J. Phys. Chem. 1992, 96, 7841-7844
ductance of OH- is somewhat higher than that of ea;, the predicted conductance a t 100 ns is only 8% lower than that for an argon-purged system. Thii difference is slightly less than the 10% difference estimated from the ion pair yields observed in the conductivity experiments of Klever et al. a t microsecond times7 but is substantially lower than the 15% decrease in conductance reported by Anderson et al. at 100 n ~ . ~ In general, the calculations show that the scavenging of one entity in a radiation spur can have a significant effect on the yields of other intermediates, sometimes in ways that are not intuitively obvious. It is found, for example, that in N20-saturated solutions radical scavengers slightly increase the yields of OH- which escape from the radiation spurs, in contrast to the decrease in the anion yield noted in water itself. Detailed calculations such as those briefly described here are required to understand fully the various contributions to the radiation yields of secondary products in all but the most dilute solutions.
7841
(4) Schuler, R. H. J . Phys. Chem. 1992, 96, 7169. (5) Radiation chemical yields are quoted as G values in units of molecules/ 100 eV throughout this Letter. (6)Barker, G. C.;Fowles, P.; Sammon, D. C.; Stringer, B. Trans. Faraday SOC.1970, 66, 1498. (7) Klever, H.; Toth, L.; Wagner, B.; Schulte-Frohlinde, D. Ber. BunsenGes. Phys. Chem. 1976, 80, 1265. (8) Schmidt, K. H.; Ander, S. M. J . Phys. Chem. 1969, 73, 2846. (9) Anderson, R. F.;Vojnovic, B.; Michael, B. D. Radial. Phys. Chem. 1985, 26, 301. (10)Pimblott, S. M.; Laverne, J. A. Radial. Res. 1990, 122, 12. (11)Laverne, J. A,; Pimblott, S. M. J . Phys. Chem. 1991, 95, 3196. (12)Pimblott, S. M.; Laverne, J. A. Radiat. Res. 1992, 129, 265. (13) Samuel, A. H.; Magee, J. L. J . Chem. Phys. 1953, 21, 1080. (14) Buxton, G.V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J . Chem. Phys. Ref. Data 1988, 17, 513. (1 5) The rate coefficients for reaction 5 and 7 were assumed to be the minimum value consistent with diffusion-controlled reaction and that for reaction 6 was taken from: Burns, W. G.; Marsh, W. R. J . Chem. SOC., Faraday Trans. 1 1981, 77, 197. (16) Hellwege, K.-H.; Hellwege, A. M.; Schafer, K. L.; Lax, E. Lun-
dholt-Bornstein, Zahlenwerke und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Tecknik 11.7; Springer-Verlag: Berlin, 1960;p 259. (17) Schwarz, H. A. J . Phys. Chem. 1969, 73, 1928. (18) The transient conductivity of a solution is given in the units S cm-' N Gy-' by I( = p N - ' a , X , G , where p is the density of the solution in g IS Avogadro's number, G, is the radiation chemical yield of species i having an equivalent conductance of Xi, and Cis a constant (=6.242X lo')) which converts the units of absorbed energy from 100 eV g-' to Gy.
References and Notes (1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL 3513 from the Notre Dame Radiation Laboratory. (2) Janata, E.;Schuler, R. H. J . Phys. Chem. 1982, 86, 2078. (3) Zehavi, D.; Rabani, J. J . Phys. Chem. 1971, 75, 1738.
Photodissociation and Recombination Dynamics of I,- in Water and Alcohols Alan E. Johnson, Nancy E. Levinger, and Paul F. Barbara* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: July 7 , 1992; In Final Form: August 13, 1992)
The photodissociation and subsequent recombination, vibrational relaxation, and cage escape of I; in solutions have been investigated for the first time. The experiments involve a synthesis-pump-probe sequence of three ultrashort laser pulses, beginning with 1,- as a photoprecursor for IC. Compared to published results on I2 in nonpolar solvents, our data on IC, which is restricted to polar solvents, indicate extraordinarily fast vibrational relaxation and very efficient geminate recombination, which we attribute to stronger solute/solvent interactions in the case of 12-. Preliminary results suggest that I; in water undergoes geminate recombination and vibrational relaxation in less than 3 ps, which is less than 10 vibrational periods and the shortest vibrational relaxation time measured for a diatomic.
brational relaxation3 and might also dramatically alter the energetics of this reaction. Second, the photodissociation of 12involves only dissociative state^,^ as compared to I2 where predissociative, dissociative, and quasi-bound states are important. Therefore, the details of the initial events after photoexcitation should be very different for these two systems. Another motivation for studying I< in solution is that data on this reaction in liquids are interesting to compare the recent results on the picosecond measurements of I< photolysis in mass-selected cluster^.^ In these experiments, Lineberger and co-workers observed very fast vibrational relaxation and very low quantum yields for escape compared to I2 in solution. In this Letter, we report pumpprobe measurements on the photolysis of 12- in water, methanol, ethanol, and 1-propanol. Estimates for the time scales for recombination and vibrational relaxation and for the yield of cage escape are made from examination of the experimental results. These estimates are compared to published data on I2 in nonpolar solvents and 12- in clusters.
