Ring-Closing and Dehydrogenation Reactions of Highly Excited cis

Feb 10, 2011 - It stands to reason that the stilbenes excited to electronic states of higher energy .... If there is a time lapse between the excitati...
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Ring-Closing and Dehydrogenation Reactions of Highly Excited cis-Stilbene: Ultrafast Spectroscopy and Structural Dynamics Jie Bao, Michael P. Minitti, and Peter M. Weber* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States ABSTRACT: The ultrafast dynamics of highly excited cis-stilbene (CS) in a molecular beam is explored using femtosecond time-resolved mass spectrometry and structuresensitive photoelectron spectroscopy. cis-Stilbene is initially pumped by a 6 eV photon to the 71B state and the reaction is followed by ionization with a time-delayed 3 eV probe pulse. Upon excitation, cis-stilbene rapidly decays to the 31B state, where it undergoes a ring-closing reaction to form 4a,4b-dihydrophenanthrene (DHP). Whereas 14% of the ionized CS molecules dissociate one hydrogen atom to form hydrophenanthrene, the ionized DHP molecules completely dehydrogenate in the ion state to produce hydrophenanthrene and phenanthrene with a 1:1 ratio. We determined the lifetimes of the 71B state and the 31B state of CS to be 167 and 395 fs, respectively.

’ INTRODUCTION To chemists and laser spectroscopists alike, cis-stilbene is a most fascinating model of a photoreactive molecule,1-4 as it can react in several interesting ways.4-7 The frequently studied photoisomerization between trans-stilbene (TS) and cis-stilbene (CS) involves a conical intersection (CI) between the 11B and S0 states respectively, with a perpendicular geometry that is in between the pure trans- and cis-structures.3,8-10 With a lifetime of about 160 fs at the perpendicular minimum on the 11B state, the molecule crosses a conical intersection to the S0 surface to form either TS and CS with a 1:1 branching ratio.11,12 This photoisomerization process, which is utilized in numerous naturally occurring processes, represents not only an efficient way of harvesting photon energy but also of converting it to mechanical forces.13 As a result, there is an intense interest in developing molecular motors, machines, photo switches, and even nanoscale robots based on these photoactive compounds.14,15 Unlike the trans- form, cis-stilbene has a second reaction pathway that leads to a closed-ring product, 4a,4b-dihydrophenanthrene (DHP).16-18 Upon excitation to the 11B state, about 70% of the cis-stilbene moves toward the cis-trans CI and about 30% proceeds to a cis-DHP CI. From those intersections, cis-stilbene and DHP are reformed in about a 2:1 ratio.11,12 This additional pathway makes the photochemistry of cis-stilbene considerably more complicated than that of trans-stilbene. Even after extensive study,15-17,19,20 our knowledge is limited because DHP is difficult to detect, because the electronic states involved are very short-lived, and because DHP itself is short-lived on account of subsequent dehydrogenation reactions. It stands to reason that the stilbenes excited to electronic states of higher energy than those typically studied will exhibit interesting photochemical dynamics as well. Yet our understanding of the photochemical reactions on such higher excited states, of their pathways, transition points, and life times, remains quite rudimentary. As part of a series of studies of photochemical reactions on the highly excited states, we present here our results on cis-stilbene excited to an electronic state that lies significantly higher than the commonly studied 11B state. r 2011 American Chemical Society

Specifically, we trigger the reaction using 6 eV photons that bring cis-stilbene to the 71B state, and then monitor the progress of the reaction with ultrafast time-resolved mass spectrometry and structurally sensitive photoelectron spectroscopy. Interestingly, our studies show that highly excited cis-stilbene behaves quite differently than on its 11B state: it decays rapidly to the 31B state, where only the ring-forming reaction to DHP is seen. We have also captured the dehydrogenation reactions of DHP to hydrophenanthrene (HPT) and phenanthrene (PT), but those processes take place on the ion surface subsequent to ionization.

