7897
J. Phys. Chem. 1995, 99, 7897-7903
Femtosecond Time-Delayed Photoionization Studies of Ultrafast Internal Conversion in 1,3,5-Hexatriene? Carl C. Hayden" and David W. Chandler Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551 Received: November 28, 1994; In Final Form: February 27, 1995@
This paper presents results from femtosecond time-resolved studies of the ultrafast intemal conversion in 1,3,5-hexatriene following excitation of the llB, state at around 250 nm. The intemal conversion process is investigated using time-delayed single and multiple photon ionization with 350 nm and 310 nm probe pulses. The initial step in the intemal conversion is found to be very fast (< 100 fs) as expected from absorption line width measurements. However we are able to observe the subsequent time evolution of the molecule for more than 1 ps. The intemal conversion appears to proceed through an intermediate state, likely to be the 2'Ag electronic state, that is seen to decay with a -250 fs time constant.
Introduction Ultrafast intemal conversion plays a crucial role in the photochemical processes of many organic molecules. Intemal conversion produces highly vibrationally excited molecules that are responsible for many of the reactions seen in the photochemistry of polyatomic molecules. Ultrafast intemal conversion is also frequently used as a source of vibrationally hot molecules for kinetics studies of unimolecular rearrangement reactions.'-3 However, little is known about the detailed pathways for conversion of electronic to vibrational excitation in molecules where this process is so fast that fluorescence is not observed. The electronic absorption spectra of molecules as small as acetylene and as large as fullerenes are profoundly influenced by these processes. An important example of a molecule that undergoes such ultrafast intemal conversion is 1,3,5-hexatriene. The spectroscopy of this molecule has been extensively partly in an effort to understand the electronic structure of conjugated linear polymers that are important in biological p h o t ~ c h e m i s t r y .A~ ~strong ~ electronic absorption of 1,3,5-hexatriene occurs in the ultraviolet with the origin band centered at about 39 800 cm-'. The cis and trans forms of the molecule have qualitatively similar room temperature absorption spectra in this regi0n.I Direct absorption measurements of jet-cooled samplesI0 show the line width of the origin band for the trans isomer to be about 155 cm-I, a decrease of about 50% from the room temperature spectrum. The jet-cooled spectrum of the cis isomer shows a somewhat broader line width of -250 cm-I. Estimation of the homogeneous line widths from the total resonance Raman cross section has yielded fwhm line widths of -130 cm-' for the trans isomer" and -330 cm-' for the cis isomer.I2 These line width results indicate a very short (40 fs) lifetime for the initial electronic excitation in these molecules, implying that the initial step in the intemal conversion process is very fast. An important question about the electronic structure of 1,3,5hexatriene that has received a great deal of attention is the ordering of the excited singlet electronic states'. The strong optical transition observed at around 250 nm is from the 'A, ground state to the l'B, excited states (symmetry labels for the trans-isomer are used throughout to refer to states of either * Author to whom correspondence should be addressed. + This work is supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. Abstract published in Advance ACS Abstracts, April 15, 1995. @
isomer: corresponding labels for the cis isomer are 'A1 and 'Bl). Molecular orbital theory at the Hartree-Fock level predicts the 2'A, excited state to lie above the l'B, excited state in energy.13 There have been many experimental attempts to locate the 2'A, state,l4-l7 but the forbidden nature of the 2'A, l'Ag transition has made it difficult to directly observe. Recently however, the 2'Ag state has been located experimentally for the cis isomer and found to lie approximately 5000 cm-' below the llB, excited state.I8 This ordering of the energies of the excited states is the same as that seen in the longer polyenes. Since its initial identification, the 2'A, state in the cis isomer has been studied with resonance-enhanced two-photon ionization,l 3 laser-induced fluores~ence,'~ and resonance Raman spectroscopy.20 The presence of this excited singlet state, somewhat lower in energy than the optically excited l'B, state, is likely to play an important role in its fast intemal conversion. Polyenes exhibit a variety of photophysical processes depending on their chain length and substituent groups. Longer chain polyenes fluoresce, but the fluorescence yield from hexatriene and butadiene is vanishingly small." Fluorescence in the longer polyenes following excitation to the l'B, state (or corresponding 'B state for molecules of different symmetry) can occur predominantly from the initially excited state, as is seen in jetcooled octatetraene,21s22predominantly from the lower lying 2'Ag state, as seen in diphenyl~ctatetraene,~~ or from both excited singlet states, which is the case for many of the systems studied.24 These different fluorescence