J . Phys. Chem. 1991, 95, 2917-2920
2917
Temperature Dependence of the Quenching of I*(*P,,,) by I, from 300 to 800 K Joseph I. Clinet and Stephen R. Leone*'$ Joint Institute for Laboratory Astrophysics, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: January 29, 1991)
The quenching rate of I'(2PI 2) by I2 is measured over the temperature range 297-800 K by using a time-resolved diode laser absorption/gain method. The room-temperature bimolecular quenching rate constant is determined to be k, = (2.89 f 0.06) X lO-" cm3/(molecule.s), in good agreement with several previous determinations. The rate is found to be independent of temperature over the temperature range 300-800 K within experimental uncertainty; at 800 K the quenching rate is kq = (3.02 f 0.30) X IO-" cm3/(molecule.s). The temperature dependence is consistent with a quenching mechanism involving the formation of an I3 intermediate complex. The quantum yield for I* formation from the 480-nm photolysis of I2 is measured to be 0.304 f 0.001 at 297 K and increases slightly with temperature. This quantum yield is significantly larger than that previously determined.
I. Introduction Ever since the discovery of the iodine atom laser by Kaspar and Pimentel,l the quenching dynamics of I*(zP1/2)has been the subject of considerable technological interest. The quenching rate of I* by I2 is of particular importance in an understanding of the operation of the chemically pumped, oxygen-iodine high-power I* laser,2 since I2 is present in the laser gain medium. At the high-power densities attainable in such lasers, conventional optics are often unusable. One means of Q-switching such a laser is by Zeeman-shifting the absorption of a high-temperature vapor containing I atoms, produced by thermal dissociation of 12. A knowledge of the I* I2 quenching rate as a function of temperature is then essential in the design of such optical devices. The mechanism for quenching of excited 2Pl/2halogen atoms by halogen molecules is also of fundamental interest. The quenching process has been postulated to proceed via the formation of a long-lived intermediate trihalogen complex. The temperature dependence of the quenching rate can provide important evidence in the elucidation of this mechanism. Previous measurements of the I* + Iz quenching rate have been at room temperature3-' with the exception of one study* at temperatures up to 410 K. Unfortunately, the room-temperature rate constant in the latter study disagrees with the other room-temperature determinations. Here we report an application of a diode laser absorption/gain technique to measure the I* I2 quenching rate a t the relatively high temperatures that occur in these Qswitch devices. The results of this work show that the quenching rate is essentially independent of temperature from 297 to 800 K. This unusual temperature dependence can be explained in terms of a mechanism requiring the formation of an intermediate trihalogen complex and is consistent with related low-temperature studies of halogen atom deactivation.
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11. Experimental Section A block diagram of the experimental apparatus is shown in Figure 1. The quenching kinetics of I* with I2 are studied by a time-resolved diode probe laser absorption/gain technique following pulsed laser photolysis of I2 to produce I* and I atoms. A Stern-Volmer type analysis is used to extract second-order quenching rate constants at T = 300-800 K. Quenching measurements are made in a 2.5-cm-diameter, I - d o n g quartz photolysis cell. Each end of the cell is fitted with double quartz windows with an intervening evacuated region. The cell is wrapped with a layer of ceramic fiber tape, and Nichrome heater wire is coiled about this ceramic tape layer. The heated cell is encased in an alumina fibrous ceramic block for thermal 'Present address: Department of Chemistry, Mail Stop 216, University of Nevada, Reno, NV 89557-0020. 'Staff Member, Quantum Physics Division, National Institute of Standards and Technology.
