J. Phys. Chem. 1993,97, 12561-12565
12561
Dephasing of Excited-State Wave Packets in an Oxazine Dye Qiang Hong, Ian A. D. Pexton, George Porter, and David R. Klug’ Department of Chemistry, Centre for Photomolecular Sciences, Imperial College, London SW7 ZBB, U.K. Received: June 15, 19936
We present observations of previously unreported excited-state wave packets in the methanol solvated oxazine dye LD690. We have been able to monitor the dephasing of certain excited-state wave packets despite complications caused by the presence of oscillations which originate from the ground state. We have also measured resonant impulsive stimulated Raman scattering (IRSR) from the solute without undue interference from nonresonant solvent ISRS. This allows us to demonstrate that the dominant contribution to the oscillatory features in the transient absorption experiments is from probe absorption/stimulated emission rather than from ISRS.
Introduction Femtosecond spectroscopy has made it possible to observe vibrational wave packets of excited singlet states in small molecules in the gas phase; however, there are relatively few reports of excited-state wave packets in large or small solvated molecules at room temperature.l4 If oscillatory behavior is observed, it can be difficult to determine whether the oscillations originate from ground-state populations, excited-state populations, or coherences. Even simple diatomic molecules in solution can and as there is no known generic exhibit complex method for distinguishing excited-state phenomena from those which originate from the ground state, one sometimes has to rely on a range of circumstantial evidence for making this distinction. Precise modeling of the multidimensional potential energy surfaces of a large solvated molecule is a difficult task, yet in order to understand chemical reactivity in such systems, it is useful to obtain some information about the reaction coordinate. Modeling of wave packet propagation in the oxazine dye nile blue has suggested that the contributions of excited and ground singlet states are somewhat intermingled.* Nevertheless, once electronic coherences have collapsed to populations, it is helpful if an analysis can separate, at least to some extent, excited- and ground-state wave packet contributions to the optical signal. In this paper we present transient absorption data in which the signal to noise is sufficiently good to allow us to distinguish at least three oscillatory frequencies in experiments on the oxazine dye LD690. One of these oscillations is attributed to the excited state, and the damping of this oscillation is therefore a measure of the rate of dephasing of the excited state. Another complicating factor in a pumpprobe experiment is that both pumpinduced probe scattering and pumpinduced probe absorption/stimulated emission can contribute to a transient absorption signal. For example, it has been shown that under certain circumstances even nonresonant ISRS can be sufficiently strong to scatter a large fraction of the probe beam.’ We have attempted to determine what proportion of our transient absorption data originates from ISRS rather than from probe absorption/stimulated emission. This was done by deliberately enhancing the relative contribution of ISRS to a nominally transient absorption signal which is detected far from the central probe wavelength. These results are then compared to transient absorption data in which the scattering has not been enhanced. The scattering enhanced measurements might be expected to include ISRS from solute (LD690) motion in the fluid methanol matrix. There have been numerous measurements of intermolecular dynamics in transparent liquids. Many of these measurements are based on monitoring the optical Kerr effect via time-resolved birefringence as a means of obtaining dynamic information.”l3 This method has been shown to be very effective Abstract published in Adurrnce ACS Absrracrs, November 1, 1993.
0022-365419312097-12561$04.00/0
but is most easily implemented in nonresonant measurements. The scattering enhanced data which we presnt might be capable of distinguishing the solute motion from that of the solvent. Mokhtari and Chesnoyl used resonant ISRS to monitor excitedstate wave packets in malachite green, and our scattering enhanced experiments provide similar information to the frequency shift configuration used by them.
