Second-harmonic elastic light scattering by molecular

liquid mixtures

S. Kielich, M. Kozierowski, J.-R. Lalanne

To cite this version:

S. Kielich, M. Kozierowski, J.-R. Lalanne. Second-harmonic elastic light scatteringby molecular liquid mixtures. Journal de Physique, 1975, 36 (10), pp.1015-1021.<10.1051/jphys:0197500360100101500>. <jpa-00208332>

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Submitted on 1 Jan 1975

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1015

SECOND-HARMONIC ELASTIC LIGHT SCATTERINGBY MOLECULAR LIQUID MIXTURES

S. KIELICH and M. KOZIEROWSKI

Nonlinear Optics Division, Institute of Physics, A. MickiewiczUniversity, 60-780 Pozna0144, Poland

and J.-R. LALANNE

Centre de Recherches CNRS Paul Pascal, 33405 Talence, France

(Reçu le 28 avril 1975, accepté le 14 mai 1975)

Résumé. - Nous montrons que l’étude de la diffusion bi-harmonique élastique de la lumière dansles mélanges liquides est une source de données sur les corrélations entre molécules non-centrosymé-triques (par ex. dipolaires) et centrosymétriques (par ex. quadrupolaires), entre molécules dipolaireset tétraédriques, et entre les composantes des milieux polyatomiques, et qu’elle permet de déterminerles polarisabilités moléculaires d’ordres 2 et 3 quant à leur signe et leur valeur.

Abstract. - Second-harmonic light scattering (SHS) studies of liquid mixtures are shown toprovide valuable information regarding correlations between non-centrosymmetric (e. g. dipolar)molecules and centrosymmetric (e. g. quadrupolar) ones, between dipolar molecules and tetrahedralones, as well as between the components of atomic media. Such studies moreover permit determina-tion of nonlinear second-order and third-order molecular polarizabilities as to sign and value.

LE JOURNAL DE PHYSIQUE TOME 36, OCTOBRE 1975,

Classification

Physics Abstracts2.410 - 7.124 - 8.822

1. Introduction. - Since the earliest observationsof second-harmonic scattering (SHS) of laser light10 years ago, a vast amount of work has been devotedto the study of various nonlinear processes of photonscattering in gases, liquids, and crystals [1]. Terhuneet al. [2], Weinberg [3], and Maker [4] observed elasticincoherent SHS in liquids composed of non-centro-symmetric molecules, whereas Kielich and Lalanne [5]observed elastic cooperative SHS in liquids composedof molecules having a centre of symmetry in theirground state.The theory of SHS was initially worked out with

respect to gases [6], then for molecular liquids [7, 8, 9],and recently for atomic fluids [10, 11]. In this paper,we develop the theory of elastic SHS in liquid atomicand molecular solutions similarly as done by us withregard to usual anisotropic Rayleigh scattering [12].

2. Foundations of the theory. - We consider a

medium of volume V containing N Ni molecules.i

Ni = xi N is the number of molecules of species iand xi their molar fraction. Assume as incident on Va laser light wave with electric field

of an intensity so large that not only light of thefundamental frequency ce but moreover of the multi-

ple frequencies 2 w, 3 cv, ..., is scattered. We are

concerned with SHS for which the differential cross

section, with analyzer polarisation given by thevector n, is expressed (in the wave zone and in anelectric dipole approximation) by the formula :

Above,

with kw - the wave vector of incident light andk2. - that of the SHS wave ; the absolute value ofAkj ils defined in référence [71; the symbol &#x3E;stands for statistical averaging ; and

is the distance separating two scattering moleculesat the positions r pi and rqj’The electric dipole moment induced at the fre-

quency 2 ce in molecule p of species i is given generallyby the expansion [5] :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197500360100101500

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where :

nro and n2ro being the refractive indices of the scatteringspherical sample V at the frequencies 0) and 2 co,respectively, and no that of the medium surrounding V.