characteristics reflect, first, competition between the strongly allowed radiative decay (2 ns radiative lifetime for octatetraene calculated from SO to S2 integrated abs~rption)~ of the l'B, state and intemal conversion to the 2'A, state, followed by competition between the relatively slow radiative decay of the 2'Ag state and isomerization or intemal conversion to the ground state. The complete absence of fluorescence from hexatriene has led to the suggestion that it may rapidly intemally convert directly to the ground electronic state.I0 From determinations of the homogeneous absorption line widths it is possible to infer the lifetime of the initial electronic excitation in the molecule, but such measurements do not provide information on the subsequent evolution of the excitation. With femtosecond lasers it is now possible to probe these processes directly. In this paper we describe experiments in which we observe in real time the evolution of 1,3,5-hexatrienefollowing excition of the l'B, state
0022-3654/95/2099-7897$09.00/0 0 1995 American Chemical Society
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7898 J. Phys. Chem., Vol. 99, No. 20, 1995 at -250 nm in order to learn how the internal conversion proceeds after the decay of the initial excitation. The experimental approach used for these studies is femtosecond time-delayed two-pulse photoionization. The timedelayed photoionization method was first employed to study the lifetime of triplet benzene25and has since been applied with nanosecond time resolution to many other A variety of time-delayed ionization techniques has also been used on the picosecond and femtosecond time scales to study vibrational predissociation of van der Waals's clusters,29dissociation dynamics of meta130.31and hydrogen-bonded molecular photodiss~ciation,~~ and decay dynamics of molecular Rydberg state^.^^-^^ In the experiments described here, the femtosecond excitation pulse at 250 nm excites the l'B, electronic state that undergoes ultrafast internal conversion. A second femtosecond pulse (ionization pulse), at a longer wavelength in the UV, ionizes the excited molecules. The wavelength of the second pulse is chosen such that it cannot directly two-photon ionize the ground state 1,3,5-hexatriene. As the time delay between the excitation and ionization pulses is varied, the ion yield is measured in a time-of-flight mass spectrometer. An important feature of these experiments is that increasing the ionization pulse intensity produces multiphoton ionization that is detected by ion fragmentation. By measuring ion yields from single and multiphoton ionization as a function of delay time between the laser pulses, we observe both the initial llB, state and a transient intermediate in the internal conversion process.
Hayden and Chandler
1,3,Ei-Hexatriene energy level diagram (symmetry labels for trans isomer)
cis
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CGH,++ H
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+
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III I II I IIIII
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0
Experimental Method The laser system used in these experiments is based on a colliding-pulse mode-locked (CPM) dye laser (Clark Instrumentation, Inc.) operating at 625 nm. The output of this laser consists of -50 fs pulses at a 90 MHz repetition rate. Individual pulses are amplified to 100 pJ in a multipass dye amplifier pumped at 50 Hz by the doubled output of an injection-seeded Nd:YAG laser (Continuum). Portions of this amplified pulse are focused into rotating quartz plates to generate two separate continuum pulses. A 10 nm bandwidth around 700 nm is selected by an interference filter from one of the continuum pulses and amplified by double passing it through a Bethune cell38dye amplifier pumped by the doubled Nd:YAG laser. A 10 nm bandwidth around 750 nm is selected from the other continuum pulse and amplified in a two-stage Bethune cell dye amplifier system. The resulting pulses at 700 and 750 nm each have pulse energies over 100 pJ and pulse lengths of about 100 fs following dispersion compensation in prism pulse compressors. The excitation pulse for the experiments is generated by frequency doubling the 750 nm pulses to 375 nm then sum frequency mixing these two pulses to produce a 250 nm pulse. These frequency conversions are performed in two 0.5 mm thick BBO crystals (CSK Optronics). The limited phase-matching bandwidth for the sum frequency generation in the second crystal lengthens the 250 nm pulse to around 200 fs, as measured by cross correlation with the 750 nm pulse. Experiments have been performed using ionization pulses at two different wavelengths, 350 and 310 nm. Ionization pulses at 350 nm are generated by frequency doubling the 700 nm dye amplifier output, and 310 nm ionization pulses are produced by doubling the unused portion of the amplified CPM laser at 620 nm. These pulses were each cross correlated with the 750 nm pulse and found to be about 150 fs long. The relative delay between excitation and ionization pulses was adjusted to zero and then varied under computer control with a 1 p m resolution motorized translation stage (Burleigh Instruments, Inc.).
