Published: February 10, 2011

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

ARTICLE

pubs.acs.org/JPCA

Ring-Closing and Dehydrogenation Reactions of Highly Excitedcis-Stilbene: Ultrafast Spectroscopy and Structural DynamicsJie 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 molecularbeam is explored using femtosecond time-resolved mass spectrometry and structure-sensitive photoelectron spectroscopy. cis-Stilbene is initially pumped by a 6 eV photon tothe 71B state and the reaction is followed by ionization with a time-delayed 3 eV probepulse. Upon excitation, cis-stilbene rapidly decays to the 31B state, where it undergoes aring-closing reaction to form 4a,4b-dihydrophenanthrene (DHP). Whereas 14% of theionized CS molecules dissociate one hydrogen atom to form hydrophenanthrene, theionized DHP molecules completely dehydrogenate in the ion state to produce hydro-phenanthrene and phenanthrene with a 1:1 ratio. We determined the lifetimes of the 71Bstate and the 31B state of CS to be 167 and 395 fs, respectively.

’ INTRODUCTION

To chemists and laser spectroscopists alike, cis-stilbene is a mostfascinating model of a photoreactive molecule,1-4 as it can react inseveral interesting ways.4-7 The frequently studied photoisomeriza-tion between trans-stilbene (TS) and cis-stilbene (CS) involves aconical intersection (CI) between the 11B and S0 states respectively,with a perpendicular geometry that is in between the pure trans- andcis-structures.3,8-10 With a lifetime of about 160 fs at the perpendi-cular minimum on the 11B state, the molecule crosses a conicalintersection to the S0 surface to form either TS and CS with a 1:1branching ratio.11,12 This photoisomerization process, which is uti-lized in numerous naturally occurring processes, represents not onlyan efficient way of harvesting photon energy but also of converting itto mechanical forces.13 As a result, there is an intense interest indeveloping molecular motors, machines, photo switches, and evennanoscale robots based on these photoactive compounds.14,15

Unlike the trans- form, cis-stilbene has a second reaction pathwaythat leads to a closed-ring product, 4a,4b-dihydrophenanthrene(DHP).16-18 Upon excitation to the 11B state, about 70% of thecis-stilbenemoves toward the cis-transCIand about 30%proceeds to acis-DHP CI. From those intersections, cis-stilbene and DHP arereformed in about a 2:1 ratio.11,12 This additional pathway makes thephotochemistry of cis-stilbene considerably more complicated thanthat of trans-stilbene. Even after extensive study,15-17,19,20 our knowl-edge is limited because DHP is difficult to detect, because theelectronic states involved are very short-lived, and because DHP itselfis short-lived on account of subsequent dehydrogenation reactions.

It stands to reason that the stilbenes excited to electronic states ofhigher energy than those typically studied will exhibit interestingphotochemical dynamics as well. Yet our understanding of the photo-chemical reactions on such higher excited states, of their pathways,transition points, and life times, remains quite rudimentary. As part ofa series of studies of photochemical reactions on the highly excitedstates, we present here our results on cis-stilbene excited to an electr-onic state that lies significantly higher than the commonly studied 11Bstate.

Specifically, we trigger the reaction using 6 eV photons thatbring cis-stilbene to the 71B state, and then monitor the progressof the reaction with ultrafast time-resolved mass spectrometryand structurally sensitive photoelectron spectroscopy. Interest-ingly, our studies show that highly excited cis-stilbene behavesquite differently than on its 11B state: it decays rapidly to the 31Bstate, where only the ring-forming reaction to DHP is seen. Wehave also captured the dehydrogenation reactions of DHP tohydrophenanthrene (HPT) and phenanthrene (PT), but thoseprocesses take place on the ion surface subsequent to ionization.

