2-甲基呋喃分子激發(fā)態(tài)超快非絕熱動(dòng)力學(xué)

龍金友 劉志明 邱學(xué)軍,2 張冰,*

(1中國科學(xué)院武漢物理與數(shù)學(xué)研究所，波譜與原子分子物理國家重點(diǎn)實(shí)驗(yàn)室，武漢430071；2中南民族大學(xué)電子信息工程學(xué)院，武漢430074)

2-甲基呋喃分子激發(fā)態(tài)超快非絕熱動(dòng)力學(xué)

龍金友1劉志明1邱學(xué)軍1,2張冰1,*

(1中國科學(xué)院武漢物理與數(shù)學(xué)研究所，波譜與原子分子物理國家重點(diǎn)實(shí)驗(yàn)室，武漢430071；2中南民族大學(xué)電子信息工程學(xué)院，武漢430074)

利用飛秒時(shí)間分辨的光電子影像技術(shù)研究了2-甲基呋喃分子激發(fā)態(tài)超快非絕熱動(dòng)力學(xué)。2-甲基呋喃分子吸收兩個(gè)400 nm的光子后同時(shí)被激發(fā)到n=3的里德堡態(tài)S1[1A′′(π3s)]、1A′(π3px)、1A″(π3py)、1A″(π3pz)和價(jià)電子態(tài)1A′(ππ＊)，之后被兩個(gè)800 nm的光子電離。通過母體離子產(chǎn)率隨泵浦-探測延遲時(shí)間的變化曲線測得這些里德堡態(tài)與價(jià)電子態(tài)的平均壽命為50 fs。通過解析光電子能譜中n=3的里德堡態(tài)與價(jià)電子態(tài)所對(duì)應(yīng)的組分峰的相對(duì)演化特征，觀測到了這些激發(fā)態(tài)之間的內(nèi)轉(zhuǎn)換過程，并且揭示了價(jià)電子態(tài)1A′(ππ＊)在內(nèi)轉(zhuǎn)換過程中扮演的重要“紐帶”作用。里德堡態(tài)與價(jià)電子態(tài)之間的混合，形成勢能面間的錐形交叉，導(dǎo)致了如此超快的內(nèi)轉(zhuǎn)換過程。

超快；光電子影像；非絕熱動(dòng)力學(xué)；2-甲基呋喃

1 In troduc tion

Nonadiabatic interactions thatoccurbetween adiabatic potential energy surfaces(PESs)of differentelectronic statesare notonly ubiquitous,but also essential inmany photochemical and pho-tobiologicalprocesses such as photosynthesis,photoisomerization in vision and the photostability of deoxyribonucleic acid(DNA)1. The PEScrossing between valenceand Rydberg states is one of these most fundamental nonadiabatic interactions.From the physical viewpoint,such a strong nonadiabatic coupling represents an interesting exampleof a situation inwhich the Born-Oppenheimer approximation is no more valid for the description of coupling between electronic and nuclearmotions.

In amolecule,larger principal quantum number(n)and orbital angularmomentum(l)reduce the probability of penetrating a Rydberg electron into themolecular ion core,resulting in a longer lifetime of the Rydberg state.The lifetimes of low(n=3－5) Rydberg statesof aromaticmoleculesare roughly recognized to be slower(i.e.,by 1－2 orders ofmagnitude)than those of the valence electronic excitations in the same energy domain2.The fact thatmolecular valence and Rydberg states aremuchmore likely tomix in the vacuum-ultraviolet(VUV)than they are in the ultraviolet immediately implies that the lifetimesof low-lying 3s and 3p Rydberg statesare remarkably shortened asa resultof a larger probability of coup ling of a Rydberg electron w ith valence electrons3,4.The complicated spectral features and extremely short lifetimes of thesem ixed valence and Rydberg states pose great challenges in the direct real time observation and characterization of such nonadiabatic couplings between low-lying Rydberg and valence states in polyatomicmolecules.

