- Email: [email protected]
Nanosecond laser measurements of optical absorption coefficients of electrons in polar fluids
32 (1978) 313-322 Publishing Company
ChemicalPhysics 0 North-Holland
NANOSECOND LASER MEASUREMENTS OF OPTICAL ABSORPTLON COEFFICIENTS OF ELECTRONS IN POLAR FLUIDS
G.E. HALL and G.A. KENNEY-WALLACE* Department
of Chemistry
Received 10 January
University of Toronto, Toronto M.5S IAI. Canada
1978
Direct measurements of the optical absorption coefficients of the short-lived electrons in polar fluids have been made. Laser-induced photoionization of pyrene in alcohols at 293 K generated equal numbers of electrons (e;) and pyrene positive ions (P’). The cm&e;) were derived by measuring the relative absorption intensities of es and P’, which had been independently characterised. The cmax values for e; in methanol to ldecanol fell within the range (1.0-2.0) X lo4 M-r cm-‘, the implications of which are discussed.
1. Introduction Electrons
in polar
and non-polar
fluids
[I ,2] in
temperature glasses [3], and F-centres in alkali halide crystals [4] all display a characteristically broad and structureless absorption spectruin in the visible and near IR regions. Whereas the anionic defect site which hosts the excess electron in a crystal low
is in a relatively welldefmed lattice, and this F-centre
can be described accurately by the polaron theory [S] , electrons in molecular fluids are more typical of the electronic stares of disordered systems, and the similarity in absorption spectra reveals little more than the possibility of strong electron-phonon coupling in both cases. The electronic origins of the strong absorption of electrons in polar liquids have been recently investigated through nanosecond and picosecond laser photobleaching studies [6-lo], and studies have been made of the influence of temperature, pressure, and dielectric properties of the fluid on the position and profile of the optical absorption band [ll-15]_ The data collectively point towards a molecular model, in which the electron is initially trapped within a discrete molecular cluster [15,16]., whose structure then develops under the influence of the excess electronic charge [16,17]_ .A full quantum-mechanical description of the final * Alfred P. Sloan Fellow.
electronic states and a precise-understanding of the final size (and probable distribution of sizes) of the molecular cluster are problems of current and liveiqr i&rest_ Their resolution is pertinent not only to the theory of electrons in fluids but also to the more general problem of electronic states of disordered sys-. terns and to the species formed by clustering of neutrals and ions under molecular beam [ 181 and atmospheric conditions [ 191. In this paper we report the results from a pulsed laser study of the photophysical properties of e; in normal alcohols, in which the absolute absorption cross-sections have been measured directly for the first time. The coefficients E (M-l cm-l) are compared with those available from earlier and indirect measurements, in which the product Gfie and the free ion yields G, were obtained in pulsed electronbeam studies. The absorption coefficients for electrons in this series of linear alcohols are important for assigning oscillator strengths to the electronic transitions and for calculating the radiationless relaxation rates from optical safuration data [6,10]. They also allow the determination of primary ion yields from picosecond pulse radiolysis GE data of the alcohols [ 16,201, and thus assist in the interpretation of picosecond kinetic events in the context of theories of ion-recombination [21,22]. The principal uncertainty in the time-resolved,
314
G.E. Hall, G.A. Kenney-WalIacefOpticalobsorptioncoefficients Ofelectrons in polarfluids ..
spectroscopic measurements of E for a short-lived species is usually the concentration of that species. Ground or excited-state depletion experiments (optical or chemical [2324]) have been employed from which the concentration of the transient may be inferred_ We have chosen a counter-ion technique, in which the number of solvated electrons initially created in the fluid is equal to the number of positive counter ions, the reference ions in question being optically well-characterized_ A ratio of the initial absorbances due to electrons and cations in the fluid is thus the r;ltio of their absorption coefficients. Since electrons in alcohols e.xhibit lifetimes of up to IO-6 s, a positive ion was chosen which would.survive longer than IO-5 s in the same system, in order to permit an independent study oFits absorption and kinetics in the absence of e;_ The pyrene cation fulfilled these criteria_
Fig_ I_ Schematic of P-switched ruby Iaser system: ADP, frequency-doubling crystal; F’. CuS04 filter; B, beam splitter; J, joule meter; P, pyrene sample cell; F, filters; S, shutter; L, lenses; hl, monochromator; PM, photomultiplier; RC, RC filter; TC, timing circuits; 47517912, Tektronix oscilloscope or transient digitizer; D, PIN photodiode trigger.
2. Experimental
impurities were detected. The long solvated electron and pyrene fluorescence lifetimes provided a further internal check on the purity of the chemical samples studied.
2_I. Materials
2.2. Apparatus and techrrique
Pyrcne (Eastman) was purified by recrystallizations from ethyl acetate (Fisher, certified) and column chromatography using siiica gel and n-pentane. When the resulting large, clear crystals were dissolved in the alcohols, no impurities could be detected by absorption or ffuorescence spectroscopy. Xenon (Matheson), nitrogen (Canox, dry), nitrous oxide (Matheson) and nitromethane (Eastman, spectrograde) were used as received. Sodium dodecyl sulfate (Aldrich) solutions were prepared in redistilled water. Methanol and jr-butanol (Fisher, certified) were
The time-dependent absorption spectra of all transient species generated in the nanosecond laser photolysis of the pyrene/alcohol systems were recorded from 400 nm to 700 nm over 10mg to lOa s. The 1.5 J pulse from a Korad Q-switched ruby laser was
purified by refluxing with 3 g/Q of sodium borohydfide (B.D.H.) and i-propanol with 1 g/Q CaHz, both
procedures Being followed by a fractional distillation. N-octanol (Fisher, certified) was refluxed with 3 g/Q of sodium metal and 3 g/Q of sodium borohydride and distilled under reduced pressure. In all cases, only the middle fraction of the distillate was taken for the.laser photolysis experiments_ Ethanol (U.S.I.) was used as received. The alcohols were analyzed for water and ofganic impurities by gas chromatography. Water content was in all cases G 0.05% by weight and no organic
directed into an angIe-tuned ADP crystal, which generated frequency-doubled pulses at 347 nm of typic,& ly loo-150 mJ in energy and 20 ns (fwhrn). The W
laser pulses were passed through a saturated CuSO4 solution filter, to remove the 694.3 nm component, and into the aperttired sample ceil, as shown in fig. 1. Electrons were then photoinjected into the alcohol via the two:photon UY photoionization of pyrene [ZS] . Samples, contained in 1 cm2 quartz cells with teflon stopcocks, were deoxygenated with N, prior to experiment. Transient absorptions were monitored by a pulsed (150 W dc) Xe arc lamp, triggered before the laser pulse and focused through the sample at right angles to the excitation laser pulse. The transmitted light signal, passed through appropriate fdters and onto the slits of a Jarrell-Ash, 0.3 m grating tionochromator, with 2 bandpass of 2 & from 400700 nm, was recorded on a RCA lp28 phijtomultiplier tube. Four stages of gain were used with the tube,
G.E.,HoII.CA; Kenney~lVallace/Opficnl obsorpfiorzcoefficientsof electronsin polar fluids
the photocurrent being drawn from the 5th dynode into an impedance matched 50 S2RC filter. The high frequency component (> 20 kHz) contained the transient absorption and emission signals; the Iow frequency component, sampled a few microseconds before the laser fired, measured the background light level, lo. These output signals were coupled into a Tektronix 475 (250 MHz) oscilloscope and recorded on Polaroid 410 film, or stored and analyzed in a Tektronix 7912/PDP-11 trsnsient digitizer system. The time response of the detection system was f 2 ns. Ai wavelengths less than 550 nm, pyrene fluorescence significantly modified the absorption waveforms. Thus, the fluorescence was first observed with the analyzing light off and then subtracted from the combined absorption plus fluorescence signal observed from the same sample with the light on. The 79 121 PDP-i I system permits extremely accurate data acquisition and electronic sigwl processing and allowed us to readily obtain the true timedependent absorption signal. The latter was then anaIyzed hito its component absorptions from the various ions and excited states as described below.
3. Results 3.1. Kinetic scheme In the photoionization process, electrons and pyrene radical cations are necessarily produced in identical numbers..Any subsequent ionic reactions are either slow on the nanosecond time scale of observation or else do not affect the ratio of electrons to cations. We consider the following reactions in photolytic pyrene (P) systems: P + 2hv + (P” f e;),
(1)
(Pi + eJ + ‘P*, 3P*,
(2)
(P’+e;)+P++e[,
(3)
e, + P+ + ‘P*, 3P*,
(4)
e; +P-+P-,
(5)
e; +ROH+H+RO-,
(6)
2e; + HZ + 2RO-,
(7)
P+ t p-.+ Pz*.