Materiab and Methods
Pulse Production. The femtosecond transient absorption spectrometer used for these experiments comprises the following. A colliding-pulse mode-locked dye laser generates pulses with an autocorrelation of 90 fs (fwhm) and energies of 0.16 nJ at 622 nm. These pulses are amplified to 3 rJ using a ‘bow time” amplifier. The energy for this amplification is provided by the 6.5-kHz,5 11-nm beam from an Oxford Lasers copper vapor laser. The amplified pulses are weakly focused into a flowing water cell which produces a small amount of spectral broadening through self-phasemodulation. This spectral broadening allows the pulse to be shortened by subsequent recompression in an anomalously dispersive prism delay line. The pulses are split into two parts, one part to form the excitation beam and one part to form the probe beam. After reamplification, the excitation pulses had energies of 1 pJ, which was typically reduced to 0.05 pJ before reaching the sample. The probe pulses were kept at least 20 times weaker than thoseused to excite the sample. Groupvelocity dispersion in both the excitation and probe beams was controlled using two anomalously dispersive prism delay lines when necessary. The pulses had cross-correlation fwhm of 83 f 2 fs at the sample, suggesting pulse durations of 59 fs assuming Gaussian profiles; i.e., the pulse period was of the order of the highest oscillatory frequencies under investigation. The shortest crosscorrelations yielded the most clearly resolved 60-fs oscillations, which supports the view that there was very little chirp in either pump or probe pulse at the position of the sample during experiments. Transform limited pulses were maintained by shaping the pulse spectrum within the prism recompression stages of the apparatus using appropriate apertures and by adjusting the degree of anomalous dispersion. The time bandwidth product of our pulses was calculated by fitting both spectra and crosscorrelations to Gaussians. Temporal wings on the pulses were removed by adjusting the pulses to yield the shortest crosscorrelations, calculating the time bandwidth product, changing the apertures, and then checking that the cross-correlations did not broaden. The time bandwidth products of both pump and probe were typically 0.52, which also indicates that neither pump nor probe beams were significantly chirped at the sample. The spectrum of our probe pulse is shown in Figure 1. Both excitation and probe beams were parallel-polarized to better than 95%. The excitation beam was focused to a 270-pm waist in the sample, and the smaller probe beam was aligned with 0 1993 American Chemical Society
Hong et al.
12562 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 0
-20
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% -60
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b
Wavcl.npth/m
h2
Figure 1. Spectrum of the probe pulses used in the transient absorption experiments. The spectrum of the pump pulses are essentially identical to that of the probe. The two positions of Xz (see text) marked on the figurewere for the two pieces of scattering enhanceddatashown in Figure 4.
the aid of a pinhole to interrogate the excited volume. Excitation and probe beams were crossed in the sample a t an angle of 3O. Samples were placed in spinning cuvettes with 100-pm-thick glass windows and a 180-pm path length. The optical density of the samples was 0.4 at 618 nm. Use of a cuvette allows us to perform experiments on samples which cannot be used in a free flowing jet of liquid. Detection of Optical Transients. Two detection schemes were used. The first scheme was a conventional pumpprobe arrangement. Three photodiodes were used: one to monitor probe intensity (via a reference arm), one for pump intensity, and one to monitor the intensity of the probe transmitted through the sample at a detection wavelength selected by a post sample monochromator set to 2-nm resolution. The pump beam was modulated using a 3.25-kHz chopper, and the signal photodiode was monitored using a lock-in amplifier. Ratiometers were used to normalize intensity fluctuations in the pump and probe beams. The second detection arrangement was designed to enhance the relative contribution of pump-induced scattering of the probe. This was done by placing a slit between the prisms used to compensate for dispersion of the probe beam. This allowed us to reduce probe intensity a t the monochromator detection wavelength by a factor of 100, while leaving the rest of probe pulse essentially unaffected. The overall change in spectrum of the probe is minor, as the modified part of the probe spectrum is separated from the central wavelength by more than the pulse bandwidth. Cross-correlations using the spectrally modified probe pulses exhibited broadening of less than 3 fs. By reducing the probe intensity at the detection wavelength, we reduce the pumpinduced change in the number of photons falling on the detector due to changes in transmission of the probe at this wavelength. However, any pump-induced scattering will be largely unaffected and will therefore increase in contribution relative to the changes in transmission. As the probe intensity is not reduced to absolute zero, transmission changes will still be contribute to the overall signal, but its relative contribution will be reduced. This can be expressed as follows. The total signal detected at the monochromator setting A2 is where SIand SZare given by
Sz(X2)
a
P.E,
x ~ ( E ~ E ~ E ~ ~ , ) E(3)~ ~
E, is the pump field, Eprl is the probe field centered a t XI, and E,, is the probe field at XZ. E b is the local oscillator which is
1600
2400
3200
rir/r.mto.esond.