insulation. The temperature of the gas within the cell is monitored by thermocouples inserted into small quartz fingers that protrude into the interior of the cell. Temperature measurements are made near one end window and at the middle of the cell. Sample mixtures are prepared from iodine crystals which are resublimed immediately prior to use in a glass vacuum apparatus. The iodine partial pressure is maintained below its room-temperature vapor pressure. High-purity (>99.99%) argon is used as a buffer gas to thermalize the I* and I velocities after photolysis. Capacitance manometers are used to measure gas pressures. The relative accuracy of the manometers was checked by using pressure measurements of the expansion of known volumes. A transient population of I* is generated by pulsed laser photolysis of I2 at 480 nm. The photolysis laser pulse is generated by a dye laser which is pumped by the third harmonic of a Nd:YAG laser operating at 10 Hz. A telescope is used to increase the dye laser beam diameter to 7 mm. The dye laser beam is directed along the central axis of the photolysis cell. Typical photolysis laser pulse energies are 4-5 mJ. The transient population of I* vs I is measured by diode laser gain/absorption on the iodine 2Pl/2-2P3/ztransition near 1315 nm. This technique has been described in detail e l ~ e w h e r eand ~ ~ 'is~ only summarized here. The probe laser consists of a CW 1300-nm GaAsInP laser diode which is frequency-locked by optical feedback from a confocal Fabry-Perot cavity. The laser wavelength is fine-tuned by scanning the cavity length. The bandwidth of the probe laser is 900 K) in which IBr is thermally dissociated to produce iodine atoms. The absorption of the probe beam by iodine atoms in the oven is monitored by a Ge photodiode to provide an absolute frequency reference. The laser is servo-locked to the center of the iodine 2Pl/2(F= 3)-2P3/2(F=4) transition.I0 The diode laser probe beam is propagated nearly collinearly with the photolysis beam along the axis of the photolysis cell. The two laser beams intersect near the center of the cell at a small ( I ) Kaspar, J. V. V.; Pimentel, G. C. Appl. Phys. Left. 1964, 5, 231. (2) McDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. .. Phys. Leu. 1978, 32, 469. (3) Hofmann, H.; Leone, S . R.J. Chem. Phys. 1978. 69, 641. (4) Burde, D. H.; McFarlane, R. A.; Wiesenfeld, J. R. Chem. Phys. Lett. 1975, 32, 296. (5) Grimley, A. J.; Houston, P. L. J . Chem. Phys. 1978, 68, 3366. (6) Burrows. M. D. J. Chem. Phvs. 1984. 81. 3546. (7) Boriev, I. A.; Gordon, E. B.;-Efimenko, A. A.; Chernin, S. M. Opr. Spekttosk. 1985, 59, 277. (8) Deakin, J. J.; Husain, D. J. Chem. SOC.,Faraday Trans. 2 1972,68, 1603. (9) Hess, W. P.; Leone, S. R. J. Chem. Phys. 1987,86, 3773. (IO) Cline, J. 1.; Taatjes, C. A,; Leone, S . R. J. Chem. Phys. 1990, 93, 6543.
0022-3654/91/2095-2917%02.50/0 0 1991 American Chemical Society
2918 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Cline and Leone I
r;=To vacuum line
.-c
etector
Ihi Assembly LPuised
dye laser
;,-l-GComputer +pG-j Figure 1. Schematic diagram of the experimental apparatus.
angle. Upon exiting the cell, the probe beam is directed through a red-pass filter which blocks the blue photolysis beam and then through an interference filter which has a band-pass centered at 1315 nm. The diode laser beam is detected with a reversed-biased Ge photodiode detector. The preamplified detector signal is recorded by a digital oscilloscope. The risetime of the entire detection system is less than 50 ns. Background transient signals measured with the diode laser tuned off the iodine atom transition are subtracted from the on-resonance transients. This procedure remova any artifacts resulting from thermal lensing in the sample and the radio-frequency interference from the photolysis laser pulse. The fractional gain is always small, so that the signals are linear. The experimental procedure consists of measuring the time dependence of the probe laser power transmitted through the cell following the photolysis laser pulse. Measurements of this type are made at a series of I2 partial pressures in an inert bath gas of Ar atoms. Attempts to measure the kq for T 1 900 K are plagued by poor signal-to-noise. Checks were made to ensure that the decrease in signal was not due to saturation of the photodiode detector by blackbody radiation. It is found that a t such high temperatures there is sufficient thermal dissociation of I, to result in substantial attenuation of the probe laser by I atom absorption. While the equilibrium concentration of I atoms at T < 1000 K is small and should not affect the validity of the measured rate constant, we only report k, for T I 800 K because of the poor quality of the higher temperature data. 111. Results We treat the quenching of I* using the following kinetic scheme:
-
I2 + hu (480 nm) I*
-+ 1 /Tmd
(1)
hv (1.315 pm)
(2)
-+ + + + -
I*
I*
I
I* + I -I+I
+ I, Ar
I*
I* + I
k,
I
kl
+ M - kr
I
I,
(3)
I
Ar
(4)
21
I,*
(5)
+M
(6)
The production time for I* by the 5-ns dye laser pulse (eq 1) is very short on the time scale of the quenching dynamics. Emission of I* to its electronic ground state is much slower than the quenching rate in these experiments (rmd 0.1 s) so that eq 2 can be ignored." Similarly, the bimolecular rates (computed from the known rate constantsI2) for I* quenching by Ar (eq 4) and I (eq 5 ) are more than IO4 times smaller than the quenching process in eq 3 and can be neglected. Finally, the I* + I recombination rate (eq 6 ) is sufficiently slow to be observable only in the absence of quenching species,I3 so that the only significant
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(11) Comes,F. J.; Piontek, S.Chem. Phys. Lett. 1976, 42, 558. (12) Husain, D.;Donovan, R. J. Adu. Photochem. 1971, 8, 1 . (13) Abrahamson, E. W.; Husain, D.; Wiesenfeld, J. R. Trans. Faruduy SOC.1968,64, 833.
