Chemical Physics Letters 468 (2009) 134–137

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Chemical Physics Letters

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A time resolved study of S1/S2 electronic coupling in naphthalene

Raúl Montero, Asier Longarte, Roberto Martínez, Maria N. Sánchez Rayo, Fernando Castaño *

Departamento de Química-Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apart. 644, Bilbao 48080, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 October 2008In final form 2 December 2008Available online 10 December 2008

0009-2614/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.12.014

* Corresponding author. Address: Departamento deCiencias, Universidad del País Vasco, Barrio SarrienaSpain. Fax: +34 94 464 85 00.

E-mail address: f.castano@ehu.es (F. Castaño).

The relaxation dynamics of jet cooled naphthalene (NPH) has been studied by mass-resolved transientionization, following excitation to the S1(Lb) and S2(La) states and further single/multi photon ionizationat a variety of probe wavelengths. Time-dependent signals were collected from the parent C10Hþ8 and theC10Hþ7 fragment ions. The relaxation transients include two lifetimes of 30 and 800 fs. The short decaysuggests the presence of a conical intersection nearby the La state surface minimum that is crossed inone single passage by most part of the population, whereas the long lifetime is tentatively assigned eitherto internal vibrational redistribution (IVR) within the Lb state, or to the decay of coherent recurrences.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Naphthalene (NPH) molecule has been used as benchmark tomodel the electronic structure of aromatic molecules for more thanhalf a century. According to Hückel molecular orbital theory, NPHHOMO and LUMO are the p4–p5 and the p�6—p�7 doubly degenerateorbitals, respectively. Each of the two lowest singlet excited states,known as Lb and La in Platt’s nomenclature [1] due to the orienta-tion of the transition dipole moment, are a linear combination ofthe doubly degenerate HOMO and LUMO states. The small energygap between La and Lb states (being Lb more stable by 0.5 eV) re-sults in a strong vibronic interaction that rules the photophysicsof the molecule. The La S0 transition has a substantial oscillatorstrength, while Lb S0 is only weakly allowed and mostly contrib-uted by the vibronic coupling with the La state via b1g symmetryvibrational modes. The interaction with S3 (located 1.86 eV overLb) through ag modes has also been postulated to lend some inten-sity to the Lb S0 transition. It is commonly accepted that the rel-ative energy of La and Lb states and the extent of their couplingdictate the photophysics properties of naphthalene and its substi-tuted derivatives.

A considerable experimental and theoretical work has been de-voted to understand the Lb/La coupling in naphthalene and some ofits substituted derivatives, such as naphthol [2,3], aminonaphtha-lene [3–5] and methylaminonaphthalene [4,5]. The major sourceof experimental data has been the gas phase fluorescence spectros-copy. Detailed discussions on the spectroscopic consequences ofthe vibronic interaction are found in papers by Stockburger et al.[6,7], Beck et al. [8,9], and Behlen et al. [10,11]. The anomalous

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Química-Física, Facultad des/n., Lejona (Vizcaya) 48940,

intensity pattern observed in fluorescence emission of single vib-ronic levels of the Lb state reveals the interaction with the nearbyS2(La) and S3(1B3u) states (the energy ordering of these states is stilla matter of debate). The experimental results on NPH have beenmodelled in a number of theoretical papers [12,13], providing asatisfactory description of its electronic structure and the interac-tions between the lowest singlet excited states.

This letter presents a study of the Lb/La non-adiabatic couplingusing time resolved spectroscopy in the femtosecond time scale.To our best knowledge only Schmitt et al. [14] have previously re-ported on the dynamics of naphthalene Lb/La states in the ultrafastscale, providing a lifetime for La lower than 100 fs. Surprisingly, theexcited state dynamics of naphthalene radical cation has receivedgreater experimental and theoretical attention than the neutralNPH itself [15–17], likely due to its relevant role in astrophysics.In this study we have investigated the relaxation of the NPH low-est singlet excited states by pump–probe femtosecond ionizationspectroscopy. The Lb and La states of naphthalene were preparedby exciting with 304 and 267 nm laser pulses respectively andtheir time evolution probed by single or multiphoton ionizationat a number of wavelengths. The information collected in theparent ion mass channel is complemented with the transientsrecorded at the C10Hþ7 fragment channel. The relaxation of the La

state includes a 30 fs lifetime compatible with the internal conver-sion to Lb via a conical intersection. The ultrafast rate of the processsuggests a barrier less mechanism that places the crossing point inan energy range accessible by vertical excitation from S0. Some ofthe previously observed features of naphthalene electronicspectroscopy will be discussed in the light of our results.

