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J. Phys. Chem. 1996, 100, 6406-6411
ARTICLES Ultrafast Dynamics of Chlorine Dioxide Photochemistry in Water Studied by Femtosecond Transient Absorption Spectroscopy Yong Joon Chang and John D. Simon* Department of Chemistry and Biochemistry and Institute for Nonlinear Science, UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0341 ReceiVed: NoVember 7, 1995X
The ultrafast dynamics of chlorine dioxide (OClO) photochemistry in aqueous solution was studied by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 395 nm, the transient absorption dynamics were probed at 12 different wavelengths ranging from 350 to 700 nm. The transient absorption features observed in the visible wavelengths are assigned to correspond to the vibrationally hot photoisomer ClOO*. The spectral dynamics reveal the vibrational relaxation of this molecule in its ground electronic state. The total vibrational energy relaxation occurs within ∼9 ps. The dynamics of the formation of chlorine atom was examined by measuring the absorption dynamics in the 350-390 nm range. The time constant for Cl formation is calculated to be ∼200 ps. The data show that the dominant pathway for Cl formation is via the isomerized ClOO molecule. No fast component for Cl is detected, indicating that the ClOO molecule reaches thermal equilibrium before dissociating into Cl and O2.
I. Introduction The study of chemical reaction dynamics in the condensed phase is complicated by the interaction of the reacting species with the surrounding medium. Much effort is currently being expended to understand the nature of these intermolecular interactions and to determine their role in influencing various photophysical and photochemical processes. In particular, the investigation of photodissociation and recombination reaction of small molecules has provided a wealth of information regarding the effects of solute-solvent frictional coupling on the dynamics of cage recombination and the subsequent process of vibrational energy relaxation (VER).1 Using time-resolved spectroscopic techniques, detailed studies of the VER process have been reported for a number of diatomic 6-11 13 CN-,12 and Nand triatomic molecules, e.g., I2,2-5 I2, 3, in a wide variety of solvents. In addition, theoretical studies of VER have also been reported for small molecules, e.g., CH318-20 The experimental and theoretical Cl,14,15 I2,16-18 and I2. work reveals that solute-solvent electrostatic interactions play a central role in affecting the time scales of the VER process. Specifically, the rate of VER for I2 in polar solvents (e.g., in water, τVER ≈ 1 ps) is more than 2 orders of magnitude greater than that for I2 in nonpolar solvents (τVER ) 50-200 ps). The present study reports the femtosecond dynamics accompanying the photodissociation reaction of chlorine dioxide (OClO) in water solution. In contrast to the molecules mentioned above, the photoreactivity of OClO is more complicated; there are four possible competing reaction pathways, and the partitioning between them depends on the solvent.21 The four distinct reaction pathways are
OClO(X2B1) + hν f ClO(2Π) + O(3Pg)
(1)
OClO(X2B1) + hν f Cl(2Pu) + O2(3Σg)
(2)
* Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6406$12.00/0
OClO(X2B1) + hν f Cl(2Pu) + O2(1∆g)
(3)
OClO(X2B1) + hν f ClOO f Cl(2Pu) + O2(3Σg ) (4) In the gas phase, a number of different spectroscopic studies22-30 show that the photoexcitation of OClO in the 270-475 nm region gives rise to wavelength-dependent photochemistry. In particular, dissociation into ClO + O is the dominant pathway and is facilitated by excitation of the asymmetric stretch of OClO. The products Cl + O2 are also observed, and the relative yield of this channel is increased if energy is deposited into the bending vibration of OClO.28 The maximum quantum yield of the Cl + O2 channel in the gas phase is ∼4%. The intermediacy of ClOO along the reaction pathway that generates Cl + O2 is still controversial. In the cryogenic solid matrix, the situation is very different. The photoproducts Cl and O2 are exclusively produced from the intermediate, ClOO, which is formed by either direct isomerization of excited OClO or ultrafast cage recombination of the dissociated products ClO + O. The unit quantum yield combined with the theoretical calculation of OClO31 and ClOO31,32 support the conclusion that the first step is an excited state isomerization reaction. A number of different matrix isolation studies utilizing infrared33,34 and electron spin resonance35 techniques have confirmed this mechanism. Our group has studied the photochemistry of OClO in solution.36-40 The partitioning of the photoproduct channels and the recombination dynamics in liquids are different from those found either in cryogenic matrices or in the gas phase. The quantum efficiency of the various reaction channels are also solvent dependent. Specifically, the quantum yield of the direct, symmetric C2V elimination of Cl from OClO is strongly sensitive to solvent polarity, increasing in quantum efficiency by over 1 order of magnitude from water (0.5%) to CCl4 (7%). The dominant photoproducts in water are still ClO + O (pathway 1, Φ ∼ 0.9), and 70% of these products undergo © 1996 American Chemical Society