’EXPERIMENTAL METHODS

The experimental setup for this study has been described previ-ously.21-24 Briefly, cis-stilbene was heated to 80 �C, seeded into astream of helium carrier gas and expanded through a 100 μm nozzleorifice and a 150 μm skimmer. The molecular beam is crossedperpendicularly by the ultrafast pulsed lasers. Multiphoton ionizationcreates photoelectrons and photoions that were collected by a time-of-flight photoelectron spectrometer and a time-of-flight mass spec-trometer, respectively. The ultrafast laser pulses were produced by aregeneratively amplified laser system with a near-IR tuning rangebetween 760 and 840 nm, operating at a 5 kHz repetition rate, andpulse durations of about 100 fs. For these experiments, the systemwasoptimized to operate at 828 nmwith pulse energies of approximately200 μJ in the infrared. The output beam was upconverted with BBOcrystals to the second (2ω, 414 nm, 3.0 eV) and fourth (4ω, 207 nm.6.0 eV) harmonics. Through appropriate focusing the intensities atthe interaction regionwere kept on the order of 1012W/cm2 and1010

W/cm2 respectively. cis-Stilbene was first pumped by a 4ω laser pulseand then given a controlled time to allow any reaction to take place.The photoionization from the excited state by a 2ω laser pulse pro-duces the photoelectrons and photoions. By varying the time delay ofthe probepulsewith respect to the pumppulse, information about the

Received: October 4, 2010Revised: January 5, 2011

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molecular energy flow and the fragmentation dynamics is obtained.The contributions to the time-dependent photoelectron and photo-ion signals from single pulse, direct ionization processes were separ-ately measured and subtracted from the pump-probe signals toobtain the pure two-color signals.

The energetics for the involved molecular species were calculatedusing 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 calculationswere performed with the Gaussian 03 package.25

’RESULTS AND DISCUSSION

The mass spectrum (MS) of cis-stilbene, summed over all timedelays, is shown in Figure 1. The dominant peak aroundmass 180 isexpanded in the inset. Three mass peaks, at 180.2, 179.2, and 178.2,are almost equally intense in this time-averaged spectrum. Becausecis-stilbene parent ions have a mass of 180.2, the 179.2 and 178.2peaks must stem from dehydrogenation products where one or twohydrogen atoms are lost, respectively.

cis-Stilbene can isomerize to a fully cyclic structure by closing abond between the two aromatic rings, resulting in 4a,4b-dihydrophe-nanthrene (DHP), which has the hydrogen atoms of the 4a and 4bcarbon atoms in trans position. A closed structure with the hydrogensin cis positions is also possible (cis-DHP), but the ring distortionselevate the energy of that molecular form. Table 1 lists the calculatedenergies of the molecular ground state and the ground state of therespective singly charged ions. Loss of a hydrogen atom is possiblefrom the fully cyclic structure, resulting in hydrophenanthrene(HPT), or from the open CS structure, resulting in dehydrogenatedcis-stilbene (DH-CS).Because those ions are closed-shellmolecules,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 asequential generation of CS, CS-H and CS-2H: the parent CS ionsarrive first, followed by CS-H; the fully dehydrogenated product CS-2H arrives last. Because the molecules absorb two probe photons inthe ionization step (Discussion below), there is sufficient energy tofragment the molecules in additional ways. Specifically, some of themolecules loseCH3 toproducemass 165.3,whichhas the same temp-oral profile as mass 179.2. The C-C bonds between the ethylenicgroup and the phenyl groups can also be broken to produce C6H5

(mass 77.1) and C8H6 (mass 102.1), whereas the phenyl groups canfurther decompose to various fragments such as C4H3 (mass 51.1)and C3H3 (mass 39.1). These fragments have a similar time-dependence as the mass 178.2 ions.

The photoelectron spectrum (PES) of cis-stilbene, Figure 3, showsthree 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. Thephotoelectron spectrum also features a broad background, also with a

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

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

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

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

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very short lifetime, suggesting an involvement of a valence state. Onthe basis of the energy levels and the oscillator strength computed byMolina et al.,26 the broad excitation is attributed to the direct popu-lation of the 71B state. Molina’s calculation details the molecularorbital configuration for this excitation, showing it to be a core excit-ation, primarily involving a transition fromHOMO-1 toLUMOþ1.Because of its core excitation nature, this state is expected to decayvery rapidly by relaxing an electron from the HOMO toHOMO-1.This leads to an excited state with a hole in the HOMO and an elect-ron in LUMOþ1, which Molina’s work identifies as the 31B state.The excitation energy may further relax to the ground state throughother intermediate states, resulting in vibrationally hot ground-statemolecules.

