating that vibrationally excited CH3 (which is transparent at 216.36 nm) is indeed f ~ r m e d . ~At , ~the pressures employed in the present study, negligible vibrational relaxation would occur during the photolysis pulse. A sequential two-photon absorption via vibrationally excited CH3 leads to a linear de(7) Bass, A. M.; Laufer, A. H. Int. J . Chem. Kinet. 1973, 5, 1053. (8) Pilling, M. J.; Robertson, J. A,; Rogers, G. J. Int. J. Chem. Kinet. 1976, 8, 883. (9) Pilling, M. J.; Smith, M. J. C., unpublished results.
-
+ C2H5 k k
2CH3
2C2H6
(R4)
The formation of propane was not discussed in either paper. The results from the present study indicate that CHI and C2H4 can be formed by at least two channels, in contrast to the above mechanism, one of which involves absorption of one photon and leads to the positive zero pulse energy intercept (cf. Figure 2), the second involving the overall absorption of two photons resulting in a linear dependence of the fractional product yields on pulse energy. At this stage, it is simpler to assume that both channels yield the same radical products, CH3, T H 2 , H, and N2, which are those proposed by Bass and Laufer7 and by Pilling et a1.8 This fragmentation requires an energy of 565 kJ mol-’, so that direct, single-photon photolysis at 193 nm (620 kJ mol-’) is a candidate for the pulse-energy-independent channel: CH3N2CH3
CH3N2CH3 % 2CH3
4
(R1)
k6
H
pendence of Y(CH4) and Y(C2H4) on pulse energy, as observed experimentally, provided that there is no significant relaxation during the laser pulse. There is no experimental evidence to distinguish between the alternative two-photon mechsnisms (direct two-photon absorption or secondary photolysis) other than to note that the fractional yields of minor products are greater in the white light photolysis of az~methane,’,~ despite the reduced power, suggesting that direct two-photon photolysis is of minor importance. Simulations were performed based on the following mechanism using the FACSIMILE numerical integration program:1°
H
+H +M
--. + ks
H2
M
Reasonable fits to the methane and ethene yields were obtained, although it was necessary to vary k2 and k3 somewhat. The fits were based initially on the variation of two parameters relating the yields from the minor photolysis channels P2 and P3 to those of the main channel P1. The simulations tended to overestimate Y(C2H4)and to underestimate Y(CH4) although the general trends in the experimental data were reproduced. Satisfactory fits could only be obtained by substantially increasing kz (e.g. by 60% at 10 Torr and 300 K) and reducing k3 (by 40% under the same conditions). These variations lie somewhat outside the literature uncertainties, suggesting some deficiency in the mechanism. The following independent observations provide more telling evidence against this initial mechanism: (i) Propane is formed via reactions R5 and R6 in low yields, an order of magnitude smaller than those observed experimentally. The simulated yields of propane were also found to increase with increasing pulse energy, since they depend on the initial concentration of H atoms produced via the one-photon process, P3, and the two-photon process (reaction P1 combined with reaction P2). The experimental results indicate that propane is formed primarily via a one-photon process (Y(C3H8)is independent of pulse energy), with a considerably greater yield than that simulated by using Pl-P3 and Rl-R8. (ii) The rate of decay of hydrogen atoms, generated by the photolysis of hydrogen iodide at 248 nm, was found to increase in the presence of azomethane, which is transparent at this wavelength, showing that hydrogen atoms react with azomethane. The reaction was studied in greater detail by photolyzing azomethane itself, at low pulse energies and high azomethane concentrations ((2-12) X 1014molecules ~ m - ~such ) , that the H atom decay was dominated by reaction with azomethane. Reaction (10) Chance, E. M.; Curtis, A. R.; Jones, I. P.; Kirby, C. R.; FACSIMILE: A Computer Program for Flow and Chemistry Simulation, and General Initial
Value Problems; H.M.S.O.,London, 1977,No. C13.
320 The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
I
/ 'I
!
000
./
I-
I
I
I
I
-1 00
0.51 /t/
L/
Baggott et al.
