244 1
J . Phys. Chem. 1990, 94, 2441-2446
the absorption path length, I, is the incident light intensity, and It is the transmitted light intensity, x and xo are then given by eq A4 and A5, respectively. x = -OD tl
the respective first- and second-order rate coefficients. Let x = [XI at time t and xo = [XIa t f = 0; then dx -- = 2k2x2 + klx dt dx = 1 0 x 2 k 2 x 2+ k l x
-Lfdf
xo =
Let
(OD),=, €1
Substituting for x and xo in eq A 2 gives eq A6
n = 2k2/kl
tl(OD),,o + n(OD)(OD),=o tl(0D) + n(OD)(OD),,o
then
nx2 + x
[In XI:, - [In (1
+ nx)]*, = -k,t
thus In
[
= klt
The optical density of the solution (OD) at time t , assuming only species X absorbs, is given by the Beer-Lambert Law (eq A 3 ) . t is the extinction coefficient (base 10) of species X, 1 is IO
OD = log - = txl
4
('43)
1
= k,t
(A6)
The value of n is adjusted so that a plot of the left-hand side of eq A6 versus time is linear and passes through the origin. Since the second-order component is most significant in the early part of the decay it is important that the fit is optimized in this region. k , is then given by the slope and 2k2 = n k , . In the present study the linear least-squares correlation coefficient was always greater than 0.990 over more than 3 half-lives for the best fit to the data and rose to 0.999 for those experiments with the highest signal to noise ratio. Obviously the value of n is particularly sensitive to errors in t, which is consistent with the normal expectation for second-order rate constants obtained from absorbance measurements. Indeed, results are frequently expressed as the ratio k / t to eliminate this dependence. The results obtained by using the above method were verified by transferring the data from a number of experiments to a mainframe computer where a three-parameter nonlinear leastsquares fitting procedure was used to determine xo, 2k2, and k l . Registry No. SO,, 14808-79-8;CI-, 16887-00-6;CIY, 12595-89-0; C1, 22537-15-1; H20,7732-18-5.
Kinetic Isotope Effects in the Electronic Quenching of OD/OH(A22+,~=0) at 296 f 4 K R. D. Kenner,+F. P. Capetanakis, and F. Stuhl* Physikalische Chemie I , Ruhr- Universitat Bochum, 4630 Bochum 1, Federal Republic of Germany (Received: June 30, 1989; In Final Form: October 10, 1989)
Electronic quenching of OD/OH(A2Z+,v=O)by a number of collision partners has been investigated at 296 f 4 K. The excited hydroxyl radicals were generated in the pulsed photolysis of nitric acid in the presence of a large amount of Ar as a relaxing gas. The current results are in agreement with most values in the recent literature and show that there is no isotope effect for many collision partners. However, a significant isotope effect has been found for quenching by CO, NO, and CF2C12.
Introduction The hydroxyl radical, in both its first electronically excited state, A 2 P , and its ground state, X2n,is one of the best studied simple radicals. The connecting ( A X) transition generates a spectrum with relatively wide rotational spacing, which is easily accessible in the UV.' By theoretical methods, potential surfaces of OH in collisions with simple molecules can be mapped with increasing The current interest in this radical stems partly from the key role it plays in combustion4 and air chemistry.s Our interest in the properties of OD(A) originates from a systematic study of the collisional properties of electronically excited hydrides which is currently underway in several laboratories including ours6 In contrast to the knowledge accumulated for O H ( A ) , not much was known about the collisional properties
-
'Present address: 3417 Patricia Ave. Apt. 26, Montreal, Quebec, H4B
1Y9,Canada. 0022-3654/90/2094-2441$02.50/0
of OD(A) when the present work was initiated. During this study, we became aware of the extensive work on the quenching of O D ( A ) performed by Vaghjiani and Ravishankara.' While the current results are in very good agreement with those of ref 7, comparisons with the cross sections for quenching of O H ( A ) given in the literature suggested an isotopic dependence of the quenching (1) Watson, W. W. Asfrophys. J . 1924,60, 145. (2) (a) Farantos, S. C.; Vegiri, A. J. Phys. Chem. 1988,92, 2719. (b) Vegiri, A.; Farantos, S.C.; Papagiannakopoulos, P.; Fotakis, C. In Selecfivity in Chemical Reactions; Whitehead, C., Ed.; Reidel: Dordrecht, 1988. (3) Staemmler, V. Private communication. (4) Combusfion Chemistry; Gardiner Jr., W . C., Ed.; Springer-Verlag: Berlin, 1984. (5) Warneck, P. Chemistry of the Natural Atmosphere; Academic: London, 1988. (6) Crosley, D. R. J . Phys. Chem. 1989,93, 6273. (7)Vaghjiani, G.L.; Ravishankara, A. R. J . Chem. Phys. 1987,87,7050.