-
L 1
0
1
2
3
4
5
Time (p)
-io
o
io
20
30
40
50
60
70
ao 90
Time (ps)
Figure 2. Experimental transient showing diode laser power transmitted through the experimental cell following the photolysis laser pulse at t = 0. The sample gas is 12.3 Pa of I2 in Ar at 1.34 kPa total pressure. Experimental points are shown by dots; the solid curve is a fit to eq 13 with 4 = 0.304 and k,' = 0.086 ps-'. The inset shows the behavior of the experimental signal at very early times. The first data point at r = 10 ns shows gain as a consequence of incomplete collisional thermalization of the I atom velocities and the resulting distortion of the Doppler line shape of the transition.
mechanism for I* quenching is eq 3. Figure 2 shows a typical experimental transient. The dots represent experimental data points, and the solid line represents the fit to the first-order kinetic model as described below. The photolysis laser pulse occurs at t = 0. We define the experimental signal, S(t), as the difference between the probe beam power, P(t), before and after the photolysis laser pulse: S ( t ) = P(r) - P(t
0.08
-
0.06
-
0.02
0'04
61
1
5
1
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1
4.5
1
H I
E
I
0.00 0.0
,
1
O
1
I
I
000
0
200 300 400 500 600 700 800 900
Figure 3. Stern-Volmer plot of the pseudo-first-order quenching rate k,' = 1 / T as a function of I2 partial pressure. The slope gives the bimolecular quenching rate constant, k,. Experimental data are shown at four different temperatures. Representative error bars ( f l u ) are shown for the
data points taken at 448 K. TABLE I: Temperature Dependence of Bimolecular Quenching Rate, k , and I* Ouanbm Yield, 6,at 480 nm, with I? k,, lo-" cm3/
T,K 297 448 496 534 602 667 800
(molecu1e.s) 2.89 f 0.06 2.65 f 0.2 2.73 f 0.1 2.64 f 0.2 3.16f 0.3 2.92 f 0.8 3.02 f 0.3
4 0.304f 0.001 0.339 0.004 0.347 f 0.003 0.344f 0.001 0.349 f 0.009 0.342f 0.006 0.367 f 0.008
*
'Errors are f l u computed from a least-squares analysis of the Stern-Volmer plots (for k,) or of the experimental transients (for 6).
480 nm.) The experimental signal amplitude at t = 0 is determined by the quantum yield, 9. As the I* is collisionally quenched to its ground state, the signal exponentially approaches a final (negative) value determined by the total number of I atoms produced in the photofragmentation. The ratio of the t = 0 vs t = signals can be used to obtain an absolute measurement of the quantum yield. Diffusion of the iodine atoms out of the probe volume also affects the experimental signal. This leads to an apparent decay of the experimental signal on a time scale substantially longer than the quenching dynamics, but fast enough to affect the measured quenching rate, especially at high temperatures. In Figure 2 this effect is slightly noticeable as a slight positive slope in the experimental transient for t > 60 ps. The diffusion rates of I* and I from the probe volume are assumed to be equal and are treated phenomenologically as a first-order decay of [I](r) and [I*](t) with rate constant kd, so that eq 11 becomes S(t) 0: e+'[39
exp(-k,[I,]t)
- 11
(13)
Experimental signals of the type in Figure 2 were fit to eq 13 in order to obtain the pseudo-first-order quenching rate constant, k,' = k [Q], and the quantum yield, 9. The value of kd is determind prior to the analysis by first fitting the long-time behavior of the experimental transients. A Stern-Volmer type quenching analysis is used to extract the bimolecular quenching constant, k,, a t a given T. Within experimental error, all the Stern-Volmer plots show a zero intercept. Table I and Figure 3 summarize the results of the analysis at seven temperatures from 300 to 800 K. The bimolecular quenching rate constant shows no significant temperature dependence. IV. Discussion Quenching Rare. The bimolecular rate constant, k,, has been previously measured by several researchers at room temperature (see Figure 4). The measurements of Hofmann and Leone3 (k,
T (K)
Figure 4. Temperature dependence of the bimolecular quenching rate constant, k Results are shown from several studies: 0 , this work; A, ref 4; M, reP'5; 0,ref 8; A, ref 6;V, ref 7; 0, ref 3.