’ EXPERIMENTAL METHODS The experimental setup for this study has been described previously.21-24 Briefly, cis-stilbene was heated to 80 °C, seeded into a stream of helium carrier gas and expanded through a 100 μm nozzle orifice and a 150 μm skimmer. The molecular beam is crossed perpendicularly by the ultrafast pulsed lasers. Multiphoton ionization creates photoelectrons and photoions that were collected by a timeof-flight photoelectron spectrometer and a time-of-flight mass spectrometer, respectively. The ultrafast laser pulses were produced by a regeneratively amplified laser system with a near-IR tuning range between 760 and 840 nm, operating at a 5 kHz repetition rate, and pulse durations of about 100 fs. For these experiments, the system was optimized to operate at 828 nm with pulse energies of approximately 200 μJ in the infrared. The output beam was upconverted with BBO crystals to the second (2ω, 414 nm, 3.0 eV) and fourth (4ω, 207 nm. 6.0 eV) harmonics. Through appropriate focusing the intensities at the interaction region were kept on the order of 1012 W/cm2 and 1010 W/cm2 respectively. cis-Stilbene was first pumped by a 4ω laser pulse and then given a controlled time to allow any reaction to take place. The photoionization from the excited state by a 2ω laser pulse produces the photoelectrons and photoions. By varying the time delay of the probe pulse with respect to the pump pulse, information about the Received: October 4, 2010 Revised: January 5, 2011 Published: February 10, 2011 1508

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Table 1. Computed Wnergies of the Molecular and Ionic Ground States of cis-Stilbene and Its Reaction Productsa

Figure 1. cis-Stilbene mass spectrum, summed over all time delays. The insert shows the expanded mass 180 region.

molecular energy flow and the fragmentation dynamics is obtained. The contributions to the time-dependent photoelectron and photoion signals from single pulse, direct ionization processes were separately measured and subtracted from the pump-probe signals to obtain the pure two-color signals. The energetics for the involved molecular species were calculated using density functional theory (DFT) at the B3LYP/6-311þG(d,p) level. For each species, energies of both the molecular ground state (S0) and the ion ground state (D0) were calculated. All calculations were performed with the Gaussian 03 package.25

’ RESULTS AND DISCUSSION The mass spectrum (MS) of cis-stilbene, summed over all time delays, is shown in Figure 1. The dominant peak around mass 180 is expanded in the inset. Three mass peaks, at 180.2, 179.2, and 178.2, are almost equally intense in this time-averaged spectrum. Because cis-stilbene parent ions have a mass of 180.2, the 179.2 and 178.2 peaks must stem from dehydrogenation products where one or two hydrogen atoms are lost, respectively. cis-Stilbene can isomerize to a fully cyclic structure by closing a bond between the two aromatic rings, resulting in 4a,4b-dihydrophenanthrene (DHP), which has the hydrogen atoms of the 4a and 4b carbon atoms in trans position. A closed structure with the hydrogens in cis positions is also possible (cis-DHP), but the ring distortions elevate the energy of that molecular form. Table 1 lists the calculated energies of the molecular ground state and the ground state of the respective singly charged ions. Loss of a hydrogen atom is possible from the fully cyclic structure, resulting in hydrophenanthrene (HPT), or from the open CS structure, resulting in dehydrogenated cis-stilbene (DH-CS). Because those ions are closed-shell molecules, their energy is only about 1.5 eV above that of the CSþ ion. Finally, loss of two hydrogen atoms leads to phenanthrene (PT). The time-resolved mass spectrum (Figure 2, solid lines) shows a sequential generation of CS, CS-H and CS-2H: the parent CS ions arrive first, followed by CS-H; the fully dehydrogenated product CS2H arrives last. Because the molecules absorb two probe photons in the ionization step (Discussion below), there is sufficient energy to fragment the molecules in additional ways. Specifically, some of the molecules lose CH3 to produce mass 165.3, which has the same temporal profile as mass 179.2. The C-C bonds between the ethylenic group and the phenyl groups can also be broken to produce C6H5 (mass 77.1) and C8H6 (mass 102.1), whereas the phenyl groups can further decompose to various fragments such as C4H3 (mass 51.1) and C3H3 (mass 39.1). These fragments have a similar timedependence as the mass 178.2 ions.