Ultrafast Dynamics of OClO Photochemistry in Water recombination to regenerate OClO on a nanosecond time scale. The cage recombination process is also solvent dependent. For example, no recombination was observed on this time scale in 1-propanol, acetonitrile, or toluene solutions. Understanding the role of photoisomer ClOO in OClO photochemistry is difficult because of its kinetic instability. Once formed, ClOO quickly dissociates into Cl and O2. On the basis of orbital symmetry considerations, the ground state O2 (3Σg) correlates to the ground (2A′′) and two doublet excited states (2A′, 2A′′) of ClOO. On the other hand, the photoreactive excited state of OClO (2B2) correlates to the singlet excited state of O2 (1∆g). Experimental data clearly show that ground state O2 (3Σg ) is dominant. In water, photolysis of OClO generates 1 38 This Cl + O2 where the ratio of O2 (3Σg )/O2 ( ∆g) is ∼20:1. suggests that ClOO is responsible for most of the Cl production in the liquid phase. Recent observation of transient absorption features in the blue region (400-450 nm) in water, 1-propanol, and toluene on the picosecond time scale has been attributed to the intermediate ClOO.40 However, the time resolution of these earlier experiments was not sufficient to probe how this intermediate is formed and whether or not the kinetics exhibited by this transient are related to those recorded for the Cl photoproduct. In the present work, we have extended the time resolution of the transient absorption study of aqueous OClO photochemistry into the femtosecond domain. The aim of the current study is to gain an insight into the initial photophysical and photochemical events following the photodissociation of OClO in water. In particular, we desire to address the role of ClOO in the photochemistry. II. Experimental Section The transient absorption data were obtained using a Ti: Sapphire-based oscillator and regenerative amplifier laser system. The oscillator provides the seed pulses (∼1 nJ/pulse, 82 MHz, 790 nm) that are amplified by a regenerative amplifier. The output pulses are generated at 1 kHz with ∼350 µJ/pulse and 130 fs fwhm. The optical pulses used in the experiment are obtained by first frequency doubling the amplified output in a β-barium-borate (BBO, type-I, 2 mm thick) secondharmonic crystal. A 50:50 beam splitter is then used to separate the second-harmonic output into the pump and probe optical arms. In the probe arm, the 395 nm light is focused into a 1 cm path length quartz cell containing deuterated water, thereby generating a white light continuum. The probe wavelengths (ranging from 350 to 590 nm) were obtained by spectrally filtering the continuum using interference filters (10 nm bandwidth). The probe wavelengths of 650 and 700 nm were obtained by filtering the continuum generated by the fundamental (790 nm) instead of the second harmonic. The typical energies of the pump and probe pulses were 7 and 0.5 µJ, respectively. The linearity of the transient absorption signal was checked by varying the pump energy from 1 to 7 µJ. The time zero of the experiment was calibrated at each probe wavelength using either laser dyes [Coumarin 152 in methanol (410-460 nm) or DCM in methanol (480-700 nm)] or 9,10dibromoanthracene in toluene (340-390 nm). The probe transient absorption signal was detected using a silicon photodiode. The diode output was sent to a lock-in amplifier (EG&G Model 5302), which was referenced to a mechanical chopper that was placed in the pump arm. For each probe wavelength, 4-10 scans were collected and averaged. Each scan took approximately 15 min. For the 650 and 700 nm probe wavelengths, up to 25 scans were collected and averaged due to the small signal. The reproducibility of the data was checked
J. Phys. Chem., Vol. 100, No. 16, 1996 6407
Figure 1. Transient absorption data of OClO in water at 295 K; photoexcitation at 395 nm and probe at 12 different wavelengths covering a spectral range of 350-700 nm. The probe wavelength used for each data is labeled.
