Abstract Raman spectroscopy has enjoyed a dramaticimprovement during the last years: The interference bythe fluorescence of impurities is virtually eliminated, thesample preparation is considerably easier as for infraredspectroscopy and many applications in routine analytics,quality control and process control in various branches ofindustry are now possible. It is shown that the up-to-datenear-infrared Raman spectrometers now meet most de-mands for a modern analytical instrument concerning ap-plicability, analytical information and convenience. It canbe anticipated that Raman spectroscopy will catch up in-frared spectroscopy, the current workhorse of vibrationalspectroscopy.

1 Introduction

In spite of its potential abilities Raman spectroscopy hasuntil recently not been substantially used in analytical lab-oratories, it had been mainly applied to academic prob-lems. Routine vibrational spectroscopy was the task of in-frared spectroscopy. The reason was that the fluorescence,even of minute traces of impurities could overlay and hidethe Raman spectrum completely. In 1986 the use of thelaser line at 1064 nm was demonstrated successfully [1]as well as the benefit of recording Raman spectra by in-terferometers. This introduction of the so-called NIR FTRaman spectroscopy has changed vibrational spectros-copy considerably. One can now expect that Raman spec-troscopy excited in the NIR range at 1064 nm will be auniversal method of vibrational spectroscopy – for routineanalyses, for the solution of scientific problems as well asfor production control and quality assurance. It may evenbe applied more often than infrared spectroscopy since thesample technique is easier and the information suppliedcan be more significant. The old advice can now be fol-

lowed easily: Evaluate the combined complementary in-formation of infrared and Raman spectroscopy!

Several books have been published dealing with theNIR FT Raman spectroscopy [2–5]. In the followingchapters the different aspects of the optimization are de-scribed. In order to make this test more readable, con-densed statements are given with specific references tomore exhaustive explanations.

1.1 Overcoming the old handicap of the Raman effect: the fluorescence!

The interactions of photons with molecules can be de-scribed by molecular cross-sections, Fig.1 [5]. The cross-section for UV or fluorescence spectroscopy is just oneorder of magnitude smaller than the gas-kinetic cross-sec-tion of collisions. The cross-section for infrared spec-troscopy is smaller by about 2 additional orders of magni-tude. The cross-section of Raman spectroscopy is evensmaller by another 10 orders of magnitude. Thus, Ramanspectroscopy seems to have the worst starting conditionsas a method of optical spectroscopy

However, photons are measured by detectors, which,in addition to a signal which is proportional to the photonflux, produce noise – as a consequence of mainly thermaleffects. The detectors are qualified by the NEP, the noiseequivalent power, which is the light flux necessary to pro-

Bernhard Schrader

Raman spectroscopy in the near infrared – a most capable method of vibrational spectroscopy

Fresenius J Anal Chem (1996) 355 :233–239 © Springer-Verlag 1996

Received: 1 December 1995 / Revised: 27 December 1995 / Accepted: 4 January 1996

LECTURE

B. SchraderInstitut für Physikalische und Theoretische Chemie, Universität Essen, D-45117 Essen, Germany

Fig.1 Typical molecular cross-sections: ΣCOLL collision cross-section, ΣUV, ΣIR, ΣRA cross-section for UV, IR, and Raman spec-troscopy (reproduced from [5] with kind permission of VCH Ver-lagsgesellschaft, Weinheim)

duce a signal of the same magnitude as the noise [5]. InFig.2 the NEP of different detectors is shown, togetherwith the relative number of light quanta which are neces-sary to produce the NEP, drawn as sloping lines. For atypical “classical” detector of Raman spectra, the photomultiplier (PMT), the number of photons to produce theNEP is up to 10 orders of magnitude smaller than those ofdetectors employed for infrared spectroscopy. This fullycompensates the smaller cross-section of Raman spec-troscopy. Consequently, infrared and Raman spectra havea limit of detection of the same order of magnitude [5].