a

All energies are relative to the ground-state energy of cis-stilbene. b The energy of a H atom (-0.500 Ha) is added for mass 179.2 species. c The energy of an H2 molecule (-1.165 Ha)27 is added for phenanthrene. When two hydrogen atoms are added to PT instead, energies of 12.25 and 4.65 eV result for D0 and S0, respectively.

Figure 2. Top: Time-resolved transients of cis-stilbene’s major MS peaks (solid lines) and the PES peaks (symbols). For easier visual comparison, all transients are normalized to the same vertical scale. Bottom: An expanded view of the same data for the region of time delays between -0.25 to 0.6 ps.

The photoelectron spectrum (PES) of cis-stilbene, Figure 3, shows three major peaks with binding energies (at time zero) of 2.41, 2.03, and 1.55 eV, respectively. They all have very fast decay times. The photoelectron spectrum also features a broad background, also with a 1509

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Scheme 1. Excitation and Reaction Schemes for cis-Stilbenea

Figure 3. Photoelectron spectrum of cis-stilbene. The pump-probe photoelectron signal (one-color signals subtracted) is plotted against the delay time, with the color representing the natural logarithm of the photoelectron intensity. The binding energy (BE) of the photoelectrons is the difference between the energy of the ionizing photons and the energy of the ejected electrons.

very short lifetime, suggesting an involvement of a valence state. On the basis of the energy levels and the oscillator strength computed by Molina et al.,26 the broad excitation is attributed to the direct population of the 71B state. Molina’s calculation details the molecular orbital configuration for this excitation, showing it to be a core excitation, primarily involving a transition from HOMO-1 to LUMOþ1. Because of its core excitation nature, this state is expected to decay very rapidly by relaxing an electron from the HOMO to HOMO-1. This leads to an excited state with a hole in the HOMO and an electron in LUMOþ1, which Molina’s work identifies as the 31B state. The excitation energy may further relax to the ground state through other intermediate states, resulting in vibrationally hot ground-state molecules. To further understand the molecular reaction pathways, we take a close look at the time dependencies of the three major mass peaks and the three major photoelectron peaks. Although the dynamic processes are very fast and the transient signals are very similar to each other, Figure 2 shows that there are important differences between them. The following features are worth noting. First, the mass peak of the cis-stilbene parent (MS 180.2) arrives the earliest, before all other peaks, including the photoelectron peaks. Second, this mass peak also decays much faster than all the other peaks. Third, the other two mass peaks arrive in a sequential fashion. Fourth, the three PES peaks appear in a less distinct, but noticeably time-delayed manner. And last, the cis-stilbene parent ions approach a level below zero after about 1 ps, whereas the signals of the other two mass peaks stay above zero. These pump-probe signals have the one-color signals subtracted, giving rise to the possibility of negative signals. Based on these experimental results, photochemical processes as illustrated in Scheme 1 are proposed. Upon photoexcitation, CS is elevated to the core excited state 71B state. When immediately ionized by one 3 eV probe photon, one photon suffices for the ionization but subsequent absorption of a photon can fragment the molecule. If there is a time lapse between the excitation pulse and the ionization pulse, the molecule may convert to the 31B surface, where it closes the ring to form DHP that is ionized by absorption of two photons. The ensuing fragmentation leads to HPTþ and PTþ. As outlined in the Appendix, we performed a quantitative modeling of the ionization and photodynamics pathways. From the usual deconvolution of the temporal profiles from the instrument function, we obtain the dynamics time scales (Figure 4). It is noteworthy that in addition to the time constants, the fit quantitatively accounts for the