I
I
0.00
I
I
I
200
1.00
I I 3.00
i o 3t/s
I
I
I
I
I
I
I
I
2.0 4.0 6.0 8.0 10-14[Azomethanel molecule cm-3 Figure 3. Dependence of the first-order rate constant k, for H atom decay on azomethane concentration. Total pressure (argon diluent) = 28 Torr, T = 296 K.
between CH3 and H was included in the analysis, although its contribution was only 5 1 0 % even under the most extreme conditions (low azomethane concentration). Figure 3 shows a plot of the apparent first-order rate constant vs. azomethane concentration at a total pressure (argon diluent) of 28 Torr and a temperature of 290 K, where k2 = 1.6 X lo-'' cm3 molecule-I s-'! A weighted linear least-squares fit gives a second-order rate azomethane of (3.41 f 0.22) X cm3 constant for H molecule-' s-l where the error limits represent two standard deviations. The rate constant is independent of pressure. A similar cm3 molecule-' s-l) was found rate constant ((3.5 f 0.6) X from an analysis of the (HI + azomethane)/248-nm system. In the end product analysis experiments, reactions R5,. R7, and R8 are of minor importance and azomethane competes primarily with methyl radicals (reaction R2) for reaction with hydrogen atoms. It might, therefore, be expected that the methane yield would decrease as the azomethane concentration was increased. This was confirmed by simulation. However, a fourfold increase in the initial azomethane concentration was found to have no effect on the final product distribution. Such behavior would arise if (a) methane were a product of the H azomethane reaction or (b) methane were produced by a mechanism not involving hydrogen atoms or (c) both mechanisms a and b operated. The measured rate constant for H + CH3N2CH3is far too large for an abstraction reaction and is more compatible with addition to the -N=N- bond. A subsequent H atom shift would then lead to methane formation: H CH3N2CH3 CH3NHNCH3 CH4 N2 CH3 (R9)
+
000
&+* I
I
-10
+
+
+
I
I
100
I
I
I
I
200 300 10' tis
Figure 4. Time dependence of the H atom fluorescence: (a, top) 30 Torr total pressure; (b, bottom) 200 Torr total pressure Azomethane pressure = 2.1 mTorr. Fractional photolysis 12%.
-
+
+
I
0.00
'"..
I
'tL
OO
1
2
+
H abstraction from azomethane by methyl radicals is far too slow at room temperature to produce significant quantities of methane in the present experiments,I2 while the insensitivity of Y(CH4) to a SO-fold change in pressure argues against a contribution from abstraction by vibrationally excited CH3. Reactions R2 and R9, of H atoms with methyl radicals and azomethane radicals, respectively, remain the only feasible reactiue routes to significant yields of methane, the insensitivity of Y(CH,) to changes in [CH3N2CH3]suggesting that reaction R9 is the predominant channel in the reaction of H with azomethane. There remains, however, the possibility of a direct, photolytic route to CH,:
Direct photolysis to methane has been observed in the photodissociation of acetone, which is isoelectronic with azomethane, (11) Allara, D. L.; Shaw, R. J . Phys. Chem. Ref. Data 1980, 9, 523. (12) Durban, P. C . ; Marshall, R. M. Inr. J . Chem. Kinef. 1980, 12, 1031.
Figure 5. Simulations of the time dependence of the H atom concentration under conditions comparable with those in Figure 4: 0 -, 30 Torr total pressure, [HI, = [CH,], = 4.5 X 10" ~ m - 0~ ---, ; 30 Torr total pressure, [CH,], = 0.0. 0 --, 200 Torr total pressure, [HI, = [CH,], = 4.5 X 10" 0 200 Torr total pressure, [CH,], = 0.0. The subscript zero refers to zero time. e-,
at 184.913and at 193 nm,I4 in yields amounting to a few percent of the photolysis products. Diazomethane, formed in reaction P4a, would not be detected by the present chromatographic analysis. Reactions Rl-R8 produce C3H8at comparatively long times, as [CH3]falls and H C2H4 becomes competitive with H CH,. The rate constants for reactions R5 and R9 are comparable, while [CH3N2CH3]>> [C2H4]. Thus the inclusion of reaction R9 in the mechanism reduces the simulated yield of propane still further. (iii) Figure 4a shows the time dependence of the H atom resonance fluorescence intensity following laser flash photolysis of 1.7 mTorr of azomethane at a total pressure of 27.6 Torr. The signal increases initially and then decreases as H reacts, primarily with CH3 and CH3N2CH3. The initial rise may be ascribed to
+
+
(13) Lin, L.J-T; Ausloos, P. J . Photochem. 1972, 1, 453. (14) Lightfoot, P. D.; Pilling, M. J., to be submitted for publication.
