J. Phys. Chem. 1980, 84, 1145-1150
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Picosecond Dynamics of Electrons in Fluids and Laser-Induced Electron Transfer G. A. Kenney-Wallace,” G. E. Hall, L. A. Hunt, and K. Sarantidist Department of Chemistry, Universiw of Toronto, Toronto, Canada M5S 1Al (Received July 16, 1979)
The dynamics of the quasi-free-bound transition of electrons in polar fluids will be shown to be correlated to the dynamical molecular structure of the host fluid in response to the sudden perturbation presented by the electron. In very low dipole density systems, we will show how picosecond time-resolved spectroscopyreveals several stages of clustering about the electron, the first of which corresponds to the onset of cluster formation in these liquids as observed independently via the spin-lattice relaxation times of internal motion in the same systems. Once stabilized in a configurationallyrelaxed cluster,the electron can be excited to bound electronically excited states from which nonradiative relaxation occurs on a picosecond time scale or faster. In the special case of electrons in alcohols, laser-induced optical excitation at high fluxes promotes the electron into a highly reactive state which competes effectivelywith vibronic relaxation. Isotope effects indicate that the dynamics of this electron transfer occur through the vibration of the OH bond.
In liquids aind amorphous solids the energetics and the time dependence of many intramolecular and intermolecular electron transfer processes require that the electron-medium interactions play an important role in the outcome of this event. In a wide range of liquids, strong local interactiions lead to electron localization in a cluster of molecules, and these excess electron states present the simplest case in which to explore the influence of molecular dynamics of the medium on subsequent photodetachment and electron transfer processes. In this paper we first discuss the picosecond dynamics associated with the electron localization step in dilute polar fluids in whiclh the electron becomes solvated in a preexisting trapping site. Secondly, we describe results in which we observe for the first time a laser-induced,bond selective, electron transfer process during photoexcitation of electrons in alcohols a t 298 K, under high laser intensities.
A. Dynamics of Electron Solvation There is a continuing debate over the role of the long range vs. the short range electron-medium interactions in the electron localization and solvation mechanism. The question as to whether or not the longitudinal relaxation time T~ = Td(lE,,p/Cs), which is the Debye relaxation time modified by the ratio of the high frequency to static dielectric constants of the medium, should correctly predict the response of a polar liquid to the sudden presence of an excess electron has been discussed at 1ength.l On the other hand, the theory of dielectric relaxation of polar fluids and dielectric relaxation measurements have also been critically assessed as an approach to understanding molecular motion in dense fluids.2 The chronological sequence represented by transitions from the quasi-free electronic state (e f)to a localized and ultimately a fully solvated and configurationally relaxed e; has been described3 in teirms of relaxation of the long-range dipolar field followed by configurational relaxation in the vicinity of the trapping center, reminiscent of molecular vibrational relaxation. In this approach (I) TL describes the first step and the local equilibrium polarization is established lasta3 Hence molecular reorganization of the medium and configurational relaxation occurs from the “outside in”.* The opposite view (11) sees the initial localization of e,[ by a *A. P.Sloan Foundation Fellow; to whom correspondence should be addressed. ‘Department of Chemistry, University of British Columbia, Vancouver. Canada. 0022-3654/80/2084-1145$0 1.OO/O
configurational fluctuation or a preexisting trap in the liquid, following which gross molecular reorganization occurs from the “inside out”. The influence of the excess charge extends radially and reorganizes those molecules in the immediate vicinity of the trapping site into a discrete cluster: which also undergoes configurational relaxation. However, the parameters of the theories, which frequently treat the liquid as a structureless continuum, and those variables under control in experiments, are too often disparate functions of the local liquid structure. We present here new electron solvation data in the context of the liquid structure of the host fluid, which indicates that, for certain classes of fluids, mechanism I1 is the correct approach.