among the consecutive scans as well as by repeating the experiment on different days using different samples. The aqueous OClO solution was prepared as described previously.41,42 The sample concentration was adjusted to an optical density of about 0.2 at 395 nm in a 2 mm cell. The water used was HPLC grade (Fisher Scientific). A flowing system comprised of a 4 L OClO solution in a dark bottle, Teflon tubing, and a 2 mm path length quartz flowing cell at the pump/probe overlap region was used to introduce fresh sample for every laser excitation. The sample solution was changed to a fresh solution for each probe wavelength change. The sample degradation was checked by comparing the absorption spectra before and after the sample irradiation. No significant degradation was observed during the 4-10 scans collected for each probe wavelength. III. Results and Discussion The transient absorption data for 12 different probe wavelengths ranging from 350 to 700 nm are presented in Figure 1. Longer scans (up to 620 ps) were recorded for the probe wavelengths 350, 390, 400, 410, and 440 nm and are plotted in Figure 2. The data can be partitioned into three spectral regions: 440-700, 400-410, and 350-390 nm. In particular, for the 440-700 nm region, the transient signal rises and then completely decays on a fast time scale. In the 400-410 nm region, the ground state bleach is observed followed by an increase in absorption and then a decay of the signal to a constant nonzero level. Below 400 nm, the features observed in the 400-410 nm region are further complicated by a slowly rising absorption signal. This can be clearly seen in the 350 nm long scan shown in Figure 2. This slow rise has a time constant of ∼200 ps and has previously been assigned to correspond to the Cl atom radical formation.40 The transition
6408 J. Phys. Chem., Vol. 100, No. 16, 1996
Figure 2. Longer scan transient absorption data of OClO in water at 295 K shown for probe wavelengths at 350, 390, 400, 410, and 440 nm.
responsible for this absorption is the Cl-water complex charge transfer transition, which absorbs throughout the wavelength range from 320 to 390 nm.43,44 In the region of the probe wavelengths from 440 to 700 nm, an absorption initially rises and decays back to zero. With decreasing probe wavelengths, the rise and decay times of the transient absorption increase; this is easily seen because the temporal width of the absorption profile gets broader. These data indicate a systematic blue-shift of the transient absorption spectrum with increasing time following photolysis. We assign these spectral dynamics to a transient species that is undergoing VER. As will be discussed below, this VER process is also manifested in the data obtained using a probe wavelength of 350 nm. The VER process of this transient intermediate covers a total energy range of relaxation of more than 14 300 cm-1. We now consider what chemical species is undergoing VER following the initial photolysis of OClO. The possible molecular species are the various photoproducts of OClO chemistry: 1 2 2 2 O2* (3Σg , ∆g), ClO* ( Π), OClO* ( B1), and ClOO* ( A′′) (the asterisk indicates that the product can be formed vibrationally hot). The photoproduct O2* is formed either from (1) a direct mechanism involving the photodissociation of OClO to form Cl (2Pu) + O2 (1∆g) or (2) isomerization of OClO* (2B2) to ClOO*, which then thermally dissociates into Cl (2Pu) + O2 40 (3Σg ). In a previous picosecond transient absorption study, we found that 90% of the photoexcited OClO molecules fragment into ClO + O and the remaining 10% generate Cl + 1 O2 [9.5% O2 (3Σg ) and 0.5% O2 ( ∆g)]. The dissociation
Chang and Simon -1 45 which is energy of ground state O2 (3Σg ) is 41 263 cm , more than enough to cover the energy range of the observed vibrational relaxation. However, at energies lower than the probe wavelengths used, there are low-lying, singlet excited states of O2, 1∆g, and 1Σ+ g . The energy gaps between these ground state are 7882 and 13 121 cm-1, states and the 3Σg 45 respectively. The depopulation from these excited states to the ground state only occurs by forbidden singlet-triplet radiative transition, which takes place on a microsecond time scale. On the basis of these energetic considerations, it is highly unlikely that O2* is the species undergoing VER in the 440700 nm region. The photoproduct ClO* is also an unlikely candidate on the basis of earlier gas phase studies22 that showed that the nearUV excitation of OClO produced ground state ClO (2Π) in V ) 3-6. This corresdponds to a vibrational excitation of ∼2600-5100 cm-1 (ωe ∼ 854 cm-1),45 which is not consistent with our observed vibrational energy relaxation of over 14 300 cm-1. The remaining two possibilities are OClO* (X2B1) and