The term scheme, Fig.3, demonstrates that IR and Ra-man spectroscopy both excite vibrational states, but bydifferent mechanisms. The photons of the visible spec-trum are usually employed for classical Raman spec-troscopy. In competition, however, for molecules with ab-sorption bands in the visible range, they may produce ex-cited electronic states. They may return to the groundstate while emitting fluorescence. According to Fig.1 thecross-section of this process is by about 10 orders of mag-nitude larger than that of the Raman effect. Thus, the

broad fluorescence spectrum of impurities in a very lowconcentration – even of less than 10–6 – may completelyoverlay and mask the weak Raman spectrum. This is thereason why Raman spectroscopy could mainly be appliedonly to purified samples and not generally be employedfor problems of routine analysis.

Hirschfeld and Chase [1] demonstrated in 1986 thatRaman spectra can be excited by the radiation emitted bythe Nd:YAG (yttrium aluminium garnet, doped withneodymium) laser at 1064 nm. Its light quanta have only46% of the energy of those emitted by the Ar+ laser at 488 nm. Now the probability of producing fluorescence isvery low since electronically excited states of most mole-cules cannot be reached by these quanta.

Figure 4 shows, in a diagram linear in wavenumbers,the position of the infrared spectrum and the different Ra-man spectra in the optical spectrum. All spectra extendover a range of 4000 cm–1. Raman spectra have usuallybeen excited in the visible range with the laser lines of theAr+ laser at 488 and 515 nm, and of the HeNe laser at 623nm. GaAs diode laser emit at 780 nm in the near infrared(NIR) range. It may be seen that the Raman spectrum ex-cited by the Nd:YAG laser at 1064 nm marks an ex-tremum, this Raman spectrum cannot be shifted substan-tially further as it would penetrate into the range of the in-frared (IR) absorption spectrum.

In fact, as Fig.5 shows, this Raman spectrum is alreadysituated in the range of the typical NIR absorption spec-trum, composed by the overtones and combinations offundamental vibrations in the infrared range. Fortunately,the wavelength of the exciting radiation, 1064 nm, lies ina minimum of the absorption spectrum of water, the mainconstituent of all samples of biological origin. Thestrongest band in the NIR absorption spectrum of waterhas a linear decadic absorption coefficient of 10 cm–1, thatmeans a transmission of 10–10 for a sample with a thick-

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Fig.3a–d Term scheme of transitions to an excited vibrationalstate in the electronic ground state. a Infrared absorption. b Ramanscattering excited with the line at 488 nm of the Ar+ laser. c Com-peting fluorescence process. d Raman scattering excited with theline 1064 nm of the Nd:YAG laser, the quanta of which have only46% of the energy of those at 488 nm (reproduced from [5] withkind permission of VCH Verlagsgesellschaft, Weinheim)

Fig.2 Normalized detectivity D* and equivalent number of lightquanta for different detectors for optical radiation. FUV, UV, VIS,NIR, MIR, FIR: far ultraviolet, ultraviolet, visible, near, medium,and far infrared region (reproduced from [5] with kind permissionof VCH Verlagsgesellschaft, Weinheim)

Fig.4 The visible, near, middle and far infrared region of thespectrum drawn in a scale linear in wavenumbers. The infrared(IR) and far-infrared (FIR) spectrum is recorded by absorption oflight from a continuous spectrum in the range of λ = 2.5 … 100µm X= v 4000 … 100 cm–1 and λ = 100 … 1000 µm X= 100 … 10cm–1. Raman spectra are excited by monochromatic radiation,emitted by different lasers in the visible (VIS) or near-infraredrange (NIR). Molecules emit Raman lines with a frequency differ-ence ∆v to that of the exciting frequency v0 between 0 and + 4000or – 4000 cm–1. Usually only the Raman spectrum which is shiftedto smaller wavenumbers, the “Stokes” Raman spectrum, is recorded.Its range is indicated by bars for different exciting lines: Ar+ laserat 488 and 515 nm, HeNe laser at 623 nm, GaAs laser at 780 nm,and Nd:YAG laser at 1064 nm (Reproduced from [5] with kindpermission of VCH Verlagsgesellschaft mbH, Weinheim)

ness of 1 cm. It is situated in the Raman spectrum at about2500 cm–1, a range where usually no Raman lines occur.Therefore, Raman spectra can be recorded in the NIRrange without considerable obstruction by the NIR ab-sorption bands.