Experimental Section and Results The experiments on electron solvation in very dilute alcohol-alkane systems have been carried out in collaboration with C. D. Jonah of ANL, and a full description of the stroboscopic single pulse, picosecond electron beam facility has been given elsewhere.6,eAlcohols and alkanes were obtained as distilled in glass (Burdick and Jackeon) spectroscopic quality, and used as received. The butanols and 2-decanol were diluted quantitatively to mole fractions xROH as low as 3 X lov3in order to follow the electron trapping process at average dipole densities ((Pd)) 1 loig (OH dipoles c ~ n - ~over ) , three orders of magnitude less than the typical pure fluid value. Samples were flowed during measurement under a pressure of helium. It is worth noting that we cannot perform siniilar transient absorption studies in such dilute systems in picosecond laser spectroscopy with Nd glass lasers because the low repetition rate Hz)prevents us from using signal averaging techniques to enhance the signal to noise on the very weak signals. However, recent developments in subpicosecond laser spectroscopy and synchronous pumping technique^,^ in which we use a continuous train (40-MHz repetition rate) of 120-ps pulses from a modelocked argon-ion laser to pump an extended cavity dye laser, from which emerges another continuous train of typically 2-ps pulses? will permit us to examine these weak transient absorptions very effectively. The dye laser pulses can be compressed to subpicosecond times and amplified to generate a 10-Hz train of gigawatt The data obtained from dilute solutions of t-BuOH and 2-decanol in n-hexane are shown in Figure 1. Trace a in Figure l a corresponds to the transient absorption observed at 740 nm in t-BuOH following a 20-ps electron pulse, while trace b is the signal at 600 nm displaced along the 0 1980 American Chemical Society
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Kenney-Wallace et al.
studies of alcohols in alkanes carried out by infrared absorption and ultrasonic experimentation.ll
Discussion It seems clear from the above picosecond data and previous results5 that electrons ejected into a high (&) t A i alcohol-alkane system at 298 K are initially trapped in a preexisting site, namely, an alcohol multimer. The fact that we do not see any spectral shifts nor changes in the decay kinetics of the transient signal in pure n-hexane until Flgure 1. Electron solvation in f-BuOH and 2decanol and their mixtures in n-hexane, see text for discussion. Left: absorption signal in t-BuOH a threshold dipole density of alcohol is reached suggests ( t-C4) at (a) 740 nm; (b) 600 nm; (c) xRCm = 0.27; (d) xRoH = 0.03. that below this critical density electrons are localized in Right: (a) signals from t-BuOH and 2decanoi normalized and suthe alkane. Not until sufficient numbers of dimer or higher = 7 X lo-*; (c) . . ., perimposed; (b) 2decanoi in n-hexane, xRon alcohol species are present is there an effective competition xRCm = 7 x 10-3. between ion recombination and electron trapping in an alcohol site. Small alcohol clusters are necessary preretime axis for clarity. Only the amplitudes of the signals quisite for trapping electrons in ROH in these systems and vary and, when (b) is normalized to (a), the time depenthus the localization and solvation process does not occur dence of (b) is exactly the same, namely, 54 ps from in a totally disoriented environment. The latter has been for e; analysis of the exponential growth. Since the A, in t-BuOH is 1200 nm at 298 K,9 these observations apply proposed for the presolvated electron environment in low temperature alcohol glasses.12 As the alcohol concentration to the high-energy side of the spectrum and are consistent increases so does the number of clusters and the size of with previous conclusions5on the similarity of the solvation When t-BuOH is diluted the clusters, until at XROH > 0.1 in primary alcohols, and profiles at wavelengths.5 X 1021 It is not our intent to analyze here the existing theories the dynamics appeared to simulate those of the pure tfor electron trapping since we have recently undertaken BuOH. The amplitude of the absorption signal in these that task in a more comprehensive discussion of new t-BuOH-n-hexane systems remained essentially constant electron solvation data in a wide range of very dilute alover the range 0.05 C xROH 5 0.4, following which the cohol-alkane systems.14 Instead we conclude this disabsorbance increased linearly in the xROH to about twice cussion by reiterating the theme of this paper, Le., the the initial ~ a 1 u e . l ~ importance of the role of the dynamical statistical structure The absorption peak for a stable e; in 2-decanol occurs of the fluid. Evidence from picosecond spectroscopy at 800 nm and coincides with the value for 2-octanol restudies of e; solvation5J3J5and electron mobility meaported earlier, as expected on the basis of local structure surements16 in very dilute alcohol-alkane liquids clearly argument^.