0 1990 American Chemical Society
2442 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 TABLE I: lntralaboratory Comparisons of the Cross Sections for Quenching of OH(AzL+)and OD(A2L+)" uq/
M H2 N2 0,
D2O N20
co2
ref 8b 7.4 f 1.8 8.3 f 2.1 3.4 f 0.9 3.5 f 1.5 13.9 f 3 12.0 f 3
ref 9c 4.6 f 1.5 6.3 f 2.4 5.8 f 2.1 4.4 f 1.5
m2
ref lod
ref I I d ~~~~~
92.4 f 12 85.7 f 5.4
90.2 f 8.4 85.7 f 5 . 4 70.7 f 4 . 2 63.7 f 5.4 69.0 f 8.1 69.4 f 3.9
Upper entry of each pair is uqoH,and the lower entry is uqoD. All uncertainty limits are 3u. b N f= 1. CEquilibriumpopulation at 320 K. d N ' = 0. (I
cross sections for some collision partners. Previous intralaboratory comparisons of the quenching of OH(A) and OD(A)8-11had shown no such effects for the species investigated with the possible exception of N 2 0 ; see Table I. Since many of the cross sections for quenching of OH(A) required for comparison with the current results for OD(A) had not been measured directly, and to eliminate possible contributions from differing systematic errors in measurements made in different laboratories, we have also measured the cross sections for quenching of thermalized populations of OH(A) at room temperature. Comparison of the current results shows that there is no isotope effect within the experimental uncertainty for most collision partners with the notable exceptions of CO, NO, and CF2CI,. In addition, we present a comparison of rate constants for quenching of OD(A) measured with two different transient recorders under otherwise unchanged experimental conditions and compare the cross sections for quenching of OD(A) with those of the corresponding second-row deuteride, SD(A2Z+).12 Experimental Section Electronically excited hydroxyl radicals were generated by the photolyis of nitric acid in the unfocused beam of an ArF laser (193 nm, pulse duration 15 ns, repetition rate 2 Hz). The apparatus was derived from that previously used for the time-resolved photolysis of HNO3.I3J4 The experimental conditions were similar to those in the recent work on CH(A)ISand PH(A)I6 from this laboratory. Rotationally resolved spectra of the fluorescence emission from the OD(A,u'=O+X,u"=O) transition were obtained with a I-m monochromator; low-resolution spectra of both the OH and OD(A-X) transitions were obtained with a 0.25-m monochromator. For all spectra, the output of the photomultiplier was processed by using a boxcar averager with gated integrator, and the results were digitized and stored in a microcomputer for later analysis. For the kinetic measurements on both excited hydroxyl radicals, an interference filter (Amx = 309.0, fwhm = 7.0 nm, 20% transmission) was used. (8) German, K. R. J . Chem. Phys. 1976, 64, 4065. (9) Lengel, R. K.; Crosley, D.R. J. Chem. Phys. 1978,68, 5309. (IO) Copeland, R. A.; Crosley, D. R. Chem. Phys. Leu. 1984, 107, 295. ( I I ) Copeland, R. A.; Dyer, M. J.; Crosley, D. R. J . Chem. Phys. 1985, 82, 4022. (12) Tiee, J . J.; Ferris, M. J . ; Wampler, F. B. J. Chem. Phys. 1983, 79, 130. (13) Papenbrock, Th.; Haak, H . K.; Stuhl, F. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 675.
(14) Kenner, R. D.;Rohrer, F.; Papenbrock, Th.; Stuhl, F. J . Phys. Chem. 1986, 90, 1294. (15) Heinrich, P.;Kenner, R. D.; Stuhl, F. Chem. Phys. Leu. 1988, 147, 57s. (16) Kenner, R . D.;Pfannenberg, S.; Stuhl, F. Chem. Phys. Leu. 1989, 156. 305.
