J. Phys. Chem. 1!494,98, 7009-7013
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Femtosecond Kinetic Measurements of Excess Electrons in Methanol: Substantiation for a Hybrid Solvation Mechanism C. Phpin, T. Goulet, D. Houde,' and J.-P. Jay-Grin Dtpartement de mtdecine nucltaire et de radiobiologie, Facultt de mgdecine, Universitt de Sherbrooke, Sherbrooke (Qutbec), Canada JIH 5N4 Received: February 24, 1994; In Final Form: April 25, 1994'
Multiphoton ionization of neat methanol at room temperature (294 K) with 2-eV laser pulses (-300 fs) is used to study electron solvation. For the first time the experimental conditions allowed a quantitative as well as a qualitative study of the formation and evolution of the solvated electron in methanol. The kinetics exhibits a continuous blue shift of the spectral peak. This shift from 1.57 to 1.92 eV is found to be exponential with a characteristic time of 13.6 f 0.6 ps. A simultaneous stepwise process is evidenced by the concurrent first-order (6.1 f 0.6 ps) absorption decay in the infrared and growth in the visible. Our proposed hybrid model of electron solvation involves two electron-solvent configuration states (a weakly bound and a strongly bound one) in both of which relaxation occurs via a continuous shift and between which there is a stepwise transfer mechanism. About 50% of the electrons are found in the strongly bound state within the first picosecond, suggesting that they are trapped directly into this state.
Introduction Ever since its identificationas a genuine chemical species,1the solvated electron, e-$, has not only aroused great interest due to its predominant role in major fields like photochemistry, radiation chemistry, and electrochemistry but has also been at the center of many different if not conflicting views in regard to the chain of primary events that lead to its formation in polar solvents. The first theoretical studies, based on a continuous dielectric description of the medium, predicted that electron solvation resembles the solvation of larger anions, which is characterized by a continuous shift of the absorption peak while the anion's surroundingsadjust to the presence of an electric charges2 Lowtemperature solvation kinetics obtained with nanosecond pulses in liquid34and glassy7.8 alcohols seemed to confirm the existence of such a shift. However, a stepwise mechanism (such as electron migration from shallow to deep traps) could not be excluded because the first-order decay observed in the infraredcorresponded quite well with the absorption buildup in the visible.3 The role of the molecular structure of the solvent was established by the correlation that was found mostly from picosecond e~periments3-~'2 between the solvation time ( T ~invarious ) alcohols and the dielectric rotation of the monomer molecules around the electron (7132). The model that emerged from those studies and which became the dominant one thereafter described electron solvation as a the result of two consecutive events. The first one consists of the localization of the electron into a preexisting trap of the solvent (e- is a seeker). The second one involves a stepwise process through which the electron manages to create the final structure in which it is solvated (e- is a digger).'2 The model suggests that this last event is related to molecular motions (because of the correspondence between T~ and 7 ~ 2 but ) did not describe the actual mechanics of the stepwise process. Subsequent femtosecond laser studiesof the hydrated electron13 by direct biphotonic ionization strongly reinforced the idea of a stepwise transition that seemed to account for the observed results. Long et aI.14 also brought support to this idea when they reported an isosbesticpoint near 820 nm. Concurrently,adiabaticquantum molecular-dynamicalsimulations of electron solvation in water suggested that the stepwise process should involve something other than a mere solvent r e l a x a t i ~ n . ~On ~ Jthe ~ basis of the apparent discrepancybetween the experimental and simulatedobservations, *Abstract published in Advance ACS Abstracts, June I, 1994.
Rossky and SchnitkerI5 indicated that the early dynamics of hydration should require consideration of mechanisms beyond the adiabatic regime and postulatedthat the intermediatetrapped electron is, in fact, an excited state of the fully hydrated electron to which it evolves. Subsequent nonadiabatic quantum-classical simulationsof electronsolvation dynamicsin waterI7-19generally supported this two-state hydration model. In spite of the success of the stepwisetransition model, a number of observationsremained unaccounted for. First, the kinetics of low-temperature alcohols are neither first order nor wavelength independent. Second, many authors have noted that hydration kinetics in the near-infrared are difficult to explain with a strictly stepwise mode1.20*21These difficulties are directly related to the controversies surrounding the existence of an isosbestic point and of an "encounter pair" species in water.1420 Third, in a recent series of picosecond laser experiments on electron solvation in several alcoholsat room temperature, Mataga and co-workersaa suggested that two characteristic times are needed to describe the solvation kinetics that follows trapping in those media. Their results also conveyed the idea that the last and longest step of electron solvation consists of a continuous blue shift analogous to bulkier anion solvation and to what is observed at low temperatures. Obviously, no consensus has been reached on a single model that could account for the wealth of information that has been gathered on electron solvation. Since the controversies often stem from an unresolved convolution of the studied kinetics with the pump and probe pulses, we proceeded to perform the first largescale study (400-1350 nm) of electron solvation in an alcohol with sub-picosecondtime resolution. Wechose to focus on roomtemperature methanol, whose electron solvation had never been temporally resolved and which constitutes the simplest alcohol and the one closest to water. Experimental Section
In our experiment, electrons are directly ejected from the solvent molecules by a multiphoton ionization process with 2-eV laser pulses, which was shown to provide a high yield of solvated electron^.^^ At least 3 photons are needed to reach the threshold of 4.7 eV, above which solvated electrons are generated in liquid methanol.2s The CPM (colliding pulse mode-locked) laser followed by several amplifier stages that produce these short (-300-fs fwhm) and powerful (-0.5 mJ) exciting pulses has