2. Experimental

The experiments were carried out in a time-of-flight (TOF) massspectrometer using a 1 + n0 pump-probe ionization scheme [18],

-250 0 250 500 750 1000

0

1

Ion

curre

nt(a

.u.)

Time (fs)

Fig. 1. Transient of NPH+ ion collected by pumping at 304 nm and probing with800 nm pulses. The dashed line is the 1 + 40 non-resonant ionization signal ofethylene. Experimental data are shown as circles and decay fittings by lines.

R. Montero et al. / Chemical Physics Letters 468 (2009) 134–137 135

where n0 varies from 1 to 3 according to the probe wavelength. The1 kHz train of femtosecond pulses centred at 800 nm wasgenerated by a commercial Ti:Saphire oscillator-regenerativeamplifier system (Coherent) and further split in three beams. Onebeam drives an OPA (Opera, Coherent) tuneable in the 300–2600 nm range and used either as pump or probe depending onthe experiment. A second beam is tripled to 267 nm and employedonly for pumping purposes. Finally, in most experiments the fun-damental (800 nm) beam was used as the probe pulse. Thepump–probe delay is controlled by a linear translation stage with150 ps total equivalent travel and a resolution of 17 fs. Pump andprobe beams are appropriately focused and crossed into the ioniza-tion region of the TOF, reaching intensities of 109 and 1011 Wcm�2

respectively. The magic angle configuration was kept for the line-arly polarized beams except for the 332 nm probe and for thepolarization anisotropy r(t) measurements.

Naphthalene, purchased from Aldrich and used without furtherpurification, was heated up to 55 �C and seeded in Ar at 2 atm ofpressure before expansion through the 0.5 mm diameter nozzleof a pulsed electromagnetic valve (General valve). The supersonicjet formed was skimmed before entering the ionization region ofthe linear time-of-flight (TOF) spectrometer. The selected portionof the beam interacts with the femtosecond laser pulses yieldingions that are accelerated by appropriate electrostatic lenses alongthe 1 m TOF tube that ends in a 18 mm diameter dual MCP detec-tor. The resulting mass spectrum was monitored in a digital oscil-loscope and the ion signal intensities of selected masses (up to amaximum of three), integrated for each valve shot with a boxcarintegrator (Stanford SR250). The DC output of the boxcar togetherwith a signal proportional to the pump-probe delay was fed into anA/D converter and stored in a computer. Typical transients contain3 � 105 total shots yielding an average of 3 � 103 points at each de-lay time.

In order to set accurately the starting zero time, Dt = 0, and theinstrumental response function of the system the non-resonant1 + 40 transient of ethylene was recorded simultaneously to theproblem signal. Pulse widths (FWHM) of 100–180 fs (varying withthe wavelength) and 60 fs were measured for pump and probe,respectively.

-2000 0 2000 4000 6000 8000

0

10 600 1200

0

1

NPH+

NPH+-1

Ion

curre

nt

Time (fs)

b

NPH+

NPH+-1

a

Fig. 2. Short (a) and long (b) time scale transients of NPH+and NPH+-1 recordedfollowing pumping at 267 nm and probing with 800 nm. The dashed line is the1 + 40 non-resonant ionization transient of ethylene. Experimental data are shownas circles and decay fits by lines.

3. Results

The NPH La and Lb excited electronic states were prepared bypumping the Lb S0 transition with 304 nm pulses and the La S0

with 266 nm pulses. For probing, we tried single or multiphoton ion-ization at a number of wavelengths, looking for the larger sensitivityto the relaxation dynamics. Time-dependent signals were collectedat the parent (C10Hþ8 ) and the fragment C10Hþ7 ion mass channels,which are the only sizable intensity peaks appearing in the massspectra. The dynamical information contained in the fragment ionchannel is determined by the fragmentation mechanism. Bearingin mind the photostability of naphthalene La and Lb states, theC10Hþ7 ion is likely produced by fragmentation of the parent ion dur-ing the probe absorption. Thus, the fragment transients reflect theevolution of the fragmentation cross-section on the La/Lb PES andcontain information on the neutral NPH relaxation dynamics [19].