To further understand the molecular reaction pathways, we take aclose look at the timedependencies of the threemajormass peaks andthe three major photoelectron peaks. Although the dynamic pro-cesses are very fast and the transient signals are very similar to eachother, Figure 2 shows that there are important differences betweenthem. The following features are worth noting. First, themass peak ofthe cis-stilbene parent (MS 180.2) arrives the earliest, before all otherpeaks, including the photoelectron peaks. Second, this mass peak alsodecaysmuch faster than all the other peaks. Third, the other twomasspeaks arrive in a sequential fashion. Fourth, the three PES peaksappear in a less distinct, but noticeably time-delayedmanner. And last,the cis-stilbene parent ions approach a level below zero after about 1ps, 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 asillustrated in Scheme 1 are proposed. Upon photoexcitation, CS iselevated to the core excited state 71B state. When immediatelyionized by one 3 eV probe photon, one photon suffices for theionization but subsequent absorption of a photon can fragment themolecule. If there is a time lapse between the excitation pulse and theionization pulse, themoleculemay convert to the 31B surface, whereit closes the ring to form DHP that is ionized by absorption of twophotons. The ensuing fragmentation leads to HPTþ and PTþ.

As outlined in the Appendix, we performed a quantitative model-ing of the ionization and photodynamics pathways. From the usualdeconvolution of the temporal profiles from the instrument function,weobtain the dynamics time scales (Figure 4). It is noteworthy that inaddition to the time constants, the fit quantitatively accounts for the

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

Scheme 1. Excitation and Reaction Schemes for cis-Stilbenea

aCS is initially excited to its 71B state. One probe photon is sufficient toionize from there, however some of the cis-stilbene ions can furtherabsorb an additional probe photon, leading ultimately into a dehydro-genation channel to form HPT (about 14% of the CS ions convert toHPT at this point, see below). The 71B state decays very rapidly (τ1 =167 fs) to its 31B state, on which CS quickly cyclizes to formDHP. DHPfurther 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 (395fs) and ultimately relaxes back to the ground state via a conicalintersection on the 11B state. The decay time constants and the productratios resulted from a fitting analysis as described in detail in theAppendix.

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

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intensities of the one-color signals as well as the two-color signals.Weare therefore able to obtain the branching ratios of the variouspathways.

Upon ionization of CS without time delay, the ground-state (D0)CSþ ions may absorb an additional probe photon, giving the ionsufficient energy for dehydrogenation. Although the direct dehydro-genation product should be dH-CSþ as has been depicted inScheme 2, our calculation suggests that a ring closing reaction fromdH-CSþ toHPTþ is likely to take place on the ion surface (below).The loss of the hydrogen radical removes a significant amount ofvibrational energy from the molecular ion, so that it no longer hasenough energy to further dehydrogenate to form PTþ. Therefore,only HPTþ results at this step. As a net effect of the absorbed extraprobe photon, it is estimated based on the quantitative fit (Table 2)that about 14% of CSþ dehydrogenates to formHPTþ. Meanwhile,CSþmight also close a ring to formDHPþ. However, our data prov-ides no evidence to prove or disprove such reaction. Because CSþ

and DHPþ have the same mass, mass spectrometry will not revealthis reaction. Nor can photoelectron spectroscopy show it becausethe reaction would take place after ionization.

The 71B state of CS quickly decays to the 31B state with a decaytime of 167 fs. On the 31B state, CS very rapidly closes the ring toform DHP as shown in Schemes 1 and 2. However, because therapid reaction along the steeply slopedmolecular energy surface qui-ckly converts electronic energy to vibrational energy, the moleculenow requires two probe photons to ionize. This ionization is facil-itated by an intermediate set of Rydberg states, which are obser-ved as three distinct peaks in the photoelectron spectrum. Ionizationfrom the highly vibrationally and electronically excited DHP resultsin DHPþ ions that are unstable and dehydrogenate to HPTþ andPTþ. As discussed in before, onceHPTþ is formed it cannot furtherdehydrogenate to form PTþ because of its limited total energy afterthe first dissociation. However, DHPþ is able to generate PTþ in aparallel process by dissociating an H2 molecule. The quantitative fitdetermines this branching ratio to be 49% for HPTþ and 51% forPTþ.