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
Excimer Laser Photolysis of Azomethane
321
TABLE II: Photolysis Channels and Subsequent Radical Reactions Considered in the 193-nm Photodecomposition of Azomethane (All Rate Constants Have Units of cm3 molecule-' s-I) channel
PI P2 P3 P4 P5 R1 R2 R3 R4 R5 R6 R7a R7b R8 R9 RlOa RlOb
reactants
CH3NICH3 CH3 CH3N2CH3 CH3N2CH3 CH3NZCH3 CH, + CH, CH3 + H 'CH, + CH, T H 2 + 3CH2 H + C2H4 CH3 + C2H5 H + C,H, H + C2H5 H+H+M H + CH3N2CH3 CH3 + CH2N2CH3 CH3 + CH,N,CH,
products
2CH3 + N2 3CH2+ H CH, + 3CH2+ H CHP + CH2N2 CH2N2CH3 + H C2H6 CH4 C2H4 + H C2H2 + H2 C2H5
C3Hs 2CH3 C2H6
H, + M CH4 + N2 + CH3 C2HS + CH3 + N2 CH4 + C2H4 + N,
notes
+ N,
a
k, = 5.8 X lo-" at 300 K3J' k , = 1.2 X lo-" at 10 Torr and 1.0 X at 566 Torr and 300 Kslc k3 = 5.0 X k4 = 5.3 X k , = 4.7 X at 10 Torr and 1.2 X lo-" at 566 Torr, 300 Kd k , = 4.9 x 10-"e k7b= 0.0 at 10 Torr k7a = 1.0 X k7a = 5 x lo-", k7b = 5 X lo-'' at 566 Torrf and 1.6 X lo-', at 10 and 566 Torr, respectively, at 300 K k8 = 2.83 X k9 = 3.5 x 10-'2t kl0 = 4 x 10-"h
'If ICH2 is produced in the photolysis of CH, radicals, it will be deactivated rapidly to 'CH,. For example, at a total pressure of 10 Torr (Ar), the lifetime of 'CH, with respect to deactivation is -0.5 ps (using a rate constant of 6.0 X lo-', cm3 molecule-' s-' for 'CH, Ar15) while, for an initial yield of CH3of 5 X lOI4molecule cm3 (typical of the maxima concentration employed), the effective half-life for decay of CH, via R3 is -40 ps. bThe small pressure dependence3 of this rate constant has been neglected. 'Pressure-dependent rate constants for this reaction have been determined recently by Brouard et using direct, time-resolved techniques. dThese rate constants have been estimated by using the pressuredependent rate data of Kurylo et a1.I8 which were adapted for an argon bath gas using the data of Michael et aI.l9 and fitted to a modified Lindemann falloff curve.20 e Rate constant determined by Coles et aL2' using photolysis of azomethane/azoethane mixtures and end product analysis. 'The literature values for reactions R7a and R7b are not very precise and do not provide direct information on the yields of the two channels. The rate C2H4 reaction over a range of constants used here were determined directly (but approximately) from end product analysis studies of the H pressures.22 These data are preliminary, but the reaction is of minor importance here. gSee text for details of the measurement of this rate constant. Because of the presence of this reaction the steps R5, R7, and R8 consume negligible quantities of H atoms and, in consequence, the precise values of k , and k9 are not critical. *This value was assumed by comparison with alkyl radical recombination/disproportionation reactions (see, for example, ref 1, p 245). The mechanism assumes that CH2N2CH3reacts only with CH, and, since the CH3 radical concentration is high, the precise value of the rate constant is unimportant, provided that it is sufficiently high to lead to reaction.