Fig. 1 depicts the transient collected for the parent ion withpump and probe at 304 and 800 nm, respectively and includesthe ethylene non-resonant signal for comparison purposes. Theblack dots stand for the experimental data while the solid linesare the computed exponential fits. No significant signal of theC10Hþ7 fragment is observed when pumping with 304 nm. TheNPH+ transient is dominated by a prominent peak centred atDt = 0 that matches the ethylene reference signal. At longer timesthe signal exhibits a broad background with a lifetime too long to

be accurately measured with our delay line (150 ps). The transientis fitted to the sum of two contributions: a non-resonant compo-nent with s = 0 and a constant ionization from the populated S1

state that extends to very long delay times (s =1). The promi-nence of the non-resonant 1 + 30 ionization may be attributed tothe small oscillator strength of the Lb S0 transition, 0.002 [20],while the background ionization is assigned to the long lifetimeof the Lb state (300 ns) [10]. Therefore, as expected no fast relaxa-tion is observed in NPH when excited to the bottom of the Lb sur-face well.

The behaviour of the system is substantially different uponexcitation to the La state (4.45 eV). Fig. 2 shows the transients ofthe parent and the fragment ion following excitation at 267 nmand subsequent multiphoton probing with 800 nm light. Thetime-dependent signals of both ions were fitted to multi-exponen-tials with the same lifetimes, namely: s1 = 30 fs, s2 = 800 fs ands3 > 100 ns. For the fragment ion an additional s0 = 15 fs contribu-tion was required to fit the rising wing of the signal. These lifetimesare transient times between different PES locations along the relax-ation pathway. From the exponential coefficients of the fitting, theprobe ionization/fragmentation cross-sections at each of thoselocations are calculated. Lifetimes and ionization/fragmentation

-200 0 200 400 600

0.0

0.1

0.2

Anis

otro

py r(

t)

Time (fs)

NPH+

NPH+-1

Fig. 4. Polarization anisotropy decays of NPH+ and NPH+-1, collected by pumping at267 nm and probing at 800 nm. Experimental data are shown as circles and decayfits by lines.

136 R. Montero et al. / Chemical Physics Letters 468 (2009) 134–137

cross-sections are collected in Table 1 for the entire set of pumpand probe wavelengths used in the study.

The sensitivity of the ionization/fragmentation process to thedynamics has been investigated using a number of probe wave-lengths. Fig. 3 shows the time-dependent signals of the parentand fragment ions recorded pumping at 267 nm and probing eitherwith 400 nm or 332 nm pulses. No lifetimes (Cf. Table 1) other thanthose already obtained with the 800 nm probe are observed,although the ionization cross-sections show a strong dependencewith the wavelength. A detailed discussion of this behaviour is pro-vided below.

Additional information on the probing process is obtained fromthe polarization anisotropy function r(t), recorded by probing with800 (Fig. 4) and 400 nm (not shown) pulses:

rðtÞ ¼ Ik � I?Ik þ 2I?

ð1Þ

where I|| and I? are the intensities when pump and probe polariza-tions are either parallel or perpendicular. The maximum polariza-tion anisotropy of the parent and the fragment ions are 0.1 and0.18 respectively. Positive polarization anisotropies are expectedwhether the laser probe meets a resonance with dipole momentparallel to that of the pump laser absorption. The fragment ionhigher value indicates that the probe laser reaches an additionalresonance. To model r(t), the individual parallel and perpendiculartime-dependent signals were fitted and convoluted with thecross-correlation function. The anisotropy shows a 30 fs decay,which is too fast to be originated by molecular rotation and matchesthe s1 lifetime already found in the parent and fragment ionstransients.

Table 1Lifetimes (si) and ionization cross-sections (si) extracted from the exponential fits ofNPH+ and NPH+-1 transients collected at a number of pump and probe wavelengths.The error bars are the standard deviations computed from five scans.