The molecule decays out of the 31B state with a lifetime of 395fs. While we cannot follow the details of the reactions subsequentto this decay, one might speculate that CS reaches the 11B stateand then decays further to the ground state via the well-knownconical intersection linking the two surfaces.11 We pick up thesignal of the vibrationally hot species on the ground state againbecause it can be ionized by multiple probe photons, as does the

cold CS molecule. However, the extra vibrational energy leads ahigher dehydrogenation probability of the molecule in its ionstate compared to the coldmolecule. This process is revealed by adepletion of the relevant species, the details of which will bediscussed later.

With the ionization and energy relaxation pathways establishedand quantitativelymodeled, we can now proceed to discuss severalfurther observations and conclusions:1. Computations and Fractional Ion Yield Interpretations.

The interpretations summarized in Schemes 1 and 2 are supportedby and in agreement with the computational studies of the relevantspecies (table 1). First, the dehydrogenation reaction from theinitially created CSþ to HPTþ takes place only after the absorptionof an additional probe photon simply because there is a lack of energyfor the reaction to take place otherwise. The ionization potential ofHPT (9.89 eV) is well above the energy of one pump photon andone probe photon combined (8.98 eV), substantiating the need forthe 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 hypothesisis made on the basis of the computed energies of dH-CS andHPT,which suggest no significant barrier for dH-CSþ to react to theclosed-ring product of HPTþ on the ion surface. Given the very longflight time of the generated ion to reach our detectors, it is thereforeassumed that HPTþ is the detected species. As a result, a ring closingreaction of dH-CSþ to HPTþ is proposed as shown in Scheme 2.However, one may also postulate a reaction pathway for this processwhereby 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 resultsindicate that this ring-closing reaction with a subsequent dehydro-genation reaction is not likely. On the basis of the numerical fits(Appendix) used to quantitatively model Scheme 1, about 14% ofCSþ converts toHPTþ on the ion surface, whereas no PTþ is prod-uced there. However, when reacting fromDHPþ on the ion surface,49% of HPTþ and 51% of PTþ are produced, whereas DHPþ

completely disappears. Whereas this large difference could beattributed to the additional probe photon absorbed by DHP, thestructural difference between CSþ and DHPþ may play a roleas well. If CSþ were to react to DHPþ first and then dehydro-genate, this structural effect could not be involved. However,we consider this evidence weak and suggest that further analysisis required.Third, as revealed in Scheme 1, our fits suggested no PTþ

generation fromCSþwhen ionization is out of the 71B state. Thisis even though three possible pathways for PTþ generationsmight be proposed, namely to generate PT on the 71B state, togenerate PTþ via the reaction to HPTþ, and to generate PTdirectly from CSþ after the additional photon absorption. Giventhose options, why is little or no PTþ generated in this process atall? First we note that the generation of PT on the 71B state doesnot seem likely as the lifetime for the 71B state (167 fs) is tooshort for such reaction. Second, generating PTþ subsequent tothe generation of HPTþ is not possible because the sequentialgeneration of HPTþ and PTþ implies the generation of twohydrogen radicals, which requires more than 12 eV of energy(Table 1). Therefore, absorption of a third probe photon wouldbe necessary, which greatly reduces the overall probability of sucha process. Finally, to form PTþ directly from CSþ, a H2 moleculehas to dissociate at the same time as the ring closes. However,having so much energy in the vibrational manifold, the molecule

Scheme 2. cis-Stilbene Reaction Summary

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is more likely to lose one hydrogen atom quickly, leaving noopportunity 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 reactionsoriginating with CSþ, a sequential reaction to HPTþ and then toPTþ is energetically not possible. In addition to the very high energyrequirement for the generation of two hydrogen radicals, the firstejected hydrogen atom is likely to take with it a large amount ofenergy, which further reduces the energy available for the generationof PTþ. Consequently, HPTþ and PTþ must be formed in parallelprocesses, and PTþ is created through the dissociation of a H2

molecule.Given the geometric separationof the twohydrogen atomsin DHP, such a dissociation might seem surprising. We propose twopossible explanations. The first one involves the formation of cis-DHP during the ring closing reaction of CS. As shown in Table 1, ifinstead of formingDHP (with the hydrogen atoms in trans position)on the 31B surface, the reaction leads to cis-DHP where the twohydrogen atoms are at the same side of the molecular plane, it wouldbe much easier to generate H2. However, we consider the possibilityof such process not to be very high because, at least in the molecularground state and the ion state, cis-DHP is about 0.4 eV higher energythan DHP (Table 1). Additionally, the structural similarity betweenthe geometries of CS and DHP is much higher than that of CS andcis-DHP, suggesting a higher barrier for the reaction to cis-DHP. As aresult, the production to DHP is both thermodynamically andkinetically preferred.How is the dissociation of molecular hydrogen from DHP