Fem tosecond time-resolved photoelectron imaging(TR-PEI) isuseful in probing thesenonadiabatic interactionson real time in polyatom ic molecules.TR-PEI could measure both the kinetic energy and angular distribution of the photoelectrons simultaneously aswell as their correlation asa function of timeand have been successfully applied in recentyears to a variety ofmolecular systems5,suggestiveof an ideal fingerprintsensor for investigating ultrafastnonadiabatic interactions involving changes in electronic characterswith nuclearmotions in complexmolecules.

2-Methyl furan,has served asexcellentprototype system for studying nonadiabatic dynamics involving thenatureof Rydbergvalence interactions.Compared with thewell-documented furan molecule6－9,the prom inent differences are that the substitution effectof an H by amethylgroup on the neighboring position of oxygen in the furan ring leads to notonly a small redshiftof the first VUV absorption spectrum but also the presence of several extra Rydberg transitions that are forbidden in furan.The first broad and diffuse VUV absorption band of 2-methyl furan in the energy range5.00－9.91 eV arises from themixing of electronic transitions from the ground state S0to the Rydberg S1[1A′′(π3s)],1A′(π3px),1A′′(π3py)and1A′′(π3pz)statesand valence1A′(ππ*)state. These Rydberg transitions that appear together with valence transitionsparticularly complicate the vibronic structures in the firstVUV absorption spectrum of 2-methyl furan,suggesting that muchmore complex nonadiabatic interactionsmightexist in 2-methy l furan.A lthough extensive experimental10－20and theoretical12,14,20－22studies have been performed on assignments and characterizations of the firstVUV absorption spectrum of 2-methyl furan,ultrafastobservables have not yet been explored experimentally.

In the presentwork,we investigate thenonadiabatic dynamics of 2-methyl furan asan exampleof a system with strong Rydbergvalence interactionsby fem tosecond time-resolved photoelectron imaging and femtosecond time-resolvedmass spectroscopy.The Rydberg and valence statesof 2-methy l furan in the red edge of its firstVUV band areoptically excited by two-photon absorption at400 nm,their dynamicalevolution is then interrogated by twophoton ionization at 800 nm.The electronic relaxation processes havebeen directly observed in real-timeby the time-dependences of the photoelectron spectra.And thecoupled Rydbergand valence componentsarealso successfully extracted and discussed.

2 Experim en talm ethods

Theexperimentalsetup employed in the presentwork hasbeen described elsew here23.The liquid samp le of 2-methyl furan (A laddin,98%),seeded in helium buffer gas at a background pressure of 2×1.01325×105Pa,is expanded through a pulsed valve to generatea pulsedmolecular beam.Thebeam isskimmed and introduced into the ionization chamberwhere it is intersected perpendicularly w ith the linear polarized pump and probe laser beams.The generated photoelectrons were extracted and accelerated by the electrostatic immersion lensand then projected onto a two-dimensional(2D)detector.Each image is accumulated over 40000 laser shots.Three-dimensional(3D)distribution reconstructions are performed by the basis-set expansion(BASEX) forward convolutionmethod24.The details of our femtosencond laser system have been described elsewhere25.Briefly,the femtosecond laser seed pulse is generated by a self-mode-lock Ti: sapphire oscillator pumped by a CW second harmonic of an Nd: YVO4laser,and then amplified by an Nd:YLF pumped regenerative amplifier to generatea1 kHz pulse train centered at～800 nm of 45 fspulsewidthw ithmaximum energy of～1m J·pulse－1. The second harmonic pulsewasgenerated in a 0.5mm thick BBO (BaB2O4)crystal and the centralwavelengthwasspectroscopically measured tobe400nm with abandwidth of～6 nm.Inourpumpprobeexperiments,the pump pulse(400 nm)energy isattenuated to be less than 1μJ·pulse－1and theoptimalprobe pulse(800 nm) energy is controlled to be around 30μJ·pulse－1.The pump and probebeams are recombined collinearly ata dichroicmirror prior to being softly focused on themolecular beam with a spherical plano-convex lens(focal length(f)=250mm).