0 fs 15 ± 5 fs 27 ± 6 fs 750 ± 100 fs >100 ns

304/800 mn NPH+ 0.7 – – – 0.6267/800 mn NPH+ – – 1 0.43 0.42

NPH+-1 – 0.45 1 0.42 0.53267/400 mn NPH+ – – 1 – 1

NPH+-1 – – 0.1 – 1267/332 mn NPH+ – – 1 0.08 0.06

NPH+-1 – 0 1 0.18 0.16

-2000 0 2000 4000 6000 8000

0

1-200 0 200 400 600

0

1

NPH+

NPH+-1

b

Time (fs)

Ion

curre

nt (a

.U.)

NPH+

NPH+-1

a

Fig. 3. Transients from the NPH+ and NPH+-1 ion channels, recorded by pumping at267 nm and probing either at 400 nm (a) or at 332 nm (b). Experimental data areshown as circles and decay fittings by lines.

4. Discussion

The lifetimes of NPH S1(Lb) and S2(La) states in gas phase havebeen determined to be 300 and 60 ns respectively [10]. Conse-quently, at the time scale used in this work, no significant popula-tion reaches the ground state and only dynamical informationabout the S1 and S2 states is obtained.

The transients measured following pumping at 304 nm to the Lb

state do not contain any dynamical feature, as it is expected fromthe vibronic resolved spectrum [6–11]. However, these measure-ments substantiate the relevant contribution of the non-resonantprocess that efficiently competes with the 1 + 30 resonant ioniza-tion. At the stated pump wavelength the interaction with the La

state does not severely perturb the Lb vibrational levels and thecoupling is manifested in the anomalous intensity pattern of thefluorescence excitation and emission spectra [6,8,13].

Pumping NPH with 267 nm pulses yields the S2(La) state, withsome 1500 cm�1 energy excess over its band origin(35900 cm�1). The transients of the parent and fragment ions ex-cited at this wavelength and probed at 800, 332 and 400 nm havebeen fitted using the same set of decay constants. However, the rel-ative weights of the components vary with the probe wavelengthand are not the same in the parent and fragment ion transients.The different sensitivity of the probing process aids to understandthe origin of the observed dynamical features. The s0 = 10 fs relax-ation time required to fit the raising wing of the fragment tran-sients is interpreted as vibrational motion out of the Frank–Condon vertical region where the system is excited to. This contri-bution does not appear in the decays of the parent ion. Thes1 = 30 fs lifetime is associated to the internal conversion fromthe initially prepared La state to the lower PES of the Lb state.The ultrashort lifetime, which is shorter than a vibrational period,suggests that the relaxation occurs through a conical La/Lb surfacecrossing. The conical intersection (CI) is presumably located some1000 cm�1 above the La well, as confirmed by the severely per-turbed vibrational spectrum observed for this region [8].

A key aspect to be addressed is the wave packet branching ratioat the CI. The time-dependent polarization anisotropy measure-ments contain relevant information regarding this aspect of thesystem time evolution. The r(t) function of the parent and fragmentions reaches a maximum of 0.1 and 0.18 respectively, decaying inboth cases with a lifetime of 30 fs. This relaxation time matches thedynamics observed at the magic angle configuration and it is toofast to be related to rotation. The positive anisotropy is attributedto the resonant absorption of photons from the La state PES, whilst

R. Montero et al. / Chemical Physics Letters 468 (2009) 134–137 137

the higher value of the fragment is due to resonant absorption ofextra probe photons in the ion that leads to fragmentation. The30 fs relaxation lifetime of the anisotropy decay represents the exitfrom the La PES, and since the function drops to zero we can statethat most of the population crosses to the Lb PES state in a singlepassage. This interpretation is corroborated by the photoelectronexperiments of Schmitt et al. [14], where the band correspondingto the ionization of La state into the ground state of the ion (D0) dis-appears within the cross-correlation time.

It is worth noting that the ionization cross-sections from the La

and Lb states and hence, the sensitivity to the dynamics depends onthe probe wavelength. The 332 nm pulses have been found to bethe most sensitive probe for the La ? Lb relaxation. At this wave-length the ionization from the La state is accomplished with a sin-gle photon and the energy excess is small (�1500 cm�1 over D0).When the system relaxes to the Lb state, the stored vibrational en-ergy shifts the Frank–Condon window for ionization to higherenergies and the ionization cross-section notably diminishes. Incontrast, when the molecule is probed with 400 nm pulses the ini-tial spike that accounts for the La ? Lb relaxation disappears. In thiscase two 400 nm photons are required to reach the IP from the La