= (3.1 f 0.5) X lo-" cm3/(molecule.s)), Burde et a1.4 (k, = (3.6 f 0.3) X lo-" cm3/(molecule.s)), Grimley and Houstons (k, = (3.8 f 0.24) X lo-" cm3/(molecule.s)), Burrows6 (k, = (3.0 f 0.1) X lo-" cm3/(molecule.s)), and Boriev et al.7 (k, = ( 5 f 1) X lo-" cm3/(molecule.s)) are in approximate agreement with the value from the present study, k ,= (2.89 f 0.06) X cm3/ (molecu1e.s). Deakin and Husain* made two measurements of the room-temperature quenching rate constant and found k, = (0.41 f 0.08) X IO-" and (1.08 f 0.8) X lo-" cm3/(molecule.s), significantly smaller than the other studies. The only previous measurement of the temperature dependence of the rate constant is part of the latter study by Deakin and Husain. In that work k, was measured for temperatures up to 410 K and was found to decrease slightly with temperature. An Arrhenius treatment of the temperature dependence gave a small negative activation energy, E,, = -3.3 f 1.1 kcal/mol. Within experimental error, we observe no temperature dependence of the rate constant and a zero activation energy for I* I2 quenching. This result is consistent with models developed previously that suggest a mechanism involving a long-lived I3 intermediate The I3 mechanism for quenching of I* by I2 is supported by a variety of evidence. Comparison of the I* Iz quenching rate constant to that of related processes suggests a reactive collision process involving the breakup of an intermediate trihalogen species. The quenching rate for reactive processes of this type is typically at least an order of magnitude larger than that observed for simple nonreactive E-T,R,V processes with atomic and molecular collision partner^.^^,^^ Rate constants for deactivation of I* by other halogen and interhalogen molecules are similar to that for 12. Hofmann and Leone3 and Burrows6 found rate constants for room-temperature deactivation of I* by Brz, IBr, ICl, and BrCl of (5.2 f 0.3) X lo-", (6.6 f 0.3) X (2.3 f 0.2) X lo-", and (2.7 f 0.2) X lo-" cm3/(molecule.s), respectively. An important exception is the small value6 of (2.0 f 0.1) X lodt4 cm3/(molecule.s) for the rate constant for quenching by Clz. This result can be attributed to the instability of C1-containing trihalogen complexe~.~ I* is known to react with other diatomic halogen and interhalogen species to give products that result from atom exchange. In particular, H o u ~ t o n and ' ~ Wiesenfeld and Wolk16*17 studied the reaction of I* Br2 in detail and estimated that less than 25% of the deactivating collisions result in deactivation to I + Br2 products. The major deactivation process is the reactive collision producing IBr Br* and Br, the Br* channel being predominant. Wiesenfeld and Wolk suggest that these products result from the breakup of a bent trihalogen intermediate complex which has no
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+
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(14) (15) (16) (17)
Houston, P. L. Adu. Chem. Phys. 1981, 47 (2), 381. Houston, P. L. Chem. Phys. Leu. 1977,47, 137. Wiesenfeld, J. R.;Wolk, G. L. J . Chem. Phys. 1978, 69, 1797. Wiesenfeld, J. R.;Wolk, G. L. J . Chem. Phys. 1978, 69, 1805.
