D11AS2-RI92 478 EXCITED STATE ENERGETICS AND DYNAMICS OF LARGE i/MOLECULES COMPLEXES AND CLUSTERS(U) TEL-AVIV UNIV

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4th Periodic Re2ort

1. Title: EXCITED-STATE ENERGETICS ANDDYNAMICS OF LARGE MOLECULES,COMPLEXES AND CLUSTERS

2. Pri:ncipal Investigator: Professor Joshua Jortner

3. Associate Investigator: Professor Uzi Even

4. Contractor: School of ChemistryTel Aviv UniversityTel Aviv 69978Israel

5. Contract No.: DAJA45-85-C-0008

The Research reported in this document has been made, possible through the support and sponsorship of the

U.S. Government through its European Research Officeof the U.S. Army. F,,. . ,d 1 r

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6. PRO RESS IN TECHNICAL APPROACH

New techniques for spectroscopy in supersonic expansions

and in the development of specific supersonic sources weredeveloped.

6.A Vacuum Ultraviolet Absorption'Spectroscopy in SupersonicExpansions. We have combined the techniques of vacuum ultra-violet (VUV) spectroscopy together with planar supersonicjets, which allows for the interrogation of absorption spectraof large molecules cooled in supersonic expansions in the nearVUV region.* The experimental setup consists of a high pres-sure Xe lam , CaF 2 optics, a vacuum ultraviolet spectrographand a nozzl slit (0.27x90 mm, repetition rate 9 Hz and gaspulse dura ion 300 Usec). The characteristics of this spectro-scopic set p are: (i) Energy range 6-10 eV. (ii) Spectralresolutio 0.1 A. (iii) Routine measurements of high-energyabsorpti spectra. (iv) Interrogation of fluorescence excita-tion sp ctra of the parent molecule or its photoproducts withlimiti g quantum yields of Y > 10- 4 .

6.B Deveopment of Conical Nozzles r Supersonic Jets. Conicalnozzles (nozzle opening angle(a = 30Q, and nozzle diameterD = 0.3 mm) were constructed and used in conjunction with amagnetic pulsed valve. The use of conical nozzles considerablyenhances clustering in supersonic expansions, facilitatingstudies of large van der Waals complexes and clusters.

7. ACCOMPLISHMENTS OF OBJECTIVES

7.A Energetics of Rydberg States of Jet Cooled Molecules. VUVabsorption spectra of benzene, benzene-D 6 and naphthalenecooled in planar supersonic expansions were measured over therange 2000-1600 A, providing evidence on energetics, linebroadening and interference effects. , .- figio, ror

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7.B Rydberg States of Anthracene. Several Rydberg transitionsof jet-coo ed anthracene were observed in the spectral region1900-2000 A. These Rydbergs are superimposed on a broad"background" of ww* transitions. Up to now no fluorescencefrom Rydb rg states of polyatomic molecules has been reported.The 1997 A n=3 Rydberg of anthracene reveals fluorescencewith a quantum yield of -. 5%. This fluorescence originatesfrom internal conversion R "d* S, followed by S1 -- * So emission,as documented by the spectral distribution of the dispersedfluorescence.

7.C Intramolecular Relaxation of Rydberg States. Information onintramolecular dynamics of extravalence excitations of benzenewas obtained from lineshape analysis. The lineshape of the3Pxv(O) Rydberg is Lorentzian, whose homogeneous width resultin The lifetime T = 0.19±0.02 psec for CH 6 and T = 0.22±0.02psec for C6D6. Semiquantitative information on the lifetimesof some Rydbergs of anthracene was obtained, which fall in therange of ", 0.1 psec. These lifetime data imply that (i) therelaxation of the Rydberg is characterized by moderate energygap~s), and Cii) the electronic relaxation rate of the Rydbergis considerably less efficient than that of the intravalenceexcitation in the same energy domain.