QQ22-3654/94/2098-7QQ9~04.5Q/Q 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
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Figure 1. Variation of the observed absorbance with time in liquid methanol at 294 K for analyzing wavelengths of 500, 800, 1000, and 1350 nm. The kinetic traces have bcen normalized so that the absorption spectrum observed at 100 ps corresponds to that obtained in radiolysis.27 Time zero
was determinedwith the Raman emissionand absorptionthat are observed at some wavelengths when the pump and the probe pulses are synchronizedaZ6 The solid lines are fitted through the data points (see text). The inset shows the shape of the kinetic traces at early times (for clarity, only the fits are shown). been described previously.24.26 Focusing optics permitted an irradiance >1013W/cmZinsidea 5-mm Suprasil quartz cell filled with ACS Certified Spectranalyzed grade CH30H at -294 K. The short duration of the pulse prevented the occurrence of dielectric breakdown in the sample even at those high intensities. At this irradiance level, the pulse intensity does not seem to influence the kinetic traces besides their amplitude. In fact, no measurable difference in the curve profiles was found when the pump intensity was decreased over an order of magnitude at 580 nm. Some of the data were recorded on intensified silicon double diode arrays (1024channels) from Princeton Instruments coupled to a grating spectrophotometer. Other single-channel traces between 700 and 1100 nm in 50-nm intervals were collected with silicon photovoltaic diodes while nitrogen-cooled germanium photodiodes were used for the measurements a t 1225 and 1350 nm. Some improvements were added to the original setup, including a chopper controlled by the acquisition program which blocked the pump pulse at regular intervals in order to Correct for drifts resulting from varying experimental conditions over long periods. Due to the probe refracting optics, a translation of the focal point takes place while the wavelength is varied. This factor causes an uneven pumpprobe interaction volume in the sample from one end of the studied wavelength domain to the other. In order to eliminate this effect, every kinetic trace was normalized at 100ps with theradiolysisspect~m?~ This operation is justified by the fact that the spectral shape is not found tovarysignificantlY after that time delay. It was shown previously that in water the hydrated 'pectrum matches the radiolysis one after only a few picosecond^.^^ Results Figure 1 displays some of the kinetic traces that we measured in liquid methanol at 294 K. These results clearly show that the
absorbance increases rapidly in the first picosecond and that its subsequent evolution differs widely from one wavelength to the other. One could relate qualitatively the rise in the visible to the decay in the infrared, but a deeper analysis of the kinetic traces shows that the characteristic evolution time of the absorbance varies with wavelength. For example, one notes in Figure 1 that the decay a t 1350 nm is faster than that at 1000 and 800 nm. In order to quantify this trend, we performed independent fits on each kinetic trace with a purely stepwise model that incorporated the effect of electron*ation and electron-radical recombinations.28 As seen in Figure 1, each fit was excellent. However, the fitting parameters tl and t2 (corresponding respectively to the average rise and decay times of the infrared species) were found tovary monotonicallywith the probe wavelength. This systematic variation, which is shown in Figure 2, demonstrates that a strictly stepwise model cannot account for the entirety of our kinetic results. In fact, this model predicts t l and t z to be wavelength independent. A similar variation of t2 with probe wavelength was observed in methanol by Hirata and Matagaz30vera narrower spectral range. From the fits of the various kinetic traces that were measured, we constructed a sequence of absorption spectra a t differenttime delays. Three of those spectra (with delays of 5 , 10, and 100 ps) are shown in Figure 3. On the one hand, the most energetic part of the spectrum (Figure 3a) displays a maximum that shifts continuous~ytoward the blue. we found that, for all delays after ps, this part ofthe spectrum can be closely fitted by a ~ ~ Lorentzian (GL)function, which is often used to describe the shape of solvated electron spectra,27 on the other hand, the infrared part of the spectrum (Figure 3b) displays a t early times a significant departure from the GL functions that are used to fit the absorbance in the visible. The evolution of this excess infrared component can be described in terms of a global exponential decay combined with a concomitant spectral deformation which is reflected in the wavelength dependency of t2 (see
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The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7011
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exponential law with a characteristic time of 13.6 & 0.6 ps. (4) The excess infrared part of the spectrum seems to undergo an energy shift similar to that observed in the visible (tl and 12 are wavelength dependent up to 1350 nm). (5) The continuous spectral shift alone cannot explain all the kinetics. The species associated with the excess infrared absorption is also found to undergo a first-order decay whereas the species associated with the G L function experiences a first-order growth. (6) The characteristic time of this first-order growth in the visible is well determined and equals 6.1 f 0.6 ps. The corresponding firstorder decay in the infrared is more difficult to separate from the continuous spectral shift, but a value of 6 ps for its characteristic time would be consistent with the data. (7) Finally, we note that the area of this latter visible component has already attained about half of its final value at I ps. This is also reflected in the fact that the absorbance rises rapidly and significantly within the first picosecond for all observed wavelengths (see inset of Figure 1). From those observations, we are led to conclude that, within 1 ps, the quasi-free electrons (rqf) become distributed in two localized electron-solvent configuration states: a weakly bound one (e-wb) and a strongly bound one (eib). The e-,b is found to transform into e-8b by a stepwise mechanism, but in the course of this transformation, the transient spectra of both species relax via a continuous blue shift. The corresponding scheme of our proposed hybrid model of electron solvation is shown in Figure 5.