then possible? According to the recently popularized concept ofhydrogen roaming,28 when the C-H bonds accumulate largeamounts of vibrational energy, their stretching mode may getparticularly excited such that the bond can extend to about 3-4 Å. At such large distances, the hydrogen atoms roam aroundthe molecule in unpredictable paths. In DHP, at such large bonddistances it would be easy for one H atom to reach the other. As aresult, the roaming atom concept might provide a framework tounderstand the dehydrogenation of DHPþ to PTþ. Whereas amore detailed computational analysis of this scenario seemswarranted, our experiments are unambiguous in the observationof 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 thethreemajorMS andPES peaks in Figure 2 shows that their lifetimesare not the same. Especially the earliest peaks, theMS180.2 and BE2.41 eV, have a different dynamics: themass peak comes earlier anddecays much faster than the PES peak. Noting the very similaroverall dynamics of all three PES peaks, and especially that theyhave the same lifetimes, one infers that they all come from the sameexcited 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 slightbut detectable delays between the onsets of the three peaks, whichare inconsistent with the assumption that they are a vibronicprogression within the same state. Thus, multiple electronic statesmust be involved. We note that this conclusion is in contrast to theobservation of vibrational progressions in trans-stilbene.21

To account for these detailed observations in the photoelectronspectrum ofCS, we attribute the generation of the three PES peaksto a resonant multiphoton ionization out of 31B via differentmolecular Rydberg states. While ionization out of a highly excitedvalence states such as the 71B state has a very broad FC envelope,rendering the transition broad and featureless, the ionization out ofa Rydberg state leads to a sharp peak at the binding energy of theRydberg electron.23,29,30

The binding energy of electrons in Rydberg states can beapproximated by the equation

EB ¼ ERyðn-δlÞ2

where EB is the binding energy, ERy is the Rydberg constant(13.6 eV), n is the principal quantum number, and δl is a constantcalled quantum defect that depends on the angular momentum, l.For smaller values of l, the Rydberg electron is closer to the posi-tively charged ion core, resulting in a larger shift of the Rydbergenergy 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 levelsthat are reasonably assigned to the 3pz, (3px, 3py), and 3d orbitals.The appearance of the peaks in the photoelectron spectra is ascribedto two-photon ionization out of the 31B state via Rydberg 3pz,

Table 2. Summary of the Data-Fitting Model and the Fit Resultsa

process no.

1 2 3

ionization out of ... ... 71B ...out of 31B baseline shift

overall ion yield (R1 þ β1 þ γ1)A0e-t/τ1 (R2 þ β2 þ γ2)A0τ2/

(τ2-τ1)(e-t/τ2-e-t/τ1)

(R3 þ β3 þ γ3)A1/

(1 þ e-(t-t1)/τ3)

requirements Ri þ βi þ γi = 1

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 Constantsτ1/fs τ2/fs τ3/fs t1/fs A0 A1

167 395 209 744 3.39 0.369aAll processes are convoluted with a Gaussian instrument function (σ = 142 fs) characterizing the laser pulses. Peak yields in the mass domain weredeconvoluted to remove overlapping contributions. The three σ errors of the fitted parameters are 10% of their values.

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(3px, 3py), and 3d states respectively. This explanation is consistentwith the computational results of Molina et al.26

3. Time-Resolved Structural Dynamics of the Ring ClosingReaction of cis-Stilbene. As the three PES peaks are assignedto Rydberg states of the CS/DHP system, there remains nospectral signature that could correspond to the dehydrogenationreaction from DHP to HPT and PT. One would expect thatdifferent PES peaks, corresponding to HPT and to PT, wouldhave to appear in sequential manner. However, no such newpeaks have been found. Additionally, it seems unlikely that thedehydrogenation (especially the ejection of a H2 molecule)would occur within the short lifetime of the excited state. Wethus conclude that the dehydrogenation must occur on the ionsurface after the ionization.It is likely that as theCS ring closes to formDHP, a coherent wave