Eq. (2) involves the 3-rd rank tensor B;rCr pi’ Q,i),which describes the effective nonlinear 2-nd order

polarizability of the p-th molecule of species i withthe position rpi and orientation Q PL .. Insertion of (2)into (1) leads to the product B2, B2wqj, a 6-th ranktensor which, to start with, can be unweightedlyaveraged by taking recourse in general invariant for-mulae derived by one of us [13] (recently, these for-mulae have been re-written in rotation matrixform [14]). By (1), we hence obtain, for the horizontaland vertical component at vertically polarized incidentlight,

where we have introduced the following molecularquantities, characterizing SHS by many-componentsystems :

U denoting the symmetric unit tensor of rank 2.The parameter (6) characterizes the quasi-isotropicproperties of the scattering medium, and is thus thecounterpart of the isotropic scalar component inlinear scattering [12], whereas (7) accounts for theanisotropic properties of SHS.By (4) and (5), we obtain for the depolarization

ratio :

Likewise, by (1) we derive, for scattering at the

angle ’1J = 0, the following reversal ratio of circu-larly polarized light [9] :

3. Molecular-statistical analysis. - The nonlinearpolarizability tensor of eq. (2) depends on the pro-

perties of the molecule pi in the ground state as wellas on the orientations and electric fields F(r, t) of itsneighbours. The time- and space-fluctuations of themolecular field F(r, t) affects the tensor B;f bothdirectly and indirectly. It directly lowers the intrinsicsymmetry of the molecule by inducing in it highernonlinearities, in accordance with the expansion :

where the tensor b;r now describes its nonlinearsecond-order polarizability at F(r, t) = 0 and thefourth-rank tensor cpw describes linear variations inbpw due to the field F(r, t).The field F(r, t), as such, is related with dispersional

interactions between the molecules as well as electro-static and inductional interaction of molecular multi-

poles, as given by the expression [15, 16] :

where

is the field existing at the centre of molecule p of

species i due to the electric multipoles of molecule sof species k. The interaction of the two molecules isdescribed by the tensor (1)1r(n)pi,sk of rank n + 1.

Indirectly, the molecular field F(r, t) affects the

expansion (10) by way of translational fluctuations,which cause spatial redistribution of the molecules inregions of short-range order [9, 12]. The latter effect,as well as many-body distortion effects, play anessential role in atomic substances [10, 11].

With regard to (10) and (11), the molecular para-meters (6) and (7) can be written for a many-compo-nent system as follows :

We shall now discuss the various terms for severalmodels and approximations.

3. 1 No MOLECULAR FIELD. - The first two termsof (12) and (13) describe additive, incoherent SHS byindividual molecules with the parameters :

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The parameters (14) and (15) differ from zero for

non-centrosymmetric molecules only [6]. Their appli-cation to molecular symmetry groups proceeds as inthe single-component case.The second and higher terms of the expansions (12)

and (13) account for coherent light scattering onmolecules statistically correlated as to their positionsand mutual spatial orientation [7, 8], and express thenon-additivity of scattering by the components of themixture. In a first approximation, neglecting themolecular fields, we obtain for the two-moleculecorrelation parameters :

In this approximation, the parameters (16) and (17)differ from zero for non-centrosymmetric moleculesonly if the statistical averaging &#x3E; is performed witha correlation function dependent on molecularorientation.

3.2 INFLUENCE OF THE MOLECULAR FIELD IN THE

ABSENCE OF ANGULAR CORRELATIONS. - Averaging,performed with the (angular correlation-independent)radial .correlation function, causes the two-moleculeparameters (16) and (17) to vanish. The contributionsfrom the molecular field (11), inherent in the expan-sion (10), have now to be taken into account. Thenon-zero result thus obtained is, obviously, propor-tional to the square F2 (in the linear approximation,it vanishes on isotropic averaging). On assumingmoreover for simplicity the nonlinear polarizabilitytensor

as isotropic, we obtain :

and F(iJ = 0. This approximation, consequently, yields no contribution to the anisotropic parameter (7).In the same way, we derive for the quasi-isotropic parameter of three-molecule correlations :

where Pn+1 is a Legendre polynomial of order n + 1.

Here, too, FT2w(ijk) = 0. Clearly, the anisotropic para-meters Fr2w(i.j), FT 2w(ijk) will differ from zero in the absenceof angular correlations only if anisotropy in thetensor cpw is taken into account [5] or if, in the expan-sion (10), higher nonlinearities are taken, thus thequadratic nonlinearity dpw : F(r, t) F(r, t) etc., or

other components, due to the action of the field

gradient etc. [16, 17].

