J . Phys. Chem. 1984, 88, 6084-6087
6084
the mechanism of ferroin-catalyzed oscillations may be much different from cerium oscillations. Ganapthisubramanian and NoyesZoput forward the proposal that the oxidation of organic substrate may involve a Fe(1V) species. However, the simple FKN mechanism, which leads to great discrepancies between models and experiment for the cerium system, leads to a simple and adequate model for the ferroin-catalyzed oscillating reaction.
for the ferroin system this term is the following: K8BX/ho(C-
X);in the Oregonator the corresponding term is kBX. As a result the Oregonator does not contain the important parameter C and permits the ceric ion concentration to exceed the total concentration of cerium in the system. It was suggested earlier12,28,29 that due to the presence of ligands (28) Noyes, R. M. J . A m . Chem. Os. 1980, 102, 4644-4649. (29) Ganapthisubramanian, N.; Noyes, R. M. J . Phys. Chem. 1982, 86, 5158-5162.
Registry No. CHBr(COOH)2, 600-3 1-7; bromate, 15541-45-4; fer-
rain, 14708-99-7.
Kinetics of I, Following ArF Laser Excitation: Thermal Dissociation of the A’(2,) State Joel Tellinghuisen,* Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
Andrew R. Whyte, and Leon F. Phillips Chemistry Department, University of Canterbury, Christchurch. New Zealand (Received: August 14, 1984)
The quenching of the lowest excited state of I, by 12,Ar, and N2 is studied by a delayed absorption probe technique following excimer laser photolysis. The resulting rate constants (units cm3/s; T = 296 f 3 K) are kl, = 8.3 (1.0) X lo-”, kAr = 2.8 (2) X and kN2= 7.2 (7) X lO-I4. The latter two are interpreted as thermal dissociation, but the I, self-quenching is tentatively ascribed to the process 12(A’) + I2 I3 + I.
-
Introduction One of the simplest termolecular chemical reactions is the three-body recombination of atoms to form a diatomic molecule, and one of the most studied such reactions is the iodine atom recombination.’s2 I
+I +M
-+
I2
+M
(1)
This reaction was first studied quantitatively by Rabinowitch and Wood in 1936.3 When these authors extended their gas-phase experiments to inert liquid solvents: they observed very efficient recombination, which they attributed to the “cage effect”, whereby photodissociated I atoms are forced back together by a solvent cage. Recently the cage effect has come back into prominence, as it has become possible to study it dynamically by using picosecond laser technique^.^-'^ At the same time the gas-phase studies have been extended to an inert gas density regime comparable to that for liquids, permitting observation of the cage effect in the gas An interesting point about reaction 1 is that most experiments have yielded only phenomenological rate constants, since they have involved indirect measurements-namely the detection of changes (1) H. S.Johnston, “Gas Phase Reaction Rate Theory”, Ronald Press, New York, 1966. (2) R. K. Boyd and G. Burns, J . Phys. Chem., 83, 88 (1979). (3) E. Rabinowitch and W. C. Wood, J . Chem. Phys., 4, 497 (1936). (4) E. Rabinowitch and W. C. Wood, Trans. Faraday SOC.,32,547, 1381
( 1936).
(5) T. J. Chuang, G. W. Hoffman, and K. B. Eisenthal, Chem. Phys. Lett., 25, 201 (1974). (6) C . A. Langhoff, B. Moore, and M. DeMeuse, J. Am. Chem. Soc., 104, 3576 (1982). (7) D. F. Kelley and P. M. Rentzepis, Chem. Phys. Lett., 85, 85 (1982). (8) D. F. Kelley, N. A. Abul-Haj, and D.-J. Jang, J. Chem. Phys., 80,4105 (1984). (9) P. Bado, C. Dupuy, D. Magde, K. R. Wilson, and M. M. Malley, J . Chem. Phys., 80, 5531 (1984). (10) P. Bad0 and K. R. Wilson, J . Phys. Chem., 88, 655 (1984). (11) J. Troe in “High Pressure Chemistry”, H. Kelm, Ed., Reidel, Dordrecht. 1978. (12) J.-C. Dutoit, J. M. Zellweger, and H. van den Bergh, J . Chem. Phys., 78, 1825 (1983).