ClOO* (X2A′′). We first consider the energetics of the ground electronic states. The bond dissociation energies31 for these molecules with respect to the fragments ClO (X2Π) + O (3P) -1 and Cl (2P) + O2 (X3Σg ) are 20 203 and 697 cm , respectively for OClO and 21 454 and 2647 cm-1, respectively for ClOO. Thus, the binding energies with respect to the ClO + O channel make it plausible for the vibrational energy relaxation of up to 14 300 cm-1 to occur on the ground electronic state of either OClO or ClOO. However, the binding energies calculated with respect to the Cl + O2 channel indicate a shallow potential well. The potential energy surface is largely repulsive with respect to this dissociation channel for a vibrationally hot OClO or ClOO molecule undergoing VER in the ground electronic state. This means that for either molecule, the VER process involves the normal modes that do not couple to the Cl + O2 reaction channel. Otherwise, we would observe Cl products concomitant with VER, which, as discussed later, is not observed. The assignment of the transient to OClO* is discounted for the following reasons. If the transient absorption features in the 440-700 nm region are due to OClO* undergoing VER, then we can only conclude that the 400 nm data shows the initial bleach of OClO followed by the recovery of over 96% of the photoexcited OClO back to the thermally equilibrated ground state level with a time constant of ∼9 ps. This would then require that the net quantum yield for photochemistry is less than 4%. This is not correct. Picosecond studies40 clearly show that 90% of the photoexcited OClO fragments into ClO and O (of which 70% recombines on a nanosecond time scale to regenerate OClO) and 10% fragments to Cl and O2. The initial quantum yield for chemistry is 100%. In addition, if only 4% of the photoexcited OClO undergoes reaction, then the data obtained at the probe wavelength of 350 nm cannot be explained. Using the published extinction coefficients for aqueous Cl44 and OClO46 (2500 L mol-1 cm-1 and 900 L mol-1 cm-1, respectively, at 350 nm), the Cl contribution to the signal at this probe wavelength is in excess of 25%. However, if the total quantum yield is 4%, even if all of the reactive OClO* molecules form Cl, the Cl portion of the transient absorption signal in the 350 nm scan could only comprise 10% of the total signal. A third argument that precludes OClO being the probed transient species is the observation of the rise and partial decay of transient absorption at a 400 nm probe. The small decay component at 400 nm is connected to much more prominent decay features observed at 410 nm and redder wavelengths which correspond
Ultrafast Dynamics of OClO Photochemistry in Water to the vibrational population loss toward lower vibrational levels. The presence of this decay feature at 400 nm indicates that VER is still occurring and that the thermally equilibrated ground state has not yet been reached. But, the photolysis wavelength is ∼395 nm, and therefore we expect that 400 ( 10 nm must then probe the thermally equilibrated ground state of OClO. We therefore conclude that OClO* is not the transient species responsible for the observed VER. We now turn to ClOO*. Determining the role of ClOO* in OClO photochemistry has proved difficult because of its kinetic instability toward dissociation into Cl and O2. However, the results from theoretical studies31,32 predict that the ground and the excited doublet states of ClOO are nearly isoenergetic with those of OClO, indicating that there should be UV-visible electronic transitions (330-620 nm for the 2A′′ excited states and 250-830 nm for the 2A′ excited states) present in ClOO. As discussed above, the VER process covers an energy range of 14 300 cm-1. It is plausible that this process is occurring in the X2A′′ state of ClOO. In addition, previous studies21,38 indicate that most of the Cl generated from the OClO photochemistry is due to the thermal decomposition of the ClOO intermediate. As is shown in Figure 2, the 350 nm probe scan clearly shows the formation of Cl. The long time constant (200 ps) for Cl formation reflects the fragmentation of ClOO. By comparing the data obtained at probe wavelengths of 350, 400, and 410 nm (Figure 2), we observe the expected correlation between the ClOO loss and the Cl formation. Both the 400 and 410 nm scans show a gradual decay in the transient absorption signal with time; the time constant of this decay is equivalent to the time constant calculated for the Cl absorption rise reflected by the 350 nm data. Thus, we can conclude that the transient absorption signals observed in the spectral region from 440 to 700 nm is due to the vibrational cooling of ClOO* molecules that are formed directly from electronically