J. Phys. Chem. 1991, 95, 6585-6593 above) are slowed because of the low density of states in the exit channel. This is analogous to the role of the low density of states at the central barrier in the double-well model for slow reactions." However, the properties of the central barrier can be deduced only indirectly, while the properties of the product-like transition states in the present reactions are known from the thermochemistry. In this sense, the present results give direct support for the postulated entropy effects in the double-well model. The present work and other recent result^'^.'^ extend the treatment of ion-molecule reactions to energy surfaces with (19) Sunner, J. A.; Hirao, K.; Kebarle, P. J . Phys. Chem. 1989,93,4010.
6585
varying complexity. In the present model, fast reactions are accounted for by a singlewell model. Slow reactions are described by a doublewell model.'' Some slow reactionsin hindered systems proceed through more complex surfaces with several free energy barriers and ~ e l l s . ' ~ In J ~all the models, entropy (or densityof-states) factors at the barriers play significant roles.
Acknowledgment. I thank Dr. W. Tsang and Dr. L. W. Sieck for helpful discussions, and Dr. C. Lifshitz for suggesting the work on multichannel reactions. This work was supported in part by a grant from the Division of Chemical Sciences, Office of Basic Energy Sciences, US.Department of Energy.
Electronic Quenching of CH(A*A), NH(Asn), NH(c'n), and PH(Asn) between 240 and 420 K R. D. Kenner: S. PCannenberg,*P. Heinrich, and F. Stuhl* Physikalische Chemie I, Ruhr-Universitdt Bochum, 0-4630 Bochum, Federal Republic of Germany (Received: February 22, 1991; I n Final Form: May 30, 1991)
Rate constants for electronic quenching of thermally equilibrated populations of CH(A), NH(A), NH(c), and PH(A) in = 0 by nine collision partners between 240 and 420 K have been measured. These data have been augmented by using literature values of electronic quenching rate constants for these and other diatomic hydrides. This body of data is then used to examine the general applicability of the various classical models and methods used by previous authors in individual cases. u
Introduction
The first- and second-row diatomic hydrides form a family of radical species that are important in a number of practical situations, i.e., combustion and air They are also relatively simple species for which detailed quantum mechanical calculations can be performed.'~~The rates of quenching of the electronically excited states of these species are often of interest in monitoring their concentrations by using laser-induced fluore~cence.~This paper presents the results of a study of the rates of collisional electronic quenching of several members of that family over the temperature range 240-420 K. To get an overview of the temperature dependence of the quenching rates of electronically excited diatomic hydrides, a group of %andard" collision partners with a variety of properties was chosen. That group consists of three nonpolar diatomics (H2, N2, and OJ, one nonpolar triatomic (CO,), two weakly polar molecules (CO and N20),two highly polar molecules (NH, and H20), and one representative alkane (C2H6). A few measurements were also made with C2H4 and some other hydrocarbons. A number of different hydrides can be generated in the 193-nm laser photolysis; CH(A2A), NH(A311), NH(cIn), and PH(A311) were chosen for this study. To achieve immediate rotational and translational equilibrium, the experiments were performed in a very large excess of Ar. In addition to these species, OH(A22+) is also conveniently generated, but some data are already available for quenching of that species between 200 and 400 K.68 Some data are also available for the rates of quenching of CH(A), NH(A), and OH(A) at temperatures greater than 900 Ke15 and BH(A'II),I6 SiH(A*A)," and SD(A22)I8at room temperature. Some of the data in the current work have been presented previousIy.19 'Current address: CSIRO Division of Applied Physics, P.O.Box 218, Lindfield, N.S.W. 2070. Australia. a Current address: Inno-Tec GmbH & Co. KG, Universitiitsstrasse 150, D-4630 Bochum, Federal Republic of Germany.
Experimental Section Apparatus. The electronically excited states of the hydrides
investigated were generated by photolysis of appropriate parent (1) Combustion Chemistry; Gardiner Jr.. W. C., Ed.; Springer: Berlin, 1984. ( 2 ) Armosphlrische Spurenstofle und ihr physikalisch-chemisches Verhalten; Becker, K. H., Lbbel, J., Eds.; Springer: Berlin, 1985. (3). (a) Vegiri, A.; Farantos, S.C.; Papgiannakopoulos, P.; Fotakis, C. In Sclectivlty in Chemical Reactions; Whitehead, J. C., Ed.; Kluwer: Dordmcht, 1988; NATO AS, Ser. C 245; p 393. (b) Vegiri, A.; Farantos. S.C. Mol. Phys. 1990,69, 129. 141 Jonas. R.: Staemmler. V. Z . Phvs. D 1989. 14. 143. (Si Copeland,' R. A.; Wi& M.L.; Rinsberger, K.J;; Crosley, D. R. Appl. Opt. 1988, 27, 3679. (6) Copeland, R. A.; Crosley, D. R. J . Chem. Phys. 1986,84, 3099. (7) Jefferies, J. B.; Copeland, R. A.; Crosley, D. R. J. Chem. Phys. 1986, 85. 1898. (8) Kenner, R. D.; Capetanakis, F. P.; Stuhl, F. J . Phys. Chem. 1990,94, 2441. (9) Garland, N. L.; Crosley, D. R. Chem. Phys. Len. 1987, 134, 189. (10) Crosley, D. R.; Rensberger, K.J.; Copeland, R. A. In Selectiuity in Chemical Reactions; Whitehead, J. C., Ed.; Kluwer: Dordrecht. 1988; NATO, AS, Ser. C 245, p 543. (1 1) Garland, N. L.; Crosley, D. R. 21st Symp. In?. Combust. 1986, 1693. (12) Garland, N. L.; Jefferits, J. B.; Crosley, D. R.; Smith, G . P.; C o p land, R. A. J. Chem. Phys. 1986,84,4970. (13) Rensberger, K. J.; Copeland, R.A.; Wise, M.L.;Crosley, D. R. 22nd Symp. In?. Combust. 1988, 1867. (14) Fairchild, P. W.; Smith, G . P.; Crosley, D. R. J . Chem. Phys. 1983, 79, 1795.