^ An interesting contrast is provided by the has revealed the importance of local liquid structure in the 2-decanol system, where the absorption signals in the pure dynamics of the quasi-free-bound transition. Thus the fluid indicate that the e; solvation time is comparable to questions posed above as to whether the relaxation (or that in t-BuOH, as shown in Figure lb, where their repolarization wave of the medium) flows in toward the spective traces for 600 nm are normalized and superimeventual localization site, or flows out from the initial posed in trace a. However, the threshold at which 2-delocalization site, depends on the identity of the strong can01 molecules affect the early kinetic events in n-hexane electron-medium interaction that promotes the electron and promote electron trapping in ROH is now considerably trapping and solvation sequence. As noted before,4bthe lower than t-BuOH, namely, XROH = 7 X or (P)d = 3 crucial experiment must still be done, that is, to examine X 1019,the same value as l-butanol. The plateau region the validity of the first localization mechanism, I. All appeared at about xROH = 7 X In many respects this picosecond experiments have been on liquids which must kinetic profile over the full dilution range followed that respond during electron solvation in a manner outlined in of l-butanol in the same alkane solvent. The structural the second mechanism, 11. differences between t-BuOH and 2-decanol lie in the What of the actual localization mechanism in low ( P d ) identity of the substituents surrounding the OH moiety systems? We propose that since electron scattering from of the alcohol. Thus the facility with which clustering can density and coiifigurational fluctuations must be a general occur is expected to vary significantly both in terms of the feature of all electron-fluid interactions that the notion solute-solvent interactions prior to electron ejection and of a critical density ( p d ) is tantamount to a critical density in medium reorganization following electron trapping. fluctuation, and that there is a minimum amplitude These ideas are supported by 13C Fourier transform NMR fluctuation required to promote electron localization. We measurements of the T1spin-lattice relaxation times in are exploring the quantitative implications of this hyalcohols and alcohol-alkane systems,1° where the internal pothesis in subpicosecond experiments on electron traprotational freedom of each C atom in the alcohol chains ping in liquids and dilute gases. can be studied as a function of (Pd). The critical dipole In high density fluids both molecular dynamics (midensities observed in picosecond studies of electron solcroscopic) and hydrodynamics (macroscopic) can be used vation correspond to the region where T1data reflect the to describe molecular motion. Their domains of validity onset of clustering in the same systems.1° These concenare quite different, The theoretically most pertinent detration regions also correspond to the results of association
Picosecond Dynamics of Electrons in Fluids Electron
Mobility
and
( q u a s i f r e e - localized
F l u i d Density
state t r a n s i t i o n )
I
1
Figure 2. Electron mobility and fluid density in helium and alcohol alkane systems, in comparison to threshold absorption data from picosecond studies of electron solvation. See text for references and discussion.
scription of the liquid depends on the time of the observation as well as the local molecular interaction potentials, inertial friction, and so forth, At subpicosecond times where the liquid is often treated as a lattice considerable short-range order exists, and since density and configuration fluctuations are frozen-in, this order is quite extensive in hydrogen bonded or otherwise associated liquids. Only electronic polarization and configuration relaxation via multiphoiion processes can take place. Over long times, the averaging out of local order via rotational and translational motion leads to a liquid that resembles the isotropic state usually ascribed to it and one may treat the liquid more ,as a gas and expand in density functions to derive transport properties. It has been demonstrated that after about 10 collisions a molecular dynamics calculation will yield comparable dynamical results1’ to a hydrodynamic calculation which includes the correct boundary conditions.l* This time interval, when liquid theorists suggest we move from a microscopic to a macroscopic description, is precisely the period when electrons become localized and eventually solvated in the liquid. We have already shown that in pure liquid primary alcohols, the solvation times can be treated within a “hydrodynamic” f r a m e ~ o r k . ~However, ”~ this does not imply that microscopic effects are averaged out as indeed the data presented here for low ( p d ) have shown. Figure 2a shows the experimental mobility data of Northby and Sanders and calculations for electron localization in helium at a critical density (see ref l b for details). In Figure 2b we have included our absorption datal4 on the mobility data of Baxendale and co-workers for l-propanol/isooctane.16 It is important, that theories of electron localization and solvation, to be quantitatively predictive, take the correct microscopic (approach to the liquid in which the electron is trapped. Only then can we draw with confidence the same arrow indicating agreement between theory and experiment on electron localization in these fluids.