Kenner et al. All experiments were performed at room temperature (296 f 4 K). Pressures were measured with capacitance manometers. The relative spectral sensitivity of the detection system was calibrated by means of a standard D2 lamp (Optronics). For some experiments the fluence of the photolyzing laser was reduced by calibrated dielectric reflectors. For most of the collision partners in the experiments on quenching of OD(A), time-resolved experiments were performed in two sequential runs using two different transient recorders: Krenz Model TRC 4080 (called TR1 in the following) and LeCroy Model 9400 (TR2). For the experiments on quenching of OH(A), only the latter was used. The frequency response is 26 and 125 MHz for TR1 and TR2, respectively. In all cases 500 singlefluorescence decays were accumulated to yield the decay curve. All gases were slowly flowed through the photolysis cell to prevent the accumulation of photolysis products. The nitric acid was mixed with a large excess of Ar in all kinetic runs. To obtain well-defined mixing ratios (>IO0 ppmv), a part of the Ar flow was first saturated with D N 0 3 / H N 0 3 at 297 f 1 K followed by two dilution steps as described previ~usly.~'The vapor pressures of the isotopic nitric acids were assumed to be the same. The values of the mixing ratios were verified for DNO, by titration using NaOH. The quenching gas was added to this flow. All flows were calibrated in each experiment by measuring the rate of pressure rise in a calibrated volume. The partial pressures of the species in the quenching cell were determined from the relative flows and the total pressure. All fluorescences analyzed in this investigation appeared simultaneously with the photolysis laser pulse and, after the first 100 ns, decayed exponentially. Therefore a decay rate, 7-I. is defined and is given by the following equation: 7-l
=
+ k,[P] + k,[M] + k,[Q]
T ~ - ~
(1)
In eq 1, 70 is the zero-pressure lifetime, P, M, and Q refer to the parent molecules, bath gas, and added quencher, respectively, the k's are the bimolecular rate constants for loss of the excited species, and the square brackets indicate number densities. Quenching rate constants, k were determined via (1) by keeping [PI and [MI constant anrvarying [Q]. All decays were recorded with a dwell time of 20 ns. The lifetimes were varied from 150 to 555 ns and the decays followed for 3-47. In average, 11 different [Q] were used for each determination. For the kinetic experiments, DNO, was produced in the reaction of D2S0, (purity >98%, rest D20, 99.5% D) with KNO3.I8 For some spectroscopic experiments, commercially prepared 68% DNO, in D 2 0 (99.75% D) was used. All other chemicals used in this investigation were commercial products of high purity and were used as received except NO, which was fed through a trap cooled to 120 K to remove NOz. They are as follows (with supplier if other than Messer Griesheim and given minimum purity in percent): HNO, (Riedel de Haen, loo%, fuming), N, (99.999), Ar (99.999), O2 (99.998), CO (99.999), NO (99.98), CHI (99.995), c0, (99.995), N 2 0 (L'Air Liquide, 99.99), CzH6 (99.995), C2H4 (99.6), C3H8(99.95), n-C,H,, (99.5), i-C4Hlo (99.5), CF2CI2(98.5), and CF3H (98.5). Results For OD(A) produced in the ArF laser photolysis of DNO,, the dependence of the fluorescence intensity, In,on the photolysis laser fluence, IL,was determined from the slope of a plot of In (I") as a function of In (IL)to be 1.7. The relative nascent vibrational population of the OD(A) was determined from the integrated intensities of the (1,O) and overlapping (0,O) and (1,l) bands of the OD(A-+X) fluorescence spectrum obtained by using very low pressures of neat DNO,. The relative transition probabilities from Crosley and LengelIg were used in the data reduction. We find (17) Papenbrock, Th.; Stuhl, F. In Physico-Chemical Behauiour of Afmospheric Pollutants; Angeletti, G., Restelli, G., Eds.; D. Reidel: Dordrecht, 1987. (18) (a) Stern, S. A,; Mullhaupt, J. T.; Kay, W. B. Chem. Reo. 1960, 60, 185. (b) Kay, W. B.; Stern, S. A. Ind. Eng. Chem. 1955, 47, 1463.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2443
Electronic Quenching of OD/OH(A2Z+,~=O)
0
10
Pressure /Pa
30
20
Figure 2. Decay rates for the quenching of 0D(A2Zt,u=O) measured with the transient recorders TR1 (A) and TR2 (B) as a function of pressure of I'-C4HI0.The indicated error limits represent 3o. For curve A, the right and for curve B the left ordinate is valid.
TABLE II: Rate Constants for the Quenching of OD(A*Z+,v=O) at 300 K
M 0
2
1 Time / ps
3
N2
co
Figure 1. Semilogarithmic plot of decays of OD(A2Zt,u=O)recorded by the different transient recorders TRl (A) and TR2 (B). In each case, a mixture of 1.4 Pa of DN03 and 13 kPa of Ar was photolyzed and the signal from 500 single runs was accumulated. The dwell time was 20 ns per data point. The background has been subtracted before plotting.
~28% of the OD(A) is generated in u = 1. The nascent rotational population of OD(A,u=O) was determined from a plot of the logarithm of the emission intensity divided by the line strength as a function of the rotational energy. The plot is relatively linear, and the nascent rotational population can be described by a temperature of ~ 2 8 0 0K. (The lower rotational levels, up to J = 27.5, can be fitted by a higher temperature of ~ 3 9 0 0K.) Spectra of both OH(A) and OD(A) measured under the conditions used in the kinetic experiments (13.3 kPa of Ar, 100-ns delay between the photolysis laser pulse and opening of the gated integrator) show that the rotational populations are well-described by Boltzmann populations at about 300 K. The relative populations for u = 1 relative to u = 0 for these conditions are 0.03 for OH(A) and
