MONOLAYER SURFACE RAMAN AND FLUORESCENCE SPECTROSCOPY OF THE ANTHRACENE CRYSTAL

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MONOLAYER SURFACE RAMAN AND FLUORESCENCE SPECTROSCOPY OF THE

ANTHRACENE CRYSTAL

M. Orrit, J. Bernard, J. Turlet, Ph. Kottis

To cite this version:

M. Orrit, J. Bernard, J. Turlet, Ph. Kottis. MONOLAYER SURFACE RAMAN AND FLUORES-

CENCE SPECTROSCOPY OF THE ANTHRACENE CRYSTAL. Journal de Physique Colloques,

1983, 44 (C10), pp.C10-479-C10-482. �10.1051/jphyscol:19831096�. �jpa-00223554�

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JOURNAL DE PHYSIQUE

Colloque CIO, suppl6ment au n012, T o m e 44, dkcembre 1983 page Cl0-479

MONOLAYER SURFACE RAMAN AND FLUORESCENCE SPECTROSCOPY O F T H E ANTHRACENE C R Y S T A L

M. Orrit, J. Bernard, J . M . Turlet and Ph. Kottis

Centre de Physique MoZ6cuZaire Optique e t Hertzienner, UniversitS de Bordeaux 1, 33405 Talenee Ceder, France

R6sumC : L'observation du spectre d'excitation de la fluorescence de l'exciton de surface d'un cristal d'anthracsne non recouvert et recouvert d'un film d'azc- te, nous a permis de mettre en gvidence un mode de phonon de surface d'gnergie plus basse que le mode correspondant dans le volume. La forme de raie de cette structure et son comportement lorsque la surface est recouverte, sont analysCs en termes de diffusion Raman rgsonnantede surface,derelaxations intra-surface et vers le volume et de couplage entre la surface d'anthracsne et le film dSposS.

Abstract : We report evidence for of a surface lattice phonon with lower energy than its bulk counterpart,by observation of the excitation spectrum of surface exciton fluorescence for uncoated and N2-coated anthracene crystal (001) face.

Theline-shapeofthissurfacestructureanditsbehaviouronsurfacecoat~ngareanalysed in terms of surface resonant Raman scattering, intrasurface and surface-to-bulk relaxation processes and coupling between the anthracene surface and the N2 film.

I

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INTRODUCTION.

Organic molecular crystals with bulk electronic excitons of the Frenlcel type, may present also monolayer surface and sub-surface collective excitations (Site Shift Surface Excitons) due to the specific environment (lacking or different van der Waals interactions) seen by surface molecules.Their existence on a parti-cular face of the crystal, is simply reLated to the dominant intraplane character of the excitation energy exchange sum I(k), for a wave-vector direction normal to this face.

At the opposite of Surface Polaritons, that are non-radiative evanescent modes exten- ding one wave length deep into the crystal (macroscopic surface), Surface Excitons are radiative bidimensional Frenkel excitons, localized essentially on the first few monomolecular layers of the crystal {

11.

+ +

In the anthracene crystal, organized as a stack of (a,b) planes very weakly coupled by coulombic dipolar interactions, such surface and sub-surface exciton resonances have been observed for the first singlet-singlet transition (1.25000 cm-l){21 As the exciton-photon coupling is quite strong for this transition and correlatively the light penetration into the crystal very shallow, the polariton dispersion behavior leads, at low temperature, to a high and broad bulklreflectivity of the (001) face,on which surface exciton resonances appear as interference figures. (See I, I1 and 111 on the upper spectrum on Fig. 1). The surface origin of these structures, that do not appear in the reflectivity spectra of the other crystal faces, has been unambiguously established by observing their reversible low energy shift upon coating of the crystal surface with a transparent layer of frozen gas {21.

Due to their bi-dimensional character, surface excitons coupled to the radia- tion field, have a high radiative unstability (r '1. picosecond) that overcomes intrasurface and surface-to-bulk non radiative relaxation processes ( 3 ) . This accounts for the very weak fluorescence lines I and I1 (see lower spectrum on Fig. 1) observed on the high ener y side (+ 207 cm-I and + 6 cm'l) of the bulk fluorescence origin ;E'( = 25093 cm-

B

) , at the same energy as surface reflectivity structures, and e x h i b i e ~ n ~ the same behavior upon surface coating {41. Line I (EoO = 25300 cm-I), in- terpreted as the collective emission from the lowest Davydov component of surface sb

* ( L . A . 283 du C.N.R.S.)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831096

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JOURNAL DE PHYSIQUE

Fig. 1

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Reflectivity and fluorescence spectra of the (001) face of a sublimation grown anthracene monocrystal at 1.7 K.

exciton on the first monomol.ecular layer, is of particular interest for surface stu- dy, since it is 200 cm-'far apart from any bulk emission.