0022-3654/84/2088-6084$01.50/0
in the absorption by ground-state I2 molecules. Such experiments can say little about the mechanism by which the atoms initially recombine. For example, the ground state of I2 accounts for only 1/16 of the statistical degeneracy of two ground-state I(2P312) atoms, so most I I collisions occur on one of the other nine Hund’s case c electronic states which correlate with the lowest atomic limit. Theoretical calculations, principally classical trajectory studies, have done much to enhance our understanding of the recombination p r o c e ~ s . ’ ~ - ’One ~ of the results of such studies is that weakly bound excited states play only a minor role in the recombination. However, since two of the relevant excited states of 12-A’(2,311) and A(lU3II)-are now known to be moderately bound,I6 these states could be important in the recombination mechanism. In the present work we have carried out what we believe to be the first gas-phase kinetics study of the lowest excited state in 12, the A’ state. Specifically we have prepared this state by ArF laser excitation of I, in N2 and Ar buffer gases at pressures of 50-900 torr and measured the depopulation rate of the A’ state by a delayed laser-probe absorption technique. The resulting rates appear to be first order with respect to all three gases. The N2 and Ar results are interpreted as thermal dissociation. The I2 results, on the other hand, force us to the speculative conclusion that quenching occurs via the reaction 12(A’) I, I3 I (2)
+
+
-
+
Spectroscopic Background Iodine absorbs light very strongly in the vacuum-UV region X(’2,’). Spectrovia the charge-transfer transition D(O,+) scopic analysis of the D state” shows that, at the 1930-A wavelength of the ArF laser, the absorption originates mainly in u” = 1 and terminates in u’= 149. At low pressure the D state depopulates very rapidly (7 = 15.5 nsl*) through fluorescence back
-
(13) (14) (15) (16) ( 198 3). (17)
A. J. Stace, J . Chem. SOC.,Faraday Trans. 2, 77, 2105 (1981). G. Burns and A. W. Young, J . Chem. Phys., 72, 3630 (1980). D. J. Nesbitt and J. T. Hynes, J . Chem. Phys., 77, 2130 (1982). K. S. Viswanathan and J. Tellinghuisen, J . Mol. Spectrosc., 101, 285 J. Tellinghuisen, Can. J . Phys., in press.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol, 88, No. 25, 1984 6085
Letters D,v=14( 50
-
40
l,‘I-
\I
13
’$
4
0 -
m
3
Q.
Y
3
u
10
a
Figure 2. Transient absorption spectrum (inverse transmittance) obtained less than 100 ns after ArF laser excitation of I2 ( P = 0.2 torr) in Ar ( P = 1 atm). Also shown are the ~ a l c u l a t e dband ~ ~ ~origins ~’ for absorption bands (red degraded) of the D’ A‘ system originating from u’‘ = 0 and 1. The lengths of the lines are proportional to the Boltzmann-weighted Franck-Condon factors. (The strongest bands for u’ 2 2 are at least a factor of 3 weaker than the weakest band shown.)
-
-
0 I
3000
2950
0
m
wavelength using the ArF laser at low power as source, for several I, pressures, with N, and Ar used to pressure broaden the discrete absorption lines. Our value was a somewhat smaller 5.5 ( 5 ) X io3 L/(mol cm).