trans
Ionization Potential
nm
-
II I
II II
21Ag
250 nm
Ground State 1'A,
Figure 1. Energy level diagram for 1,3,5-hexatrieneexcitation and ionization. Ionization and dissociative ionization thresholds shown are from ref 38. Energetics for single and three-photon ionization of the excited molecule are illustrated, although absorption of the third photon may occur after ionization.
The experiments are performed in the ionization region of a time-of-flight mass spectrometer. The vacuum system attained a background pressure of less than 1 x lo-* Torr. The sample is introduced through a precision leak valve adjusted to produce a pressure of -5 x lo-' Torr in the vacuum chamber. Ions formed are accelerated to 2000 eV in a static electric field and detected by a dual microchannel plate detector. Mass spectra are recorded with a high-speed transient digitizer. The yields of ions at particular masses are recorded as a function of relative excitation-ionization pulse delay by gated integrators triggered at appropriate times to detect the microchannel plate signal from ions in the desired range of masses. The samples of 1,3,5-hexatriene (Aldrich) were used as obtained and contained an unknown mixture of cis and trans isomers.
Results In all of the experiments the 1,3,5-hexatriene is initially excited with an unfocused 250 nm pulse with an energy of about 2 pJ. The intensity of the excitation pulse is kept low so that it produces very few ions by itself. To observe the initial internal conversion process, we use an unfocused ionization pulse at either 350 or 310 nm to create parent ions. The ionization pulses are not resonant with any transitions in the unexcited sample and also are at wavelengths long enough that two-photon ionization of the sample is not energetically possible. Therefore, they produce little ionization by themselves. As can be seen from the energy level diagram in Figure 1, the photon energy of the excitation pulse at 250 nm plus the photon energy of the ionization pulse at 350 nm just exceeds the ionization
J. Phys. Chem., Vol. 99, No. 20, 1995 7899
Ultrafast Internal Conversion in 1,3,5-Hexatriene
o.2v
5 0.4
0.4
"-*
0
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-0.2 -500
1 I
0
I 1
1 1
500
1000
I
* * * **********
0.2
*
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1500
Time Delay (fsec)
1
Figure 2. Normalized parent ion yield vs time delay between the 250 nm excitation pulse and the unfocused 350 nm ionization pulse.
potential39 of 1,3,5-hexatriene by about 0.3 eV. In Figure 2 the ion yield for the parent 1,3,5-hexatriene mass is shown as a function of the time delay between the excitation and ionization pulses. A short spike of ion production is observed only when the two pulses overlap in time. The ionization laser wavelength is near the energetic threshold for ionization of the excited state so only the initially excited electronic state is ionized. The observed signal peak is symmetric about zero delay with a width comparable to the excitation laser pulse width, indicating a very short initial state lifetime. This result is consistent with the estimated excited state lifetime from spectroscopic measurements of -50 fs. A significantly longer lifetime would produce an asymmetry in the ion yield vs time delay plot. Essentially identical results are obtained when the same experiment is performed using the 310 nm ionization pulses. The absence of any significant ion yield when the ionization pulse is delayed indicates that a single 3 10 nm photon does not have enough energy to efficiently ionize the intemal conversion products. The total energy in the neutral molecules after excitation is fixed, so it is energetically possible to ionize them with 310 or 350 nm light at any time delay. However, the products of the intemal conversion process are vibrationally excited molecules, and as the vibrational energy in the neutral molecule increases, it is expected that the photon energy needed for efficient ionization will increase. The effective ionization energy increases because the ionization process must provide additional energy to yield vibrationally excited ions that have favorable Franck-Condon factors with the vibrationally excited neutral molecules. This effect has been previously discussed for the photoionization of vibrationally excited triplet-state molecules following intersystem crossing.26 similarly, to observe internal conversion products in the experiments reported here, more photon energy is needed in the ionization step than was needed to ionize the initially excited molecules. In the present experiments we accomplish this by increasing the intensity of the -8 pJ ionization pulse by focusing with a 1 m lens into the ionization region so that multiphoton absorption becomes probable. The intensity is kept low enough that multiphoton ionization of the unexcited sample is negligible. A time delay scan between the excitation pulse and focused 350 nm ionization pulse while monitoring the parent ion yield is shown in Figure 3. The 1,3,5-hexatriene can now be ionized for more than a picosecond after excitation but eventually evolves to a condition where it is no longer efficiently ionized. The time delay scan taken while monitoring parent ion yield has contributions from both multiphoton (above threshold) ionization of the excited
0.8 -
(a) Parent mass peak
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7900 J. Phys. Chem., Vol. 99, No. 20, 1995 I
I
Hayden and Chandler I
1
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I
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I
500
,
~