F m e 2. Pump intensity dependence of scattering e n h a n d data. The data in this figure have already been linearly normalized against pump intensity; therefore, any change in signal between the traces indicates a nonlinear dependenceon pumpenergy. The three t r a w shown correspond to pump intensitiesdiffering by a factor of 2. The trace with the largest peak corresponds to a pump intensity 4 times that used for the trace with the smallest peak. While the coherent coupling peak at t = 0 is clearly dependent on the square of the pump intensity, it is qually evident that the fast decay phase of the ISRS is linear in pump intensity. The first few oscillations alsoexhibit nonlinearitywith pump intensitydue to being within the pumpprobe temporal overlap. effectively E,z as long as the field at XZ is sufficiently strong to linearize the signal. The data which we present are the pumpinduced changes to the total signal S obtained through lock-in detection of the probe beam. The total signal at XZ is the combination of two signals S1 and SZ, one of which is the standard transient absorption signal (SI). ISRS is that proportion of the polarization created by EpEpEpll which is scattered to Xz. This signal is linearized by the presence of a finite local oscillator at XZ (see Figure 1) to produce signal Sz. We show in Figure 2 that Sz is indeed linear in the intensity of the pump beam. The local oscillator is effectively that portion of the probe at Az, i.e., Eprz. As the probe is centered at XI, Epr2 depends on factors such as spectral shape of the probe, detection bandwidth, etc. It can be seen from eqs 2 and 3 that any change in the strength of Eprz will have a greater effect on SI than on S,. Therefore, by changing Epr2,it is possible to change the ratio of ISRS detected at XZ to probe absorption/stimulated emission detected at XZ. Both SIand S2 should be linear with respect to both pump and probe intensity as Eprl a Epr2 for a given probe spectrum. We find that most regions of our data are linear with respect to both pump and probe intensity in either transient absorption data (where SIdominates) or scattering enhanced data (where SZis relatively large). Signals within the p u m p probe temporal overlap are not linear in pump energy, however, and it is also possible to enter other nonlinear regimens for SI. Figure 2 demonstrates that the scattering dynamics are linear to pump intensity, while the data within the pumpprobe overlap are proportional to the square of the pump intensity. SZis expected to carry information about any process which shifts the polarization created by pump and probe from XI to Xz. Low-frequency and/or overdamped intermolecular modes such as those observed in OKE experiments will contribute to ISRS, as will intramolecular wave packets. Although S1 does not carry any scattering information, Sz will always be present as long as Eprzacts as the local oscillator and the sample contains Ramanactive vibrational modes, but the relative contributions of SIand Sz to the total signal will depend on the amplitude of Eprz. In our scattering enhanced experiments we modulate Epr2 without affecting Eprlby the using of an aperture in the probe prism delay line. As Xz is separated from XI by greater than the probe bandwidth, the effect of modulating Eprzon the duration of the probe is small. Outside of the pumpprobe temporal overlap, the ratio of the wave packet amplitude to slowly-decaying background is found
Excited-State Wave Packets in an Oxazine Dye
The Journal of Physical Chemistry, Vol. 97. NO. 48, 1993 12563
I
200
250
300
350
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Vibrational frequencyluavenumbers
Figure 3. Contour plot of Fourier transform of a time-resolved ISRS signal plotted against detection wavelength. The linear relationship of the contours is due to the linear relationship between Stokes shift and detection wavelength which is expected for an essentially continuous distribution of oscillators. The distribution is not truly continuous, however, which is why the contours do not continue further. The finite length of the pump and probe pulses also limits the maximum oscillatory frequency which can be detected.
to be the same no matter what the amplitude of Epr2(see Discussion). When Epr2is large, the vast majority of the pumpinduced slowly-decaying backgroundoriginates fromSl (Le., probe absorption/stimulated emission). This is confirmed by the observation that the slowly-decaying background essentially has the spectrum of an LD690 ground-state bleach/stimulated emission. This implies that nearly all of the oscillatory (wave packet) dynamics must also originate from SI.In other words, although ISRS probably does contribute to the oscillations, its contribution is much smaller than that from probe absorption/ stimulated emission. Modulation of the SI:&ratio is probably most useful for studying the dynamics of vibrational features with frequencies of the order of the inverse of the probe pulse durations (as in our experiments). If the probe pulses have too high a bandwidth, then reducing the probe intensity at the detection wavelength will distort their temporal profile. However, if the pulses are too long (with a concomitantly narrow bandwidth), then thedynamics will not be resolved. However, an inherent disadvantage of this method is that the observed dynamics of the scattering might be