(8)
315
Reaction (1) is the. two-photon photoionizatidn process, which has been shown to occur by nvxm of successive one-photon absorptions via the intermcdiate excited singlet state [25] _The electron is not ejected. from the highly excited pyrene with high kinetic energy, but evidently is solvated iu a region close enough to the geminate cation to make geminate recombination, reaction (2), competitive&b diffusion to homogeneously distributed ions, r&ction (3). Although some of the solvated electrons do recombine homogeneously with other pyrene cations, reaction (4), the dominant reactions ofe, are the attachment to pyrene or the reaction with the solvent itself, reactions (5) and (6). At the low concentrations of e; generated (< I@ M), bimolecular reaction of es is negligible. The system is regenerated by the transfer of charge from the anions in the solution to the pyrene cation in reaction (8). We have also observed, through fluorescence and absorption spectra, the formation of a stable photoproduct in low quantum yield after repeated photolysis of a single sample. It is presumably a consequence of solvent reaction with the pyrene radical anion or cation [26] _The absorption intensities and kinetics of CL and P” were not appreciably affected by the accumulation of this product; nonetheless, samples were replaced regularly as a precaution against spurious photochemical interference. In summary, the decay of solvated electrons is dominated by reactions (5) and (6) and can be adequately described by an exponential decay, except for times less than 100 ns, when a faster decay attributed to reaction (2) appears. This fast component is increasingly evident in less polar solvents. Pyrene radical cations decay according to second order kinetics, primarily through reaction (8), except for the faster geminate recombination in reaction (2) that occurs at early times. 3.2. Spectra of transients The species directly produced during the laser photoionization of the pyrene[alcohol system are *P*, P’ and e,, whiIe 3P*, P- and Pz appear as a consequence of secondary reactions. The observed nanosecond and microsecond transient absorption spectra in pyrene/&ohol solutions are shown in fig. 2a and are spectroscopically and kinetically consistent with the
316
G.E. Hall, GA kemze&WaI&e/Optical absorption coefficients of electrons in Polarfluids
cl
5 -2.6-
-3.0
1 0
’
1 400
’
1 800
’
1 1200
’
1 1603
’
2000
n set
b
’
I
I
I
*iD*orPllo”
5pecrra
---.-.
..
...”
.
.
.
.
.
.
.
pyrcne ‘pyrene
-
PyrelIe?
-----
Pyrene-
.Fig_ 2. (a) Transient absorption spectra of pyrene in methanol: o immediately after 20 ns laser pulse and l 1.6 ps later. (b)
Composite absorption spectrum of species formed in laser photolysis of pyrene solutions*.
presence of the above species, all of which have been studied previously [15,26-30]* and generate the composite spectrum shown in fig. 2b. We will focus on the absorptions at 590 nm, 450 nm, and 415 nm from which most of the reported data analyses were taken. At 590 nm, the dominant contribution to the absorbance is from solvated electrons. Addition of known electron scavengers, such as N20 or CHSNOz, accelerated the decay of the absorption at 590 nm leaving about 10% of the original absorption, which we attribute to the pyrene cation. This same residual absorption is observed several microseconds after laser photolysis, when the e; have decayed in the pure pyrenejalcohol systems. After subtracting this constant absorbance from the 590 nm signals, the decays
‘g-3.2Q + -3.6 -
-4.0
1 0
1
1
1 400
1
1 600
1
I 800
1
n set Fig. 3. Decay kinetics of ei at 590 nm in: (a) 4.9 X IO-* M pyrenej0.1 M sodium dodecyl sulfate/water; @) 5.5 x low5 M pyrene/methanol; (c) 6.1 5 10m5M pyrenejn-butanol. Insets show ac and de components of observed signal. as shown in fig. 3 for water, methanol and butanol. The deviations’ at short times increase in solvents of decreasing polarity, as expected were exponential,
* Ref. Cl51 - e;;ref. [27] - P; ref. [28] - 3P*;ref. [29] P+,.and ref. [30] - P:
1 200
G.E_Hall. G.A. Kenney-Wallace/Opticalabsorption coefficients of electrons in polar fluids
for enhanced geminate recombination. At 415 nm the dominant absorption is due to 3P*. We observe initially a rapid formation, the absorbance following the integrated pulse profile, which we attribute to reaction (2). A slight increase in the signal during the next several hundred nanoseconds is consistent with Slow (ki, = 7 X lo5 s-l) [31] intersystern crossing from the pyrene singlet excited state. The subsequent decay of the 415 nm absorbance follows second order kinetics, consistent with triplettriplet annihilation occurring over tens of microseconds as shown in fig. 4. At 450 nm, P+ is the principal absorbing species, although there are also contributions from IP* and e;. The e; component can readily be calculated from the ratio of ex using published es spectral lineshapes and the e; absorbance at 590 nm. When this is subtracted from the observed signal at 4.50 nm, the remainder is due to singlet and cation. The first-ordtr rate constant for the decay of the singlet absorption is obtained directly from the fluorescence decay, and the second-order decay rate for reaction (8) can be obtained directly from microsecond measurements at 450 nm after the ‘P* is gone as in fig. 4. Thus, we may reliably deduce the relative intensities of the cation and singlet components of the 450 nm signal with a least-squares curve-fitting procedure.
0
O-
,
I
4
!
I
8
p set
,
I
12
, 16
Fig. 4. Decay kinetics of P-at 490 nm, P+at 450 nm and ‘P* at 915 nm. San&k
5.5 X 10” M pyrene/methanol_
317
3.3. Test of the analysis To test the adequacy of the data analysis technique we repeatedly calculated the ratio of e; JP” absorbance from measurements on pyrenelmethanol solutions. The results can be summarized as follows. Changing the pyrene concentration by a factor of two, the laser pulse energy by a factor of three, and altering the decay kinetics of tP* by saturation with Xe did not affect the calculated es/P’ ratios, despite absolute changes in the ionic concentrations, variations in the relative amounts of singlet and cation, and changes in the decay kinetics of the individual species involved. Fig. 5 shows the analysis of two such 450 nm absorption signals. The points were measured from the experimental oscilloscope photographs shown in the insets. The component due to solvated electron absorption was obtained from the observed absorption at 590 nm and the ratio of solvated electron absorptions at 450 nm and 590 nm: The rate constant for lP* decay was obtained from the pyrene fluorescence measured at 415 nm and the rate of cation decay was obtained from a plot of l/A versus time at 450 nm in the time period 3-10 ps. The relative magnitudes of the exponential lP* absorption and the hyperbolic P” absorption were computed by a least-squares curve-fitting procedure. The sum of the three components is shown as the solid line which passes through the experimental points. Data for t < 100 ns were excluded from the curvefitting, since geminate recombination reaction (2) altered the anticipated kinetics at those times. The magnitude of the geminate decay was measured from the es absorption signal and a proportional decay was assumed for the cation. The dashed lines indicate the corrections at early times implied by geminate recombination. Note that the ratio of e;/P+ absorptions is unaffected by the magnitude of this geminate recombination, since the stoichiometric equivalence of e;/P’ is preserved. Table 1 shows the ratio of e; absorption at A,, to P+ absorption at 450 nm for methanol/pyrene solutions under the conditions described. The observed invariance of the es/P+ absorbance ratios under this variety of experimental conditions, and the good theoretical tit, support the validity of the data analysis technique, which was then applied to the laser photolysis data from other pyrene/akohol systems.
-.
318.
G-E_ Hall. G.A. Kenney-WaUace/Optical I
a
I
I
absorption coefficients
of electrons in polarfluids
Table 1 Analysis of e; and P+abscrbance ratios a).in methanol
I
[PI
uv laser
00
energy (mJ)
5.5 x 9.4 x 5.5 x 5s x 5.5 x
10-s 10-s 10-s 10-e h) 10-s
170 150 130 130 50
0.090 0.118 0.064 0.064 0.018
0.54 0.52 0.53 0.55 0.47
t f r f *
0.03 0.01 0.08 0.03 0.03
a) Values cited are averages of several kinetic analyses; errors are one standard deviation. h) Xenon saturated.
3.4. Calibration of cation absorptim
coefficient
The measurement of e,/P+ absorbance ratios in a series of solvents generates a series of relative ab-
?