3. 3 INFLUENCE OF THE MOLECULAR FIELD IN THEPRESENCE OF ANGULAR CORRELATIONS. - In the pre-sence of orientational molecular correlations the

non-vanishing contributions are those linear in themolecular field F. However, for the sake of simplicity,we shall assume the molecular species i as dipolar(albeit without higher multipoles) and the species j ascentrosymmetric, bJro = 0. We now get, for the two-molecule angular correlations :

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with the notation :

These formulae describe a curious situation in which the fields of electric dipoles Jlpi of a non-centrosymme-tric molecular species induce nonlinear electric moments c;j. F qj,pi in the molecules of a centrosymmetricspecies thus destroying their centre of symmetry and rendering SHS of a cooperative kind possible [5]. Weare dealing here with a mixed SHS due to the intrinsic absence of a centre of symmetry and to the removalof one by a molecular electric field.

The contributions (20) and (21) are easily seen to vanish on isotropic averaging. However, averaged witha two-molecule angular function correlating a dipolar molécule and a quadrupolar one e j :

they become, in a satisfactory approximation :

Now let us see what happens when the species i is dipolar but the species j tetrahedral. By (16) and (17),we now get :

0pi,qi(3k). (k = 1, 2, 3) denoting the angle between the permanent dipole and the k-axis of molecular coordinates,attached to the tetrahedral molecule.

On averaging, performed isotropically or with thefunction (27), the anisotropic factor T2w(i,j) vanishes ;for dipole-tetrahedral octupole interaction, it willdiffer from zero only in the fourth approximation ofthe distribution function expansion (~ ju3 Dl/k3 T 3)and will thus be practically negligible.

Furthermore, taking into account the molecularfield of the dipoles and a nearly isotropic tensor cJw,we get :

On averaging the preceding expression with the

angular correlation function for interaction betweenthe permanent dipole and the dipole induced in the

tetrahedral molecule :

(with aj-linear optical polarizability), we obtain in thefirst, nonvanishing approximation :

The preceding formulae for specific molecularsymmetries prove that temperature-dependent studiesof SHS in liquid mixtures are a source of informationnot only concerning the two- and three-moleculedistribution function (19) but, primarily, permit one

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to determine the signs of the tensor elements ofnonlinear second-order and third-order polarizabili-ties. This information is essential to our knowledgeof molecular structure.

3.4 ATOMIC DISPERSIONAL INTERACTIONS. - In thecase of SHS in atomic media, we have by (6) and (7) :

/

with F now denoting the field of London dispersionalinteractions. The tensor cfro is defined as in eq. (18).By (8) and (9) we find that Dv2ro = 1/9 and R2ro(O) = 0for all orders of atomic interaction.We see moreover, from (28), that even in the limit

Ak. r pi,qj -+ 0 the molecular factor of quasi-isotropicscattering already differs from zero for two-bodyinteraction of different atoms. For i = j the factor (28)vanishes in such cases [10], since a pair of identicalatoms is still a centrosymmetric element.The molecular quantities (6) and (7) are accessible

to discussion by the method of statistical perturbationcalculus [17] for polar molecules with arbitrary sym-metries and interaction energies, as done previouslyfor the molecular polarisation of many-componentsystems [18].

4. Discussion and conclusions. - For a binary sys-tem, the expansions (12) and (13) are convenientlywritten as a series in powers of x (= X2 - the molarfraction of the soluté ; xi = 1 - x - that of the

solvent) [19] :

where the expansion coefficients :

are accessible to direct determination by measurementsof B2w or r’2wThe coefficient Qo of (29) describes the properties

of the solvent alone and, with regard to (12), is givenby :

The other coefficients of (29) describe the solution ;by (12), they can be approximately expressed as :

Similar expressions can be written for the r 2w’S.Knowing the properties of the pure solvent, eq. (29)

can be re-written in the form of the variation :

For an infinitely dilute solution, (31) becomes :

Thus, the infinitely dilute solution is characterized

by the quantity Ql, defined by (30b). Concentratedsolutions markedly diverge from additivity i.e. from

linearity in x, so that higher terms of (31), like Q2,Q3, ..., have to be taken into account.

Let us consider the case when the solvent moleculesare centrosymmetric and those of the solute noncen-trosymmetric. We now have in the absence of corre-lations for the solvent, by (14) and (15) :

In the dilute solution, the predominant role belongsto molecules which, by interacting with one another,cause cooperative light scattering as given by eq. (18),unless one takes anisotropy of nonlinear polariza-bility into account. In particular, for quadrupolemolecules one has by (18) and (19) :

where 01 is the electric quadrupole moment of thesolvent molecule.For highly symmetric e.g. octahedral solvent mole-

cules the first non-zero electric moment is the hexa-

decapole 01, and one has by (18) :

Likewise, by (19), a contribution is derived for tripleradial correlations.