excited OClO. In contrast to the simple rise and decay of transient absorption signals observed in the 440-700 nm region, the 400 and 410 nm data exhibit an initial bleach. At 410 nm, the signal then goes positive followed by a decay back to a negative signal. The data at 400 nm is similar although the signal never actually goes positive. The bleach occurs because these wavelengths probe the 2B1 f 2A2 of OClO. Since the photochemistry is irreversable, a bleach is observed upon photolysis. The subsequent dynamics can be accounted for by the VER of the ClOO* photoproduct. For probe wavelengths from 350 to 390 nm, the data are similar to that observed at 400 nm. But there is an additional feature reflecting the slow formation of Cl from ClOO. This can be seen clearly in the long scans presented in Figure 2 and has been discussed in detail in the previous picosecond study.40 Following the photoexcitation of OClO at 395 nm, a temporal lag in the appearance of the transient absorption rise is observed throughout the probe wavelength range covered in our experiment. The time zero of the experiment was accurately determined using a combination of laser dyes (stimulated emission, Coumarin 152 in methanol and DCM in methanol, 410-700 nm range) and 9,10-dibromoanthracene in toluene (ground state bleach, 350-400 nm). After corrections were made for any index of refraction mismatches, the delay times, ∆t, in the appearance of the initial transient absorption signals ranged from 0.74 ps for 700 nm to 1.73 ps for 350 nm. The delay times for all wavelengths studied are listed in Table 1. These delays in the appearance of transient absorption signal relative to photolysis indicate that the initial photophysical and photochemical events leading to the observed vibrationally hot
J. Phys. Chem., Vol. 100, No. 16, 1996 6409 TABLE 1: A List of Parameters for the Fitting of Experimental Data Using Eqs 7, 9, and 10; Also Listed are Values for tmax, t1/e, and ∆t, As Discussed in the Text probe λ (nm)
c1
700 650 590 550 510 480 460 440 410 400 390 350
1 1 1 1 1 1 1 1 0.64 0.44 0.38 0.23
c2
c3
0.17 0.27 0.29 0.37
1/k1 ≈ 1/k2 1/k3 tmax (ps) (ps) (ps)
c4
0.19 0.29 0.31 0.02 0.30 0.10
1.0 1.4 1.7 2.7 3.8 4.5 5.2 6.7 7.2 9.2 9.3 9.1
1.8 2.1 2.5 3.0 4.2 5.4 7.0 8.7 11.6 12.2 198 12.8 198 13.2
t1/e (ps)
∆t (ps)
4.0 5.1 6.0 8.2 12.3 14.8 17.3 20.4 24.0 29.0 30.0 32.7
0.74 0.74 0.49 0.74 0.49 0.74 0.99 1.17 1.72 1.74 1.72 1.73
ClOO (X2 A′′) molecules are optically undetected by the probe wavelengths used. From the measured values of ∆t, these undetected processes must occur on a time scale faster than about 0.7 ps. All photochemistry of OClO is thought to occur from the 2B2 excited state.21 However, because of selection rules, the initial excitation populates the 2A2 state. The 2B2 state becomes populated via an internal conversion from the 2A2 to 2A (spin-orbit coupling) and then to 2B (vibronic coupling). 1 2 From the experimental line widths of the rovibronic transition in the high-resolution jet-cooled spectra,26 the levels of the 2A2 excited state exhibit lifetimes ranging from 200 fs to 20 ps. These data are believed to reflect the time scales for the 2A2 f 2A process; no information of the 2A f 2B internal conversion 1 1 2 is available. Our experimental result for the longest wavelength probed at 700 nm indicates that the upper limit for the time scale of electronic relaxation and photoisomerization to ClOO* (X2 A′′) in water is ∼0.7 ps.47 To quantify the VER process, we have analyzed the data using the following kinetic model. A three-level system consisting of (A) an ensemble of vibrationally hot ClOO* molecules in the ground electronic state, occupying higher vibrational levels than those that are probed by the probe wavelengths, (B) intermediate species representing an ensemble of vibrationally relaxing ClOO* molecules that are probed at various probe wavelengths, and (C) the population of lower level vibrational levels of ClOO. These three levels are assumed to be connected by a consecutive mechanism as follows: k1
k2
A 98 B 98 C
(5)
Because of the narrow bandwidth of the probe interference filter, we expect that k1 ≈ k2. However, these rates will depend on the wavelength probed. Using the consecutive kinetic model, the rise and decay of the transient absorption profile observed in the probe wavelengths of 440 to 700 nm are modeled to correspond to the rise and decay of the intermediate species B undergoing VER. The expression for the time-dependent transient absorption signal, r1(t′),48 is then given by
r1(t′) ) [B]t′ )
( )
k1 (e-k1t′ - e-k2t′) k2 - k1
(6)
Equation 6 must be convolved with the pump-probe crosscorrelation function, G(t). The resulting response function, R1(τ), given by eq 7 is then fit to the experimental data.