200
400
600
800
I
I
n set )b
I
I
I
sorption coefficients for solvated electrons in those solvents. However, to obtain absolute es absorption coefficients, the Pf coefficient must be known absolutely. The optical properties of ions such as the pyrene radical cation are insensitive to the solvent. We have taken the value of 1.84 X IO4 M-l cm-l at hmax for the hydrated electron eas to be wellestablished [11,32] and measured the ratio e,/P’ in art aqueous system to obtain an absolute value for (P’). Pyrene is insoluble in water, so we emeArnax ployed a micellar system, sodium dodecyl sulfate in water, as a solvent for the aqueous control experiments_ The absorbance data are presented in table 3 end the emax for P” was determined to be 2.05 X IO4
M-l cm-l. Although Pf has been previously studied under a variety of conditions, the lack of quantitative absorption data from well-characterized systems indicated the need for this determination of emax.
.I 0
3.5. Absoption
zoo
400 nsec
Fig. 5. Computer analyses of typical 450 nm absorption signals for pyrene/methanol system. Insets show raw data: fluorescence and absorption + fluorescence. (a) 9.4 X 10m5M pyrene, 1.50 ml laser pulse energy, Nz saturated. @) 5.5 x low5 M pyrene, 130 I+ laser pnlae energy, Xe.saturated. See text for details.
ratios in alcohols
The measured eJP+ ratios derived from analysis of the absorption data in the alcohols are also sum-
marized in table 3. The ratios of e; absorption at 450 nm and X,,(e;) to the wavelen&b of e; observation (usually 590 mu) were taken from the published literature lineshapes, as shown in table 2, and used to adjust the +, to eArnax values. Taking the absorption coefficient of P” at 450 nm
G.E. Hall, G.A. Kenney-WaltacelOptical absorption coefficients of electrons in polar fluids
319
Table 2 Ratio of es absorption coefficients from spectral lineshapes f450lE
Hz0
meOH etOH CprOH n-buOH n-octOH n-decOH .-
hmax
essolQm
--_
720 630 700
[11,32] [12,15,33] [12,33,34]
0.32 0.58 0.56 0.54
0.63 0.92 0.97 0.98
820 660 680 680
r331 1151 (151 [151
3.6. Discussion The absolute absorption coefficients of e; in a wide range of n-alcohols fall within 1.0 to 2.0 X lo4 M-l cm-‘_ In the longer chain alcohols octanol and decanol, the considerable degree of ion recombination which occurs in the nanosecond time domain leads to larger uncertainties in the analysis of the kinetics, and this is reflected in the limits we quote on Table 3 AbsoluWabsorption coefficients of e; in alcohols -‘h,2,(“,)
(lo4 M-l cm-l) 0.84 t 0.05 0.53 2’0.03 0.43 k 0.03 0.60 r 0.05 0.70 i 0.14 1.0 f 0.10 0.68 t 0.15
a) From refs. [11,33].
Ref.
0.617 0.95 0.88
include those associated with pulse-to-pulse fluctuations in the UV laser energy, and uncertainty in the curve fitting analysis of P+ absorption. Explicitly tzot included is any error in the value of ch,,(e&). Since our E,, values were determined relative to the hydrated electron data, future, albeit unlikely, reassessment of the Em&e;) will naturally cause a concomitant scaling of our emJPf) and thus the absolute coefficients for es.
H20/NaLS meOH etOH i_prOH n-buOH n-octOH n-decOH
Amazy(nm)
0.188 0.55 0.55
to be 2.05 X lo4 M-1 cm-l, the absolute absorption coefficients of e, in the alcohols were then c&ulated to be the values given in table 3. The errors specified
AA man(e&1450tp+)
ax
1.84 r 0.05 a) 1.15 r 0.07 094 + 0.07 1.31 r 0.10 1.53 r 0.30 2.0 f 0.20 1.4 f 0.3
the absorption coefficients. The EA,, values in alcohols are somewhat lower than those determined indirectly for electrons in ammonia and amines [35], where emax = 3 X IO4 M-l cm-l, and several ethers [36] for which E,, = (2.5 + 0.5) X lo4 M-1 cm-l_ Hentz and Kenney-Wallace [1.5] have measured optical absorption spectra of electrons solvated in alcohols of.varying dielectric properties and structure, including the series of n-alcohols. The remarkable fact that the band maxima and profiles are nearly superimposable in the Jr-alcohols, despite wide variations in the dielectric constants, reflects the importance of the short-range, charge-dipole interactions in determining the binding and transition energies of e; in this discrete molecular cluster. The absorption bands of e; in other liquids can be assigned to spectral groups based on the strength of the dipole involved, namely -OH, -NH, -CH, and the molecular structure of the fluid [ 15,371. The new absorption coefficients reported here imply that the oscillator strengthsfI of the transition responsible for this relatively strong optical absorption are also comparable. The increase in ema.. on moving to the longer alcohols, is consistent with the higher E~,~ observed in the similarly non-polar ethers. Since the lineshape clearly influences the determination of the oscillator strength for this unusually broad band, an evaluation Offi fore; in the different liquids can be made from [31] :
However, precise values for fi are still dependent on a more exact measurement of the large absorption wings on the high and low energy side of the alcohol
spectra. Furthermore, in the theoretical interpreta-
G.E. Hall, G.A. Kenney-Wallace/Opticalabsorption coefficients of electrons in polar fluids
320
ticmofthe es
.
spectrum; it is not necessarily the case that the two extrema correspond to the same electronic transitions, although recent [8,10] laser saturation studies in liquid alcohols and ammonia present evidence for a homogeneously broadened band. Estimates of the fifor the alcohols thus fall in the range 0.5 to 0.8, encompassing the values [36] of 0.66 for e; in water, 0.75 in ammonia, and 0.6-0.7 in the liquid ethers, for this one-electron transition_ Previously published values for absorption coefficients of solvated electrons in alcohols are summarized in table 4_ These measurements all used electron-beam pulse radiolysis and combined the results of independent chemical or conductance measurements of the 100 eV ion yield, G, and the product GE. The large discrepancies among previous measurements primariIy arise from disagreements over G values. Our counter-ion measurement technique has the advantage that it is completely dissociated from any controversy over the determination of G values. In a single experimental configuration, we can measure directly the absorption coefficients for electrons solvated in a series of fluids relative to a known counter ion_ The absolute scale for these coefficients in the present work was fried by accepting the established value 1.84 X lo4 M-r cm-l for e,-,max of the hydrated electron [11,32] _Inspection of tablCs 3 and 4 reveals that our determinations of eArnaxfor es in methanol and ethanol agree with the previously reported values that cluster around IO4 M-l cm-l rather than those around b5 X IO4 M-l cm-1 _Absorption coefficients of electrons in the longer chain alcohols have not been previouslyyreported_ The discrepancies between the indirect determinations of E,, in table 4 are usually not in the data for GE,,,= but in the determinations of the G values. However, knowledge of the absolute absorption COefTable 4 Comparisonswith absorp:ion coefficientsderivedfrom GE data fore; in ROH System
e (M-’ cm-‘)
ref. 1331 ref. [40] ref. [391 ref. 1411 this work meOH 17000 etOH 15000 Z--prOH 14000
10000 .10200 17000 9400
11500 9400 13100
ficients Emmax for a series of liquids does not in itself generate a unique set of G values, which then could be reliably compared with theories of ion-recombination. The ieason lies m the intrinsic nature of the G value. As the ionizing radiation analogue of the photochemical quantum yield, the G value of a-chemical species is defmed as the number of that species generated per 100 eV absorbed radiation. Thus the G value is a quantitative parameter reflecting the Outcome of a complex sequence of primary and secondary ionization, excitation, and the subsequent competition between relaxation, geminate ion recombination and solvation for the transient charged species. In turn, the G values of the ions, radicals and excited states in the picosecond time domain determine the scale of subsequent chemical reactions. The techniques and difficulties associated with the determination of G values have been extensively outlined elsewhere [22,32,38], but the salient points for our discussion concern the competition between electron solvation and recombination, and the nonhomogeneous kinetics of the e; species,.during the lo-l2 to 1O-8 s time domain when GE measurements are taken. First and foremost is the fact that the initial IR spectrum observed at the earliest times (= 10 ps) for electrons in alcohols gragually shifts towards the visible spectrum characteristic of stable e; [16,17] _The shifting spectrum has been assigned to the orientational relaxation of the local dipolar molecules about the initial electron trapping site. Thus to derive picosecond G values from these early absorbance measurements, one requires not only the ck for the stable e; spectrum, but also a quantitative means of interpreting the IR absorption. The earliest times at which unambiguous G values can be determined from optical measurements are the times at which the fully-relaxed spectrum is present, which varies from lo-l1 to IO-9 s in the alcohols under . study, By then, geminate recombination of ions has been proceeding at different rates for different time intervals in each alcohol, and a simpk comparison of these G values at the same time cannot reveal information of fundamental significance. Thus, at the present time, the general trends in picosecond G values from one liquid to another can serve only as a useful, qualitative basis on which to model the early events. Comparison of G(eJ values cannot be made solely in terms of the bulk dielectric and viscous properties
G.E. Ha& G.A. Kenney-iValiacefOptica1absorption coefficients of electrons in polar fluids
of the fluid because of a host of complications, which arise from-the consideration of the foliowing important issues in the theories of ion-recombination and nonhomogeneous kinetics; (1) the spatial distribution of ion-pairs at the time of electron localization; (2) the extent of recombination preceding the ultrafast, initial electron trapping step; (3) the case of the non-isolated (two or three ion pairs per spur) ion-pair and the concomitant nonhomogeneity in recombination; (4) the time-dependent Coulomb screening or dielectric relaxation of the local medium and its influence on the recombination rate; (5) the changing mobility of the electron during the quasifree-localized-solvated time sequence; (6) the possibility of resonant hole migration in some liquids. These are important and fundamental questions concerning the individual and collective behavior of ions and molecules in the condensed phase, which we introduce as a caution and a reminder that the G values are inextricably related to the microscopic electronic properties and dynamical molecular structure of the fluid. In summary, we have made the first direct determinations of the optical absorption coefficients for electrons in fluids. In a series of six alcohols of significantly varying dielectric properties, the emax values, while showing an increase in the more nonpolar systems, fall into the range (l-O-2.0) X 10J M-l cm-l_ Where comparison can be made, these measurements are consistent with the lower values of cr,,ax estimated indirectly from pulse radiolysis studies. Not only will these coefficients be of value to future photophysical studies of electrons in fluids, and to a wide range of experiments in chemical kinetics involving e; through photoionization and electron transfer reactions, but they will also. serve as another important and quantified parameter in the theoretical models of excess electron states.