7.D Interference Effects between Extravalence and IntravalenceMolecular Excitations. We have searched for Rydberg-valenceinterference effects in large molecules, which are expected tobe exhibited in asymmetric Fano-type lineshapes in absorption.For "isolated" Rydbergs, which are superimposed on a irn* tran-sition in benzene, naphthalene and anthracene, the absorptionlineshapes are symmetric Lorentzians. No line asymmetry andno antiresonances characteristic of Fano profiles were foundin that case. The absence of interference effects in this casereveals the manifestation of random interstate coupling of Rwith the w* manifold. The random coupling erodes all inter-ference effects. The situation is different for nearly lyingRydbergs, where pronounced R-R-valence interference effectswere observed in the absorption spectri of jet'-cooled naphtha-lene in the spectral region 1600-1650 A, providing informationon the homogeneous contribution to high-energy molecular coupl-ing phenomena.

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7.E Rotational State Dependence'ofIntramolecular Dynamics. Rota-

tional effects on interstate coupling are of considerable cur-rent interest. Absolute fluorescence qpantum yields from photo-selected rotational states were measured for the electronicorigins of the S, state of pyrazine. Strong rotational statedependence was observed providing novel information on inter-state coupling for the intermediate level structure. Thisunique information cannot be extracted from time-resolved decaylifetimes.

7.F The Coupling betweenIntrastateVibrational EnergyRedistributionand Interstate Electronic Relaxation. We have documented someuniversal characteristics of the decay lifetimes and fluorescencequantum yields from the S, manifold of large molecules, whichoriginate from the coupling between intrastate vibrational energyredistribution and interstate electronic relaxation. The time-resolved total fluorescence decay excited by a psec laser from theS state of jet-cooled 9CN-anthracene exhibits nonexponential decayin the energy range EV = 1200-1740 cm-1 above the S, origin, whichdoes not originate from dephasing but rather manifests the effectsof intrastate intermediate level structure for vibrational energyredistribution on intersystem crossing.

7.G Coriolis Rotation-Vibration'Couplingand Intramolecular Dynamics.We have demonstrated that rotational effects play a central roleon intramolecular vibrational energy redistribution in electronic-vibrational excitations of large molecules. Strong rotationaleffects on the fluorescence quantum yields from vibrational states(above 1000 cm-1 ) in the S, manifold of 9-cyanoanthracene wereobserved, which demonstrate that Coriolis interactions serve asthe dominant coupling leading to intramolecular vibrational energyredistribution (IVR). In contrast to common wisdom, whichattributed IVR in large molecules to anharmonic interactions, wehave shown that rotational effects play a central role stronglyenhancing interstate electronic relaxation.

7.h Lesonances in Mediated Intersystem Crossing. The mechanisms ofdirect and of mediated intersystem crossing from the first excitedsinglet manifold of anthracene and some of its deriyatives wereexplored by the study of the internal and the external heavy atomeffect on the fluorescence quantum yields. Pronounced mode selec-tivity was observed in the vibrational energy dependence of theemission quantum yield in 9-bromoanthracene and 9.10-dibromo-anthracene

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7.1 Fluorescence Quantum*Yields for Highly-Excited States of LargeMolecules. Fluorescence quantum yields Y from high electronicexcitations of naphthalene, anthracene and tetracene in the energyrange 5.0-6.5 eV have been recorded. Y exhibits an exponentialdependence on the excess vibrational energy with the slope decreas-ing with increasing size of the molecule, i.e., the vibrationaldensity of states. These data are characteristic of internal con-version from S1 to So. These results are of interest regardingrecent astrophysical implications of high-energy photophysics oflarge aromatic hydrocarbons in outer space.

7.J Photoisomerization Dynamics of Trans-Stilbene and'of Cis-Stilbene.Time-resolved fluorescence lifetimes from photoselected states oftrans-stilbene were recorded by the techniques of picosecond spec-troscopy in jets using a mode-locked dye laser and a fast photoncounting system. Decay lifetimes as short as 100±30 psec were

__recorded. Extensive information on the energy dependence of theisomerization rates of alkyl stilbenes was obtained, providinginformation on the role of intramolecular vibrational distribu-tion on the photochemistry in an isolated molecule. The absorp-tion spectrum of jet-cooled cis-stilbene is broad and no vibra-tional structure could be resolved due to the congestion ofbroadened low-frequency vibrational excitation. From the lowquantum yield Y $ 2xl0 "4 we infer a S, lifetime of T $ 0.4psec, which indicates ultrafast relaxation of this molecule.