B. Laser-Induced Electron Transfer Localized electron states in liquids and in crystalline and amorphous solids are well characterized optically, and the origin of their characteristically skewed, broad, and intense visible-IR absorption bands has been a long standing problem of significance to both theory and experiment of the electronic states of disordered systems.lg Just as in electron localization mechanisms, where liquid structure plays a crucial role, attention has now focussed onto the short-range rnolecular structure about the electron in developing a unified treatment of electronic and vibrational motion to explain both the e; absorption spectrum and electron tranrifer processes.%= Both long- and short-range interaction potentials have been constructed through the
The Journal of Physical Chemistty, Vol. 84,No. IO, 1980 1147
coupling of the electron to the low-frequency solvent modes and high-frequency intramolecular vibrational modes, respectively. Only recently has there been any direct experimental evidence concerning the role of bound-bound and bound-continuum electronic transitions in the spectrum of e; in liquids and the origins of the spectral broadening, so we will briefly review these data as a context for our experiments on laser-induced electron transfer in alcohols. Photophysical studies of electrons in polar liquids indicate that the electronically excited states of e; relax nonradiatively on a picosecond t i m e s ~ a l e . ~ Pulsed ~*~~-~~ laser saturation studies revealed that for electrons in primary alcoholsNand in amine liquids%the characteristic e*- absorption bands appear homogeneously broadened on a picosecond time scale. This does not rule out a degree of spectral broadening arising from the contributions of high frequency fluctuations in the ground state structure of the molecules comprising the cluster in which the electron is localized, or even a distribution of cluster sizes with a rapid cross-relaxation time.24 Spectral inhomogeneity due to site-bsite variations in the solvation complex of ions in matricesn and spectral narrowing of the emission from highly excited organic molecules, whose subpicosecond radiation relaxation from upper states is faster than molecular site indicate that site effects ctm be observed. However, homogeneous photobleaching was observed across 1.5 eV, the fwhm value of the band in the case of e; in primary alcohols while other evidence from chemical kinetics indicated that, at least at transition energies up to 1.7 eV, the photoexcitation occurred into a bound electronic ~ t a t e . 2We ~ may summarize by saying that from all the present spectroscopic evidence pertaining to electrons in polar liquids in general, and to alcohols specifically, the maximum oscillator strength in the absorption band corresponds to a bound-bound transition, which is homogeneously broadened at times 210-l’ s and from which nonradiative relaxation to the ground state occurs on a picosecond timescale. During attempts to measure the nonradiative relaxation time of e;* following pulsed laser excitation and depletion of ground state absorbers, full recovery of the ground state population was not observed at high incident laser fluxes (>20 MW cm-z) because a highly reactive chemical channel appeared to open up for e,*.z4J6 We observed that 4, the fraction of electrons lost, displayed the same frequency dependence as the absorption cross section of the ground state e;.24 From these resulta and earlier dataz6* it was speculated that the reactive channel ultimately leads to the formation of RO- and hydrogen atoms. We present here the preliminary results from a detailed study of (i) the efficiency of this reactive channel as a function of laser intensity, and (ii) isotope effects in the postulated electron transfer mechanism.
Experimental Section Our present experimental arrangement for kinetic laser saturation (KLS) spectroscopy with a Q-switched ruby laser has been described in detail previouslyz4and is illustrated in Figure 3. The e; were generated via the two-photon, nanosecond laser-induced photoionization of pyrene in alcohol at 347 nm30 and then subjected to a laser-saturating pulse of up to 700 mJ per pulse at 694 nm (peak powers -45 MW or a laser fluence of 0.9 J cm-2) some 50 ns later. At a number density of typically 10l6 ~ m -the ~ , kinetic lifetime of e; was lo4 s, and photon/e; ratios ranged up to lo3. Deuterated alcohols were obtained at a purity of 99.9% from Baker laboratories. N