+
+
reaction R3 which is faster than either reactions R 2 or R9 under the conditions used. Simulations (Figure 5 ) confirm the validity of this conclusion; when reaction R3 is removed from the reaction scheme, a monotonic H atom decay is found. At high pressures (-200 Torr) no short-time buildup in [HI was observed (Figure 4b). At these pressures reaction R2 is much closer to its highpressure limit and the H atom decay occurs on a time scale similar to its production via reaction R3, thus masking the buildup. This interpretation is confirmed by simulation (Figure 5 ) . These observations are fully compatible with a photolysis mechanism in which H and CH, are produced in similar yields. Reducing the pulse energy, however, while maintaining the other conditions constant (azomethane pressure = 1.7 mTorr, total pressure = 30 Torr), led not only to a diminished, but observable H atom signal, but also to a reduction in, and eventual elimination of, the buildup. This could only be reproduced in the simulations by reducing the photolysis yield of CH2 relative to H as the fractional photolysis falls, indicating that H is produced in greater yield than CH, in the minor single-photon channel. These observations demonstrate that, while reaction P2 provides a satisfactory mechanism for the overall two-photon process, the one-photon route(s) to minor products must produce, in addition to methane and ethene, far higher ultimate yields of propane. In terms of initial products, it must produce more H than CH,. A mechanism based on the photolysis channels Pl-P4 is incapable of reproducing this behavior. A one-photon photolysis channel P5 is therefore proposed, which replaces reaction P3, a feasible route being provided by H atom elimination: CH3N2CH3-k CH2N2CH3
+H
(P5)
The H atoms produced lead excusively to CH4 via reactions R2 and R9, the other H atom reactions being of very minor importance. The present data do not allow us to determine quantitatively the relative importance of channels P4 and P5. Some qualitative information was adduced by adding ethene as an H atom scavanger in concentrations such that reaction R5 occurred much
more rapidly than reactions R 2 and R9. It was found that the methane yield was reduced, thus demonstrating the occurrence of an H atom producing channel (reaction P5) but was not completely eliminated, providing support for the occurrence of reaction P4. It may be concluded that both reactions P4 and P5 operate in the photolysis at a 1% level, although the low yields prohibit a more quantitative assessment. The CH2N2CH3radical provides suitable routes for the production of propane throughout the range of fractional photolysis and of ethene via the single-photon channel, via its reactions with CH3:
-
CH3 + CH2N2CH3
+
CH3 + CH2N2CH3
+
C2HS+ CH,
+ N2 CH4 + C2H4 + N2
(RlOa) (RlOb)
Reaction RlOa, which may be thought of as a recombination reaction, ultimately results in the production of propane via reaction R6. The adduct initially produced in reaction RlOa rapidly dissociates because of the large exothermicity: CH,
+ CH2N,CH3
-
+
C2HSN2CH3
+
C2HSN2CH3 C2H5 CH3
+ N2
AH = -370 kJ mol-' AH = 140 kJ mol-'
The large excess of CH3 radicals present converts C2H5to propane (the disproportionation/recombination ratio for CH3 + C2H5 is only 0.02 at 300 K2' and disproportionation may, therefore, be (15) Ashfold, M. N. R.; Fullstone, M. A,; Hancock, G.; Ketley, G. W. Chem. Phys. 1981, 55, 245. (16) Pilling, M. J.; Robertson, J. A. Chem. Phys. Left. 1975, 33, 336. (17) Braun, W.; Bass, A. M.; Pilling, M. J. J . Chem. Phys. 1970, 52, 5131. (18) Kurylo, M. J.; Peterson, N. C.; Braun, W. J . Chem. Phys. 1970, 53, 2776. (19) Michael, J. V.; Osborne, D. T.; Suess, G. N. J . Chem. Phys. 1973, 58, 2800.
( 2 0 ) Troe, J. J . Phys. Chem. 1979, 83, 114. (21) Coles, M. A,; Davis, A,; Pilling, M. J., unpublished results.
