Circular polarization attachments for FTIR spectrometers

Journal of Molecular Structure, 175 (1988) 407-410

407

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

CIRCULAR POLARIZATION ATTACHMENTS FOR FTIR SPECTROMETERS

D. KOLEV I, E.H. KORTE 2, B. JORDANOV I, D. TSANKOV I IBulgarian Academy of Sciences I Institute of Organic Chemistry, 1113 Sofia (Bulgaria) 21nstitut fur Spektrochemle und A~gewandte Spektroskople, 4600 Dortmund (FRG)

ABSTRACT Two devices (retarders) for converting linearly polarized IR radiation into circularly polarized one t based on total internal reflection t are described. One of them uses two totally reflecting KBr prisms while the other one works with a single ZnSe prism. Tests for the quality of the circular polarization are given.

INTRODUCTION Circular polarizers (retarders)

for the IR region could be constructed on the

basis of linear birefringence, total internal reflection and selective reflection by cholesteric liquid crystals, A linearly polarized beam, entering a birefringent medium at normal incidence is split into ordinary and extraordinary beams which emerge from this medium with a phase shift (ref. i) 6 = 21vdhn

(1)

where v is the wavenumber, d - thickness of the medium, 5n=n -n - the differeno e ce of the refractive indices of the ordinary (n o) and extraordinary (n e) beams, Eq. i shows that circular polarization

[6-(k+-1)w/2] is periodlcally produced at

those frequences for whicJl 4vdAn becomes an integer number, This method is the basis of the photoelastic modulators widely used in the CD spectroscopy, The total internal reflection of linearly polarized radiation causes a phase shift between the parallel (Rp) and perpendicular (Rs) components of the amplitudes of the electric vector of the reflected beam (ref, i) so that

(Sp/R 8) - ei~t~,~a

(2)

and

tan(6/2) - (cos~ s/s~n2¢-:--~2)/ In eqs, i

and

sln2¢

(3)

2 ~ is the angle of incidence, ais the azimuth of the electric

vector of the linearly polarized beam and n is the refractive index of the totally reflecting material. Three parameters np ~ and u govern the ellpticlty of the emerging radiation. If circular polarization is produced for a given fre-

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408

quency then moving away from it causes a slight eliptical distortion [nff(v)], The selective reflection by cholesteric liquid crystals or induced choleste-

ric solutions can also be used for producing circularly polarized IR radiation in narrow spectral intervals (150 - 200 cm"1) in which the selective reflection occurs (ref. 2).

TOTALLY REFLECTING RETARDERS Only two works were noticed in the literature ( refs, 314) in which the total intenlal reflection has been used for IR-CD spectroscopy, The selection of materials and geometry for the design of totally reflecting IR retarders is based on eqs. (i) and (2). Limitting factors are the maximum phase shift 6 m and the corresponding maximum angle of incidence 0m obtained for a-45 ° from the extremum condition d[tan(6/2)]/dO - 0 following from eq. (3) sin20m - 2n/(l + n 2) ;

tan(6m/2) - (I - n2)/2n

The transparent IR materials could roughly be derided into two classes for the purpose of the total reflectlng devices. The first one (A) includes materials with refractive index up to 2.4 while materials with refractive index above this value belong to the second goup (B). In generalp retarders made of materials of class A like KBrp NaC1 and CsBr can not produce 90" phase shift for one reflection since 6 <90 ° holds for all of them. Two reflections are needed in orm

der to attain 90 ° phase shift. On contraryp materials belonging to class B canbe used for retarders yielding 90* phase shift for one reflection only. ZnSe s KRS-5 s Ge w 5i a n d others represent this class. In the present article we describe a KBr attachment with two reflections and a ZnSe attachment with a single reflection. Fig. 1 represents these devices, It

!

II

Fig. i. Totally reflecting retarders with two KBr prisms (I) and with a single ZnSe prism (ll). 141, MI' - plane mirrors m M2 - colimator, M2' - focussing mirror P - polarizerp 5 - sample.