Acknowledgement We are grateful to the National Research Council of Canada for fmancial support. Acknowledgement is also made to the Research Corporation and to the
321
Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. References [l] J. Jortner and N.R. Kestner,edr., Electrons in fluids (Springer, Berlin, 1973). [2f N.S. Kestner, in: Electron-solvent and anion-solvent interactions, eds. L. Kevan and B. Webster (Elsevier, Amsterdam, 1976) ch. 1. [3] L. Kevan, Advan. Radiat. Chem. 4 (1974) 181; DC. Walker, J. Chem. Phys. 67 (1977) 2399. [4] L. hfollenauer, Optics Lett. 1 (1977) 1647; W.F. Fowler, ed., Physics of color centers (Academic Press, New York, 1968).
[S] S. Pekar, 1. Phys. (USSR) 10 (1946) 341,347; L. Landau, Soviet. Phys. 3 (1933) 664. [6] G.A. Kenney-Wallace and DC. Walker, J. Chem. Phys. 55 (1971) 447. [7] P. Rentzepis, R. Jones and J. Jortner, J. Chem. Phys. 59 (1973) 776. [8] D. Huppert and P.M. Rentzepis, J. Chem. Phys. 64 (1976) 191, and references therein. [9] A. Bromberg and J.K. Thomas, J. Chem. Phys. 63 (1975)
2_124. [lo] G.A. Kenney-Wallace and K. Sarantidis,Chem. Phys. Letters 53 (1977) 495. [ll] B.D. Michael, E.J. Hart and R.H. Schmidt, J. Phys. Chem. 75 (1971) 2798. [12] R.R. Hentz, Farhataziz and EM. Hansen, J. Chem. Phys. 75 (1971) 4974; K.N. Jha, G.I. Bolton and G. Freeman, J. Phys. Chem. 76 (1972) 3876: F.-Y. Jou and G.R. Freeman, J. Phys. Chem. 80 (1977) 909, and references therein. [13] J. Jortner and A. Caathon, Can. J. Chem. 55 (1977) 1801; A. Gasthon, G. Czapski and J. Jortner, J. Chem. Phys. 58 (1973) 2648: R. Olinger, S. Hahne and U. Schindewolfe, Ber. Bunsenges. Physik. Chem. 76 (1972) 349. [14] J.R. Brandon and R.F. Firestone, J. Phys. Chem. 78 (1974) 792. [ 151 R.R. Hentz and G.A. Kenney-Wallace, J. Phys. Chem. 76 (1972) 2931; 78 (1974) 514. [16] G.A. Kenney-Wallace and CD. Jonah, Chem. Phys. Letters 39 (1976) 596. (171 G.A. Kenney-Wallace, Chem. Phys. Letters 43 (1976) 529;Can. J. Chem. 55 (1977) 2009. [18] R.J. Gordon, Y.T. Lee and D.R. Herschbach, J. Chem. Phys. 54 (1971) 2393. 1191 I. Tang and AX’. Castelmain Jr., J. Chem. Phys. 62 (1975) 4576. [20] K.Y. Lam and J.W. Hunt, 3. Phys. Chem. 78 (1974) 2414;
322
[21]
1221
1231 [24]
[25] [26]
1271 [28]
-. r--
G.E. Hall. GA Kenney-Wallace/Opt2aicnl absorption coefficients of demons
C.D. Jonah, MS. Matheson, J.R. Miller and EJ. Hart, J.-Phys. Ch&m. 80 (1976) 1267; 77 (1973) 1838. A. Mosumder, in: Electron-solvent anion-solvent in-. t&actions,~eds. L. Kevan and B. Webster (Elsevier,Amsterdam..1976) p_ 139: J. Chem; Phys. 50 (1969) 3153, 3762. G-R. Freeman, Advan. Chem. Ser. 82 (1968) 339; Hia. Schwarz, J. Chem. Phys. 55 (1971) 3647; AzMozumder and J.L. Magee, Intern. J. Radiat. Phys. Chim. 7 (1975) 83. G. Dolanzmd CR. Goldschmidt, Chem. Phys. Letters 39 (1976) 320. M.B. Ledger and G.A. Salmon, Trans. Faraday SOC- 72 (1976) 883; S.G. Hadley and R.A. Keller, J. Phys. Chem. 73 (1969) 4351; R. Bensasson and E.J. Land, Trans. Faraday Sot. 67 (1971) 1904. G.E. Hall and G.A. Kenney-Waliace. Chem. Phys. 28 (1978) 205. S. A;ai, E.L. Tramba, J.R. Brandon and L.M. Dorfman, Can. J. Chem. 45 (1967) 1119; GJ. Hoytink, Discussions Faraday Sot. 45 (1968) 14; 0. Hammer&h and V.fi. Parker, Electrochim. Acta 18 (1973) 537. P-F. Holt and A.N. Hughes, JiChem. Sot. 3216 (1960). W. Heinzelmann and H. Labhart,.Chem. Phys_ Letters 4 (1969) 20.
@Ipolar fluids~
f
[ZS] T. Shida and S.-iwata,.JA& 95. (1973) 3473; [3O].N.J. Aalbeisberg, G.J. Hoytink and E.L. Mackor, j. -. Chem. Sot. 3049 (1959).. [31] J.B. BirkS, Photoph&i& of aromatic r&lecules (WileyInterscience, Few York, 1970). . [32] E.J. Hart and M. Anbar, eds., The hydrated electron (Wiley-Interscience, New York, 1970). [33] MC_ Sauer, S_ Arai and LM. Dorfman, J. i3hem. Phys. 42 (1965) 708. [34] J.F. Galvas and L&I_ D&fman, Intern. J. Radiat. Phys. Chem. 7 (1975) 227. 1 [35] J-W_ Fletcher and W.A. Seddon, Faraday Discussions Chem. Sot. 63 (1977) 18. [36] F--Y. Jou and G. Freeman, Can: J. Chem. 54 (19iS) 3693. 1371 L. Dorfman, inr Proceedings of the 5th International Congress of Radiation Research, ed. 0. Nygaard (Academic Press, New York, 1975) pp. 326-332. [38] M.S. Matheson and L.M. Dorfman, Pu!se radiolysis (ACS-MIT, New York, 1969). [39] F. Busi and M.D. Ward, Intern. J. Rad. Phys. Chem. 5 (1973) 521. [40] K. Jha, ?.I. Bolton and G.R. Freeman, J. Phys. Chem. 76 (1972) 3876. [41] D.W. Johnson and G.A. Salmon, Can. J. Chem. 55 (1977) 2030.