2920 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 activation barrier to f0rmation.l' In their model, the Br* product results from reaction on an adiabatic potential energy surface correlating to the excited-state atoms. Approximately 25-3095 of the overall deactivation yields ground-state Br atoms, suggesting that the intermediate complex is sufficiently long-lived to allow many vibrations to occur, leading to relatively efficient nonadiabatic processes. The absence of any temperature dependence of the I* I2 quenching rate can be rationalized in terms of this model. A zero activation energy is consistent with the energy of formation of the bent I3 complex. However, we would still expect a temperature dependence of T'12in the Arrhenius preexponential factor for k, by simple consideration of the temperature dependence of the bimolecular collision frequency. This would result in a factor of 1.6 increase in k, from room temperature to 800 K. Electronically adiabatic breakup of the I, complex re-forms the reactants and is not a quenching process. Only the nonadiabatic process leading to ground-state I is experimentally observable. Thus, if the I, intermediate is shorter lived at high temperatures as a result of the higher collision energy or increased vibrational and rotational excitation of the I2 reactant, the probability of nonadiabatic processes might be reduced. This effect can counterbalance the expected temperature dependence of the preexponential factor. This mechanism is supported by and theoretica12'-23studies which conclude that trihalogen species are stable and have well-defined, bent structures. Studies of I atom recombination indicate that I, is a stable species, bound by 5.3 kcal/mol, and has no barrier to formation.24 I* Quantum Yields. In addition to the quenching rate constant, the I* quantum yield is also obtained from the data analysis. The room-temperature value of the I* quantum yield for I2 at 480 nm, 4 = 0.304 i 0.001, is not in agreement with previous determinations.25q26This result confirms that I2 photofragmentation at wavelengths shorter than 495 nm (the dissociation limit of the
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(18) Mamantov, G.; Vasini, E. J.; Moulton, M. C.; Vickroy, D. G.; Maekawa, T. J . Chem. Phys. 1971, 54, 3419. (19) Lee, Y. T.; LeBreton, P. R.; McDonald, J. D.; Herschbach, D. R. J . Chem. Phys. 1969, 51,455. (20) Valentini. J. J.; Coggiolia, M. J.; Lee, Y. T. Discuss. Faraday Sot. 1977, 62, 232. (21) Ungemach, S. R.; Schaefer, H. F. J . Am. Chem. Soc. 1976,98, 1658. (22) De, B. R.; Sannigrahi, A. B. Int. J . Quantum Chem. 1981, 19, 485. (23) Castro, E. A.; Vasini. E. J. Int. J . Quantum Chem. 1984, 22, 433. (24) Bunker, D. L.; Davidson, N . J . Am. Chem. Sot. 1958, 80, 5090.
Cline and Leone I2 B311(0,+) state) does not occur exclusively on the B state electronic potential energy surface to yield I*(2Pl/2)+ I(2P3/2) as has been assumed in the past.25 This assumption would lead to a higher value of 9 = 0.5. A molecular beam photofragmentation time-of-flight study26 concluded that dissociation on the B electronic state surkace is actually the minor channel, with most molecules dissociating on the In(1,) surface which correlates to two ground-state I(2P312) atoms. The measured ratio of the probability for dissociation on the In(1,) surface to that on the BJI(O,+) surface was reported to be 1.2 f 0.2 in the molecular beam experiment. The roomtemperature quantum yield of 0.304 f 0.001 in our study corresponds to a ratio of 0.64, significantly lower than the molecular beam result and indicating a much higher I* yield. The I* quantum yield increases slowly with temperature, reflecting dissociation from more highly excited vibrational states of 12. At 800 K the quantum yield increases to 0.367 f 0.008, and the corresponding dissociation channel probability ratio defined above decreases to 0.35. Thus, it is difficult to rationalize the result in ref 26, especially since the I2 temperature in that experiment is probably lower. We have great confidence in the reliability of the absorption/gain technique and cannot explain this discrepancy. Summary. In summary, the temperature dependence of the bimolecular quenching rate of I* by I2 is measured from room temperature to 800 K. The rate is found to be independent of temperature within experimental error. This result is rationalized in terms of the stability of an I, intermediate species. Measured values of the quantum yield for production of I* in the 480-nm dissociation of I2 disagree with previous determinations of this quantity.
Acknowledgment. We gratefully acknowledge the support of the Air Force Weapons Laboratory and the National Science Foundation. S.R.L. acknowledges the Miller Institute for Basic Research in Science for a Visiting Miller Professorship at the University of California, Berkeley, during the preparation of this manuscript. (25) Burde, D. H.; McFarlane, R. A.; Wiesenfeld, J. R. Phys. Rev. A 1974, IO, 1917. Note that the definition of the quantum yield, 6, in this paper differs from our definition. (26) Oldman, R. J.; Sander, R. K.; Wilson, K. R. J . Chem. Phys. 1971. 54, 4127.