3 Resu lts and discussion

As shown in Fig.1(a),a typical time of flight(TOF)mass spectrum of 2-methyl furanwas recordedw ith the two-photon 400 nm pump and two-photon 800 nm probe at zero delay time.2-methyl furan parent ion peak of C5H6O+is clearly observed,and aminor fragment ion peak of C4H3O+becomes visible.Normally, the time-resolved photoelectron imaging experimentsare required to be conducted w ith background signals low enough to ensure minimum ionization from either beam operating independently.Wherenoted,asmentioned above,soft focus isadopted in order to avoid space charge effectand strong field effects.Consequently, nearly no background signals are generated from either beam independently.The area ratio of C5H6O+to C4H3O+is241:1 and hence the contribution to the total photoelectron signal from the fragment ion of C4H3O+could be safely neglected.

Fig.1(a)Typical Time of flight(TOF)m ass spectrum of 2-methyl fu ran recorded w ith the two-photon excitation at 400 nm and two-photon ionization at 800 nm at the zero delay tim e;(b)tim e-resolved total ion signalsof C5H6O+as a function of delay tim ebetween the pum p pulse and thep robe pulseThe circles represent theexperimentalresults,and thesolid lineshows the fitting result.

In our fem tosecond pum p-probe scheme,all electronic transitions from the ground electronic state of 2-methyl furan are optically one-photon or two-photon dipole allowed as a result of the reduction of themolecular symetry from C2vin the case of furan6,7to Csin 2-methyl furan.The origins of the singlet S1[1A″(π3s)]and1A′(ππ*)states have been documented to be at 5.47 and 5.95 eV,respectively19.The low-lyingπ-3p Rydberg seriesof1A′(π3px),1A″(π3py)and1A″(π3pz)havebeen recognised at5.73,6.02,6.06 eV,respectively19.As indicated by the notation, the1A′(ππ*)state isπ－π*valence-typeexcitation,whereas the S1stateandπ-3p Rydberg seriesareof Rydberg-type characters.For the two-photon excitation schemeused in thepresentwork,the2-methyl furanmolecule issimultaneously pumped into the S1[1A″(π3s)],1A′(ππ*)statesand the low-lyingπ-3p Rydberg series from itsground state S0(1A′)by two-photon absorption at400 nm when taking the broad excitation bandwidth(～6 nm)into account.The virtual states involved in the two-photon transition could be A′or A″states according to the symmetry of the prepared electronic state.As an exam ple,the possible two-photon transitionmatrix elements for the S1[1A″(π3s)]state could be orw ith the definition of yz in the planeof the 2-methyl furanmolecule if a A′state actsasa virtual state.In this case,the two-photon transition is induced by two-consecutive dipole transitions w ith crossed x and z directions or y and z directions.In addition,it isnoteworthy that there is no absorbance in the visible region near 400 nm.Thus the one-photon 400 nm excitation process doesnotoccur.

The photoion yieldsare recorded asa function of the delay time between the pump and probe pulses,and these provide ameasure of the lifetime of theexcited states.The time-dependent ion signal of C5H6O+is represented in Fig.1(b).The signal rapidly decays within the first200 fs.The decay profile is found to bewell reproduced only by a single exponential function convoluted w ith aGaussian thatdescribes the instrument response function.In this case,a lifetime of 50 fs is obtained and the fitting error is reasonably w ithin±2 fs.The unsatisfactory fittingsw ith two or even moreexponential functions to discern the lifetimesof the prepared excited states are likely due to the extremely short lifetimes of the prepared excited states which are largely restricted by our instrument response function of 160 fs.Thus the lifetime of 50 fsobtained in our experiment is the average lifetime of the S1[1A″(π3s)],1A′(ππ*)statesand the low-lyingπ-3p Rydberg series.

Fig.2(a)shows typical photoelectron images measured at variousdelay timewith the two-photon 400 nm pump and twophoton 800 nm probe.Each image corresponds to a slice through the 3D photoelectron scattering distributions observed at the quoted time delay.The linear polarizationsof the pump and probe lasersareboth vertical in the planeof the figure.Therings(bands) with different radii in the image stand for photoelectronswith differentkineticenergy components.In Fig.2(b),we show the timedependent photoelectron kinetic energy(PKE)distributions (PKEDs)extracted from thecorresponding imagesshown in Fig.2 (a).The photoelectron spectrahave each been normalized to the totalphotoelectron counts.Each PKED is characterized by several identifiable peakswhich are congested in the continuousenergy region of 0.05－1.00 eV.Four featured peaks w ith the central energies of 0.13,0.49,0.68 and 0.88 eV are identified in the PKEDs.Theadiabatic ionization potential(AIP)of 2-methyl furan is8.38 eV19,therefore two photonsof800 nm[(AIP-2×E400)/E800= ((8.38－2×3.1)/1.55)－2]are required to ionize the excited states.Consequently,the available energy[=hνpump+hνprobe－AIP] in thecontinuum state canbedetermined to be0.92eV for the twophoton 800 nm ionization to the zero vibrational level of the cationic ground state,and this isalso indicated by the arrow as D0in Fig.2(a).