a

CS is initially excited to its 71B state. One probe photon is sufficient to ionize from there, however some of the cis-stilbene ions can further absorb an additional probe photon, leading ultimately into a dehydrogenation channel to form HPT (about 14% of the CS ions convert to HPT at this point, see below). The 71B state decays very rapidly (τ1 = 167 fs) to its 31B state, on which CS quickly cyclizes to form DHP. DHP further absorbs two probe photons to be ionized via the Rydberg states. On the ion surface, the molecules dehydrogenate to form HPT (49%) and PT (51%). DHP decays out of the 31B state with a lifetime τ2 (395 fs) and ultimately relaxes back to the ground state via a conical intersection on the 11B state. The decay time constants and the product ratios resulted from a fitting analysis as described in detail in the Appendix.

Figure 4. Original data points and fits for the mass peaks at 180.2, 179.2, and 178.2. Plotted in discrete symbols are the original data, whereas the solid lines are the fits. All data are normalized with respect to the maximum value of the MS 180.2 peak. 1510

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The Journal of Physical Chemistry A Scheme 2. cis-Stilbene Reaction Summary

intensities of the one-color signals as well as the two-color signals. We are therefore able to obtain the branching ratios of the various pathways. Upon ionization of CS without time delay, the ground-state (D0) CSþ ions may absorb an additional probe photon, giving the ion sufficient energy for dehydrogenation. Although the direct dehydrogenation product should be dH-CSþ as has been depicted in Scheme 2, our calculation suggests that a ring closing reaction from dH-CSþ to HPTþ is likely to take place on the ion surface (below). The loss of the hydrogen radical removes a significant amount of vibrational energy from the molecular ion, so that it no longer has enough energy to further dehydrogenate to form PTþ. Therefore, only HPTþ results at this step. As a net effect of the absorbed extra probe photon, it is estimated based on the quantitative fit (Table 2) that about 14% of CSþ dehydrogenates to form HPTþ. Meanwhile, CSþ might also close a ring to form DHPþ. However, our data provides no evidence to prove or disprove such reaction. Because CSþ and DHPþ have the same mass, mass spectrometry will not reveal this reaction. Nor can photoelectron spectroscopy show it because the reaction would take place after ionization. The 71B state of CS quickly decays to the 31B state with a decay time of 167 fs. On the 31B state, CS very rapidly closes the ring to form DHP as shown in Schemes 1 and 2. However, because the rapid reaction along the steeply sloped molecular energy surface quickly converts electronic energy to vibrational energy, the molecule now requires two probe photons to ionize. This ionization is facilitated by an intermediate set of Rydberg states, which are observed as three distinct peaks in the photoelectron spectrum. Ionization from the highly vibrationally and electronically excited DHP results in DHPþ ions that are unstable and dehydrogenate to HPTþ and PTþ. As discussed in before, once HPTþ is formed it cannot further dehydrogenate to form PTþ because of its limited total energy after the first dissociation. However, DHPþ is able to generate PTþ in a parallel process by dissociating an H2 molecule. The quantitative fit determines this branching ratio to be 49% for HPTþ and 51% for PTþ. The molecule decays out of the 31B state with a lifetime of 395 fs. While we cannot follow the details of the reactions subsequent to this decay, one might speculate that CS reaches the 11B state and then decays further to the ground state via the well-known conical intersection linking the two surfaces.11 We pick up the signal of the vibrationally hot species on the ground state again because it can be ionized by multiple probe photons, as does the