x,v=l I
I
I
Experimental Section The experiments were carried out using a static system, in which known pressures of 1, and either N2or Ar were admitted to a fused silica cell 12.5 cm long by 4.0 cm 0.d. The I, was photolyzed by 1930-A ArF radiation from a Lumonics TE-861T laser, the beam being directed longitudinally through the cell. The laser was operated at a pulse rate of 33 Hz and pulse energies of 10-25 mJ, as measured with a Scientech Model 364 power meter. The beam cross section was slightly greater than 2 cm2, giving pulse energy densities of 3-10 mJ/cm2. The transient absorption measurements in the 2950-3000-A region were made with an AVCO C5000 Nz-pumped dye laser and rhodamine 590 dye; the yellow beam was doubled with an Inrad temperature-tuned ADA crystal and blocked with a Corning 7-54 filter. The UV probe beam made two longitudinal passes through the active photolysis region and then was directed into a McPherson Model 218 0.3-m monochromator, where it was detected with an EM1 9813QB photomultiplier and PAR Model 160 boxcar integrator. The signal from the integrator was digitized with a 12-bit A / D converter and recorded on an LSI 11/23 computer (DEC VT103). Further details of the experiments will be presented el~ewhere.,~
-
-
Results and Discussion Initially we attempted to detect the I,(A’) by laser-induced fluorescence. However, the prompt fluorescence produced by the ArF laser excitation was so intense as to prohibit detection any sooner than 100 ws after the excitation, at which time we saw nothing. We then turned to the absorption probe and found significant absorption at very short times following excitation. To A’ assignment, we recorded absorption within verify the D’ 100 ns of the excitation pulse at wavelengths between 2950 and 3000 A. The resulting spectrum (Figure 2) is noisy, due in large part to the long time delays required to change the wavelength of our probe laser. However, the observed structure is mostly consistent with expectations for the D’ A’ system. Transient absorption data were recorded as a function of delay time for various mixtures of 1, ( P = 30-260 mtorr) with Ar or N2 (P= 53-926 torr). The results showed a dependence on all three components (see Figure 3). The data were fitted to the expression T= (3)
-
~
~
~~~~
(18) A. B. Callear, P. Erman, and J. Kurepa, Chem. Phys. Lett., 44, 599
(1976).
(19) K. P. Lawley, M. A. Macdonald, R. J. Donovan, and A. Kvaran, Chem. Phys. Lett., 92, 322 (1982. (20) A. L. Guy, K. S. Viswanathan, A. Sur, and J. Tellinghuisen, Chem. Phys. Lett., 73, 582 (1980). (21) M . C. Sauer, Jr., W. A Mulac, R. Cooper, and F. Grieser, J . Chem. Phys., 64, 4587 (1976). (22) J. Tellinghuisen, J . Mol. Specrrosc., 94, 231 (1982). (23) J. Tellinghuisen, J . Phys. Chem., 87, 5136 (1983). (24) M. V. McCusker, R. M. Hill, D. L. Huestis, D. C. Lorents, R. A. Gutcheck, and H. H. Nakano, Appl. Phys. Lett., 27, 363 (1975). (25) A. B. Callear and M. P. Metcalfe, Chem. Phys. Lett., 43, 197 (1976). (26) R. J. Donovan, B. V. O’Grady, L. Latin, and C. Fotakis, J . Chem. Phjw., 78, 3727 (1983). (27) J. Tellinghuisen, A. R. Whyte, and L. F. Phillips, to be submitted for publication. (28) L. M. Julien and W . B . Person, J . Phys. Chem., 72, 3059 (1968).
-
-
with f(t)
= A&‘
(4)
6086
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 t(ps)
0 r
1
2
3 I
Letters cm-’ (10 kT). A possible alternative explanation is electronic quenching to high vibrational levels of the X state. However, this process provides a path for Iz-assisted recombination into the ground state at a rate an order of magnitude larger than the accepted value.29 Thus, we are led to the tentative conclusion that I2 self-quenching occurs via I3 formation (reaction 2). Furthermore, since there is no evidence of back-reaction, I, must be energetically more stable than Iz(A’). On the longer time scale our vacuum-UV measurements suggest that the I3 produced in (2) dissociates to Iz I. However, these experiments must be repeated with care to confirm the dissociation yields under a wider variety of conditions. We are also examining the temperature dependence of the quenching, which should help clarify the mechanism. We have considered several possible effects which could introduce errors into our apparent rates, including thermal heating products by the ArF laser and reactions of form I,(A’) I,(A’) and 12(A’) + I products. We failed to observe any ArF laser power dependence in the results, which tends to dismiss all of these. However, we calculate that the thermal heating could introduce a small systematic error (- 10%) at our lowest buffer gas pressures, since the thermal dissociation is so strongly T dependent (-4%/K at room temperature). For our initial yields of I,(A’) (