In the infinitely dilute case, with regard to (30b),(14) and (15), it suffices to consider for the solutethe quantities

1020

only, since the probability of correlation betweenthe scarce solute molecules, separated by very nume-rous molecules of the solvent, is small. In this situation,however, one has to consider correlation betweena non-centrosymmetric solute molecule and its cen-trosymmetric neighbours. Neglecting angular corre-lations in a first approximation, one has by (18) :

where M2 is the dipole moment of the solute molecule.The quantity B2w(21) is defined similarly.

If correlations between the solute and solventmolecules are anisotropic, one has instead of (38),by eq. (20a) and (2 la) :

Hence, the variation OB2w(x) in an infinitely dilutesolution is described by the quantities in (30b) withthe terms (33), (34), (36) as well as (38) and (39).On the other hand, the variation of the anisotropicparameter is, in this approximation :

where the non-zero contributions are given by (37)and (40).One can likewise discuss the inverse case of a solute

consisting of quadrupolar centrosymmetric molecules

and a solvent with tetrahedral ones e.g. CCI4, forwhich by (14) and (15) one has in the absence ofcorrelations :

In the présence of radial correlations only, r2w(11) = 0,and by eq. (18) one has :

where Q 123 is the octupole moment component of thetetrahedral molecule [17]. On taking into accountinteraction between molecules of the solute and

solvent, one has :

Since the octupolar contributions are generallymuch smaller than the quadrupolar ones, the infinitedilution method suggests itself for the determinationof the quadrupole moments of molecules from eq. (44).The values thus obtained will surely be more accuratethan those previously determined from SHS studiesof pure liquids [20].For concentrated solutions, higher terms of (31)

have to be taken. By eq. (30c) etc., they now involvealso parameters related with correlations betweenmolecules of the solute.The preceding example proves that the investigation

of the quantities B2.(x) and F2.(X) vs. concentrationpermits to study the correlations existing not onlybetween molecules of the same species, but, in thefirst place, between molecules of different species.If the dependence of B2w(x) and r 2w(X) on the concen-tration x is linear, one deals exclusively with aninfluence of the solvent on the molecules of the solute

amounting to at least 10 %.

References

[1] KIELICH, S., Nonlinear Rayleigh and Raman Light Scatteringby Atomic and Molecular Substances, Conference on theInteraction of Electrons with Strong ElectromagneticField, Balatonfüred, 11-16 September, 1972 (InvitedPapers, C.I.E.S.E.F. Budapest, Hungary, 1973, pp. 279-336) ;

BOGAARD, M. P. and ORR, B. J., in M.T.P. Review of Mole-cular Structure and Properties, Series II (Ed. BuckinghamA.D., Butterworth, London) 1974.

[2] TERHUNE, R. W., MAKER, P. D. and SAVAGE, C. M., Phys. Rev.Lett. 14 (1965) 681.

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[7] BERSOHN, R., PAO, Y. H. and FRISCH, H. L., J. Chem. Phys.,45 (1966) 3184.

[8] KIELICH, S., Acta Phys. Pol. 33 (1968) 89.

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(1974) 231; KOZIEROWSKI, M., Thesis, Pozna0144, 1974.[10] GELBART, W. M., Chem. Phys. Lett. 23 (1973) 53.[11] SAMSON, R. and PASMANTER, R. A., Chem. Phys. Lett. 25

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[13] KIELICH, S., Acta Phys. Pol. 20 (1961) 433.

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[14] McCLAIN, W. M., J. Chem. Phys. 57 (1972) 2264 ;HEALY, W. P., J. Phys. B 7 (1974) 1633.

[15] JANSEN, L., Phys. Rev. 112 (1958) 434.

[16] KIELICH, S., Physica 31 (1965) 444; Acta Phys. Pol. 27 (1965)395 ; 28 (1965) 459.

[17] KIELICH, S., in Dielectric and Related Molecular Processes

(Ed. Davies M., Chem. Soc. London) 1972 Vol. 1,Chapter 7.

[18] KIELICH, S., Mol. Phys. 9 (1965) 549; Acta Phys. Pol. 28(1965) 95 ; 27 (1965) 305.

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