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cold CS molecule. However, the extra vibrational energy leads a higher dehydrogenation probability of the molecule in its ion state compared to the cold molecule. This process is revealed by a depletion of the relevant species, the details of which will be discussed later. With the ionization and energy relaxation pathways established and quantitatively modeled, we can now proceed to discuss several further observations and conclusions: 1. Computations and Fractional Ion Yield Interpretations. The interpretations summarized in Schemes 1 and 2 are supported by and in agreement with the computational studies of the relevant species (table 1). First, the dehydrogenation reaction from the initially created CSþ to HPTþ takes place only after the absorption of an additional probe photon simply because there is a lack of energy for the reaction to take place otherwise. The ionization potential of HPT (9.89 eV) is well above the energy of one pump photon and one probe photon combined (8.98 eV), substantiating the need for the absorption of a additional probe photon to generate HPT. Second, instead of having dH-CSþ as the dehydrogenation product of CSþ, HPTþ is proposed in Scheme 1. This hypothesis is made on the basis of the computed energies of dH-CS and HPT, which suggest no significant barrier for dH-CSþ to react to the closed-ring product of HPTþ on the ion surface. Given the very long flight time of the generated ion to reach our detectors, it is therefore assumed that HPTþ is the detected species. As a result, a ring closing reaction of dH-CSþ to HPTþ is proposed as shown in Scheme 2. However, one may also postulate a reaction pathway for this process whereby CSþ reacts first to form the closed-ring product DHPþ, which then undergoes a dehydrogenation reaction to form HPTþ. While lacking enough evidence to discount this process, some results indicate that this ring-closing reaction with a subsequent dehydrogenation reaction is not likely. On the basis of the numerical fits (Appendix) used to quantitatively model Scheme 1, about 14% of CSþ converts to HPTþ on the ion surface, whereas no PTþ is produced there. However, when reacting from DHPþ on the ion surface, 49% of HPTþ and 51% of PTþ are produced, whereas DHPþ completely disappears. Whereas this large difference could be attributed to the additional probe photon absorbed by DHP, the structural difference between CSþ and DHPþ may play a role as well. If CSþ were to react to DHPþ first and then dehydrogenate, this structural effect could not be involved. However, we consider this evidence weak and suggest that further analysis is required. Third, as revealed in Scheme 1, our fits suggested no PTþ generation from CSþ when ionization is out of the 71B state. This is even though three possible pathways for PTþ generations might be proposed, namely to generate PT on the 71B state, to generate PTþ via the reaction to HPTþ, and to generate PT directly from CSþ after the additional photon absorption. Given those options, why is little or no PTþ generated in this process at all? First we note that the generation of PT on the 71B state does not seem likely as the lifetime for the 71B state (167 fs) is too short for such reaction. Second, generating PTþ subsequent to the generation of HPTþ is not possible because the sequential generation of HPTþ and PTþ implies the generation of two hydrogen radicals, which requires more than 12 eV of energy (Table 1). Therefore, absorption of a third probe photon would be necessary, which greatly reduces the overall probability of such a process. Finally, to form PTþ directly from CSþ, a H2 molecule has to dissociate at the same time as the ring closes. However, having so much energy in the vibrational manifold, the molecule 1511

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Table 2. Summary of the Data-Fitting Model and the Fit Resultsa process no. 1

2

1

ionization out of ... overall ion yield

3 1

... 7 B

...out of 3 B

baseline shift

(R1 þ β1 þ γ1)A0e-t/τ1

(R2 þ β2 þ γ2)A0τ2/

(R3 þ β3 þ γ3)A1/

(τ2-τ1)(e-t/τ2-e-t/τ1)

(1 þ e-(t-t1)/τ3)

Ri þ βi þ γi = 1

requirements fractional ion yield Ri (MS 180.2)

0.86

0

-1

βi (MS 179.2)

0.14

0.49

0.50

γi (MS 178.2)

0

0.51

0.50

Time Constants τ1/fs

τ2/fs

τ3/fs

t1/fs

A0

A1

167

395

209

744

3.39

0.369

a

All processes are convoluted with a Gaussian instrument function (σ = 142 fs) characterizing the laser pulses. Peak yields in the mass domain were deconvoluted to remove overlapping contributions. The three σ errors of the fitted parameters are 10% of their values.