biased by the choice of detection wavelength. The pumpinduced scattering by a vibrational mode effectively causes a wavelength shift of the probe pulse. The wavelength shift is directly related to the vibrational frequency which causes the scattering. This is illustrated in Figure 3 for pump-induced nonresonant scattering from optical glass. Figure 3 shows detection wavelength plotted against the Fourier transform of the time-resolved dynamics, i.e., the vibrational frequency. In this experiment diode arrays were used instead of single photodiodes. Figure 3 shows that there is a linear relationship between detection wavelength and the vibrational frequency detected with maximum amplitude. Despite biasing of the observed dynamics by the detection wavelength, scattering by a given vibrational frequency can be detected fairly far away from the central scattering wavelength, as illustrated by the contour lines of Figure 3. The scattering enhanced arrangement (Epr2 reduced by a factor of 100) is used in our studies of LD690 in an attempt to monitor resonantly-enhanced scattering from the solute molecule while surpressing other resonant features such as absorption of the probe or stimulated emission. Scattering from samples of pure
solvent was too weak to be detected and was therefore a t least 100 times weaker than that from the solute. In all of the above detection schemes, an iris was used to ensure that only light transmitted within the divergence of the probe beam was allowed to fall onto the detectors. All of the data presented in this paper are the result of between 4 and 10averaged scans of the delay line. Signal averaging was approximately 0.1 s per point per scan, giving a total integration time of 0.4-1 s per point along the time axis. Noise levels in both experimental arrangements were of the order of 5 X 10-5 AOD a t 1-Hz bandwidth. Each piece of data comprised 1000 points each separated by 3.3 fs. Controls. All samples were placed in cuvettes with glass windows. Windows of 100-pm thickness were used in order to reduce ISRS from the cuvette windows which can sometimes be large enough to significantly distort data in femtosecond p u m p probe experiments.I4 In all of the data presented here, the glassinduced oscillations were at lepst 10times smaller than thesignals from the solvated dye itself. ISRS from the solvent also contributes to the signals, but any pumpinduced scattering from the solvent was a t least 100times smaller than the smallest signals in either the transient absorption or the scattering enhanced experiments. These controls were performed by exchanging dye samples for pure solvent without moving the optical cuvettes, which ensured consistent overlap of the pump and probe beams. Experiments were performed both with the cuvette spinning rapidly enough to exchange the excited volume between flashes or with the cuvette stationary. The data produced by either of these arrangements were found to be identical, presumably due to the combination of a relatively low level of excitation and diffusion of the excited molecules away from the excitation area. Cross-correlations were performed for each experiment in order to determine t = 0 and to ensure that the pulses remained transform limited. A 200-pm-thick BBO crystal was used for the crosscorrelation measurements. Cross-correlations were typically 8085 fs (fwhm), and t = 0 was reproducible to better than the resolution of our delay line (3.3 fs). We estimate however that t = 0 for the cross-correlations is only accurate to t = 0 in the LD690 experiments to about 10 fs, due to the need to7eplace the BBO crystal with the sample. All signals which we report were linear with respect to both pump and probe pulse energies over a decade of intensity. Signals were also linear to solute concentration, which was typically 2.4 X 10-5 M. LD690 was purchased from Exciton, and spectroscopic grade Methanol was from Aldrich. Data Analysis. Data were analyzed using linear predictive fitting,I5 and the results were checked using a combination of Fourier transforms for oscillatory components and Marquardt least-squares analysis for the nonoscillatory components. Data were only analyzed from times after the pumpprobe overlap as determined by cross-correlation measurements. The results from linear predictive fitting are shown in Table I. In performing the fits, it is important to take account of factors such as the biexponential decay of the high-frequency mode@) (-600 cm-I), the presence in some of the data of a very low-frequency (-60 cm-'l), and in the scattering data the presence of significant nonoscillatory components. The results of the fits are only given for the data which are presented in Figure 4, but reproducibility was assessed by comparing a number of identical data sets. Results and Discussion Figure 4a-d shows transient absorption and scattering enhanced data at 649 nm (red side of the probe) and 612 nm (blue side of the probe). Transient absorption data are also shown for 640 nm. Pumpprobe cross-correlations are also shown with each piece of data to help locate t = 0. The results of linear predictive fits to the data are shown in Table I. The main findings are as follows. Two high-frequency modes are present, one of 4 9 0 cm-1 and one of -570 cm-l. The amplitudes of these components
12564 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
Hong et al.