(15) Smith, G. P.; Crosley, D. R. J . Chem. Phys. 1986,85, 3896. (16) Douglass, C.H.; Rice, J. K. J . Phys. Chem. 1989, 93, 7659. (17) Nemoto, M.; Suzuki, A.; Nakamura, H.; Shibuya, K.; Obi, K. Chem. Phys. Lett. 1989, 162, 467. (18) Tia. J. J.; Ferris, M.J.; Wampler, F. B. J . Chem. Phys. 1983, 79, 130. (19) (a) Vlahoyannis. Y.P.; Hontzopoulos, E.;Vegiri, A,; Farantos, S.C.; Fotakis, C.; Browarzik, R. K.; Heinrich, P.; Kenner, R. D.; Rohrer, F.; Stuhl, F. 10th Inr. Symp. Gas Kinet., Swansea, Wales, 1988. (b) Kcnner, R. D.; Pfannenkrg, S.;Heinrich, P.; Stuhl, F. J9rh Int. ConJ Phorcchem., University of Michigan, Ann Arbor, MI, 1990.
0022-3654/91/2095-6585$02.50/00 1991 American Chemical Society
6586 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991
Kenner et al.
TABLE I: Rite Coastants, k,/10-" c d s-', for the Collisional Quenching of CH(A'A) between 240 and 415 K ' Q 243 f 3 K 273 f 3 K 296 f 3 K 415 f 5 K H2 0.59 f 0.023 0.94 f 0.06 1.88 f 0.08
co2
NZ
0.01 f 0.001 0.02 f 0.002
co
6.93
CZH4 C2H6 0 2
N20 NH3 H20
0.022 f 0.006d 0.04 f 0.004b'' 0.04 f 0.002 5.9 f O.Sd 6.2 f 0.5
0.47
21.7 f 1.4 12.9 f 0.4 1.95 f 0.08 1.92 f 0.09 0.19 f 0.01
1.86 f 0.06 1.89 f 0.11
3lf2 8.1 f 0.5'
12 f Id 14 f 2 1.7 f 0.2d 1.85 f 0.07 0.23 f 0.02d 0.24 f 0.04 34 f 3d 8.8 f 0.8d
0.08 f 0.004 0.14 f 0.03 7.1 f 0.6 15 f 0.7
1.5 f 0.1 0.46 f 0.06
lit! 0.90 0.08 (N) 1.10 f 0.02 (W) 0.026 f 0.002 (W)
*
5.2 f 0.3 (N) 19 f 1 (N) 11.0 f 0.5 (N)
1.6 f 0.1 (N) 1 -08 f 0.07 (W) 0.46 f 0.10 (N) 0.39 f 0.03 (W)
34 i 5 7.8 f 1.6
'Uncertainty limits 3u unless stated otherwise. *Value incorrectly reported as (0.4 i 0.04) X IO-" cm3s-I in ref 20. '253 K. dReference 20. 'N, ref 22, 300 K, 20; W, ref 23, 297 K. molecules diluted in a large excess of Ar (PAr1 14 Wa), and their concentrations followed by the intensity of their fluorescence. The parent molecules, observed transitions and wavelengths were as follows: CH3NH2 and for a few experiments CH3COCH3,CH(A,u=O X,u=O) at 431 nm; CH3NH2and for a few experiments X,u=O) at 336 nm; HN,, NH(c,u=O NH3, NH(A,u=O a,u=O) at 325 nm; PH,, PH(A,u=O X,u=O) at 342 nm. The parent molecules were photolyzed in the unfocused beam of an ArF laser at 193.3 f 0.3 nm (Lambda Physik EMG 100). The resulting photofragment fluorescence was monitored at right angles to the photolysis beam by using a 1-m monochromator. The exit slit of the monochromator was set to give a bandpass of about I nm, and the entrance slit was varied to control the detected intensity. The position of the transmitted bandpass was not critical because rotational equilibrium was observed to be reached very quickly. A photomultiplier (EM1 9789QB) was used, the output of which was connected to a 125-MHz digital oscilloscope (LeCroy 9400) interfaced to a microcomputer. The photolysis cell could be heated by using a resistive wire heater or cooled by passing methanol from a cryostat through a 6-mm capillary. The wire and the capillary were wound around the inner cell. The temperature in the cell was measured with a thermocouple mounted in the cell near the photolysis volume. The temperature variations in the cell were estimated to be less than f l K in the photolysis volume. The cell was evacuated by either a rotary or an oil diffusion pump. The pressures were determined by three capacitance manometers of various ranges. In all experiments, the gases were slowly flowed through the cell to prevent accumulation of photolysis products. In some experiments, when the cell was heated or cooled, the flow rate was varied to ascertain that the temperature in the cell was independent of the flow rate. The flow rate of each gas (except H 2 0 and CH3COCH3)was controlled by mass flow controllers. These controllers were individually calibrated immediately before each experiment by measuring the rate of pressure increase in a calibrated volume. For H 2 0 and CH3COCH3,a saturator and dilution system that was described previouslyzowas used. k u s e of the vapor pressure of HzO, it was not possible to make measurements with that collision partner below 253 K. The chemicals used in this investigation, with the exception of HN3 and H20, were obtained from commercial sources and used as received. The HN, was synthesized as reported earlier,21and the H 2 0was distilled. The other chemicals, with supplier if other than Messer Griesheim, and minimum reported purity in percent given in parentheses are: Ar (99.995), PH, (99.9), CH,NH2 (99.01, Hz (99.999), Nz (99.999), C02 (99.995, Messer Griesheim; technical, Linde), CzH4 (99.95), C2H6( 9 9 . 9 3 , O2(99.995), N 2 0