Figure 2). It should be noted that for clarity the effect of electron recombination has been subtracted from the actual time evolution of the spectra. Figure 4 shows the variation with time of the area of the GL function used to fit the spectra in the visible as well as the time evolution of the energy shift. Those two phenomena are found to occur with significantly different rates. The faster one, the rise of the area of the GL function, has a characteristic time of 6.1 f 0.6 ps, which correlates well with the observed decay in the infrared. As for the spectral blue shift, it takes place less rapidly, its characteristic time being 13.6 f 0.6 ps. This kind of shift has already been observed in previous electron solvation studies,22J3 but here the time resolution of the experiment is much shorter than the observed phenomenon and a truly quantitative description of the evolution of the shift can be given. The exponential character of the continuous shift is interesting in that it was predicted and observed for the solvation of anions.2*29g30 To our knowledge, it has never been evidenced in the case of electron solvation. Concerning the normalized area of the GL function, one can note in Figure 4 that it is -0.5 at 1 ps. Assuming that the total oscillator strength of the corresponding species remains constant during the shift, this means that about half of theelectrons are found in deeply localized states within the first picosecond. This observation, which is consistent with recent measurements of the transient absorptionat 497 nm invarious alcohols?' supports the existence of the two distinct electron solvation channels (a fast and a slow one), which was advocated by Jonah and coworkers. 2,32
Discussion To summarize our findings, we make the following seven observations. (1) We show that a purely stepwise process cannot account for the measured kinetic traces. This is clearly illustrated by the systematic variation of the kinetic evolution times t l and t2 with wavelength. The absence of an isosbestic point in the time-dependent (and recombination corrected) spectra of Figure 3 supports this conclusion. (2) We show that the most energetic part of the electron absorption spectrum (which is well described by a GL function) settles down via a continuous blue shift of its maximum that varies from 1.67 eV (at 5 ps) to 1.92 eV (at 100 ps). (3) We find that this spectral displacement follows an
The mixture of stepwise and continuous relaxation which emerges quite clearly from our analysis owing to a good time resolution (compared to the observed phenomena) appears to reconcile the seemingly contradictory results that have accumulated in the literature over more than 20 years. On the one hand, the existence of a continuous shift was prominent in lowtemperature alcoholse and was probably responsible for the anomalously slow solvation rates that were found by Chase and Hunt9 between 900 and 1050 nm in various alcohols around room temperature. It was also recently observed in the long-time behavior of electron solvation kinetics in ambient-temperature butanol.22 On the other hand, the good correspondence between the rise of the transient absorption below 600 nm and its decay above 1050 nm3,9J0J2originates partly from the stepwise process which is dominant at those wavelengths because of the large differences in the absorptivity of eb, and e-,b. One should note however that this correspondence does not hold quantitatively over a wide range of wavelengths since the continuous shift always affects somewhat the kinetics. In water, the first femtosecond studiesl3J4 promoted the idea of a strictly stepwise hydration process, which could elegantly account for most of the observed kinetics. Later, though, it became clear that the hydration kinetics displayed a complex behavior in the region of the controversial isosbestic point (820 nm).zo.21 Messmer and Simon33attempted to reinterpret the results of Migus et al.13 in terms of a linear combination of a continuous and a stepwise relaxation. They also introduced a channel of direct hydration to account for a structure that they identified in the 0-ps spectrum of ref 13. The existence of such a channel seems questionable since it leaves no room for any relaxation of the medium (even at long range) around the newly localized charge. In fact, it appears to us that the structure in question is located around 850 nm and not at the maximum of the e-.q absorption spectrum (720 nm). In the light of our present results, the rapid displacement of this structure from 850 to 720 nm could perhaps be seen as direct evidence of a continuous spectral shift in water. It should be noted here that molecular-dynamics simulations which supported the existence in water of a stepwise electronic transition between two bound states of the electron's cavity did not exclude a concomitant continuous relaxation of the solvent. They merely showed that this latter relaxation would be
7012 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
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Figure 3. Transient absorption spectra of excess electrons in liquid methanol for three delay values between the pump and the probe pulscs ( 5 , 10, 100 ps). The solid line is a combination of Gaussian and Lorentzian functions that describes the spectrum of the fully solvated electrons.*’ The two other curves were obtained by shifting and renormalizing the solid curve (as is recommended in ref 29,we varied the characteristicenergy width of the curve with the same function that was used for the absorption maximum). In panel b an excess absorption can be seen in the infrared at early
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