According to the previous spectroscopic studies10－22,the four featured peakswith the central energies of 0.13,0.49,0.68 and 0.88 eV are respectively assigned to be ionized from theS1[1A″(π3s)],1A′(π3px),1A″(π3py)and1A″(π3pz)Rydberg states. More interestingly,ionization from the1A′(ππ*)state isalso expected to yield a photoelectron bandwith theenergy around 0.67 eV,which overlapswith the above featured peaks.Asan added support for our assignments,the energies and quantum defectsof Rydberg states can be obtained by26

Fig.2(a)Tim e-resolved photoelectron im agesm easu red as a function of thepum p-p robe delay timeThe linearpolarizationsof thepump and probe lasersarealigned verticalin the planeof the figure(b).Time-resolved photoelectron kinetic energy distributions extracted from the corresponding images shown in Fig.2(a)asa function of the pum p-probe delay time.Asguided by the vertical dashed dot lines,four featured peakswith the centralenergiesof 0.13,0.49,0.68 and 0.88 eV are respectively assigned to be ionized from the S1[1A″(π3s)],1A′(π3px),1A″(π3py)and1A″(π3pz) Rydberg states,which are respectively labeledby 3s,3px,3pyand 3pzfor sim plicity.Additionally,ionization from the1A′(ππ*)state is also identified to yield aphotoelectron bandw ith theenergy around 0.67eV,which overlapsw ith theabove featured peaks.Theavailableenergy for the two-photon 800 nm ionization is indicated by thearrow as D0.

where T(Rydberg)and hνpris theenergy of the Rydberg statesand the probe photon,respectively,IP is the ionization potential,n is the principalquantum number,δis thequantum defect,and R is the Rydberg constant,13.606 eV.Hence,the quantum defect values for the delay timesof 0 fsare respectively calculated to be 0.86,0.72,0.63 and 0.52 for the four featured peaks w ith the central energies of 0.13,0.49,0.68 and 0.88 eV with the assumption of principal quantum numbers n=3.The quantum defect isa constant thatdependson the symmetry and typesof the Rydberg orbital.Formolecules composed of second-row atoms, typicalδvaluesare0.9－1.2 for s orbital,while theδvalues of p orbitalare about0.3－0.5,andδvalues of d orbital are about027. Giuliani etal.19obtained the quantum defectvaluesof 0.84,0.73, 0.60 and 0.58 for the S1[1A″(π3s)],1A′(π3px),1A″(π3py)and1A″(π3pz)Rydberg states,respectively,and found that thequantum defectvalues for 3p Rydberg orbitals seemed to be a little bithigh and explained this could be due to the Rydberg-valence interaction.This interaction could bemore important in thismolecule than in furan since the 3p state isnow much closer in energy w ith the valence1A′(ππ*).Therefore,quantum defect values further suggest that the assignments of the four featured peaks seem consistentwith the previouswork.19

Inspection of the PKEDs of 2-methyl furan in Fig.2(b),the intensities of the PKEDs rapidly decreasew ith increasing delay time,which is coincidentwith the short lifetimeof 50 fs for the parent ions.Upon amore detailed inspection of the time-dependent behavior between 0 and 39 fs,the intensitiesof the featured peaksin each PKEDmonotonously decay ina similarmanner.The energy positions of these featured peaks do not change with the delay time,however,the relative changes in the peak intensities among these featured peaks are not apparent.By analogy to the case of furan,similar decay channels could be correlated to these featured states in 2-methyl furan.As discussed above,the S1[1A″(π3s)],1A′(ππ*)statesand the low-lyingπ-3p Rydberg series of1A′(π3px),1A″(π3py)and1A″(π3pz)statescould besimultaneously excited from itsground state S0(1A′)by two-photon absorption at 400 nm.Hence,internal conversionsamong these featured states are likely to occur.