is more likely to lose one hydrogen atom quickly, leaving no opportunity for the formation of a H-H bond. Schemes 1 and 2 suggest a parallel reaction from DHPþ to either HPTþ or PTþ. As explained above in the context of the reactions originating with CSþ, a sequential reaction to HPTþ and then to PTþ is energetically not possible. In addition to the very high energy requirement for the generation of two hydrogen radicals, the first ejected hydrogen atom is likely to take with it a large amount of energy, which further reduces the energy available for the generation of PTþ. Consequently, HPTþ and PTþ must be formed in parallel processes, and PTþ is created through the dissociation of a H2 molecule. Given the geometric separation of the two hydrogen atoms in DHP, such a dissociation might seem surprising. We propose two possible explanations. The first one involves the formation of cisDHP during the ring closing reaction of CS. As shown in Table 1, if instead of forming DHP (with the hydrogen atoms in trans position) on the 31B surface, the reaction leads to cis-DHP where the two hydrogen atoms are at the same side of the molecular plane, it would be much easier to generate H2. However, we consider the possibility of such process not to be very high because, at least in the molecular ground state and the ion state, cis-DHP is about 0.4 eV higher energy than DHP (Table 1). Additionally, the structural similarity between the geometries of CS and DHP is much higher than that of CS and cis-DHP, suggesting a higher barrier for the reaction to cis-DHP. As a result, the production to DHP is both thermodynamically and kinetically preferred. How is the dissociation of molecular hydrogen from DHP then possible? According to the recently popularized concept of hydrogen roaming,28 when the C-H bonds accumulate large amounts of vibrational energy, their stretching mode may get particularly excited such that the bond can extend to about 34 Å. At such large distances, the hydrogen atoms roam around the molecule in unpredictable paths. In DHP, at such large bond distances it would be easy for one H atom to reach the other. As a result, the roaming atom concept might provide a framework to understand the dehydrogenation of DHPþ to PTþ. Whereas a more detailed computational analysis of this scenario seems warranted, our experiments are unambiguous in the observation of the existence of the reaction. 2. Origin of the PES Peaks. It is tempting to assign by association the three distinct peaks in the photoelectron spectrum

to the three MS species CS, CS-H, and CS-2H, respectively. However, already the comparison of the transient behavior of the three major MS and PES peaks in Figure 2 shows that their lifetimes are not the same. Especially the earliest peaks, the MS180.2 and BE 2.41 eV, have a different dynamics: the mass peak comes earlier and decays much faster than the PES peak. Noting the very similar overall dynamics of all three PES peaks, and especially that they have the same lifetimes, one infers that they all come from the same excited state. If the three peaks were from a vibrational progression, one would expect them to have exactly the same time dependence. However, as shown in the lower panel of Figure 2, there are slight but detectable delays between the onsets of the three peaks, which are inconsistent with the assumption that they are a vibronic progression within the same state. Thus, multiple electronic states must be involved. We note that this conclusion is in contrast to the observation of vibrational progressions in trans-stilbene.21 To account for these detailed observations in the photoelectron spectrum of CS, we attribute the generation of the three PES peaks to a resonant multiphoton ionization out of 31B via different molecular Rydberg states. While ionization out of a highly excited valence states such as the 71B state has a very broad FC envelope, rendering the transition broad and featureless, the ionization out of a Rydberg state leads to a sharp peak at the binding energy of the Rydberg electron.23,29,30 The binding energy of electrons in Rydberg states can be approximated by the equation EB ¼

ERy ðn-δl Þ2

where EB is the binding energy, ERy is the Rydberg constant (13.6 eV), n is the principal quantum number, and δl is a constant called quantum defect that depends on the angular momentum, l. For smaller values of l, the Rydberg electron is closer to the positively charged ion core, resulting in a larger shift of the Rydberg energy level. This is observed as a greater quantum defect value. In the case of CS, the observed quantum defects of 0.63 (2.41 eV), 0.42 (2.03 eV), and 0.04 (1.55 eV) correspond to the energy levels that are reasonably assigned to the 3pz, (3px, 3py), and 3d orbitals. The appearance of the peaks in the photoelectron spectra is ascribed to two-photon ionization out of the 31B state via Rydberg 3pz, 1512

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Figure 5. Time-dependent center position the 3pz Rydberg peak.