R1(τ) ) ∫ dt G(τ - t)r1(t′)
(7)
The transient absorption features at 400 and 410 nm are further complicated by the presence of the initial bleach, the rise and
6410 J. Phys. Chem., Vol. 100, No. 16, 1996
Chang and Simon
Figure 3. Plots of tmax (O) and t1/e (0) vs probe νj are shown.
decay of absorption signal, and the growth of absorption signal. The bleach is modeled as a step function, h(t) [h(t) ) 1 for t > 0 and h(t) ) 0 for t e 0]. The rise and decay of signal is still given by r1(t′), and the growth of long time signal is given by the r2(t′), eq 8,which takes into account that these probe
r2(t′) ) [C]t′ ) {1 + [1/(k1 - k2)]}(k2 e-k1t′ - k1 e-k2t′) (8) wavelengths are sensitive to the time-dependent concentration increase of species C. Equation 8 must also be convolved with the pump-probe correlation, giving R2(τ). The response function for fitting 400 and 410 nm data is then given by
R(τ) ) c1R1(τ) + c2R2(τ) - c3H(τ)
(9)
where H(τ) represents the convolved function of h(t). For the 390 and 350 nm data, R(τ) also includes a function that represents the formation of Cl; the convolved kinetics term is represented by R3(τ) in eq 10.
R(τ) ) c1R1(τ) + c2R2(τ) - c3H(τ) + c4R3(τ)
(10)
Specifically, R3(τ) represents the convolved function of r3(t′) where
r3(t′) ) 1 - e-k3t′
(11)
The data were fit with floating parameters k1, k2, and k3 using the Levenberg-Marquardt nonlinear least-squares method. As expected, k1 ≈ k2, and the value varies with probe wavelength. The best fit parameters are listed in Table 1, and the fits are overlaid with the data in Figure 1. The time constants associated with the rise and the decay of the transient species (1/k1 ≈ 1/k2) gradually increase from about 1 ps at 700 nm to a maximum of 9.3 ps at 390 nm. The time it takes for the maximum of the transient absorption signal to be reached (tmax) as well as the time corresponding to the first moment of the subsequent decay (t1/e) are listed for each probe wavelength in Table 1. In Figure 3, the plots of tmax and t1/e vs probe energy (cm-1) are shown. The slopes are nonlinear and decrease with increasing energy. This is consistent with the prediction that vibrational relaxation near the top of the potential well is considerably faster than that in the lower, more harmonic region of the well.16 As can also be seen in Figure 3, the value of tmax becomes nearly constant for E > 25 000 cm-1. The value of t1/e for this energy range also differs from the trend observed at redder wavelengths. This reflects that the transient absorption signal around 410 nm is due to thermally equilibrated ClOO. Further
evidence for this process is given by the concomitant decrease and the increase in the pre-exponential factors, c1 and c2, respectively, in going from 410 to 350 nm, indicating the systematic shift in the ClOO population from excited vibrational levels to the lowest levels. The numbers for 1/k2 indicate that the ClOO vibrational energy relaxation to the lowest levels takes approximately 9 ps in water. This experimentally obtained vibrational relaxation time for the triatomic radical ClOO can be contrasted with those of other systems that have been studied. Vibrational energy relaxation of small (diatomic and triatomic) molecules in condensed-phase solutions have been extensively investigated both theoretically and experimentally. Neutral molecules such as I2 in halogenated methane, hydrocarbon, and rare gas solutions are found to undergo vibrational relaxation with a time constant on the order of 50-200 ps.2-5 The vibrational relaxation of ionic molecules (e.g., I2 ) in polar solvents such as water have received theoretical18-20 and experimental6-11 attention. The long-range Coulombic interactions cause a dramatic increase in the vibrational relaxation rate of the solute. 