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fragment ion yield. The solid line is the parent ion yield with the unfocused 350 nm ionization pulse from Figure 2, included here for reference. addition to the much larger parent mass peak, extensive fragmentation is observed with ion peaks seen at flight times corresponding to removal of 1-5 carbons from the molecule. In this manner we can distinguish between ions produced by single and multiphoton ionization. Fragment ions can only have been produced by absorption of more than one photon, in addition to the excitation photon, because the fragmentation process is not energetically accessible in absorption of a single probe photon. It is important to note that the vertical scale for the signal is the same for the two mass spectra. Thus, the total fragment ion production when the ionization pulse immediately follows the excitation pulse is much greater than the parent ion yield when the ionization pulse precedes the excitation pulse. This shows that the fragment masses observed must come from multiphoton ionization of the evolving, excited neutral molecule and not from fragmentation of ions produced by the excitation pulse. Scans of ion yield vs time delay between the 250 nm excitation and focused 350 nm ionization pulses were taken while monitoring each of the major fragment ion masses observed in the mass spectrum. Figure 5 shows a time delay scan taken detecting fragment ions containing four carbon atoms. The parent ion yield as a function of delay was measured at the same time with a separate gated integrator. The parent ion yield is superimposed with the fragment ion yield in Figure 5, and since these two measurements were taken simultaneously there is no possibility of time delay drift between the plots. By comparison of these plots, it can be seen that the rise time of the fragment ion yield is delayed from the rise of the parent ion yield and the peak fragment ion yield occurs at a time delay of about 150 fs. The plot of parent ion yield vs time delay with the unfocused 350 nm ionization pulse is included in Figure 5 to emphasize that at the time delay where the fragment ion yield peaks (150 fs), the parent ion yield from single photon ionization is already small. Thus, molecules that are excited then single-photon ionized almost immediately (< 100 fs delay) do not efficiently absorb additional photons and produce fragment ions at the ionization intensities used in this experiment. It is only after the initially excited neutral molecules have evolved for a short time that they can be efficiently multiphoton ionized to produce fragment ions. The time delay plots of the fragment ion yield also clearly show
I 1
I I
500
1000
1500
Time Delay (fsec)
Time Delay (fsec) Figure 5. Ion yield vs time delay for 250 nm excitation pulse and 350 nm ionization pulse. The open squares show the parent ion yield with the focused ionization pulse as in Figure 3, and the closed circles, which were recorded simultaneously, are the four-carbon
I
I
0
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-0.2 -500
2
Figure 6. Ion yield vs time delay for 250 nm excitation pulse and focused 350 nm ionization pulse. The two-carbon ion fragments are
detected. 8 . 6
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0
500
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1500
2000
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Time Delay (fsec) Figure 7. Ion yield vs time delay for 250 nm excitation pulse and the
focused 310 nm ionization pulse. The open squares are the parent ion yield, and the closed circles are the four-carbon fragment ion yield. Note the longer time scale on the figure to illustrate that the fragment ion yield does not retum to the baseline. that the excited neutral molecules continue to evolve for more than 1 ps, after which time we are no longer able to efficiently ionize them. We have taken time delay scans as described above, while detecting ions from each of the prominent groups of ion peaks seen in the time-of-flight mass spectrum of Figure 4. For example, the ion yield as a function of time delay while detecting the two-carbon fragment ions is shown in Figure 6. It can be seen that the results for this fragment ion are very similar to those for the four-carbon fragments shown in Figure 5. In fact, within the time resolution of these experiments, each of the fragment ions observed has the same ion yield vs time delay as that of the four-carbon fragments. This result suggests that all of the fragment ions come from a common neutral precursor since the relative yields of the various fragments are not dependent upon when in the time evolution of the neutral molecule ionization occurred. We have also performed equivalent experiments to those described above using 310 nm as the ionization laser wavelength. The ion yield vs time delay plot for the four-carbon fragments using 310 nm ionization is shown in Figure 7 (note the change in scale for the time delay). As can be seen, the observed time evolution is essentially the same whether 3 10 or 350 nm ionization laser pulses are used. This is true no matter which fragment mass is detected. The one difference between
Ultrafast Intemal Conversion in 1,3,5-Hexatriene the data taken with 350 nm ionization and that with 310 nm ionization is that at the shorter wavelengths weak ionization of the final, intemally converted products is observed. In these data no time evolution of the product is observed after about 2 ps as indicated by the constant ion yield for longer time delays. In longer time scans final product ionization is seen to persist for as long as we have measured, out to 6 ps.