(3px, 3py), and 3d states respectively. This explanation is consistent with the computational results of Molina et al.26 3. Time-Resolved Structural Dynamics of the Ring Closing Reaction of cis-Stilbene. As the three PES peaks are assigned to Rydberg states of the CS/DHP system, there remains no spectral signature that could correspond to the dehydrogenation reaction from DHP to HPT and PT. One would expect that different PES peaks, corresponding to HPT and to PT, would have to appear in sequential manner. However, no such new peaks have been found. Additionally, it seems unlikely that the dehydrogenation (especially the ejection of a H2 molecule) would occur within the short lifetime of the excited state. We thus conclude that the dehydrogenation must occur on the ion surface after the ionization. It is likely that as the CS ring closes to form DHP, a coherent wave packet travels along the 31B potential energy surface. As the reaction proceeds, molecules are transferred onto the ion state by the resonant two-photon ionization via the Rydberg states. Since the Rydberg state binding energy depends on the molecular structure we have been able to use their time dependence to map out molecular motions.23,34-36 In the present case, one would expect that the binding energy of the observed photoelectron peaks changes as the ring closes. Indeed, a close inspection of the photoelectron spectrum, Figure 3, shows that the peak positions do depend on time, Figure 5. Thus, it is evident that the ultrafast time-resolved photoelectron spectrum captures the wave packet as it evolves along the ring-closing coordinate from CS to DHP. Thus, the structural dynamics motions of the system during the ring-closing process are captured by the time-resolved Rydberg spectrum. 4. Further Analysis of the Transients. We are now ready to explain some additional features mentioned previously in the discussion of Figure 2. a). Mass Peak of the cis-Stilbene Parent Arrives before All the Other Transients. cis-Stilbene parent ions are generated as soon as the pump beam excites the molecule to the 71B state. Because the 71B is a valence state, the photoelectrons generated from its ionization are distributed throughout the high binding energy region of the PES in Figure 3. The photoelectron peaks of Figure 2 arise from the two-photon resonant ionization out of the 31B state, which result from the decay of the 71B state. It is clear that the PES peaks are delayed relative to that of the CS parent mass peak. As for the relative delay between the MS 180.2 peak and the other two main peaks in the MS, it can further be ascribed to the decay of the originally populated 71B state (which gives rise to MS 180.2), to the 31B state (which gives rise to the other mass peaks). Even though the dehydrogenation reactions take place on the ion surface after the formation of the DHP ion, the fragment mass peaks must come later then the parent mass peak of cis-stilbene.

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b). Parent Mass Peak Decays Much Faster than the Other Peaks. Ionization of CS out of its 71B state requires only one additional probe photon. However, as the molecule quickly decays to the 31B state and slides toward its DHP configuration, a large amount of electronic energy is deposited in the vibrational manifold. This conversion of electronic to vibrational energy not only implies that DHP requires two probe photons for ionization, but it also gives rise to DHPþ with so much internal energy that the dehydrogenation reaction on the ion surface to produce HPTþ and PTþ proceeds with unit yield. Therefore, as CS converts to DHP, it quickly loses its ability to generate parent mass ions. Thus, because cis-stilbene ions are only generated while the molecule is on the 71B surface, the parent peak decays much faster than the rest of the peaks in Figure 3. The measured lifetime of 167 fs, represents the lifetime of the 71B state. c). HPTþ and PTþ Mass Peaks Arrive Sequentially after the Parent. As shown in Figure 2, the onset of the PTþ mass peak is delayed relative to HPTþ peak. Following Scheme 1, we suggest that the generation of CS out of the 71B state may be accompanied by the further absorption of a photon on the ion surface, giving rise to an early generation of HPTþ ions. Because of the generation of HPTþ in this process is faster than the generation of HPTþ and PTþ from DHP, one observes an overall earlier onset of the HPTþ signal compared to PTþ. d). Three PES Peaks Rise in a Slightly Delayed Manner. This can be explained by the potential energy landscapes of the 31B state and the Rydberg states as shown in Scheme 1. Because of the lower energy level for the 3pz surface, the first probe photon is able to reach this surface just a little earlier than the other two, resulting in an earlier PES peak. This is the same for (3px, 3py) versus 3d. e). cis-Stilbene Parent Ion is Depleted after 1 ps whereas Those of the Other Two Mass Peaks Stay Above Zero. After some time, the excited DHP relaxes back to the ground state, possibly via the 11B conical intersection, and branches into CS and DHP. Those molecules that have undergone the excitation and decay processes are hot, containing 6 eV of vibrational energy. Upon subsequent ionization with three probe photons, these hot molecules dehydrogenate more completely than the cold molecules. When subtracting the one-color ionization contributions from the cold molecules, a depletion pattern of the CS parent ion results. The time-resolved mass spectrum, Figure 2, therefore shows a negative parent signal at delay times above about 1 ps. Conversely, the dehydrogenation products show a positive offset because their signals are enlarged by this process. Another possible process responsible for the depletion could be the additional absorption of probe photons by the CSþ generated subsequent to the two pump photon-absorption. However, it is not clear at this point what percentage contribution this process has on the overall depletion effect.