18 indicate that Molecular dynamics simulations of I2 in water the relaxation rate is subpicosecond (0.6-0.7 ps). Experimental studies by Barbara et al.6,7,9 report an upper limit of 2 ps for the complete vibrational relaxation of I2 in water. The transient absorption data obtained at the probe wavelength of 350 nm exhibits a long time rise which is assigned to correspond to the formation of Cl from the OClO photodissociation reaction. A similar feature with reduced intensity is also present in the 390 nm data. The examination of orbital energy correlation diagrams for OClO indicate that there exist two possible reaction pathways for the formation of Cl radical; one being the direct, symmetric dissociation of OClO to generate Cl (2P) and O2 (1∆g) and the other involving an indirect mechanism to generate Cl and O2 (3Σg ) from the photoisomer ClOO. We have previously reported the relative importance of these two channels in aqueous solution.38,40 Taking advantage of the two distinct electronic states of photoproduct O2 generated by the two pathways, the relative quantum yield was determined by measuring the phosphorescence emission intensity of O2 1 (3Σg r ∆g) against a known standard. Only about 5% of the Cl radical formed originates from the direct Cl elimination route, indicating that most are generated from the dissociation of the intermediate ClOO. Using the 350 nm data, the time constant for the formation of Cl from ClOO is ∼200 ps. This time constant is the same as the long time decay constant at 400 and 410 nm (Figure 2). Thus, this decay, which has been assigned to relaxed ClOO, correlates with the Cl rise observed at 350 and 390 nm. IV. Conclusions The ultrafast photophysical and photochemical events following the photodissociation of OClO in water have been studied using femtosecond transient absorption spectroscopy. The photoexcitation of OClO at 395 nm generated a vibrationally hot photoisomer ClOO* within 0.7 ps. The molecule ClOO* undergoes a vibrational energy relaxation process that covers a total energy range of over 14 300 cm-1. The relaxation process is complete in ∼9 ps. The chlorine atom radical formation was also observed in the 350-390 nm probe region. Our results confirm that the dominant pathway for the formation of Cl radical from OClO* is via the photoisomerized molecule ClOO. The time constant for the formation of Cl is determined to be ∼200 ps, in
Ultrafast Dynamics of OClO Photochemistry in Water agreement with earlier picosecond transient absorption results. The molecule ClOO* relaxes to thermal equilibrium prior to dissociation. Work is in progress to measure the time-resolved transient Raman spectrum of the relaxing ClOO molecule. The Raman study will complement the results presented here and should provide a more direct probe of the ClOO vibrational relaxation process. Acknowledgment. The authors thank Dr. Peijun Cong for some experimental assistance. The support of this work from the National Science Foundation is gratefully acknowledged. References and Notes (1) For a recent review, see: Owrutsky, J. C.; Raftery, D.; Hochstrasser, R. M. Annu. ReV. Phys. Chem. 1994, 45, 519-55. (2) Harris, A. L.; Brown, J. K.; Harris, C. B. Annu. ReV. Phys. Chem. 1988, 39, 341. (3) Harris, A. L.; Berg, M.; Harris, C. B. J. Chem. Phys. 1986, 84, 788-806. (4) Lingle, R., Jr.; Xu, X.; Yu, S. C.; Zhu, H.; Hopkins, J. B. J. Chem. Phys. 1990, 93, 5667-5680. (5) Xu, X.; Yu, S. C.; Lingle, R., Jr.; Zhu, H.; Hopkins, J. B. J. Chem. Phys. 1991, 95, 2445-2457. (6) Johnson, A. E.; Levinger, N. E.; Barbara, P. F. J. Phys. Chem. 1992, 96, 7842-7844. (7) Alfano, J. C.; Kimura, Y.; Walhout, P. K.; Barbara, P. F. Chem. Phys. 1993, 175, 147-155. (8) Kliner, D. A. V.; Alfano, J. C.; Barbara, P. F. J. Chem. Phys. 1993, 98, 5375-5389. (9) Walhout, P. K.; Alfano, J. C.; Thakur, K. A. M.; Barbara, P. F. J. Phys. Chem. 1995, 99, 7568-7580. (10) Banin, U.; Ruhman, S. J. Chem. Phys. 1993, 98, 4391-4403. (11) Banin, U.; Kosloff, R.; Ruhman, S. Isr. J. Chem. 1993, 33, 141156. (12) Heilweil, E. J.; Doany, F. E.; Moore, R.; Hochstrasser, R. M. J. Chem. Phys. 1982, 76, 5632. (13) Li, M.; Owrutsky, J.; Culver, J. P.; Yodh, A.; Hochstrasser, R. M. J. Chem. Phys. 1993, 98, 5499-5507. (14) Whitnell, R. M.; Wilson, K. R.; Hynes, J. T. J. Phys. Chem. 1990, 94, 8625. (15) Whitnell, R. M.; Wilson, K. R.; Hynes, J. T. J. Chem. Phys. 1992, 96, 5354. (16) Nesbitt, D. J.; Hynes, J. T. J. Chem. Phys. 1982, 77, 2130. (17) Brown, J. K.; Harris, C. B.; Tully, J. C. J. Chem. Phys. 1988, 89, 6687. (18) Benjamin, I.; Whitnell, R. M. Chem. Phys. Lett. 1993, 204, 4552. (19) Benjamin, I.; Banin, U.; Ruhman, S. J. Chem. Phys. 1993, 98, 8337-8340. (20) Benjamin, I.; Barbara, P. F.; Gertner, B. J.; Hynes, J. T. J. Phys. Chem. 1995, 99, 7557-7567.