ChemicalPhysics 0 North-Holland
NANOSECOND LASER MEASUREMENTS OF OPTICAL ABSORPTLON COEFFICIENTS OF ELECTRONS IN POLAR FLUIDS
G.E. HALL and G.A. KENNEY-WALLACE* Department
of Chemistry
Received 10 January
University of Toronto, Toronto M.5S IAI. Canada
1978
Direct measurements of the optical absorption coefficients of the short-lived electrons in polar fluids have been made. Laser-induced photoionization of pyrene in alcohols at 293 K generated equal numbers of electrons (e;) and pyrene positive ions (P’). The cm&e;) were derived by measuring the relative absorption intensities of es and P’, which had been independently characterised. The cmax values for e; in methanol to ldecanol fell within the range (1.0-2.0) X lo4 M-r cm-‘, the implications of which are discussed.
1. Introduction Electrons
in polar
and non-polar
fluids
[I ,2] in
temperature glasses [3], and F-centres in alkali halide crystals [4] all display a characteristically broad and structureless absorption spectruin in the visible and near IR regions. Whereas the anionic defect site which hosts the excess electron in a crystal low
is in a relatively welldefmed lattice, and this F-centre
can be described accurately by the polaron theory [S] , electrons in molecular fluids are more typical of the electronic stares of disordered systems, and the similarity in absorption spectra reveals little more than the possibility of strong electron-phonon coupling in both cases. The electronic origins of the strong absorption of electrons in polar liquids have been recently investigated through nanosecond and picosecond laser photobleaching studies [6-lo], and studies have been made of the influence of temperature, pressure, and dielectric properties of the fluid on the position and profile of the optical absorption band [ll-15]_ The data collectively point towards a molecular model, in which the electron is initially trapped within a discrete molecular cluster [15,16]., whose structure then develops under the influence of the excess electronic charge [16,17]_ .A full quantum-mechanical description of the final * Alfred P. Sloan Fellow.
electronic states and a precise-understanding of the final size (and probable distribution of sizes) of the molecular cluster are problems of current and liveiqr i&rest_ Their resolution is pertinent not only to the theory of electrons in fluids but also to the more general problem of electronic states of disordered sys-. terns and to the species formed by clustering of neutrals and ions under molecular beam [ 181 and atmospheric conditions [ 191. In this paper we report the results from a pulsed laser study of the photophysical properties of e; in normal alcohols, in which the absolute absorption cross-sections have been measured directly for the first time. The coefficients E (M-l cm-l) are compared with those available from earlier and indirect measurements, in which the product Gfie and the free ion yields G, were obtained in pulsed electronbeam studies. The absorption coefficients for electrons in this series of linear alcohols are important for assigning oscillator strengths to the electronic transitions and for calculating the radiationless relaxation rates from optical safuration data [6,10]. They also allow the determination of primary ion yields from picosecond pulse radiolysis GE data of the alcohols [ 16,201, and thus assist in the interpretation of picosecond kinetic events in the context of theories of ion-recombination [21,22]. The principal uncertainty in the time-resolved,
314
G.E. Hall, G.A. Kenney-WalIacefOpticalobsorptioncoefficients Ofelectrons in polarfluids ..
spectroscopic measurements of E for a short-lived species is usually the concentration of that species. Ground or excited-state depletion experiments (optical or chemical [2324]) have been employed from which the concentration of the transient may be inferred_ We have chosen a counter-ion technique, in which the number of solvated electrons initially created in the fluid is equal to the number of positive counter ions, the reference ions in question being optically well-characterized_ A ratio of the initial absorbances due to electrons and cations in the fluid is thus the r;ltio of their absorption coefficients. Since electrons in alcohols e.xhibit lifetimes of up to IO-6 s, a positive ion was chosen which would.survive longer than IO-5 s in the same system, in order to permit an independent study oFits absorption and kinetics in the absence of e;_ The pyrene cation fulfilled these criteria_
Fig_ I_ Schematic of P-switched ruby Iaser system: ADP, frequency-doubling crystal; F’. CuS04 filter; B, beam splitter; J, joule meter; P, pyrene sample cell; F, filters; S, shutter; L, lenses; hl, monochromator; PM, photomultiplier; RC, RC filter; TC, timing circuits; 47517912, Tektronix oscilloscope or transient digitizer; D, PIN photodiode trigger.
2. Experimental
impurities were detected. The long solvated electron and pyrene fluorescence lifetimes provided a further internal check on the purity of the chemical samples studied.
2_I. Materials
2.2. Apparatus and techrrique
Pyrcne (Eastman) was purified by recrystallizations from ethyl acetate (Fisher, certified) and column chromatography using siiica gel and n-pentane. When the resulting large, clear crystals were dissolved in the alcohols, no impurities could be detected by absorption or ffuorescence spectroscopy. Xenon (Matheson), nitrogen (Canox, dry), nitrous oxide (Matheson) and nitromethane (Eastman, spectrograde) were used as received. Sodium dodecyl sulfate (Aldrich) solutions were prepared in redistilled water. Methanol and jr-butanol (Fisher, certified) were
The time-dependent absorption spectra of all transient species generated in the nanosecond laser photolysis of the pyrene/alcohol systems were recorded from 400 nm to 700 nm over 10mg to lOa s. The 1.5 J pulse from a Korad Q-switched ruby laser was
purified by refluxing with 3 g/Q of sodium borohydfide (B.D.H.) and i-propanol with 1 g/Q CaHz, both
procedures Being followed by a fractional distillation. N-octanol (Fisher, certified) was refluxed with 3 g/Q of sodium metal and 3 g/Q of sodium borohydride and distilled under reduced pressure. In all cases, only the middle fraction of the distillate was taken for the.laser photolysis experiments_ Ethanol (U.S.I.) was used as received. The alcohols were analyzed for water and ofganic impurities by gas chromatography. Water content was in all cases G 0.05% by weight and no organic
directed into an angIe-tuned ADP crystal, which generated frequency-doubled pulses at 347 nm of typic,& ly loo-150 mJ in energy and 20 ns (fwhrn). The W
laser pulses were passed through a saturated CuSO4 solution filter, to remove the 694.3 nm component, and into the aperttired sample ceil, as shown in fig. 1. Electrons were then photoinjected into the alcohol via the two:photon UY photoionization of pyrene [ZS] . Samples, contained in 1 cm2 quartz cells with teflon stopcocks, were deoxygenated with N, prior to experiment. Transient absorptions were monitored by a pulsed (150 W dc) Xe arc lamp, triggered before the laser pulse and focused through the sample at right angles to the excitation laser pulse. The transmitted light signal, passed through appropriate fdters and onto the slits of a Jarrell-Ash, 0.3 m grating tionochromator, with 2 bandpass of 2 & from 400700 nm, was recorded on a RCA lp28 phijtomultiplier tube. Four stages of gain were used with the tube,
G.E.,HoII.CA; Kenney~lVallace/Opficnl obsorpfiorzcoefficientsof electronsin polar fluids
the photocurrent being drawn from the 5th dynode into an impedance matched 50 S2RC filter. The high frequency component (> 20 kHz) contained the transient absorption and emission signals; the Iow frequency component, sampled a few microseconds before the laser fired, measured the background light level, lo. These output signals were coupled into a Tektronix 475 (250 MHz) oscilloscope and recorded on Polaroid 410 film, or stored and analyzed in a Tektronix 7912/PDP-11 trsnsient digitizer system. The time response of the detection system was f 2 ns. Ai wavelengths less than 550 nm, pyrene fluorescence significantly modified the absorption waveforms. Thus, the fluorescence was first observed with the analyzing light off and then subtracted from the combined absorption plus fluorescence signal observed from the same sample with the light on. The 79 121 PDP-i I system permits extremely accurate data acquisition and electronic sigwl processing and allowed us to readily obtain the true timedependent absorption signal. The latter was then anaIyzed hito its component absorptions from the various ions and excited states as described below.
3. Results 3.1. Kinetic scheme In the photoionization process, electrons and pyrene radical cations are necessarily produced in identical numbers..Any subsequent ionic reactions are either slow on the nanosecond time scale of observation or else do not affect the ratio of electrons to cations. We consider the following reactions in photolytic pyrene (P) systems: P + 2hv + (P” f e;),
(1)
(Pi + eJ + ‘P*, 3P*,
(2)
(P’+e;)+P++e[,
(3)
e, + P+ + ‘P*, 3P*,
(4)
e; +P-+P-,
(5)
e; +ROH+H+RO-,
(6)
2e; + HZ + 2RO-,
(7)
P+ t p-.+ Pz*.