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cm-?; Figure 4b shows $m vs. laser energy (J cm") at 694 nm, where 6 is the normalized loss of electron absorbance. At low laser energies there is a steep dependence of $m on laser intensity, but as bleaching laser energies exceed 0.2 J cm-2, the linear slope begins to curve until, at the highest laser intensities investigated, 0.9 J cm-*, the dependence appears to have reached a plateau value of 0.5. This is in marked contrast to the behavior of dye molecules of comparable concentration and absorption cross sections at 694 ntn, such as cryptocyanine and DDI in methanol, which at a laser fluence in excess of 0.4 J cm-2 exhibit values of aggq 1. Ground state recovery times of 38 and 10 ps, respectively, can be calculated for these molecules, in agreement with published values from picosecond t e c h n i q ~ e s . ~ , ~ ~ Flgm 3. Apparatus used in laser photobleaching experiments. The 347-nm UV laser pulse generates ekctrons in C(A) while the laser We conclude that whereas the ground state of the dye sahlratm pulse at 694 nm propagates (A)abng the optical delay line. molecules can be depopulated in a transient photoM, 10 M, Oscilloscope traces y a w lour mnsecufivs bansmmed pulse bleaching process, and then fully recover, the e; upon pro~esat694nm m me SecaXl configuatm M,m M.: la hob bunhg photobleaching at 694 nm have access to a highly reactive experiments on crypiccyanine (n meUunol. Low nux resun is ooserved chemical channel which is in competition with rapid viwith the attenuator In posibon A, high flux in A'. See text lor aetalls 01 the measuremenis?' bronic relaxation to the ground state in times significantly faster than 10 ps. It is also evident that at very high laser '-I fluences the onset of a further mechanism prevents the total population of e; from disappearing via photoexcited state reactions. Extrapolation of the initial slope in Figure 4b would imply that at about 0.5 J cm-2 should approach unity, whereas in our experimental configuration its actual value remains at -0.53. The two central questions are then (1)what is the mechanism through which e;* escapes into a chemical channel, and (2) what are the other contributing or competing relaxation processes that affect the value of $ a t very high laser intensities? Let us briefly comment on the second question first. The fact that the change in slope of $m occ~vsat high laser F W e 4. (a)Dependenceof the elscaon loss fdbwlng photobleachii intensities prompts us to consider whether or not another of e8- in methanol as a function of e; abswption cross section: (0) electron generating mechanism may become important. n O W l swck'um, (A) after laser saturation. (b) Dependence of $wa Since we record an absorbance which is the measure of the on the laser fluence,J cmP. average ground state population during and after photoThe depletion and recovery of the ground state e; abbleaching, it is possible that at high laser intensities the sorption spectrum were monitored with a pulsed (150 W value of $m reflects the net result of rapid electron loss dc) xenon lamp, attenuated ruby laser pulses, or a CW via photochemistry and rapid regeneration of electrons via H e N e beam. Transmitted light intensities were recorded one- or two-photon photoionization at 694 nm (1.78 eV) via a monochromator and fast photomultiplier coupled of a transient or stable species in the system. Likely through 50 R to a R7912 Tektronix transient digitizer. The candidates on energetic grounds are the excited singlet and sensitivity of these experiments was enhanced by nortriplet states of pyrene, although cross sections for these malizing each absorption signal to the intensity of the W absorptions are rather low. Photodetachment of RO-has photoionizing pulse, whose amplitude fluctuations of f 5 % a threshold in the gas phase of -1.6 e V 2but this will be were recorded independently on a fast PIN photodiode, increased in a liquid hy the solvation energies, as shown and then averaging the normalized absorbances from n for the 0, case.% Absorption of a 1.78eV photon by e;* shots of the ruby laser. Spectral and kinetic data were to yield quasi-free electrons is also conceivable but preobtained following data reduction on the PDP 11 comsumably could occur at the low laser intensities too. In puter. any event, earlier results had shown that neither ahnor the pyrene fluorescence were affected by the presence of Results and Discussion 2 M quasi-free electron scavenger molecules, whereas the The permanent loss of e; during laser photobleaching initial electron yield from the two-photon laser-induced at a given laser flux was shown to be proportional to the UV photoionization of pyrene in ethanol was decreased absorption cross section of %-, as shown in Figure 4a,which significantly in the same experiment." The electron displays the data for e; in methanol. Therefore, we may solvation time in ethanol is -18 ps."15 Clearly at present discuss the data from one wavelength as representative of there is insufficientevidence to explain these observations the response of the full absorption spectrum. We define on $, and picosecond laser experiments are currently in a,,as AA/A",where A A is the net loss in absorbance during progress to elucidate further details of the apparent fluence saturation and Ao is the prior absorbance of e; a t A. At threshold (below which $ 0)and nonlinear fluence dea constant laser flux the values of am appear to decrease pendence of $ in a range of e; alcohol systems. as the E,, of e; shifts to the red. For example, agS4 = 0.58,0.48, and 0.40 for electrons in 1-octanol, ethanol, and In addressing the first question, the mechanism of 2-propanol, where the A- are 640,700, and 810 nm, reelectron loss,we chose to study the bleaching phenomena spectively? The loss of electrons via the chemical channel in deuterated alcohols since the chemical kinetic evidence can be seen also to be a function of the laser fluence (J already cited" indicated that the loss of electrons occurred I
&,
-
-
-.