Discussion There are two different processes within the excited neutral molecule that might produce the time delay dependent ion yields observed in these experiments. One process is the evolution of vibrational excitation in the molecule that changes the Franck-Condon factors between the excited neutral molecule and ion vibrational states. This could be thought of as the propagation of a vibrational wave packet. The other possible process is the buildup and decay of population in an actual intermediate state, such as another electronic state. The evidence from the experiments reported here, combined with results from other studies, supports the hypothesis that an intermediate electronic state is populated. The time evolution of the excited neutral molecule detected using multiphoton ionization is nearly independent of both the fragment ion detected and the ionization wavelength used. This strongly suggests that an actual intermediate species in the intemal conversion process is being observed. If a vibrationally excited intermediate is formed as the excitation in the neutral molecule evolves and this intermediate undergoes multiphoton ionization, then highly vibrationally excited ions produced can subsequently fragment into the pattem of masses detected. The yield of any fragment ion as a function of time between the excitation and ionization pulses would reflect the buildup and decay of the population of the same neutral intermediate state. Thus, the ion yield vs time delay results would be independent of the ionization pulse wavelength or the fragment ion detected, as we see in our data. The appearance of the intermediate is delayed by less than 100 fs from the initial optical excitation of the molecules, consistent with the very fast decay of the initially excited electronic state inferred from line width determinations. While we have no direct evidence of this, a logical candidate for an intermediate is the 2IA, excited state. (Note again that this is the symmetry label for the trans isomer, the corresponding state for the cis isomer is actually 2lA1.) Intemal conversion from the llB, state to the 2IA, state is consistent with what is seen in other polyenes, although the process would have to be much faster in hexatriene to explain the lack of fluorescence. This pathway is quite plausible in 1,3,5-hexatriene now that the 2'Ag state is known to be located 5000 cm-' lower in energy than the l'B, stateI8 (at least in the cis isomer). The rapid decay of the intermediate seen in the present experiments would explain the inability in previous experiments to observe fluorescence from the 2IA, state following excitation of the 1lB, state. The 2IA, state will have a relatively long radiative lifetime (>50ns in longer polyenes)I9 since the transition to the ground state is forbidden. Therefore, if this state were an intermediate in the intemal conversion with a 250 fs lifetime, the fluorescence quantum yield from it would be less than 5 x In recent studies where the 2'Ag state was directly excited in the cis isomer, fluorescence was observed when the excitation was near the origin of the electronic transition, but no fluorescence was observed from excitation more than 300 cm-' above the origin.I9 Thus, the optically accessible states in the 2IA, electronic manifold in 1,3,5hexatriene become short lived as the excitation energy is increased above the transition's origin. Intemal conversion from
J. Phys. Chem., Vol. 99, No. 20, 1995 7901 the lIB, state would produce molecules in the 2'A, state with at least 5000 cm-' of vibrational energy, so a very short lifetime in that state is expected. The fast nonradiative decay of the 2IA, state in 1,3,5-hexatriene has been postulated to be due to a very low barrier for torsion about the central C-C bond.I9 We have modeled the time evolution of the fragment ion signal using simple assumptions about the buildup and decay of a neutral intermediate that is detected as fragment ions following multiphoton ionization. The intermediate population, p(r), produced from a laser pulse at t = 0 is modeled as rising due to the exponential decay of the initial state followed by its own exponential decay. Standard treatments of this situation give40
where t r is the rise time constant for the intermediate and td is the decay time constant. If we model the excitation pulse by a Gaussian with full width at half height, twe, then the time evolution of the intermediate population, P(t), produced by this finite length pulse is given by P(t) =
m
p(t - t')e-ue'
dt'
In 2 a, = 47 twe