322 The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 neglected). Reaction RlOb is required to produce CzH4 and may occur either via an adduct, with a 1,s H shift, or by abstraction followed by rearrangement of CHzNzCHz. Alternatively, the zero intensity yield of ethene could result, at least in part, from the reaction between CH2, formed in reaction P4b, and CH, (reaction R3). The overall mechanism (Table 11) was examined in two ways (Figure 2). In the first, the minor reactions R5, R7, and R8 and photolysis channel P4 were neglected and the slopes and intercepts of the fractional yield/fractional photolysis plots (cf. Figure 2) were determined by linear regression and used to estimate the relative yields of the photolysis channels P1, P2, and P5 (Table I). The model requires that the slope of the CH4 plot vs. F is twice that of the CzH4 plot, and this requirement is satisfactorily followed by the data under all conditions (Table I). A more rigorous check was made at 566 Torr and 300 K, for which the most detailed experimental information was available, by fitting the data to the full, modified kinetic scheme (reactions P1, P2, P5 and R1-RlO) using numerical integration. These calculations served to demonstrate the validity of the approximate, linear treatment and the same photolysis and rate parameters were employed. Figure 2 shows almost exact agreement between the two treatments for the yields of C2H4and CHI. The slight discrepancy in the slope of the CH4 plot is within experimental error. The slopes of plots such as that shown in Figure 2 enable the fractional yield of photolysis channel 2, F 2 / F ,to be determined, where F is the fractional photolysis of azomethane (vide supra): F 2 / F = (0.24 f 0.06)F with no apparent dependence on pressure and temperature. Typically, for our conditions, 0.01 < F < 0.12. The intercepts enable the combined contribution from reactions P4 and P5 to be assessed and lead to the conclusion that (F4 F 5 ) / F 0.01. These experiments were undertaken in order to examine the validity of the assumption, made implicitly in a recent study of methyl radical recombination,, that the recombination is unaffected by the presence of minor photolysis channels. This assumption was examined in detail by simulating the methyl radical decay for 2% photolysis (typical of, or even higher than, the experimental conditions used in ref 3) and then analyzing the resulting data with a second-order program, described previously.2 The values of the recombination rate constant returned by this analysis were found to be only 1-1.5% higher than the values input in the simulations, showing that the error introduced in the reported experimental rate data for CH3 recombination due to neglect of minor photolysis channels is well within the experimental
+
-
(22) Lightfoot, P. D.; Pilling, M. J., unpublished results. (23) Eberins, H.;Hoyermann, K.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 982.
Baggott et al. error., It should be noted that any contribution from channel P4, which was neglected in the analysis, reduces the contributions from other radicals to the CH3 decay still further. It is pertinent to note here that the 193-nm photolysis of azoethane produced large quantities of ethene and propane, in yields comparable with that of butane, indicating a much more serious effect due to photolysis channels additional to the direct formation of ethyl radicals. The photolysis was found to be cleaner at 353 nm, but the extinction coefficient of azoethane is much smaller at this wavelength. Summary The major photodissociative process in the laser flash photolysis of azomethane at 193 nm is that producing methyl radicals and nitrogen:
CH3N2CH32 CH3 + N2 + CH3
(PI)
At the low laser intensities appropriate to kinetic measurements on methyl radical reaction^,^ reaction P1 accounts for >95% of the observed photolysis products. A minor two-photon process, probably involving the absorption of a further 193-nm photon by vibrationally excited methyl radicals, formed in the initial part of the photolysis pulse, was also shown to occur:
CH,
A CHz + H
(pa
The fractional yield of channel P2, FJF, can be written as (0.24 f O.O6)F, where the errors refer to 2 standard deviations and F is the fractional photolysis of azomethane. In addition, minor (- 1%) single-photon processes occur:
+ CH2N2 CH3N2CH3-k CH3NzCH2+ H CH3N2CH3% CHI
(P4) (P5)
It was not possible, from the present data, to establish the relative importance of reactions P4 and P5. Numerical simulations showed that the presence of radicals other than methyl, at the 1% level, does not affect the suitability of the 193-nm photolysis of azomethane as a methyl radical source for real-time kinetic measurements.
Acknowledgment. Our thanks go to the Science and Engineering Research Council for the award of postdoctoral research fellowships (to J.E.B. and M.T.M.), postdoctoral support (to A.D.), research studentships (to M.B. and P.D.L.), and equipment grants (to M.J.P.). Registry No. Ethane, 74-84-0; methane, 74-82-8; ethene, 74-85-1; ethyne, 74-86-2; propane, 74-98-6; azomethane, 503-28-6; methyl, 2229-07-4; nitrogen, 7727-37-9.