packet travels along the 31B potential energy surface. As the reactionproceeds,molecules are transferredonto the ion state by the resonanttwo-photon ionization via the Rydberg states. Since the Rydbergstate binding energy depends on the molecular structure we havebeen able to use their time dependence to map out molecularmotions.23,34-36 In the present case, one would expect that thebinding energy of the observed photoelectron peaks changes as thering 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 photoelectronspectrumcaptures thewave packet as it evolves along the ring-closingcoordinate fromCS to DHP. Thus, the structural dynamics motionsof the system during the ring-closing process are captured by thetime-resolved Rydberg spectrum.4. Further Analysis of the Transients. We are now ready to

explain some additional features mentioned previously in thediscussion of Figure 2.a). Mass Peak of the cis-Stilbene Parent Arrives before All the

Other Transients. cis-Stilbene parent ions are generated as soonas the pump beam excites the molecule to the 71B state. Becausethe 71B is a valence state, the photoelectrons generated from itsionization are distributed throughout the high binding energyregion of the PES in Figure 3. The photoelectron peaks ofFigure 2 arise from the two-photon resonant ionization out of the31B state, which result from the decay of the 71B state. It is clearthat the PES peaks are delayed relative to that of the CS parentmass peak. As for the relative delay between the MS 180.2 peakand the other twomain peaks in theMS, it can further be ascribedto the decay of the originally populated 71B state (which givesrise to MS 180.2), to the 31B state (which gives rise to the othermass peaks). Even though the dehydrogenation reactions takeplace on the ion surface after the formation of the DHP ion, thefragment mass peaks must come later then the parent mass peakof cis-stilbene.

b). Parent Mass Peak Decays Much Faster than the OtherPeaks. Ionization of CS out of its 71B state requires only oneadditional probe photon. However, as the molecule quickly decaysto the 31B state and slides toward its DHP configuration, a largeamount of electronic energy is deposited in the vibrationalmanifold. This conversion of electronic to vibrational energy notonly implies that DHP requires two probe photons for ionization,but it also gives rise toDHPþwith somuch internal energy that thedehydrogenation reaction on the ion surface to produce HPTþ

and PTþ proceeds with unit yield. Therefore, as CS converts toDHP, it quickly loses its ability to generate parent mass ions. Thus,because cis-stilbene ions are only generated while the molecule ison the 71B surface, the parent peak decays much faster than therest 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 PTþmass peak isdelayed relative to HPTþ peak. Following Scheme 1, we suggestthat the generation of CS out of the 71B state may be accom-panied by the further absorption of a photon on the ion surface,giving rise to an early generation of HPTþ ions. Because of thegeneration of HPTþ in this process is faster than the generationof HPTþ and PTþ from DHP, one observes an overall earlieronset 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 31Bstate and the Rydberg states as shown in Scheme 1. Because of thelower energy level for the 3pz surface, the first probe photon is ableto reach this surface just a little earlier than the other two, resultingin 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. Aftersome time, the excited DHP relaxes back to the ground state,possibly via the 11B conical intersection, and branches into CS andDHP. Those molecules that have undergone the excitation anddecay processes are hot, containing 6 eV of vibrational energy.Upon subsequent ionization with three probe photons, these hotmolecules dehydrogenate more completely than the cold mole-cules. When subtracting the one-color ionization contributionsfrom the cold molecules, a depletion pattern of the CS parent ionresults. The time-resolved mass spectrum, Figure 2, thereforeshows a negative parent signal at delay times above about 1 ps.Conversely, the dehydrogenation products show a positive offsetbecause their signals are enlarged by this process. Another possibleprocess responsible for the depletion could be the additionalabsorption of probe photons by the CSþ generated subsequent tothe two pump photon-absorption. However, it is not clear at thispoint what percentage contribution this process has on the overalldepletion effect.

’CONCLUSIONS

In summary, we have performed ultrafast time-resolved stud-ies of CS with TRPES and MS to explore the reaction pathwaysof highly excited CS. As summarized in Schemes 1 and 2, the 71Bstate of CS is reached when pumped with 6 eV photons, and onlya single dehydrogenation reaction from CS to HPT is observedupon ionization out of the 71B state. However, the 71B statequickly decays to 31B, on which a ring forming reaction takesplace that leads to the formation of DHP. This reaction toDHP isobserved in real time by resonant two-photon ionization photo-electron spectroscopy, where the time dependence of the Ryd-

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

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berg signal intensity and binding energy maps the structuralmotions of the wave packet. The time-resolved mass spectrumshows that both HPT and PT are formed on the ion surfacedirectly from DHP via parallel reactions.