- -
-
-
(20) (a) Heinrich, P.;Kenner. R. D.;Stuhl, F. Chcm. f h y s . Lerr. 1988, 147, 575. (b) Heinrich, P.,private communication. (21) Rohrer, F.;Stuhl, F. J . Chem. fhys. 1988, 88, 4788.
(99.99), NH, (99.998, Matheson), CO (99.997), D2 (99.7), C3H6 (99.98), c-C3H6(99.0), C3H8 (99.95), t-C4H8 (99.0), i-C4Hlo (99.5) and CH3COCH3 (99.5). Data Acquisition and Error Analysis. After optimization of the
number of channels of the digital oscilloscope, over 500 decay curves were summed. The resulting curve was then converted to semilogarithmic form after subtraction of the background. The decay curve was then fitted usually over 3 or more lifetimes by using a linear regression algorithm with Poisson statistical weights. For each gas mixture, the lifetime was repeatedly measured until the agreement between measurements was within 2-3% to be certain that the flows had stabilized. For each rate constant determination, the decay rate of the photofragment fluorescence was determined with no quencher, Q,added and for 5-7 partial pressures of the collision partner. The maximum amount of added quencher resulted in an observed lifetime of approximately 150 ns. For each partial pressure, a minimum of two measurements were made. The decay rates were then plotted as a function of the partial pressure of the collision partner and fitted by using a linear regression algorithm with each point weighted by its statistical uncertainty. If not otherwise stated, the errors of the experimental data represent 3u. The fluorescence decay curve appeared to be single exponential in most cases when the first 5-10 channels (-100 ns) after the laser pulse were omitted. During that first 100 ns the decay was slower than a t later times, because the wavelength bandpass was optimized for high fluorescence intensity and therefore favored low rotational levels being populated by very fast rotational relaxation. In separate experiments it was shown that the decay times determined after the initial relaxation were independent of the spectral position of the band path. In cases of NH(A) and PH(A) quenched by NH3, the signal appeared to be slightly biexponential with an apparent late production of the excited species and only the initial faster decay was used. The capacitance manometers were compared with others in the laboratory and with a Hg barometer and were estimated to be accurate to be better than 3%. The uncertainties in the flow controllers are estimated to be about 2%. Allowing for other uncertainties, such as the calibration of the volumes used in flow rate measurements, the total uncertainty in the concentrations is estimated to be 110%.A similar value applies to the saturator and dilution system used to add H 2 0 and/or CH3COCH3. Results Rate constants were obtained from plots of the decay rate vs the pressure of the collision partner. As a typical example, Figure 1 shows the decay rate of PH(A) as a function of the partial pressure of C02for three different temperatures, 224, 300, and 420 K. Using the measured temperatures of the bath gas to convert the partial pressure to number densities, one obtains rate constants from the slopes. The rate constants obtained in this work are given in Tables I-IV.
The Journal of Physical Chemistry, Vol. 95, NO. 17. 1991 6587
Electronic Quenching
TABLE II: Rite Coostants, P,/l&" cm3 s-', for the Collisional Quenching of NH(A311).between 240 and 415 K4 Q 243 3 K 273 3 K 296 3 K 415 f 5 K H2 8.4 f 0.5 8.3 f 0.7' 8.5 f 0.3
*