The detected signal is calculated for a Gaussian probe pulse of width, tw,, assuming the rate-limiting step in the ionization is nonresonant two-photon ionization. Therefore, the ionization probability depends on the square of the instantaneous probe pulse intensity. (The assumption of two-photon ionization is not crucial to the model because the convolution with the probe pulse has a fairly small effect on the calculated result. A higher order dependence on probe pulse intensity will only reduce the effect of the convolution.) The signal as a function of time delay, t, between the excitation and probe pulses is given by
4 In 2 a, = tw;
The signal given by the model was calculated numerically, and the result is shown in Figure 8 for tr = 65 fs, td = 250 fs, tw, = 200 fs, and tw, = 150 fs. The calculation is shown with the data of Figure 5 for the four-carbon fragment ion. The fast, 65 fs, rise time is consistent with the decay time of the initially excited state inferred from line width measurements. There is no reason to presume that the actual buildup and decay of the intermediate population are truly exponential, but these experiments are not sensitive to the detailed form of the time evolution because the rise time is within the length of the excitation laser pulses used. The rise time used in the model should only be considered an estimate. To give some idea of the sensitivity of the model to the rise time, the dashed line in Figure 8 shows the calculated result with a 130 fs rise time. The 250 fs decay time is more accurately determined because it is longer than the pulse widths and we can observe the signal for several decay time constants.
7902 J. Phys. Chem., Vol. 99, No. 20, 1995
Hayden and Chandler
, * . l
-0.2 -500
0
500
1000
1500
Time Delay (fsec) Figure 8. The closed circles are the four-carbon fragment ion yield vs time delay for 250 nm excitation and the focused 350 nm ionization pulse from Figure 3. The solid curve is the result of the model calculation descibed in the text with fr = 65 fs, fd = 250 fs, tw, = 200 fs, and tw, = 150 fs. The dashed line shows the result of the calculation with fr = 130 fs while all the other parameters are kept the same.
The model used in the discussion above assumes that we are observing the rise and decay of the population in an intermediate species in the internal conversion. If no actual intermediate is involved, then the observed time evolution must simply reflect the changing Franck-Condon factors between the evolving neutral molecule and vibrational states of the ion. For multiphoton ionization with a pulse at a fixed wavelength the evolving vibrational excitation in the neutral molecule may only have favorable Franck-Condon factors with accessible ion states for short period of time. As the excitation passed through this time period, the ion yield from multiphoton ionization could increase then decrease as seen in the data. However, in this case, we expect that changing ionization wavelengths, and thus accessing a different range of ion states, would be likely to change the observed ion yield vs time delay or the distribution of ion fragment masses as a function of time delay. No such changes are seen in the data when the ionization wavelength is changed. The present experiments cannot rule out the possibility that only a particular range of ion states are accessible from multiphoton ionization of the excited 1,3,5-hexatriene molecule at either wavelength used, and hence, changing the ionization wavelength does not produce a significant change in the results. The decay time observed for fragment ion yield may reflect the decay of the intermediate electronic state, but it also places a lower limit on the time needed for the energy deposited into the molecule to become statistically distributed. Once a truly statistical distribution is established, we would expect to see no further time evolution in our experiments other than the slow (on this time scale) decay due to cyclization and unimolecular elimination of H2 to form benzene. Collisional quenching experiments by Orchard and Thrush' showed that the halfquenching pressures for benzene formation in the photolysis of 1,3,5-hexatriene varied from 0.05 to 0.7 kPa depending on the quencher. Thus, the unimolecular reaction to form benzene must occur on the nanosecond time scale. In the experiments reported here all of the observed signals show little change after a delay time of about 1.5 ps, so this is a lower limit on the time required to reach a statistical energy distribution. However, at both of the ionization wavelengths used so far we detect little signal from the intemal conversion products after 2 ps so there may be some additional evolution of the products that is not detected by these experiments. Additional experiments are needed to determine when the final statistical vibrational energy distribution is actually reached.