The Journal of Physical Chemktry, Vol. 84, No. 10, 1980 1149
Picosecond Dyniamlcs of Electrons in Fluids
TABLE I: Islotope Effects on Photobleaching Yield, @ 6 0 0 @ma
CH,OH CH,OD CD,OH CH,OH
0.30 0.04 0.32 0.6‘
@600
CH,CH,OH CH,CH,OD 65%RODinROH
b
0.6 0.2 0.4
Laser pulse of 0.44 J a Laer pulse of 0.35 J cm-2. cm-2. Laser pulse of 0.6 J cm-*.
on a time scale faster than electron solvation in the same primary alcolhols, processes which are of the same picosecond time scale as rotational Vibrational motion of the OH bond seemed a possible mechanism13 and so the photobleaching of e; in ethanol and CH3CH20D, methanol, CD30H, and CH30D was investigated over a range of laser fluences. Table I displays typical results. The values of are accurate to h0.05. Note that the yield for permanent photobleaching is identical within experimental error for electrons in CH30H and CD30H, whereas a marked decrease is observed in the value of 4 for CH30D. The magnitude of the isotope effect also depends on the laser intensity during photobleaching. Similar results are observed for e; in ethanol and its deuterated forms. Clearly although CH and CD vibrations undoubtedly contribute to the radiationless relaxation process through which population returns to the ground electronic state, it is the vibration of the OH bond that plays an intrinsic role in the electron loss mechanism. The branching ratio between relaxation and reaction has not been significantly altered by deuteration elsewhere in the molecules. In mixtures of ethanol and deuterated ethanol, little change in 4 could be observed until more than 50% of the molecules were deuterated. Thereafter the magnitude of 4 decreased linearly until it reached the value for pure deuterated ethanol. The isotope studies provide the first direct evidence that the mechanism leading to permanent photobleaching of e,- in alcohols at 298 K is a laser-induced electron transfer process, which takes place via an intramolecular vibration of the OH bond leading to dissociation and electron attachment to the fragment: e,- C hu (1.78 eV)
- e-*
RO-
+H
Electron transfer must occur to a RON molecule that is not only a member of the cluster of molecules which comprise the equilibrium molecular structure of e, in alcohols but is also part of the inner solvent shell. The results from the ethanol mixtures of ROD and ROH indicate that not until ROD coiistitute a significant fraction (namely, 50% if we assume comparable intermolecular interactions between ROH amd ROD) of the inner shell do we see the onset of the islotope effect. An intriguing possibility might be to study selective solvation phenomena of e; in alcohols and their binary mixtures in other liquids via the magnitude of the isotope effect in laser saturation studies. Whether one or more photons are involved in the laser-induced electron transfer mechanism remains to be determined by an absolute measurement of the quantum yield for photobleaching in the region where C#J has a linear dependence om laser intensity. On energetic grounds the gas phase reaction between an isolated molecule ROH and a free electroin would be endothermic to 2.8 eV.32 On absorbing a 1.78-eV photon the e-* is still in a localized state of perhaps several tenths of an electronvolt binding energy. As Table I shows that the higher the laser intensity, the greater the apparent isotope effect in 4. Photobleaching and photoshuffling studies of e; in alcohol glasses at 77 K with broad band visible light have
demonstrated that the absorption bands are inhomogeneously broadened.% Nevertheless, it has been observedB that the e; bands photobleach uniformly under light of 450-600 nm. The rate of disappearance of trapped electrons is slower in the deuterated forms of CHBOHand C,H,OH, from which it was inferred that bond scission of OH and OD to give RO-was taking place.B Although the dynamical behavior in glasses is, in general, quite different to that in liquids, any electron transfer mechanism involving electron-phonon coupling with high frequency vibrational modes should be equally pertinent. What is the identity of the OH vibration? Detailed infrared vibrational analyses of methanol and its deuterated analogues in an argon matrix have been reported for the isolated methanol molecule and its other known multimer species.35 The latter range from open chain dimers, trimers, tetramers, cyclic tetramers, and higher multimers. The OH stretching and bending modes of the methanol monomer show a slight decrease in frequency on going from the gas phase to the argon matrix (e.g., from 3682 to 3667.3 cm-l for the OH stretch (A’) and from 1340 to 1335 cm-l for the OH bending mode (A’). A significant drop occurs on deuteration, such as from 3667.3 to 2706.1 cm-’ for the OD stretch and from 1335 to 864.9 cm-l for the high-frequency bending mode. The multimer exhibits OH in-plane bending frequencies of 1407.3 and 1399 em-’ which drop to 907 cm-’ on deuteration, but they are relatively unaffected by deuteration in the methyl group. The CD30H multimer has an OH in-plane bending mode of 1345 cm-’. These in-plane bending vibrational modes appear in all the polymeric alcohol species between 1330 and 1399 cm-l, while deuteration lowers this range to 860-900 cm-l. The OH stretching modes range from 3360 to 3550 cm-l and from 2450 to 2620 cm-l in the normal and deuterated methanols, respectively. Thus there is good reason to believe that, regardless of the precise number of molecules and the structure of the alcohol cluster within which the electron is trapped, these high frequency intramolecular vibrational modes will be readily available to participate in a strong coupling interaction with the excess electron. Indeed, could we be exciting overtone OH vibrations at high laser intensities? Experiments in progress are therefore also pursuing the isotope effects in 4 following laser saturation over a range of photon energies. The role of low frequency (- 10 cm-l) solvent modes is not necessarily excluded, but they may be more important in multiphonon radiationless relaxation processes of e;*. This intramolecular coupling interaction may indeed be responsible for the line shape and spectral width of the e; absorption bands as recently proposed,21J2and clearly deformation in the intramolecular configuration ha5 occurred to the extent that under high laser intensities the OH bond breaks under the influence of the coupling with e,-*. When electron transfer processes are treated via radiationless transition theory20 it would appear that deuteration can give rise to normal or inverse isotope effects depending on whether the density of states or the Franck-Condon factors, respectively, dominate the intramolecular transition rate.2°336 Future picosecond laser spectroscopy experiments on a range of electron transfer processes will undoubtedly reveal important information to assist in the continually developing picture of the electron and its dynamical interactnons in condensed phase.