For a further analysis of the PKEDs associated with the correlated relaxation dynamicsof the S1[1A″(π3s)],1A′(ππ*)states and the low-lyingπ-3p Rydberg series,we expect to extract the spectral components that independently arise from the ionization of the corresponding S1[1A″(π3s)],1A′(ππ*)statesand the low-lyingπ-3p Rydberg series.Generally the Levenberg-M arquardtmethod28is mostly used to perform non-linear least squares fitting of the PKEDs.Themeasured PKED ateach delay time is fitted by the sum of five Voigt functions and a polynomial.The Voigt function profile(i.e.,a convolution of Gaussian and Lorentzian functions) is p referentially selected to rep roduce the S1[1A″(π3s)],1A′(ππ*),1A′(π3px),1A″(π3py)and1A″(π3pz)component spectraby assum ing the component peak centers to be fixed at0.13,0.67, 0.49,0.68 and 0.88 eV,corresponding to the ionization channels from the S1[1A″(π3s)],1A′(ππ*),1A′(π3px),1A″(π3py)and1A″(π3pz) states.Inaddition,a polynomial isunavoidably added tomatch the residual background.As an exam ple,Fig.3 shows the fitting of PKED at the delay timeof 0 fs,and the fitting residue isalso given in the bottom panel of Fig.3.The open circles represent the experimental PKED,and the blue solid line shows the sum of the fitting components which nearly reproduce the experimental PKED.Thus the time-dependentintensitiesof the five components are easily obtained by integrating theareaof each componentat differentdelay time and are show n in Fig.4.Note that the timedependent intensities of the 3s,3pyand 3pzcomponents are respectivelymultiplied by a factor of 3,5 and 2.5 tomake them more visibly comparable with those of the1A′(ππ*)and 3pxcomponents in the same p lotting.

Fig.3(a)Non-linear least squares fitting of the photoelectron kinetic energy distribution at the delay tim eof0 fs by the Levenberg-Marquardtmethod28;(b)the residue for the fitting in(a)The PKED is fitted by asum of five Voigt functionsand apolynom ial.The five Voigt functions reproduce the 3s,3px,3py,3pzand1A′(ππ*)com ponent spectra by assum ing the 3s,3px,3py,3pzand1A′(ππ*)componentpeak centers to be respectively fixed at0.13,0.49,0.68,0.88 and 0.67 eV,corresponding to the ionization from the 3s,3px,3py,3pzand1A′(ππ*)states.Thepolynomialis unavoidably added tomatch the residualbackground.See text fordetails.

As seen in Fig.3 and Fig.4,the five components appear simultaneously at the delay time of 0 fs,and the1A′(ππ*)and 3pxcomponents carrymuchmore intensities than those of the other three components,suggesting that theoptical transition strengths for the1A′(ππ*)and1A′(π3px)states are much larger than those for the S1[1A″(π3s)],1A″(π3py)and1A″(π3pz)states. The intensity profiles for the five components indicate that the S1[1A″(π3s)],1A′(ππ*),1A′(π3px),1A″(π3py)and1A″(π3pz)states decay rapidlywithin 50 fsupon theirexcitations,in fairagreement with the lifetimeof 50 fsmeasured for the parent ions.However, each of the intensity profiles could notbewell reproduced by only a single exponential decay function.The 3s component seems to decay with two different rates,i.e.,a slower rate before10 fsand a faster rateafter10 fs.This implies thatpopulation transfers from other initially excited states to the S1[1A″(π3s)]statemightoccur w ithin the first 10 fs.A sim ilar situation accounts for the 3pxcomponent,decaying with a faster ratebefore25 fsand a slower rate after 25 fs.In the case of 3pyand 3pzcomponents,it is interesting that the intensity profilesseem to behave inversely.The population transfer betw een the1A″(π3py)and1A″(π3pz)states is likely to occur due to theiroverlap in energy.More interestingly, the intensity profile for the1A′(ππ*)component exhibitsmore complex decay features.The1A′(ππ*)component carries themost intensity than thoseof theother components,and exhibitsmultiple decay rates.Thuswe speculate that the1A′(ππ*)statem ightplaya key role during the deactivation dynamicsand actas thebridge to connectwith the neighboring excited statesalthough it could not be clearly visualized as a sharp peak in the PKED.In addition, the1A′(ππ*)componentexhibitsa broad distribution,in support of the nature of a valence state.In consideration of the complex decay dynam icsamong the five components and the insufficient data points,we could not furtherextract the decay time constants for the five components by fitting each of the intensity profiles w ithmultiple exponential decay and rise functions.Thus,note that discussionsof decay time constants in Fig.4 arequalitative rather than quantitative.