409

must be noted here that the sense of polarization is altered by changing the sign of the azimuth a~ say~ from -45 ° to +45 °. The KBr retarders were prepared from old KBr dispersive prisms whose base angles were changed from 60 ° to 56 ° and their dull walls were polished. The ZnSe prism was purchased on order from Schrader Optik m Braunschwelg, FRG. Both attachments show a good throughput of about 60%.

CIRCULAR POLARIZATION TEST A reliable procedure for testing the quality of the circular polarization consists in introducing a birefringent plate into the sample space (see Fig. i) and m~ analyzer immediately behind it. Interference pattern is recorded with this experimental

set up (ref. 5). The orientation of the plate and of the analyzer

is arbitrary. Entering the blrefringent plate the circularly polarized radiation decomposes into two orthogonal to each other components along the vibration directions D' and D". On output both components turn out to be phase shifted according to eq. (i). Their interference gives rise to a linearly polarized beam whose electric vector orientation depends on the frequency. The analyzer then transmits differently each frequency thus giving rise to an interference pattern A sapfire plate of thickness 2.114 cm was used for the test. Schemes of the optical train of active elements: polarizer(P)/retarder(P0/sapfire(S)/analyzer(A) are shown in Fig. 2 for the reference and sample channel of each measurement. A part of the recorded interference spectra are also displayed there. Rotating A

RCH: P(-45) IRIS (X) /A(e)

m m

m S(~

R

P(a)

~ ~

A(e) optical trains

m

m m C

b/~

a,b

m R

c

C

c

1

a

SCH. P ( + 4 5 ) / R / S ( x ) / A ( 6 )

b

ROt: P ( - 4 5 ) IRIS (X)/A(+~90) SCH. P(+45)IRIS(x)IA(e+90)

RC~I' P ( - 4 5 ) I S ( 0 ) / A ( - 4 5 ) =-- SCH. P ( + 4 5 ) / S ( 0 ) / A ( - 4 5 ) C

C

P - polarizer

s

/t

)

AS RRC~SCH -

D I~ . . , %

"'-.

C

C

C

C

analyzer sample (sapfire) retarder reference channel sample channel ,+ P o +°°

,."

a

•

C

-1

plane of

A

1200 2000 c m Fig. 2. Circular polarization test performed with Bruker IFS-II3v FTIR spectrometer; _aD b - interference spectra of decomposed circularly polarized radiation by a 2.114 cm thick sapfire plate; c - interference spectrum of pure birefringence of sapfire.

410 by 90 ° from its initial position [ A( 0)÷A( 8+90) ] reverses the minima and maxima. Two curves are recorded in this way ( a and b in Fig. 2). The maxima and mira nima of a (minima and maxima of b) correspond to ±90 ° phase shift between the components of the electric vector emerging from the sapfire plate. The crosspoints of a and b correspond to +-180° phase shift of the same components. They coincide with the extrema of the interference curve c obtained from a pure birefringence measurement with an optical train P/S/A. The orientations of each element of the train is also shown in Fig. 2. This coincidence certifies the good alignment of the attachment. On the other hand, the slightly changing intensity of the interference extrema of curves a and b over the whole IR region 2000

-

4000 cm-I, limitted by the transparency of the sapfire D attests to the

high constancy of the produced phase shift. Thus the totally reflecting retarders could be qualified as achromatic.

ACKNOWLEDGE~NT The authors thank the Deutsche Forschungsgemeinschaft and the Bulgarian Academy of sciences for their financial support. B,J. is indebted to the Alexander yon Humboldt Foundation for a research grant.

REFERENCES 1 2 3 4 5

M. Born, Optik, Springer, Berlin 1972. B. Jordanov, D. Tsankov, J. Mol. Struet., 117 (1984) 261, R. Dudley, S. Mason, R. Peacock 0 JCS Chem. Comm., (1972), W. ¥ang, P. Griffiths t G. Kemeny, Appl. Speetrosc., 38 (1984), L.A. Nafie, M. Diem, D.W. Vidrine, JACS, i01 (1979) 496.