J. Phys. Chem., Vol. 100, No. 16, 1996 6411 (21) Vaida, V.; Simon, J. D. Science 1995, 268, 1443-1448. (22) Ru¨hl, E.; Jefferson, A.; Vaida, V. J. Phys. Chem. 1990, 94, 29902994. (23) Lawrence, W. G.; Clemitshaw, K. C.; Apkarian, V. A. J. Geophys. Res. 1990, 95, 18591-18595. (24) Glownia, J. H.; Misewich, J.; Sorokin, P. P. In Supercontinuum Lasers; Alfano, R. R., Ed.; Springer-Verlag: Berlin, 1990; pp 1-4. (25) Richard, E. C.; Vaida, V. J. Chem. Phys. 1991, 94, 153-162. (26) Richard, E. C.; Vaida, V. J. Chem. Phys. 1991, 94, 163-171. (27) Bishenden, E.; Haddock, J.; Donaldson, D. J. J. Phys. Chem. 1991, 95, 2113-2115. (28) Davis, H. F.; Lee, Y. T. J. Phys. Chem. 1992, 96, 5681-5684. (29) Bishenden, E.; Donaldson, D. J. J. Chem. Phys. 1993, 99, 3129. (30) Baumert, T.; Herek, J. L.; Zewail, A. H. J. Chem. Phys. 1993, 99, 4430-4440. (31) Peterson, K. A.; Werner, H. J. J. Chem. Phys. 1992, 96, 89488961. (32) Jafri, J. A.; Lengsfield, B. H., III.; Bauschlicher, C. W., Jr.; Phillips, D. H. J. Chem. Phys. 1985, 83, 1693-1701. (33) Arkell, A.; Schwager, I. J. Am. Chem. Soc. 1967, 89, 5999-6006. (34) Johnsson, K.; Engdahl, A.; Ouis, P.; Nelander, B. J. Phys. Chem. 1992, 96, 5778-5783. (35) Adrian, F. J.; Bohandy, J.; Kim, B. F. J. Chem. Phys. 1986, 85, 2692-2698. (36) Dunn, R. C.; Simon, J. D. J. Am. Chem. Soc. 1992, 114, 4856. (37) Dunn, R. C.; Flanders, B. N.; Vaida, V.; Simon, J. D. Spectrochim. Acta A 1992, 48, 1293. (38) Dunn, R. C.; Anderson, J.; Foote, C. S.; Simon, J. D. J. Am. Chem. Soc. 1993, 115, 5307. (39) Vaida, V.; Goudjil, K.; Simon, J. D.; Flanders, B. N. J. Mol. Liq. 1994, 61, 133-152. (40) Dunn, R. C.; Flanders, B. N.; Simon, J. D. J. Phys. Chem. 1995, 99, 7360-7370. (41) Bray, W. Z. Phys. Chem. 1906, 54, 569. (42) Babcock, L. M.; Pentecost, T.; Koppenol, W. H. J. Phys. Chem. 1982, 93, 8126. (43) Treinin, A.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 1716-1721. (44) Kla¨ning, U. K.; Wolff, T. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 243-245. (45) Huber, K. P.; Herzberg, G. In Molecular Spectra and Molecular Structure; Van Nostrand Reinhold: New York, 1979; Vol. 4, pp 490503. (46) Dunn, R. C. Ph.D. Thesis, University of California, San Diego, 1993. (47) The possibility of the formation of ClOO* from geminate recombination of photofragments ClO and O has been addressed by the picosecond transient absorption study.40 These results indicate that the geminate recombination of ClO and O gives rise to OClO rather than ClOO on a nanosecond time scale. (48) The time t′ in the kinetic equations is given by t′ ) t - ∆t, where ∆t (Table 1) is the time delay of the appearance of the transient absorption signal following photolysis.
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