We took advantage of this last point to investigate the vibration modes of surface molecules for uncoated (helium-anthracene interface) and coated (frozen nitrogen- anthracene interface) crystal (001) face, using a technique combining excitation spectra of the surface emission (line I) with gas condensation experiment. It allowed us to unveil very weak surface structures from the bulk vibronic background, while practically no surface information may be extracted from the reflectivity spectrum of the vibronic region, where numerous broad and intense bulk modes overlap. The general theory of the exciton-vibration coupling, leading in the anthracene crystal to mixed vibron and two-particle states, as well as the experimental results concerning the dominant 1400 cm-' and 390 cm-I intramolecular anthracene modes, have been discussed dsewhere {5,6). We present here, for the first time to our knowledge, the experimen-

tal evidence of a surface lattice phonon mode on an organic molecular crystal, and analyse the line shape of the surface structures in terms of specific relaxation dy- namics of the surface excitation.

11

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EXPERIMENTAL RESULTS.

All surface experiments have been done with sublimation-grown monocrystals((001) plates, a few microns thick), starting from extensively zone refined anthracene. More experimental details, as well as the description of the technique used to condense gas on the surface, may be found in {1,2,5} and in the references therein. Bulk Ra- man experiment wasperformed at 5K on a melt grown sample (2 mm thick), using the 4579.39 A ~r' line of a 171 Spectra Physics laser,normally incident on the (001) face and a U 1000 Jobin Yvon double monochromator to detect the 90" scattered beam.

At low temperature (1.7K), the b polarized reflectivity spectrum (upper spec- + trum on Fig. I) exhibits at the energy ; E: + 46 = 25138 cm-

,

the neat dip already ascribed ( 2 1 to the bulk phonon sideband Involving the 49 cm-I Ag lattice phonon mode observed in Raman spectroscopy ( 7 3 . On the same spectrum, with careful attention, one may observe at about EOO + 46

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25342 em-', a slope discontinuity that might be the surface counterpart ofSbhe bulk phonon sideband mentionned above. However no de-

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Fig. 2 - Excitation spectra of the surface emission (line I) of the (001) face of a sublimation-grown anthracene monocrystal at 4.2 K : for E//b and uncoated crystal surface (upper spectrum); for E//b and N2 coated crystal surface (middle spectrum);

for E//a and uncoated crystal surface.

The schematic profile of the b polarized structure, with the three regions A, B, and C, is represented above.

I

3W 302 388 384

J

Wave Length (nm)

finite information can be extracted from a so diffuse structure.

The b polarized excitation spectrum (upper spectrum on Fig. 2) of the surface + emission (line I), presents, at the opposite of the reflectivity spectrum, a very intense and sharp peak (A) at the energy EOO + 46 = 25344.3

+

0,5 cm-l, followed on

s b

the high energy side, by a dip ( B ) and a broad band (C). We can note already(see the inset on Fig. 2) that its bandwidth (7.2 cm-l), corresponding roughly to the excitation plus detection bandwidths, is much thinner than the excited band (18 cm-l).

Upon surface coating with frozen nitrogen, the peak (A) is drastically affected and changes to a bsoad band, shifted to lower energies (middle spectrum on Fig. 2) roughly like the b polarized emission EOO

sb' 111

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DISCUSSION.

Obviously, the location, the shape and the behavior upon surface coating of the structure EOO + 46 described above, permit to assign it to the "46 cm-l"

b (more exac-

tly 45.1 2 f.0 cm-l) Ag lattice phonon sideband associated with the surface exciton EOg. From a general view point, covering the pure electronic and vibronic region, tzls last result once again confirms the overall translational equivalence (6~200cm-~) of surface states with bulk states. However several particularities exist in the shape, the energy value and the surface coating behavior of the surface phonon structure and must be now analysed in order to find the specificity of surface mo1ecule.s.