7.K Photoisomerization Dynamics of Alkyl 'SubstitutedTrans-Stilbene.A central issue pertaining to isolated-molecule photochemistryinvolves the role of intramolecular vibrational relaxation indetermining intramolecular dynamics. We have attempted to in-crease the density of vibrational states by alkyl substitutionof trans-stil[,ene and have explored the isomerization dynamicsby pi.cosecond time-resolved spectroscopy. We have obtained thecounter-intuitive result that alkyl substitution of trans-stilbene enhances the photoisomerization rates, while generalarguments based on the role of IVR and the implications ofstatistical theories indicate that the rate should be retarded.The results can be accounted for by the modification of themolecular parameters, i.e., threshold energy, by alkyl substi-tution.

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7.L Energy-Resolved Photoisomerization Rates. The dynamics of the S1and S2 electronically excited singlet states of diphenylbutadienewas interrogated by fluorescence quantum yield measurements overthe very broad energy domain of 0-7500 cm-1 above the (false) S1origin. The issues of the lack of mode selectivity and the appli-cability of statistical theories for the description of isolated-molecule photochemistry were explored.

7.At van der Waals Complexes of Porphyrins. Excited-state energeticsand dynamics of large complexes consisting of porphyrins bound torare-gas atoms were explored. Detailed spectroscopic informationon the So - S1 and SO - S2 transitions of the free-base porphine.Ar complex was obtained, providing insight into the structure ofthis complex as well as resulting in a novel mechanism for micro-scopic solvent shifts induced by configurational distortions,which are due to complexing.

7.N Coupling between IntramolecUlar and Intermolecular Nuclear Motionin Complexes. Intermolecular vibrations of large van der Waalscomplexes, which involve the motion of the ligand relative to thelarge molecule, provide an analogue for surface vibrational motionin a finite system and constitute the precursors of phonon modesin condensed phases. Information on the coupling between inter-molecular and intramolecular vibrational motion was obtained forthe trans-stilbene-Ar complex.

7.0 Analogies between Large van der Waals Molecules and Microsurfaces.The binding of rare-gas (R) atoms to large aromatic molecules (M)on the one hand, and to the basal plane of graphite on the otherhand, represent two extremes of the interaction of R atoms withordered arrays of sp2 hybridized carbon atoms. We have exploredthe analogies between the characteristics of large van der WaalsM.Rn complexes and R atoms on graphite surfaces, focusing onstructure, packing, orientational registry effects and the nuclearmotion of R adsorbates on finite microsurfaces. The elucidationof the structure, energetics and nuclear dynamics of large M.Rncomplexes rests on semi-empirical model calculations of potentialsurfaces. These provide a quantitative account of the geometry,the existence of isomers, the dissociation energies and the fre-quencies of out-of-plane and in-plane vibrational modes for largeamplitude intermolecular nuclear motion.

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7.P Electron Localization in Clusters. Small clusters exhibit uniquephysical and chemical phenomena, which are both of fundamental andtechnological significance, and provide ways and means to explorethe "transition" from molecular to condensed-matter systems. Wehave provided a theoretical study of the structure, energetics anddynamics of an excess electron interacting with an alkali-halidecluster, which was explored by the quantum path integral moleculardynamics method. These studies establish various compositional,structural and size dependence of bulk and surface localizationmechanisms of the dynamic process induced by electron attachment.

7.Q Vibrational Predissociation Induced by Exciton Trapping in Rare-GasClusters. The dynamics of exciton trapping, vibrational energytransfer and vibrational predissociation in an electronically excitedstate of Arj 3 clusters was explored by classical molecular dynamics.This study constitutes an application of this technique for thei dynamics of electronically excited states of large systems. Newmechanisms of ultrafast (-. 10 psec) vibrational energy flow inducedby short-range repulsion were documented. In these systems vibra-tional energy redistribution does not occur and mode selective excita-tion prevails. In small (n=13) clusters the consequences of vibra-tional energy flow results in reactive vibrational predissociation,leading to the "evaporation " of Ar atoms for the cluster. In layer(n-55) clusters a transition from molecular-type reactive behaviorto nonreactive vibrational relaxation, which is characteristic ofcondensed phases, was exhibited.