’ CONCLUSIONS In summary, we have performed ultrafast time-resolved studies of CS with TRPES and MS to explore the reaction pathways of highly excited CS. As summarized in Schemes 1 and 2, the 71B state of CS is reached when pumped with 6 eV photons, and only a single dehydrogenation reaction from CS to HPT is observed upon ionization out of the 71B state. However, the 71B state quickly decays to 31B, on which a ring forming reaction takes place that leads to the formation of DHP. This reaction to DHP is observed in real time by resonant two-photon ionization photoelectron spectroscopy, where the time dependence of the Ryd1513

dx.doi.org/10.1021/jp1095322 |J. Phys. Chem. A 2011, 115, 1508–1515

The Journal of Physical Chemistry A berg signal intensity and binding energy maps the structural motions of the wave packet. The time-resolved mass spectrum shows that both HPT and PT are formed on the ion surface directly from DHP via parallel reactions. Our results also reveal that the pathway of the ultrafast reaction involving conical intersections depends strongly on the electronic state on which CS is prepared: While on the 11B state, CS is known to react to both TS and DHP, upon excitation to its 71B state, we observe that it converts to the 31B state and reacts to DHP. Both CS and DHP feature dehydrogenation reactions on their ion state, although CS only partially reacts to produce HPT, whereas DHP completely dehydrogenates to produce both HPT and PT simultaneously.

’ APPENDIX: THE NUMERICAL DATA ANALYSIS With the model as explained by Schemes 1 and 2, the transients for the mass peaks at 180.2, 179.2, and 178.2 are fit simultaneously to extract the time constants for all processes. The model along with the fit results are summarized in Table 2. Three major ionization processes related to the generation of mass peaks at 180.2, 179.2, and 178.2 are summarized in Table 2. The first is the ionization out of the initially populated 71B state (including 4ω þ 2ω and 4ω þ 2ω þ 2ω processes as explained earlier), which features a single decay with a time constant τ1, which represents the decay from 71B to 31B. The second process is the ionization out of the 31B state, which is modeled as a sequential reaction with a rise time of τ1 and a decay time of τ2, representing the generation of 31B from 71B and its decay to a lower surface. The third process accounts for the shifts in the baselines of mass peaks 180.2, 179.2, and 178.2, which stems from molecules that have undergone a cycle of excitation and decay and which can no longer give parent ions (text). To simulate the multiple consecutive decay processes from the 31B state to the ground state, a sigmoidal function is used as illustrated by the equation under column 3 in Table 2, in which t1 and τ3 collectively represents the repopulation process to the ground state. In consideration of all these processes, the overall equations used to fit the three MS peaks are as follows:31 For MS 180.2: MS 180:2ðtÞ ¼ ðt