Our results also reveal that the pathway of the ultrafastreaction involving conical intersections depends strongly onthe electronic state on which CS is prepared: While on the 11Bstate, CS is known to react to both TS and DHP, upon excitationto its 71B state, we observe that it converts to the 31B state andreacts to DHP. Both CS and DHP feature dehydrogenationreactions on their ion state, although CS only partially reacts toproduce HPT, whereas DHP completely dehydrogenates toproduce both HPT and PT simultaneously.

’APPENDIX: THE NUMERICAL DATA ANALYSIS

With the model as explained by Schemes 1 and 2, thetransients for the mass peaks at 180.2, 179.2, and 178.2 are fitsimultaneously 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 ofmass 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 explainedearlier), which features a single decay with a time constant τ1,which represents the decay from 71B to 31B. The second process isthe ionization out of the 31B state, which ismodeled as a sequentialreaction with a rise time of τ1 and a decay time of τ2, representingthe generation of 31B from 71B and its decay to a lower surface.The third process accounts for the shifts in the baselines of masspeaks 180.2, 179.2, and 178.2, which stems from molecules thathave undergone a cycle of excitation and decay and which can nolonger give parent ions (text). To simulate the multiple consecu-tive decay processes from the 31B state to the ground state, asigmoidal function is used as illustrated by the equation undercolumn 3 in Table 2, in which t1 and τ3 collectively represents therepopulation process to the ground state.

In consideration of all these processes, the overall equationsused to fit the three MS peaks are as follows:31

For MS 180.2:

MS 180:2ðtÞ ¼ðt<0ÞC1

þðt g 0Þ C2 þ R1A0e-t=τ1 þ R2A0

τ2τ2-τ1

ðe-t=τ2-e-t=τ1Þ þ R3A1

1þe-t-t1=τ3

� �

For MS 179.2:MS 179:2ðtÞ ¼ðt<0ÞC3

þðt g 0Þ C4 þ β1A0e-t=τ1 þ β2A0

τ2τ2-τ1

ðe-t=τ2-e-t=τ1Þþβ3A1

1þe-t-t1=τ3

� �

For MS 178.2:MS 178:2ðtÞ ¼ðt<0ÞC5

þðt g 0Þ C6þγ1A0e-t=τ1 þγ2A0 3

τ2τ2-τ1

ðe-t=τ2-e-t=τ1Þ þ γ3A1

1þe-t-t1=τ3

� �

The time-resolved data of MS 180.2, MS 179.2, and MS 178.2are well fit with the provided equations. The original data points,together with the fitted curves for the three MS peaks, are shownin Figure 4.

Whereas the figures in this article show only the two-colorsignals, the fits included the one-color signals arising from theionization with individual pump pulses and probe pulses, respec-tively. The constants C1 to C6 in the equations represent these

one-color ionization contributions to the signals. Whereas per-forming the experiments, the one-color ion yields were measuredindividually (we call them normal one-color yields below). Duringthe pump-probe experiments, the one-color yields of the secondpulse are inevitably modified by the first laser pulse, leading to adepletion or amplification that has a step function temporal profile.For example, at negative delay times, the probe beam comes firstand a normal one-color yield is generated. The subsequent pumppulse sees a smaller number of ground-state molecules so that itsone-color contribution is smaller than the normal one-color yield.When subtracting the normal one-color yields from the overallsignals in a pump-probe experiment, the one-color yields areactually overestimated. In most of the cases, the baseline shifts areminimal compared to the two-color process yields. However, insome cases, such as those of methylated pentadienes and amylnitrite, these processes can have a large impact.32,33 In the presentwork, we also find that these processes are not negligible, so thattheir inclusion in the data analysis was deemed necessary.

’AUTHOR INFORMATION

Corresponding Author*E-mail: peter_weber@brown.edu.

’ACKNOWLEDGMENT

This project was supported by the Army Research Officeunder the contract W911NF-06-1-0463.

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