References and Notes (1) (a) A. Mozumder in “ElectrorrSohrent and AnbrrSolvent Interactkms”, L. Kevan and 6. Webster, Ed., Elsevier, 1976, p 151, and references therein. (b) J. Jortner and A. Gaathon, Can. J . Chem.. 55, 1812 (1977).
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J. Phys. Chem. 1980, 84, 1150-1154 For a recent review see 0. A. Kenney-Wallace, Acct. Chem. Res., 11, 433 (1978), and ref lb. J. Ulstrup and J. Jortner, J. Chem. Phys., 63, 4358 (1975). K. Funabashl, I. Carmichael, and W. H. Hamill, J. Chem. Phys., 69, 2652 (1978). N. R. Kestner, J. Logan, and J. Jortner, J . Phys. Chem., 78, 2148 (1974). A. Banerjee and J. Simons, J . Chem. Phys., 68, 415 (1978). G. A. Kenney-Wallace and K. Sarantidls, Chem. Phys. Lett., 53,495 (1978); K. Sarantidis, MSc. Thesis, University of Toronto, 1977. D. Huppert and P. M. Rentzepis, J . Chern. Phys., 64, 181 (1976). A. Bromberg and J. K. Thomas, J. Chem. Phys., 63, 2124 (1975). A. P. Marchett. M. Scozzafava. and R. H. Youna. Chem. Phvs. Lett.. 51, 424 (1977); M. El. Sayed,.P. Avouris, a n d k Campion; J. Mol: Struct., 46 355 (1978). K. Choi, H. B. Lin, and M. Topp in ref 7, p 27. T. Shkia and M. Imamura, J . Phys. Chem., 78, 232 (1974). G. E. Hall and 0. A. Kenney-Wallace, Chem. Phys., 28, 205 (1978). G. Mourou, B. Drouin, M. Bergeron, and M. Denariez-Roberge, I€€€ J. Quantum Electron., QE 9745 (1973); M. Duguay and J. Hansen, Oot. Commun.. 1, 254 (1969). P.'Engelklng, G. 6. Eiiisen, andW. C. Lineberger, J. Chem. Phys., 69, 1826 (1978). U. Sowada and R. Holrovd, J . Chem. Phvs.. 70. 3586 (19791. R. May and D. C. Walker,-J: Chem. Soc., Faraday Trans.'P, 74, 1833 (1978), and references therein. A. J. Barnes and H. E. Hallam, Trans. Faraday SOC., 1920 (1971), and references therein. A. Namlki, N. Nakashima, K. Yoshihara, Y. Ito, and T. Hlgashimura, J. Phys. Chem., 82, 1901 (1978).