(8)
315
Reaction (1) is the. two-photon photoionizatidn process, which has been shown to occur by nvxm of successive one-photon absorptions via the intermcdiate excited singlet state [25] _The electron is not ejected. from the highly excited pyrene with high kinetic energy, but evidently is solvated iu a region close enough to the geminate cation to make geminate recombination, reaction (2), competitive&b diffusion to homogeneously distributed ions, r&ction (3). Although some of the solvated electrons do recombine homogeneously with other pyrene cations, reaction (4), the dominant reactions ofe, are the attachment to pyrene or the reaction with the solvent itself, reactions (5) and (6). At the low concentrations of e; generated (< I@ M), bimolecular reaction of es is negligible. The system is regenerated by the transfer of charge from the anions in the solution to the pyrene cation in reaction (8). We have also observed, through fluorescence and absorption spectra, the formation of a stable photoproduct in low quantum yield after repeated photolysis of a single sample. It is presumably a consequence of solvent reaction with the pyrene radical anion or cation [26] _The absorption intensities and kinetics of CL and P” were not appreciably affected by the accumulation of this product; nonetheless, samples were replaced regularly as a precaution against spurious photochemical interference. In summary, the decay of solvated electrons is dominated by reactions (5) and (6) and can be adequately described by an exponential decay, except for times less than 100 ns, when a faster decay attributed to reaction (2) appears. This fast component is increasingly evident in less polar solvents. Pyrene radical cations decay according to second order kinetics, primarily through reaction (8), except for the faster geminate recombination in reaction (2) that occurs at early times. 3.2. Spectra of transients The species directly produced during the laser photoionization of the pyrene[alcohol system are *P*, P’ and e,, whiIe 3P*, P- and Pz appear as a consequence of secondary reactions. The observed nanosecond and microsecond transient absorption spectra in pyrene/&ohol solutions are shown in fig. 2a and are spectroscopically and kinetically consistent with the
316
G.E. Hall, GA kemze&WaI&e/Optical absorption coefficients of electrons in Polarfluids
cl
5 -2.6-
-3.0
1 0
’
1 400
’
1 800
’
1 1200
’
1 1603
’
2000
n set
b
’
I
I
I
*iD*orPllo”
5pecrra
---.-.
..
...”
.
.
.
.
.
.
.
pyrcne ‘pyrene
-
PyrelIe?
-----
Pyrene-
.Fig_ 2. (a) Transient absorption spectra of pyrene in methanol: o immediately after 20 ns laser pulse and l 1.6 ps later. (b)
Composite absorption spectrum of species formed in laser photolysis of pyrene solutions*.
presence of the above species, all of which have been studied previously [15,26-30]* and generate the composite spectrum shown in fig. 2b. We will focus on the absorptions at 590 nm, 450 nm, and 415 nm from which most of the reported data analyses were taken. At 590 nm, the dominant contribution to the absorbance is from solvated electrons. Addition of known electron scavengers, such as N20 or CHSNOz, accelerated the decay of the absorption at 590 nm leaving about 10% of the original absorption, which we attribute to the pyrene cation. This same residual absorption is observed several microseconds after laser photolysis, when the e; have decayed in the pure pyrenejalcohol systems. After subtracting this constant absorbance from the 590 nm signals, the decays
‘g-3.2Q + -3.6 -
-4.0
1 0
1
1
1 400
1
1 600
1
I 800
1
n set Fig. 3. Decay kinetics of ei at 590 nm in: (a) 4.9 X IO-* M pyrenej0.1 M sodium dodecyl sulfate/water; @) 5.5 x low5 M pyrene/methanol; (c) 6.1 5 10m5M pyrenejn-butanol. Insets show ac and de components of observed signal. as shown in fig. 3 for water, methanol and butanol. The deviations’ at short times increase in solvents of decreasing polarity, as expected were exponential,
* Ref. Cl51 - e;;ref. [27] - P; ref. [28] - 3P*;ref. [29] P+,.and ref. [30] - P:
1 200
G.E_Hall. G.A. Kenney-Wallace/Opticalabsorption coefficients of electrons in polar fluids
for enhanced geminate recombination. At 415 nm the dominant absorption is due to 3P*. We observe initially a rapid formation, the absorbance following the integrated pulse profile, which we attribute to reaction (2). A slight increase in the signal during the next several hundred nanoseconds is consistent with Slow (ki, = 7 X lo5 s-l) [31] intersystern crossing from the pyrene singlet excited state. The subsequent decay of the 415 nm absorbance follows second order kinetics, consistent with triplettriplet annihilation occurring over tens of microseconds as shown in fig. 4. At 450 nm, P+ is the principal absorbing species, although there are also contributions from IP* and e;. The e; component can readily be calculated from the ratio of ex using published es spectral lineshapes and the e; absorbance at 590 nm. When this is subtracted from the observed signal at 4.50 nm, the remainder is due to singlet and cation. The first-ordtr rate constant for the decay of the singlet absorption is obtained directly from the fluorescence decay, and the second-order decay rate for reaction (8) can be obtained directly from microsecond measurements at 450 nm after the ‘P* is gone as in fig. 4. Thus, we may reliably deduce the relative intensities of the cation and singlet components of the 450 nm signal with a least-squares curve-fitting procedure.
0
O-
,
I
4
!
I
8
p set
,
I
12
, 16
Fig. 4. Decay kinetics of P-at 490 nm, P+at 450 nm and ‘P* at 915 nm. San&k
5.5 X 10” M pyrene/methanol_
317
3.3. Test of the analysis To test the adequacy of the data analysis technique we repeatedly calculated the ratio of e; JP” absorbance from measurements on pyrenelmethanol solutions. The results can be summarized as follows. Changing the pyrene concentration by a factor of two, the laser pulse energy by a factor of three, and altering the decay kinetics of tP* by saturation with Xe did not affect the calculated es/P’ ratios, despite absolute changes in the ionic concentrations, variations in the relative amounts of singlet and cation, and changes in the decay kinetics of the individual species involved. Fig. 5 shows the analysis of two such 450 nm absorption signals. The points were measured from the experimental oscilloscope photographs shown in the insets. The component due to solvated electron absorption was obtained from the observed absorption at 590 nm and the ratio of solvated electron absorptions at 450 nm and 590 nm: The rate constant for lP* decay was obtained from the pyrene fluorescence measured at 415 nm and the rate of cation decay was obtained from a plot of l/A versus time at 450 nm in the time period 3-10 ps. The relative magnitudes of the exponential lP* absorption and the hyperbolic P” absorption were computed by a least-squares curve-fitting procedure. The sum of the three components is shown as the solid line which passes through the experimental points. Data for t < 100 ns were excluded from the curvefitting, since geminate recombination reaction (2) altered the anticipated kinetics at those times. The magnitude of the geminate decay was measured from the es absorption signal and a proportional decay was assumed for the cation. The dashed lines indicate the corrections at early times implied by geminate recombination. Note that the ratio of e;/P+ absorptions is unaffected by the magnitude of this geminate recombination, since the stoichiometric equivalence of e;/P’ is preserved. Table 1 shows the ratio of e; absorption at A,, to P+ absorption at 450 nm for methanol/pyrene solutions under the conditions described. The observed invariance of the es/P+ absorbance ratios under this variety of experimental conditions, and the good theoretical tit, support the validity of the data analysis technique, which was then applied to the laser photolysis data from other pyrene/akohol systems.
-.
318.
G-E_ Hall. G.A. Kenney-WaUace/Optical I
a
I
I
absorption coefficients
of electrons in polarfluids
Table 1 Analysis of e; and P+abscrbance ratios a).in methanol
I
[PI
uv laser
00
energy (mJ)
5.5 x 9.4 x 5.5 x 5s x 5.5 x
10-s 10-s 10-s 10-e h) 10-s
170 150 130 130 50
0.090 0.118 0.064 0.064 0.018
0.54 0.52 0.53 0.55 0.47
t f r f *
0.03 0.01 0.08 0.03 0.03
a) Values cited are averages of several kinetic analyses; errors are one standard deviation. h) Xenon saturated.
3.4. Calibration of cation absorptim
coefficient
The measurement of e,/P+ absorbance ratios in a series of solvents generates a series of relative ab-
?