Introduction One of the best understood and most extensively studied liquid-state reactions is the photolysis of I*. In particular, the application of ultrafast spectroscopy to this reaction1V2continues to offer some of the most precise and detailed information on key processes in chemistry, including vibrational relaxation, nonadiabatic transitions, geminate recombination, and solute/solvent interactions. Since the photolysis reaction of no other diatomic has been studied in as nearly great detail, it is unknown to what extent the observed behavior of I2 reflects the specifics of the system, namely, that this reaction occurs by a collisionally induced predissociation of the optically excited state and that nonpolar solvents have been employed in most studies limiting the types of solute/solvent interactions. In this paper we report the first measurements on the photolysis of 12- as follows. 12-
-+ X = 790 nm
I-
I'
While the sizes of the reactants and products of this reaction are obviously similar to those in the photolysis of 12,the ionic case differs in two obvious respects. First, the much stronger electrostatic solute/solvent interactions for 12- should accelerate vi-
Experimental Methods Because 12- is a radical and not present in significant amounts at equilibrium in solution, it is necessary to generate it in situ. This was achieved by the photolysis of 1,-,6-8 which is stable in polar solutions. After the delay of several nanoseconds which
* To whom correspondence should be addressed.
0 1992 American Chemical Society 0022-365419212096-7841%03.00/0 , , I
-
7842 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Letters I
I
-
~. - '-
-
- 1
.L
?
a
I
I ; 8
a
::
3'I
C
I
I
I
I
I
d
0
2
4 6 time (ps)
6 time (psl
Figure 1. Pumpprobe transients of I*- in (a) water, (b) methanol, (c) ethanol, and (d) 1-propanol. IC was created by photolysis of I< using 10-15 pJ of 355-nm light (synthesis pulse). Transients were obtained 6 ns after the synthesis pulse. Pump and probe beams were both centered at 790 nm and had parallel polarizations. Pump energy was typically 5-7 MJ. Probe energy was typically less than 1 pJ. Variation of the pump energy over a factor of 8 did not affect the shape of the transient.
allows the photofragments to relax? the pumpprobe experiments on the 740-nm band of 1; were performed. As a result, the experiment requires three laser pulses shown in the scheme for the experiment below.g 1,-
photolysis
I
+
I;
pumplprobe
w2nI recombinationlrelaxation
c c c ce *
L(21-&,l/z)
cage scape
1(2P,n)
+ I-
The laser systemlo is a synchronously pumped dye laser which is amplified using the doubled output of a Nd:YAG regenerative amplifier and yields 140-fs pulses (fwhm) centered near 790 nm with 10 MJ/pulse. The fundamental light at 790 nm was used for excitation of 12- in all cases. In some cases, it was also used for the probe. For variable-color probe measurements, selected with a continuum light pulses were generated in HzO, IO-nm interference filter, and reamplified with doubled output from the Nd:YAG regenerative amplifier. The residual 1064-nm light from the regenerative amplifier was doubled and tripled in
-
-
Figure 2. Same as in Figure 1, but over a longer time scale to demonstrate the residual bleach which we assign to cage escape.
KDP to yield -100-ps pulses at 355 nm with 10-15 MJ/pulse for the photolysis of 13-. Solutions of 1-5 mM Ij- were prepared by mixing I2 with excess KI (typically 5-fold molar excess) in solution to drive the equilibrium for the reaction I-
+ I* @ 13-
toward products. KI and I2 were from Mallinkrodt and were used as received. Absolute ethanol was technical grade. Other solvents were reagent grade or better. Data were fit to a convolution of multiexponential decay functions with the instrument response function. Results and Discussion Figure 1 displays the pumpprobe data for 12-in water, methanol, ethanol, and 1-propanol probing a t 790 nm. In each case, we observed an instrument-limited appearance of a bleach followed by a recovery of the bleach which occurs on several time scales. For example, in ethanol, the recovery roughly corresponds to four time scales 100 fs, 3 ps, 30 ps, and >200 ps (residual bleach), with relative amplitudes of 70, 6 , 1, and 1, respectively. Figure 2 shows pumpprobe data, again probing at 790 nm, over a longer time scale emphasizing the multiple time scales. Interestingly, for water, methanol, and ethanol, the bleach does not completely recover at the upper limit of our time resolution. In
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The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7843
Letters
TABLE I: Vibrational Relaxation Times and Quantum Yields for Photodissociation for 11- in Various Solvents @dis
solvent
water methanol ethanol propanol
a8
P
('vib),
Ps