Fig.4 Photoelectron com ponent peak intensitiesasa function of the pump-probe delay tim eThe tim e-dependent intensitiesof the 3s,3pyand 3pzcomponentsare respectively multiplied by a factor of3,5 and 2.5 tomake them more comparablewith thoseof the1A′(ππ*)and 3pxcomponents in thesameplotting.

Quantum chemical calculationsof theexcited states in2-methyl furan,especially concerningw ith the conical intersectionsamong the excited states,or dynam ics simulations of the excited states, havenotbeen performed yet.By analogy to the case of furan7－9, similar internal conversionsamong the S1[1A″(π3s)],1A′(ππ*)states and the low-lyingπ-3p Rydberg series of1A′(π3px),1A″(π3py) and1A″(π3pz)statesare likely to dominate in 2-methyl furan.Upon the two-photon excitation at 400 nm,2-methyl furan molecules are simultaneously pumped from its ground state S0(1A′)to the S1[1A″(π3s)],1A′(ππ*)statesand the low-lyingπ-3p Rydberg series of1A′(π3px),1A″(π3py)and1A″(π3pz)states.The vibrational energies deposited for the S1[1A″(π3s)],1A′(ππ*),1A′(π3px),1A″(π3py) and1A″(π3pz)states are 0.73,0.25,0.47,0.18 and 0.14 eV,respectively.The fact that the valence state(1A′(ππ*))and the Rydberg states(S1[1A″(π3s)],1A′(π3px),1A″(π3py)and1A″(π3pz))are very close in energy and overlapwith each other issupportiveof the speculation for the high com plexity of the potential energy surfaces.Therefore,numerouspotentialenergy surface crossings, i.e.,conical intersections,probably exist among these excited states.Furthermore,theultrashortdecay time(less than 50 fs)for these excited states im plies that conical intersections aremore likely to locate in the Franck-Condon region and actas the driving force to accomplish such ultrafast deactivations of these excted states.Asdiscussed above,the1A′(ππ*)statemightplay a key role during the deactivation dynamics and intersect with the neighboring excited states.Thus,as shown in Fig.5,the deactivationsof the five excited statesmight initially continue on their own potentialenergy surface,and then rapidly internally converts to the neighboring excited states through conical intersections,and finally return to thehotground state.

A comparison w ith the nonadiabatic dynam ics of furan7－9and 2-methyl furan showsmany similarities.In both molecules,internalconversion takes place on an ultrafast timescale as themain deactivationmechanism.Theappearance of conical intersections among the potential energy surfaces effectuates such ultrafast internal conversion processes.On the other hand,due to the strong coupling of the Rydberg stateswith valence states,the lifetimes of n=3Rydberg statesare considerably shortened to be on the order of tens of femtoseconds.Of particular interest is the differencew ith regard to the nonadiabatic interactions in furan and 2-methyl furan.In the case of 2-methyl furan,the coup lings of the n=3 Rydberg states with the1A′(ππ*)valence state are much stronger than thatof the S1[1A2(π3s)]Rydberg state with the S2[1B2(ππ*)]valence state in the furan case.The n=3Rydberg transitions thatappear togetherwith valence transition in 2-methyl furan particularly complicate and dom inate the nonradiative relaxation pathways from the Franck-Condon region along the multidimensional reaction coordinateback to theground state.It isnoted thatno intersystem crossing processwith tripletstatesare observed in the currentmeasurements.Inmostcases,the triplet states also play significant contributions to photochem istry processes29,especiallywhen theenergy levelof the involved singlet and triplet states are very close30,31.However,the triplet states differ in energy as the prepared singlet state in 2-methyl furan. Moreover,there are nomolecular featureswhichwould drive an ultrafast intersystem crossing in theobserved timewindow,neither by an El Sayedmechanism asw ellasby a heavy-atom effect.