200
400
600
800
I
I
n set )b
I
I
I
sorption coefficients for solvated electrons in those solvents. However, to obtain absolute es absorption coefficients, the Pf coefficient must be known absolutely. The optical properties of ions such as the pyrene radical cation are insensitive to the solvent. We have taken the value of 1.84 X IO4 M-l cm-l at hmax for the hydrated electron eas to be wellestablished [11,32] and measured the ratio e,/P’ in art aqueous system to obtain an absolute value for (P’). Pyrene is insoluble in water, so we emeArnax ployed a micellar system, sodium dodecyl sulfate in water, as a solvent for the aqueous control experiments_ The absorbance data are presented in table 3 end the emax for P” was determined to be 2.05 X IO4
M-l cm-l. Although Pf has been previously studied under a variety of conditions, the lack of quantitative absorption data from well-characterized systems indicated the need for this determination of emax.
.I 0
3.5. Absoption
zoo
400 nsec
Fig. 5. Computer analyses of typical 450 nm absorption signals for pyrene/methanol system. Insets show raw data: fluorescence and absorption + fluorescence. (a) 9.4 X 10m5M pyrene, 1.50 ml laser pulse energy, Nz saturated. @) 5.5 x low5 M pyrene, 130 I+ laser pnlae energy, Xe.saturated. See text for details.
ratios in alcohols
The measured eJP+ ratios derived from analysis of the absorption data in the alcohols are also sum-
marized in table 3. The ratios of e; absorption at 450 nm and X,,(e;) to the wavelen&b of e; observation (usually 590 mu) were taken from the published literature lineshapes, as shown in table 2, and used to adjust the +, to eArnax values. Taking the absorption coefficient of P” at 450 nm
G.E. Hall, G.A. Kenney-WaltacelOptical absorption coefficients of electrons in polar fluids
319
Table 2 Ratio of es absorption coefficients from spectral lineshapes f450lE
Hz0
meOH etOH CprOH n-buOH n-octOH n-decOH .-
hmax
essolQm
--_
720 630 700
[11,32] [12,15,33] [12,33,34]
0.32 0.58 0.56 0.54
0.63 0.92 0.97 0.98
820 660 680 680
r331 1151 (151 [151
3.6. Discussion The absolute absorption coefficients of e; in a wide range of n-alcohols fall within 1.0 to 2.0 X lo4 M-l cm-‘_ In the longer chain alcohols octanol and decanol, the considerable degree of ion recombination which occurs in the nanosecond time domain leads to larger uncertainties in the analysis of the kinetics, and this is reflected in the limits we quote on Table 3 AbsoluWabsorption coefficients of e; in alcohols -‘h,2,(“,)
(lo4 M-l cm-l) 0.84 t 0.05 0.53 2’0.03 0.43 k 0.03 0.60 r 0.05 0.70 i 0.14 1.0 f 0.10 0.68 t 0.15
a) From refs. [11,33].
Ref.
0.617 0.95 0.88
include those associated with pulse-to-pulse fluctuations in the UV laser energy, and uncertainty in the curve fitting analysis of P+ absorption. Explicitly tzot included is any error in the value of ch,,(e&). Since our E,, values were determined relative to the hydrated electron data, future, albeit unlikely, reassessment of the Em&e;) will naturally cause a concomitant scaling of our emJPf) and thus the absolute coefficients for es.
H20/NaLS meOH etOH i_prOH n-buOH n-octOH n-decOH
Amazy(nm)
0.188 0.55 0.55
to be 2.05 X lo4 M-1 cm-l, the absolute absorption coefficients of e, in the alcohols were then c&ulated to be the values given in table 3. The errors specified
AA man(e&1450tp+)
ax
1.84 r 0.05 a) 1.15 r 0.07 094 + 0.07 1.31 r 0.10 1.53 r 0.30 2.0 f 0.20 1.4 f 0.3
the absorption coefficients. The EA,, values in alcohols are somewhat lower than those determined indirectly for electrons in ammonia and amines [35], where emax = 3 X IO4 M-l cm-l, and several ethers [36] for which E,, = (2.5 + 0.5) X lo4 M-1 cm-l_ Hentz and Kenney-Wallace [1.5] have measured optical absorption spectra of electrons solvated in alcohols of.varying dielectric properties and structure, including the series of n-alcohols. The remarkable fact that the band maxima and profiles are nearly superimposable in the Jr-alcohols, despite wide variations in the dielectric constants, reflects the importance of the short-range, charge-dipole interactions in determining the binding and transition energies of e; in this discrete molecular cluster. The absorption bands of e; in other liquids can be assigned to spectral groups based on the strength of the dipole involved, namely -OH, -NH, -CH, and the molecular structure of the fluid [ 15,371. The new absorption coefficients reported here imply that the oscillator strengthsfI of the transition responsible for this relatively strong optical absorption are also comparable. The increase in ema.. on moving to the longer alcohols, is consistent with the higher E~,~ observed in the similarly non-polar ethers. Since the lineshape clearly influences the determination of the oscillator strength for this unusually broad band, an evaluation Offi fore; in the different liquids can be made from [31] :
However, precise values for fi are still dependent on a more exact measurement of the large absorption wings on the high and low energy side of the alcohol
spectra. Furthermore, in the theoretical interpreta-
G.E. Hall, G.A. Kenney-Wallace/Opticalabsorption coefficients of electrons in polar fluids
320
ticmofthe es
.
spectrum; it is not necessarily the case that the two extrema correspond to the same electronic transitions, although recent [8,10] laser saturation studies in liquid alcohols and ammonia present evidence for a homogeneously broadened band. Estimates of the fifor the alcohols thus fall in the range 0.5 to 0.8, encompassing the values [36] of 0.66 for e; in water, 0.75 in ammonia, and 0.6-0.7 in the liquid ethers, for this one-electron transition_ Previously published values for absorption coefficients of solvated electrons in alcohols are summarized in table 4_ These measurements all used electron-beam pulse radiolysis and combined the results of independent chemical or conductance measurements of the 100 eV ion yield, G, and the product GE. The large discrepancies among previous measurements primariIy arise from disagreements over G values. Our counter-ion measurement technique has the advantage that it is completely dissociated from any controversy over the determination of G values. In a single experimental configuration, we can measure directly the absorption coefficients for electrons solvated in a series of fluids relative to a known counter ion_ The absolute scale for these coefficients in the present work was fried by accepting the established value 1.84 X lo4 M-r cm-l for e,-,max of the hydrated electron [11,32] _Inspection of tablCs 3 and 4 reveals that our determinations of eArnaxfor es in methanol and ethanol agree with the previously reported values that cluster around IO4 M-l cm-l rather than those around b5 X IO4 M-l cm-1 _Absorption coefficients of electrons in the longer chain alcohols have not been previouslyyreported_ The discrepancies between the indirect determinations of E,, in table 4 are usually not in the data for GE,,,= but in the determinations of the G values. However, knowledge of the absolute absorption COefTable 4 Comparisonswith absorp:ion coefficientsderivedfrom GE data fore; in ROH System
e (M-’ cm-‘)
ref. 1331 ref. [40] ref. [391 ref. 1411 this work meOH 17000 etOH 15000 Z--prOH 14000
10000 .10200 17000 9400
11500 9400 13100
ficients Emmax for a series of liquids does not in itself generate a unique set of G values, which then could be reliably compared with theories of ion-recombination. The ieason lies m the intrinsic nature of the G value. As the ionizing radiation analogue of the photochemical quantum yield, the G value of a-chemical species is defmed as the number of that species generated per 100 eV absorbed radiation. Thus the G value is a quantitative parameter reflecting the Outcome of a complex sequence of primary and secondary ionization, excitation, and the subsequent competition between relaxation, geminate ion recombination and solvation for the transient charged species. In turn, the G values of the ions, radicals and excited states in the picosecond time domain determine the scale of subsequent chemical reactions. The techniques and difficulties associated with the determination of G values have been extensively outlined elsewhere [22,32,38], but the salient points for our discussion concern the competition between electron solvation and recombination, and the nonhomogeneous kinetics of the e; species,.during the lo-l2 to 1O-8 s time domain when GE measurements are taken. First and foremost is the fact that the initial IR spectrum observed at the earliest times (= 10 ps) for electrons in alcohols gragually shifts towards the visible spectrum characteristic of stable e; [16,17] _The shifting spectrum has been assigned to the orientational relaxation of the local dipolar molecules about the initial electron trapping site. Thus to derive picosecond G values from these early absorbance measurements, one requires not only the ck for the stable e; spectrum, but also a quantitative means of interpreting the IR absorption. The earliest times at which unambiguous G values can be determined from optical measurements are the times at which the fully-relaxed spectrum is present, which varies from lo-l1 to IO-9 s in the alcohols under . study, By then, geminate recombination of ions has been proceeding at different rates for different time intervals in each alcohol, and a simpk comparison of these G values at the same time cannot reveal information of fundamental significance. Thus, at the present time, the general trends in picosecond G values from one liquid to another can serve only as a useful, qualitative basis on which to model the early events. Comparison of G(eJ values cannot be made solely in terms of the bulk dielectric and viscous properties
G.E. Ha& G.A. Kenney-iValiacefOptica1absorption coefficients of electrons in polar fluids
of the fluid because of a host of complications, which arise from-the consideration of the foliowing important issues in the theories of ion-recombination and nonhomogeneous kinetics; (1) the spatial distribution of ion-pairs at the time of electron localization; (2) the extent of recombination preceding the ultrafast, initial electron trapping step; (3) the case of the non-isolated (two or three ion pairs per spur) ion-pair and the concomitant nonhomogeneity in recombination; (4) the time-dependent Coulomb screening or dielectric relaxation of the local medium and its influence on the recombination rate; (5) the changing mobility of the electron during the quasifree-localized-solvated time sequence; (6) the possibility of resonant hole migration in some liquids. These are important and fundamental questions concerning the individual and collective behavior of ions and molecules in the condensed phase, which we introduce as a caution and a reminder that the G values are inextricably related to the microscopic electronic properties and dynamical molecular structure of the fluid. In summary, we have made the first direct determinations of the optical absorption coefficients for electrons in fluids. In a series of six alcohols of significantly varying dielectric properties, the emax values, while showing an increase in the more nonpolar systems, fall into the range (l-O-2.0) X 10J M-l cm-l_ Where comparison can be made, these measurements are consistent with the lower values of cr,,ax estimated indirectly from pulse radiolysis studies. Not only will these coefficients be of value to future photophysical studies of electrons in fluids, and to a wide range of experiments in chemical kinetics involving e; through photoionization and electron transfer reactions, but they will also. serve as another important and quantified parameter in the theoretical models of excess electron states.