state, providing an energy excess around 21800 cm�1 and17500 cm�1 above D0 and D1 respectively, the two ion states thatcorrelate with La under Koopman’s propensity rules. The Lb statealso correlates with D0 and D1 ion states and therefore, the largeenergy excess takes the system to a region where the ionizationprobability from either La or Lb states is quite similar. Finally, prob-ing at 800 nm has an intermediate sensitivity. It has been shownfor multiphoton ionization probing that the longer the wavelengththe more sensitivity [21,22], due to the facility of meeting selec-tively intermediate resonances and the more efficient transfer ofvibrational energy to the ion. The fragmentation cross-section alsodepends on the probe wavelength, as observed in the transients ofthe fragment ion collected at the different wavelengths. Fragmen-tation decreases from La to Lb with the 332 and 800 nm probe,whilst it increases if the 400 nm light is used.

The influence of the probe beam intensity on the transients wasstudied by increasing it up to one order of magnitude. While thereported lifetimes remain noticeably unchanged, a minor decreaseof sensitivity with the probe beam intensity was detected. This re-sult has been previously reported [23] and suggested to be due tothe ion signal strong dependence with the probe power after theelectronic/vibrational relaxation.

In addition to the 30 fs decay lifetime, the fragment and parentions have an additional component of 800 fs. The measurements re-ported do not allow us to establish unambiguously the origin of thislong lifetime and we suggest two alternative mechanisms that mayexplain it. First, the decay may be caused by internal vibrationalredistribution (IVR) within the Lb state, following internal conver-sion through the conical funnel. IVR rate constants as high as4 � 1011 s�1 have been estimated from the spectral width of singlevibronic levels at 4300 cm�1 above the Lb origin [23]. In our experi-ment, the population enters the Lb PES state with �5400 cm�1 ofvibrational energy content and hence, rate constants are expectedto be of the same order or higher. As IVR proceeds, the ionizationand fragmentation cross-section change but not in the same direc-tion. Note that while the ionization cross-section decreases withthe 800 fs component when 800 nm probe pulses are used, the frag-mentation increases (Cf. Fig. 2b) due to the efficient transfer of vibra-tional energy to the ion. Alternatively, the 800 fs lifetime may beoriginated by wave packet dynamics. The decay would reflect theelectronic recurrences of the system during the time it evolves onthe La and the Lb surfaces following the CI crossing. These populationrecurrences in both states may be washed out due to the loss ofcoherence along the different vibrational modes involved in theelectronic state coupling, resulting an exponential decay as the only

observable. As mentioned above, the vibronic spectrum of the mol-ecule in this region shows the interplay of a number of vibrationalmodes in the La/Lb states coupling [8,17]. The fact that the polariza-tion anisotropy decay does not reproduce this process, which shoulddo if a resonance is met when probing from the La state, may beunderstood considering the small contribution of this componentin the measurements collected at the magic angle.

5. Conclusions

We have measured the relaxation dynamics of isolated naph-thalene prepared in the lowest singlet excited states, Lb(S1) andLa(S2). The transients of the La(S2) state reveal its internal conver-sion to the Lb(S1) with a short decay lifetime of 30 fs. The ultrafastrate of the process points out to the existence of a conical intersec-tion in the neighbourhood of the La state minimum. Most popula-tion enters in the Lb PES through the conical funnel in a singlepassage. The resultant fluorescence pattern reflects the internalvibrational redistribution that the system undergoes after crossingto the Lb state [9].

While the experiments described here offer a comprehensivepicture of the ultrafast relaxation, some relevant aspects of theproblem require further theoretical and experimental work. Calcu-lations able of establishing the accurate location of the La/Lb CI andthe relevant vibrational coordinates, in addition to experimentaldata on the vibrational content of the population transferred tothe Lb state, are essential aspects to get a more detailed pictureof the relaxation process.

Akcnowledgments

We acknowledge the support of MEC under Grant CTQ2003-0510 and Consolider Program ‘Science and Applications of UltrafastUltraintense Lasers’ CSD2007-00013. We also thank the BasqueGovernment (BG) funding through a Complimentary Action andthe UPV-EHU Consolidated Group Program. The experimental workwas carried out at the Laser Facility of the SGIker UPV/EHU, spon-sored by FEDER, MCYT and BG. A.L. acknowledges MEC for a Ramóny Cajal contract.

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