The shape of the structure EoO + 46 in the excitation spectrum of the surface s b

emission (line I), differs greatly from the stepwise shape expected for a two particle absorption edge (see bulk reflectivity for instance). According to the model we pro- posed recently 1 6 1 , this result reflects a strong energy dependence of the relaxation inside the surface excitonic band : the peak (A) originates from the direct re-

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ClO-482 J O U R N A L DE PHYSIQUE

l a x a t i o n of t h e e x c i t i n g photon (EoO + 46) i n t o a s u r f a c e 46 cm-I phonon and a Raman s c a t t e r e d photon i n r e s o n a n c e withskhe s u r f a c e r a d i a n t s t a t e EO; ; t h i s i s b a s i c a l l y a S u r f a c e Resonant Raman S c a t t e r i n g p r o c e s s . The broad band (c$ a r i s e s from c r e a t i o n

of a ~ k s u r f a c e e x c i t o n and a f ~ 46 cmwi s u r f a c e phonon,followed by i n t r a s u r f a c e s b

r e l a x a t i o n t o t h e s u r f a c e r a d i a n t (k=o) s t a t e E O O , w h i l e t h e d i p (B) r e v e a l a quen- s b

c h i n g of t h i s i n t r a s u r f a c e p r o c e s s by any o t h e r n o n - r a d i a t i v e c h a n n e l a s f o r i n s t a n - c e s u r f a c e t o b u l k r e l a x a t i o n .

According t o t h i s a s s i g n m e n t of t h e peak A a s a Raman l i n e , t h e s u r f a c e phonon f r e q u e n c y t h a t we quoted (45.1 2 1.0 cm-l) must be compared w i t h i t s b u l k c o u n t e r p a r t e q u a l l y d e d u c e d from Ramam s p e c t r o s c o p y . I n o r d e r t o improve t h e a c c u r a c y of t h e l a s t r e p o r t e d v a l u e { 7 )

,

we r e c o r d e d t h e b u l k Ramam spectrum of a n t h r a c e n e a t 5K and found 49.35

+

0 , l cm-I f o r t h e symmetric mode Ag and 56.97

+

0 , l cm-' f o r t h e a n t i s y m m e t r i c mode Bg c o r r e s p o n d i n g t o t h e v i b r a t i o n mainly around t h e normal mole- c u l a r a x i s . ? h i s s i g n i f i c a n t l o w e r i n g of t h e v i b r a t i o n a l energy of t h e molecules on t h e c r y s t a l s u r f a c e , due t o a l a c k of van d e r Waals i n t e r a c t i o n s , i s t h e e v i d e n c e of a p o s s i b l e r e c o n s t r u r t i o n of t h e s u r f a c e monolayer, a s i t was a l r e a d y s u g g e s t e d by d i s p e r s i o n a n a l y s i s of t h e s u r f a c e e x c i t o n . { 5 , 6 }

F i n a l l y , upon N 2 c o a t i n g of t h e c r y s t a l s u r f a c e , we observed a d r a s t i c p e r t u r b a t i o n (broadeningand s h i f t t o lower e n e r g i e s ) of t h e s h a r p s u r f a c e phonon s t r u c t u r e t h a t may r e f l e c t e i t h e r an inhomogeneous broadening due t o t h e d i f f e r e n t l o c a l e n v i - ronment c r e a t e d by t h e f r o z e n N2 l a y e r , o r a n homogeneous broadening o f t h e a n t h r a - c e n e s u r f a c e phonon through i t c o u p l i n g t o t h e a c o u s t o - o p t i c a l phonons of a -N2 i n t h e 50 cm-I r e g i o n .

I V

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CONCLUSION.

I n t h i s s t u d y , o b s e r v a t i o n of s u r f a c e e m i s s i o n e x c i t a t i o n spectrum j o i n t l y w i t h s u r f a c e c o a t i n g e x p e r i m e n t , h a s proved a v e r y s e n s i t i v e t e c h n i q u e t o u n v e i l

v e r y weak s u r f a c e s t r u c t u r e s from a n i n t e n s e b u l k background. No doubt t h a t t h i s n o n d e s t r u c t i v e m e t h o d of s u r f a c e s t u d y may b r i n g i n f o r m a t i o n s of g r e a t v a l u e on t h e o r g a n i z a t i o n of t h e c r y s t a l m o l e c u l e s on t h e f i r s t few monomolecular l a y e r s , a s w e l l a s on t h e s t r u c t u r e and t h e c o u p l i n g of f i l m s d e p o s i t e d on t h e s u r f a c e .

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{7}

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