(2) J. M. Deutch in "Newer Aspects of Molecular Relaxatlon Processes", Faraday Symposium 11, Chemical Society, 1977, pp 26-32. (3) P. Rentzepis, R. Jones, and J. Jortner, J. Chem. Phys., 59, 766 (1973). (4) L. Onsager, Can. J. Chem., 55, 1819 (1977); J. Jortner, ibid., 55, 201 119771. (5) G. A.'Ken&y-Wallace and C. D. Jonah, Chem. Phys. Lett., 39, 596 (1976); 47, 362 (1977). (6) . . 0. S. Mavrogenes. C. Jonah, K. H. Schmidt, S. Gordon, G. R. T r .i m . and L. W. Cheman, Rev. Sci. Instrum., 47, 187 (1976). (7) E. Ippen and C. V. Shank, "Picosecond Phenomena", C. V. Shank, E. Ippen, and S. Shapiro, Ed., Springer-Verlag, West Berlin, 1978, p 103; J. Herltage and R. Jain, Appl. Phys. Lett., 32, 41 (1978). (8) K. Sala, G. A. Kenney-Wallace, and G. E. Hall, I€€€ J . Quantum Electron., submitted for publication. (9) R. R. Hentz and 0. A. Kenney-Wallace, J . Phys. Chem., 78, 514 (1974); 76, 2931 (1972). (10) R. Ling, G. A. Kenney-Wallace, and W. F. Reynolds, Chem. Phys. Left., 54, 81 (1978); V. Gibbs, P. Dais, G. A. Kenney-Wallace, and W. F. Reynolds, Chem. Phys., in press. (11) A. Djaranbakht, J. Lang, and R. Zana, J . Phys. Chem., 81, 2620 (1977), and references thereln. (12) S. A. Rice, G. Dolivo, and L. Kevan, J. Chem. Phys., 70, 18 (1979). (13) G. A. Kenney-Wallace in ref 7, p 214. (14) G. A. Kenney-Wallace, B. A. Garetz, and C. D. Jonah, submitted for publication. (15) W. Chase and J. W. Hunt, J . Phys. Chem., 79, 2835 (1975). (16) J. H. Baxendaie, Can. J. Chem., 55, 1996 (1977), and references therein. (17) For a discussion on this point see D. Klveison in ref 2. (18) R. Kapral, C. Hynes, and M. Weinberg, J . Chem. Phys., 69, 2725 (1978), and references therein.
Laser Photodetachment Spectra of CgFg in Nonpolar Liquids Ulrlch Sowada and Richard A. Holroyd" Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 (ReceivedJuk 16, 1979) Publlcatlon costs assisted by Brookhaven National Laboratory
Aromatic anions absorb in the visible region of the spectrum and energetic considerationssuggest that in solution photodetachment may be an important process at these wavelengths. This study reports electron photodetachment cross sections for C6F( for wavelengths between 415 and 700 nm. The anion is generated by electron attachment to solute c,4?6during an X-ray pulse. Detachment is observed as a change in conductivity induced by a subsequent light pulse from a tunable dye laser. The threshold values are reported for tetramethylsilane, n-butane, cyclopentane,n-pentane, 2,2,4-trimethylpentane,2,2-dimethylbutane,and neopentane. The thresholds are consistent with an electron affinity of 1.09 f 0.04 eV for the c6F6molecule. Comparison of the data to the absorption spectrum of C6F6-shows that for photon energies above threshold the major process is photodetachment. The relationship of photodetachment data to other properties of the excess electrons in these solvents is discussed.
Introduction The mobility of excess electrons in organic, nonpolar liquids is much larger than that of molecular i0ns.l Therefore photodetachment of electrons from negative ions dissolved in these liquids produces a conductivity change, from which photodetachment cross sections (ad) can be determined.2,3 In the gas phase the threshold for photodetachment from an anion radical is A,, the electron affinity of the neutral species. In solution, thresholds are blue shifted due to two additional energy terms: where Vois the energy of the bottom of the conduction band with respect to vacuum, and P is the difference in solvation energy of the anion and the neutral species. A comparison of photodetachment cross sections with molar extinction coefficients ( E ) permits determination of the 0022-3654/80/2084-1150$01 .OO/O
quantum yield of electron detachment by where a, is the photon absorption cross section: a, = e In (10)/(6.02 X lozo). In this study photodetachment from the hexafluorobenzene anion is investigated. This anion is unusual in that the lowest unoccupied orbital is known from an ESR study to be a CT* ~ r b i t a l .In ~ contrast, the lowest unoccupied orbital in C6H6is a r* orbital and the spectroscopy of these anions is expected to be quite different. Gas-phase studies indicate there is some uncertainty in the value of A, for C6F6,which has been reported to be 1.8 f 0.36and 1.20 f 0.07 eV.6 The neutral species has a very large cross section for attachment of low energy electrons.' Photodetachment cross sections for C6F6-are reported here for several solvents and temperatures for wavelengths in the 0 1980 American Chemical Society