Acknowledgement We are grateful to the National Research Council of Canada for fmancial support. Acknowledgement is also made to the Research Corporation and to the
321
Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. References [l] J. Jortner and N.R. Kestner,edr., Electrons in fluids (Springer, Berlin, 1973). [2f N.S. Kestner, in: Electron-solvent and anion-solvent interactions, eds. L. Kevan and B. Webster (Elsevier, Amsterdam, 1976) ch. 1. [3] L. Kevan, Advan. Radiat. Chem. 4 (1974) 181; DC. Walker, J. Chem. Phys. 67 (1977) 2399. [4] L. hfollenauer, Optics Lett. 1 (1977) 1647; W.F. Fowler, ed., Physics of color centers (Academic Press, New York, 1968).
[S] S. Pekar, 1. Phys. (USSR) 10 (1946) 341,347; L. Landau, Soviet. Phys. 3 (1933) 664. [6] G.A. Kenney-Wallace and DC. Walker, J. Chem. Phys. 55 (1971) 447. [7] P. Rentzepis, R. Jones and J. Jortner, J. Chem. Phys. 59 (1973) 776. [8] D. Huppert and P.M. Rentzepis, J. Chem. Phys. 64 (1976) 191, and references therein. [9] A. Bromberg and J.K. Thomas, J. Chem. Phys. 63 (1975)
2_124. [lo] G.A. Kenney-Wallace and K. Sarantidis,Chem. Phys. Letters 53 (1977) 495. [ll] B.D. Michael, E.J. Hart and R.H. Schmidt, J. Phys. Chem. 75 (1971) 2798. [12] R.R. Hentz, Farhataziz and EM. Hansen, J. Chem. Phys. 75 (1971) 4974; K.N. Jha, G.I. Bolton and G. Freeman, J. Phys. Chem. 76 (1972) 3876: F.-Y. Jou and G.R. Freeman, J. Phys. Chem. 80 (1977) 909, and references therein. [13] J. Jortner and A. Caathon, Can. J. Chem. 55 (1977) 1801; A. Gasthon, G. Czapski and J. Jortner, J. Chem. Phys. 58 (1973) 2648: R. Olinger, S. Hahne and U. Schindewolfe, Ber. Bunsenges. Physik. Chem. 76 (1972) 349. [14] J.R. Brandon and R.F. Firestone, J. Phys. Chem. 78 (1974) 792. [ 151 R.R. Hentz and G.A. Kenney-Wallace, J. Phys. Chem. 76 (1972) 2931; 78 (1974) 514. [16] G.A. Kenney-Wallace and CD. Jonah, Chem. Phys. Letters 39 (1976) 596. (171 G.A. Kenney-Wallace, Chem. Phys. Letters 43 (1976) 529;Can. J. Chem. 55 (1977) 2009. [18] R.J. Gordon, Y.T. Lee and D.R. Herschbach, J. Chem. Phys. 54 (1971) 2393. 1191 I. Tang and AX’. Castelmain Jr., J. Chem. Phys. 62 (1975) 4576. [20] K.Y. Lam and J.W. Hunt, 3. Phys. Chem. 78 (1974) 2414;
322
[21]
1221
1231 [24]
[25] [26]
1271 [28]
-. r--
G.E. Hall. GA Kenney-Wallace/Opt2aicnl absorption coefficients of demons
C.D. Jonah, MS. Matheson, J.R. Miller and EJ. Hart, J.-Phys. Ch&m. 80 (1976) 1267; 77 (1973) 1838. A. Mosumder, in: Electron-solvent anion-solvent in-. t&actions,~eds. L. Kevan and B. Webster (Elsevier,Amsterdam..1976) p_ 139: J. Chem; Phys. 50 (1969) 3153, 3762. G-R. Freeman, Advan. Chem. Ser. 82 (1968) 339; Hia. Schwarz, J. Chem. Phys. 55 (1971) 3647; AzMozumder and J.L. Magee, Intern. J. Radiat. Phys. Chim. 7 (1975) 83. G. Dolanzmd CR. Goldschmidt, Chem. Phys. Letters 39 (1976) 320. M.B. Ledger and G.A. Salmon, Trans. Faraday SOC- 72 (1976) 883; S.G. Hadley and R.A. Keller, J. Phys. Chem. 73 (1969) 4351; R. Bensasson and E.J. Land, Trans. Faraday Sot. 67 (1971) 1904. G.E. Hall and G.A. Kenney-Waliace. Chem. Phys. 28 (1978) 205. S. A;ai, E.L. Tramba, J.R. Brandon and L.M. Dorfman, Can. J. Chem. 45 (1967) 1119; GJ. Hoytink, Discussions Faraday Sot. 45 (1968) 14; 0. Hammer&h and V.fi. Parker, Electrochim. Acta 18 (1973) 537. P-F. Holt and A.N. Hughes, JiChem. Sot. 3216 (1960). W. Heinzelmann and H. Labhart,.Chem. Phys_ Letters 4 (1969) 20.
@Ipolar fluids~
f
[ZS] T. Shida and S.-iwata,.JA& 95. (1973) 3473; [3O].N.J. Aalbeisberg, G.J. Hoytink and E.L. Mackor, j. -. Chem. Sot. 3049 (1959).. [31] J.B. BirkS, Photoph&i& of aromatic r&lecules (WileyInterscience, Few York, 1970). . [32] E.J. Hart and M. Anbar, eds., The hydrated electron (Wiley-Interscience, New York, 1970). [33] MC_ Sauer, S_ Arai and LM. Dorfman, J. i3hem. Phys. 42 (1965) 708. [34] J.F. Galvas and L&I_ D&fman, Intern. J. Radiat. Phys. Chem. 7 (1975) 227. 1 [35] J-W_ Fletcher and W.A. Seddon, Faraday Discussions Chem. Sot. 63 (1977) 18. [36] F--Y. Jou and G. Freeman, Can: J. Chem. 54 (19iS) 3693. 1371 L. Dorfman, inr Proceedings of the 5th International Congress of Radiation Research, ed. 0. Nygaard (Academic Press, New York, 1975) pp. 326-332. [38] M.S. Matheson and L.M. Dorfman, Pu!se radiolysis (ACS-MIT, New York, 1969). [39] F. Busi and M.D. Ward, Intern. J. Rad. Phys. Chem. 5 (1973) 521. [40] K. Jha, ?.I. Bolton and G.R. Freeman, J. Phys. Chem. 76 (1972) 3876. [41] D.W. Johnson and G.A. Salmon, Can. J. Chem. 55 (1977) 2030.