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Figure 4. (a) Transient absorption signal detected at 649 nm. (b) Scattering enhanced signal detected at 649 nm. (c) Transient absorption signal detected at 612 nm. (d) Scattering enhanced signal detected at 612 nm. (e) Transient absorption data at 640 nm.
are wavelength dependent, but the linear predictive fitting procedure cannot reliably determine which of the two is the larger. However, Fourier transforms show that the 590-cm-l mode is always the larger of the two. A mode of -300 cm-I is also evident. The ratio of 600- to 300-cm-' modes is a function of detection wavelength, and the phases of the oscillationsdepend on detection wavelength as well. A very low frequency of 50-60 cm-I can be detected in some data. This low frequency makes it difficult for the linear predictive fit to establish reliable values for the slowest nonoscillatory component. Amplitudes for the slowest noncwcillatory component are best determined by other fitting methods. The most evident difference between transient absorption and scattering enhanced data is the presence of a fast (100-300 fs) nonoscillatory component in the scattering enhanced data. Figure 2 shows the power dependence of the scattering enhanced signal. These data show a roughly linear intensity dependence for the 100-300-fs exponentialcomponent and the long-lived component. The first few oscillations, however, do show some nonlinearity as they are within the cross-correlationof the pump and probe pulses (cf. Figure 4b). The ratios of wave packet amplitudes are similar in either transient absorption or scattering enhanced data at the same
wavelength. Not all wave packets are detectable at all wavelengths, however. In fact, thecontribution of the 300-cm-1 wave packet falls to a minimum well before the 600-cm-1 wave packet(s) reach a maximum on the red side of the probe spectrum (data not shown). Figure 5 is a resonant Raman spectrum of LD690 in methanol. Although both of the =600-cm-l modes are visible, the 300-cm-1 mode cannot be detected. The transient absorption measurements which we present here are rather straightforward, while the scattering enhanced measurements are primarily intended to determine how much ISRS contributes to the transient absorption measurements. The comparative measurements presented here tend to stress the sensitivity of pumpprobe experiments to the relative intensity of the local oscillator. Although the scattering enhanced measurements do increase the relative amplitude of the nonoscillatory 100-300-fs phase, probe absorption/stimulated emission still contributes to the data as the local oscillator has not been reduced to zero. Reduction of the probe intensity at A2 (&2) by a factor of 100should change the relative contributions of ISRS to absorption/stimulated emission byafactorof 10. Theratioofamplitudesofthe600-cm-1
Excited-State Wave Packets in an Oxazine Dye
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12565
TABLE I: Linear Predictive Fits to the Data of Figure 4 ~
~~
X (nm)
amplitude
dampling const (fs)
frq(cm-l)
phase ( w )
649
148.7 38.1 17.5 35 28.9 21.4 328 126.1 42.7 104.4 38.3 21.4 22 51.7 128.1 20.6 21.8 18.2
1236 2650 3366 2349 830 1329 308 1848 979 448 1100 1510 989 1105 137.5 2230 779 340
586.2 571.8
-0.18 -0.47
586.4 571.4 49.3
0.15 -0.08 0.43
586.1 566.3 306.1
1.09 1.04 -0.1
592.8 278
0.8 0.48
585.9 573.2 306.7
0.06 -0.25 0.59
649O
612
612"
640
a
Indicates scattering enhanced data.
1.70 1
100
1
450
800
Frequency (cm-1) Figure 5. Steady-state resonance Raman spectrum of LD690 in methanol.
wave packets to the amplitude of the slowest nonoscillatory component is 0.3 1 for transient absorption measurements and 0.32 for scattering enhanced measurements. This suggests tha the contribution of ISRS to the 600-cm-I oscillations in the transient absorption experiments is negligible. The amplitude ratios for the 300- to 600-cm-1 modes are 1.6:l for transient absorption a t 612 nm and 1:l for scattering enhanced signals at 612 nm. Accurate amplitude measurements are more difficult for the -300-cm-1 mode due to its rapid damping and the fact that the first two oscillations are still within the pump-probe cross-correlation and therefore show some nonlinearity to pump energy (see Figure 2). The absence of a 300-cm-1 mode in the steady-state resonance Raman spectrum of LD690 is evidence that this mode originates from the excited state. Although the Raman spectrum was taken using 488-nm excitation, some sign of the 300-cm-I mode would be expected if it did in fact originate from the electronic ground state. Wave packet analysis of steady-state resonance Raman data on Nile Blue showed a good correspondence with femtosecond transient absorption data despite the use of different excitation wavelengths.16 The behavior of the phase of the wave packets is interesting. The linear predictive fits have no trouble in reproducibly establishing the phase of the large 600-fs mode due ot its large amplitude. The accuracy of the phase for this mode is limited
by the precision with which we can establish t = 0 from the cross-correlations. We estimate this precision to be 10 fs, which translates to u / 3 for the 600-cm-I mode and u/6 for the 300cm-I mode. The limiting factor in establishing the phase of the 300-cm-l mode is its relatively low amplitude. It is clear that the phase of the 600-fs mode(s) is a function of detection wavelength, but the poor precision for the 300-cm-l mode leads to poor reproducibility in the phase (see Table I, 612-nm data). The data are clearly complicated by additional features for those times within the pumpprobe temporal overlap. The scattering enhanced data at 612 nm has a large feature centered at t = 0, which has a fwhm identical to the cross-correlation trace. Three-pulse photon echo experiments on LD690 in methanol have suggested an electronic dephasing time of 30-70 fs,I7 which would imply that all the dynamics which we discuss are due to populations of electronic states rather than coherences, particularly as the damping constant for the 300-cm-l wave packet is 300-1000 fs (see Table I). The presence of a 100-300-fs nonoscillatory component is only apparent in the scattering enhanced data which suggests that it originates from ISRS. As we mentioned above, apart from a coherence spike with a width of the order of the pulse crosscorrelations, scattering from the solvent alone is too small to be detected in our experiments. This suggests that the 100-300-fs exponential phase which we observe in our scattering enhanced data comes directly from the solute molecules. A more complete analysis of these data will be presented elsewhere.