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8. PUBLICATIONS

The following manuscripts, supported by this researchgrant, were prepared and submitted for publication:

(1) A. Amirav and J. JortnerVacuum Ultraviolet Absorption Spectroscopy in Super-

sonic ExpansionsJ. Chem. Phys. (Communication) 82, 4378 (1985)

(2) A. Amirav and J. JortnerRotational and Vibrational State Dependence on Intra-

molecular Coupling and Dynamics in the S, State ofPyrazi ne

J. Chem. Phys. (in press)

(3) J. Troe, A. Amirav and J. JortnerEnergy-Resolved and Thermalized Photoisomerization Rates

of Diphenyl-ButadieneChem. Phys. Letters l15, 245 (1985)

(4) A. Amirav, M. Sonnenschein and J. JortnerInterstate Coupling and Dynamics of Excited Singlet Statesof Isolated Diphenylbutadiene

Chem. Phys. 102, 305 (1986)

(5) U. Even, Z. Berkovitch-Yellin and J. JortnerElectronic Excitations of the Free-Base Porphine.Ar van

der Waals ComplexCanad. J. Chem. (C. Sandorfy-Special Issue) 63, 2073 (1985)

(.6) D. Bahatt, U. Even and J. JortnerCoupling between Intramolecular and Intermolecular Nuclear

Motion in a Large van der Waals ComplexChem. Phys. Letters 117, 527 (1985)

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8. PUBLICATrONS (cont'd)

(7) Klaus Rademann, Uzi Even, Shlomo Rozen and J. JortnerPhotoisomerization Dynamics of Alkyl Substituted Stilbenes

in Supersonic Jets.Chem. Phys. Lett. 125, 5-11 (1986)

C8) A. Amirav, J. Jortner, S. Okajima and E. C. LimManifestations of Intramolecular Vibrational Energy

Redistribution on Electronic Relaxation in Large MoleculesChem. Phys. Letters (in press)

(91 U. Landman, D. Scharf and J. JortnerElectron Localization in Alkali-Halide ClustersPhys. Rev. Letters 54, 1860 (1985)

(101 Dafna Scharf, Uzi Landman and Joshua JortnerEnergetics and Dynamics of ClustersProc. of the Int'l School of Physics "Enrico Fermi", Italy,1986, in: Advances in Chemical Reaction Dynamics (D. ReidelPublishing Company)

(11) D. Scharf, U. Landman and J. JortnerVibrational Predissociation Induced by Exciton Trapping in

Inert-Gas ClustersChem. Phys. Letters 126, 495-500 (1986)

(12) Dafna Scharf, Uzi Landman and J. JortnerSurface States of Electrons and Alkali-Halide ClustersProc. of the 19th Jerusalem Symposium, Israel, 1986. EdsJ. Jortner and B. Pullman (D. Reidel Publishing Company,Holland, 1986)

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(13) Samuel Leutwyler and Joshua JortnerThe Adsorption of Rare-Gas on Microsurfaces of LargeAromatic MoleculesJ. Phys. Chem.

(14) Dafna Scharf, Uzi Landman and Joshua JortnerAtomic and Molecular Quantum Mechanics by the PathIntegral Molecular Dynamics Method

Chem. Phys. Letters

(15) A. Amirav and Joshua JortnerRotation, Vibration and Electronic RelaxationProc. of the CECAM Conference, Orsay, France, 1986

(16) A. Amirav and J. JortnerResonances in Mediated Intersystem Crossing of Jet-Cooled Anthracene Derivatives

Chem. Phys. Letters

(17) A. Amirav, J. Jortner, M. Terazima and E.C. LimRotational Effects on Intramolecular RadiationlessTransitions in a Large MoleculeChem. Phys. Letters 132, 335-340 (1986)

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