An important question in this work is whether the observed signals reflect dynamics that are specific to a particular 1,3,5hexatriene isomer (cis or trans). Line widths obtained from direct absorption10and resonance Raman cross section measurements"%'2around 250 nm indicate that the decay of the the initial electronic excitation is faster in the cis isomer than in the trans isomer. Thus, there is evidence that at least for very short times after the excitation the dynamics of the two isomers are different. The experiments reported here primarily explore the evolution of the molecules after the decay of the initial electronic excitation. Previous experiments do not provide much information about this process. The two-photon i ~ n i z a t i o n and ~ ~ Jlaser~ induced fluorescence studiesI9 where the 2'Ag state of the cis isomer is directly excited have shown that there is a fast nonradiative decay mechanism from this state when it is excited more than a few hundred wavenumbers above the origin. These studies have also found that the corresponding 2'A, state of the trans isomer is not detected, so the decay rates of the isomers in these electronic states cannot be compared. The inaccessibility of the 2'A, state via single photon excitation in all-trans isomers of polyenes is consistent with the strongly forbidden nature of the transition for molecules with inversion symmetry and has been reinforced by recent onephoton resonant, two-photon ionization studies of 1,3,5,7~ctatetraene.~' In our experiments both cis and trans isomers present in the sample are excited via the transition to the l'B, state. The extinction coefficients for absorption to the l'B, state are similar' so excitation of a particular isomer should not be preferred. Photoionization of the excited molecules in either the l'B, or the 2'Ag state (assuming it is the observed intermediate in the internal conversion) is also not expected to selectively detect a particular isomer. Studies of 1,3,5hexatriene cations have observed both cis and trans isomer^^^.^^ so the structure of the ion does not preclude efficient ionization of one of the isomers. In fact, the photoelectron spectra of the isomers are very ~ i m i l a r ?indicating ~ , ~ ~ that they undergo similar geometry changes upon ionization. Previous two-photon nonresonant ionization measurements of ionization thresholds for 1,3,5-hexatriene have also clearly detected both isomers.I3 Therefore, we conclude that in our experiments both isomers are excited and likely to be detected, in contrast to the spectroscopic measurements mentioned above. Samples of mixtures of hexatriene isomers have typically been found to contain 60-70% trans isomer. Our experiments were performed using several different samples over many months with no difference in results, so there is no reason to believe the samples were unusual. As a result, the signals in these experiments are expected to be primarily characteristic of the trans isomer but with some contribution from the cis isomer. Since the signal decays on a less than 1 ps time scale, this is an upper limit on the decay time of the intermediate from both isomers. It is possible that one of the isomers has a shorter intermediate lifetime, but we see no evidence of a two-component decay in the data. These results also cannot rule out the possibility that due to a completely different internal conversion pathway one isomer produces no detectable intermediate at all, but this seems unlikely in view of the similarity of the electronic structures. The interpretation most consistent with the data is that we observe both cis and trans isomers, and, with the time resolution of these experiments, they exhibit similar behavior following excitation. Now that we have developed a method to detect the evolution of these molecules, it may be feasible to look at individual isomeric samples and see if differences can be detected. Time-resolved photoelectron spectra, which have very
J. Phys. Chem., Vol. 99, No. 20, 1995 7903
Ultrafast Intemal Conversion in 1,3,5-Hexatriene recently been measured,44 will also provide more information about the intermediate that is observed.
Conclusions These experiments show that while the initial step in the internal conversion of 1,3,5-hexatriene following excitation around 250 nm occurs in less than 100 fs, the excitation in the molecule continues to evolve for at least 1.5 ps. The intemal conversion appears to proceed through an intermediate, which may be the 2'A, excited state with its origin 5000 cm-' below the l'B, state that is optically excited in these experiments. The population of the apparent intermediate is observed to rise with less than a 100 fs delay from the excitation of the molecules and has a decay time constant of about 250 fs. The recent spectroscopic characterization of the 2'A, state should make it possible for further time-resolved experiments to determine if this state is actually involved in the internal conversion. The femtosecond, time-delayed ionization method used in these experiments has proven to be a powerful tool for studying the dynamics of ultrafast intemal conversion in gas-phase molecules. It will help to provide a much more complete understanding of these important radiationless processes.
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