Conclusions It is possible to observe coherent vibrational motion of the excitedstateofLD690alongat least onedegreeoffreedomdespite complications caused by the presence of oscillations which originate from the ground state. This has allowed us to measure the dephasing time of wave packets in the SIstate. We also find that ISRS only makes a large contribution to the data when the local oscillator is reduced in intensity. Acknowledgment. This work was supported by The Royal Society. We thank James Durrant for many helpful discussions and Chris Barnett for technical support. References and Notes (1) Chesnoy, J.; Mokhtari, A. Phys. Rev. A 1988, 38 (7), 3566. (2) Pollard, W. T.; Fragnito, H. C.; Bigot, J.-Y.; Shank, C. V.; Mathies, R. A. Chem. Phys. Lett. 1990, 168 (3,4),239. (3) Scherer, N. F.; Ziegler, L. D.; Fleming, G. R. J . Chem. Phys. 1992, 96 (7). 5544.
(4) Banin, U.;Ruhman, S.J . Chem. Phys. 1993, 98 (6), 4391. (5) Wynne, K.; Galli, C.; De Rege, P. J. F.; Therial, M. J.; Hochstrasser, R. M. In Proeeedingsof thedth InternatioMIUllrafast PhenomeM Conference; Springer-Verlag: Berlin, 1992;p 71. (6) Scherer, N.F.;Jonas, D. M.; Fleming, G. R. J . Chem. Phys. 1993, 99 (l), 153. (7) Yan,Y.-X.;Gamble,E.B.,Jr.;Nelson,K.A.J.Chem.Phys.83(11), 5391. (8) McMorrow, D.;Lotshaw, W. T.; Kenney-Wallace, G. A. IEEE J . Quantum Electron. 1988, QE-24, 443. (9) McMorrow, D.;Lotshaw, W. T. Chem. Phys. Lett. 1991, 178, 69. (10) Wynne, K.; Galli, C.; Hochstrasser, R. M. Chem. Phys. Left. 1992, 193 (1.2. 3). 17. ((1) Haitori, T.;Terasaki, A.; Kobayashi, T.; Wada, T.; Yamada, A,; Sesabe, I. J . Chem. Phys. 1991, 95 (2),937. (12) Cho, M.; Rosenthal, S.J.; Scherer, N. F.; Ziegler, L. D.; Fleming, G.R. J . Chem. Phys. 1992, 96 (7), 5033. (13) Back, R.;Kenney-Wallace, G. A.; Lotshaw, W. T.; McMorrow, D. Chem. Phys. Lett. 1992, 191 (9,423. (14) Hong, Q.;Durrant, J.; Hastings, G.; Porter, G.; Klug, D. R. Chem. Phys. Lett. 1993, 202 (3, 41,183. (15) Barkhuijsen, H.; De Beer, R.; Bovee, W.M. M. J.; van Ormondt, D. J . Magn. Reson. 1985, 61, 465. (16) Lawless, M. K.; Mathies, R. A. J . Chem.Phys. 1992,96(11), 8037. (17) Bardeen, C.J.;Shank,C. V. Chem. Phys. Lett. 1993*203 (5,6),535.