There are two reasons why in the past Raman spectrahave virtually not been excited in the NIR range: The ν4

factor states that for a given power of exciting radiationthe intensity of a Raman line is proportional to the fourthpower of its frequency. Thus, Raman lines (0…3700cm–1) excited by the Nd:YAG laser would have only 1/23to 1/75 of the intensity of the Raman lines excited by anAr+ laser at 488 nm. The second reason is the fact that theNEP of NIR detectors has been considerably larger thanthat of those in the visible, e.g. a photo multiplier. Both ef-fects cannot be compensated by an increase of the laserpower since this would destroy the sample. The increaseof the signal/noise ratio by increasing the time constantwould lead to unacceptably large recording times. Theonly way out of this dilemma is to employ spectrometersand sampling facilities which are orders of magnitudemore efficient than the “classical” Raman spectrometers.

1.2 Increasing the intensity of the Raman spectrum by making use of the Jacquinot-advantage

The radiant power Φ (in Watt) transmitted through a spec-trometer is given by [5, 6]:

Φ = Lv · Gv · ∆v2 · τ (1)

Here, the light source is characterized by its spectral radi-ance Lv, the radiant power per area, solid angle, andwavenumber (W/cm2 sr cm–1 = W/cm sr). The spectrome-ter is characterized by its spectral optical conductance Gv,the product of area and solid angle of the transmittedbeam per wavenumber (cm2 sr/cm–1 = cm3 sr) and itstransmittance τ; ∆v is the spectral bandwidth (cm–1).

Jacquinot [6] has realized that interferometers aremuch more powerful than dispersive spectrometers. Thespectral optical conductance of prism and grating spec-trometers and interferometers increases as 0.1 :1 :300 [1],when they have the same beam area at the prism, gratingor beamsplitter! This is called the Jacquinot advantage. Itis given by [5, 7]:

J = 2 π h/f (2)

Here, f is the focal length of the collimator optics and hthe slit height of the grating spectrometer (Figure 3.1–6and 3.1–10 in [5]). Typcial “classical” Raman spectrome-ters work for instance with a focal length of 1 meter and aslit heigth of 20 mm. Since the ratio h/f is usually around50, the Jacquinot advantage is roughly 300 – independentof the wavelength [5]!

Thus, by using an interferometer instead of a gratingspectrometer, the ν4 factor can be more than compensated.The entrance aperture – analogous to the entrance slit ofdispersive spectrometers – the so-called Jacquinot stop, iscircular having a radius

r = f CDDD2 / R0 (3)

with f, the focal length of the entrance optics and R0 =v/∆v0, the resolving power determined by

R0 = 2 ∆1 v (4)

with ∆1 the displacement of the moving mirror and v thewavenumber of the spectral band.

The highest radiant flux of a Raman sample is col-lected, when it is arranged in the focus of a laser beam andwhen the Raman radiation is collected within the largestpossible solid angle in a backscattering (180°) arrange-ment [5]. Hirschfeld [8] has recognized that “the circularshape of the interferometer aperture, instead of the slit-shaped one of a classical spectrometer becomes impor-tant, since a circular spot is more aberration-tolerant thana line of the same area and thus can be demagnified fur-ther” (Fig.3.5–5 in [5]). Therefore, interferometers aremost suitable for Raman spectroscopy in the NIR range.

The detectors for the Raman spectrum, excited at 1064nm, employ the – liquid nitrogen cooled – InGaAs and Gesemiconductors. Both have their most sensitive range co-incident with that of the Raman spectrum, excited at 1064nm, which extends up to 1700 nm. Also, the fiber optics,developed for telecommunication, have their highesttransmittance in this range – more than 80% per kilometer(Fig. 3.3–5 in [5])!