Fig.5 Schematic energy diagram of the ground,excited and ion ic states of 2-methyl fu ranThe valence state1A′(ππ*)and Rydberg seriesof S1[1A″(π3s)],1A′(π3px),1A″(π3py) and1A″(π3pz)statesaresimultaneously excitedby two-photonsof400 nm, as indicated by thebluew indowed area.Thesestatesare then projected to the ground ionic state by two-photons of 800 nm,resulting in the1A′(ππ*),3s, 3px,3pyand 3pzcomponentbands,respectively.Internal conversions(ICs)are likely todominateas themain deactivationmechanism for thesestates. Themagenta fence-likeband across thesestates is roughly indicativeof thepossibility of couplingsof potentialenergy surfacesamong these states,i.e.,conical intersections(CIs).

4 Conc lusions

We have used femtosecond time-resolved photoelectron imaging coupled w ith time-resolved mass spectroscopy to observe the nonadiabatic dynam ics in electronically excited 2-methyl furan.The n=3 Rydberg states(i.e.,S1[1A″(π3s)],1A′(π3px),1A″(π3py)and1A″(π3pz))and thevalence state(1A′(ππ*)) are simultaneously excited from theground state and the average lifetimeof these states ismeasured to beon the time scaleof 50 fs.Ultrafast internal conversionsamong these statesare observed and dom inated as thenonradiative relaxationmechanism.

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Ultrafast Nonadiabatic Dynamics of Electronically Excited 2-Methyl Furan

LONG Jin-You1LIU Zhi-Ming1QIU Xue-Jun1,2ZHANG Bing1,*
(1State Key Laboratory ofMagnetic Resonance and Atomic and Molecular Physics,Wuhan Institute ofPhysicsand Mathematics, Chinese Academy ofSciences,Wuhan 430071,P.R.China;2College ofElectronicsand Information, South-CentralUniversity forNationalities,Wuhan 430074,P.R.China)

Excited-state dynam ics of 2-methyl furan has been studied by fem tosecond time-reso lved pho toelectron im aging.The m olecu le 2-m ethyl furan w as simu ltaneously excited to the n=3 Rydberg series of S1[1A″(π3s)],1A′(π3px),1A″(π3py)and1A″(π3pz)and the valence state of1A′(ππ＊)by two 400 nm photons and subsequently probed by two 800 nm photons.The average lifetime of the Rydberg series and the valence state wasmeasured to be on the time sca le of 50 fs by the time-dependent ion yie ld of the parent ion.Ultrafast internal conversions among these excited stateswere observed and extracted from the time-dependences of the photoelectron kinetic energy components of these excited states in the photoelectron kinetic energy spectra. Furthermore,it is identified that the1A′(ππ＊)statem ight play an important role in internal conversions among these excited states.The Rydberg-valencem ixings,which result in numerous conical intersections,actas the driving force to accom p lish such ultrafast internal conve rsions.

U ltra fast;Photoelectron im aging;Nonadiaba tic dynam ics;2-Me thy l furan

O644

tolow,A.Annu.Rev.Phys.Chem.2003,54,89.

10.1146/ annurev.physchem.54.011002.103809

doi:10.3866/PKU.WHXB201612061

www.whxb.pku.edu.cn

Received:September14,2016;Revised:December6,2016;Published online:December6,2016.

*Corresponding author.Email:bzhang@w ipm.ac.cn;Tel:+86-27-87197441.

The projectwas supported by the National Natural Science Foundation of China(21273274,21303255,11404411).

國家自然科學(xué)基金(21273274,21303255,11404411)資助項(xiàng)目?Editorialofficeof Acta Physico-Chim ica Sinica