1.3 Increasing the efficiency of the Raman scattering sample

Since the cross-section of Raman scattering is very small(Fig. 1), the efficiency of the sample arrangement has tobe as large as possible. This can be accomplished, whenthe exciting radiation which is not converted by the sam-ple, is reflected back by a multiple reflection arrangement.This also reflects the Raman radiation which cannot becollected by the entrance optics back to the sample.

235

Fig. 5 Linear decadic absorption coefficient of H2O, D2O, Ethanol,and Cyclohexane in the near infrared region. Insertion: range ofthe Raman spectrum, excited by the Nd:YAG laser with radiationof λ = 1064 nm (reproduced from [5] with kind permission ofVCH Verlagsgesellschaft, Weinheim)

The classical rectangular or quadratic Raman cells arenot optimal. When a powerful collecting optics (collect-ing the radiation within a large steric angle) is combinedwith such a cell, the Raman radiation within only less thana half of this angle is collected (Fig. 3.5–5 in [5]). Aspherical cell allows to collect the radiation within the fullangle. The radiation which cannot be collected directlyand the remaining exciting radiation is reflected back by areflecting layer on the surface of the sphere. The exit win-dow should be covered with an anti reflection coating.Other suitable sample arrangements are discussed else-where (Fig. 3.5–8 in [5]). They make also use of the so-called Weierstrass lens [5, 9], an arrangement able tocollect the Raman radiation excited into the steric angle of2 π, (Fig.3.5–9 in [5]).

If samples have to be investigated which cannot betransported to the spectrometer – samples in productionlines, radioactive or explosive samples – optical fibershave to be used. For the transport of the exciting radiationto the sample it is advisable to use a single fiber which in-cludes an interference line filter near the sample at the endof the fiber. This eliminates the Raman lines of the fibermaterial (which is usually quartz). The Raman radiationfrom the sample can be transported to the spectrometerwith a fiber bundle having a diameter adapted to the ap-propriate Jacquinot stop [5].

1.4 The features of up-to-date Raman spectrometers

Raman spectrometers are now available, which use the ra-diation of the Nd:YAG laser at 1064 nm for excitation.They work in a spectral range which

• guarantees a maximal reduction of fluorescence,• minimizes the effect of the NIR absorption bands,• works in the optimal range of the InGaAs and Ge detectors and• in a range of highest transmittance of fiber optics.

They use interferometers which

• have a 300 fold optical conductance compared to grating spec-trometers,

• have a tolerant imaging system, and• permit an easy sampling technique.

2 Examples of Raman spectra excited at 1064 nm

The Raman spectra of the polycyclic aromatic hydrocar-bons may be regarded as a proof for the elimination of flu-orescene [10]. These substances are known to be highlycarcinogenic; the spectra of Fig.6 show samples from thelist of the environmental protection agency [11]. They flu-oresce with a large quantum yield, emitting a strong broadbanded spectrum which does not contain sharp bandswhich would permit the identification of special com-pounds. The Raman spectra of these compounds, how-ever, contain many sharp bands with different wavenum-ber and intensity, characteristic for each compound, sothat an identification and quantitative determination of thecompounds even in mixtures should be possible [12].

There are good chances to develop a procedure of separa-tion via 1- or 2-dimensional chromatography combinedwith surface enhanced NIR Raman spectroscopy to ana-lyze traces of these compounds, for instance in automo-bile exhausts. Products of the mineral oil industry, motoroil, fuels and many other products may now be analyzedwithout the danger of fluorescence. Typical vibrations ofthe skeletons, e.g. the substitution pattern of aromaticcompounds (chapter 4.1.9 in [5]) may be evaluated some-what easier compared to infrared spectroscopy.

The Raman spectrum of many compounds could not beinvestigated before, when they have strong absoprtionbands in the visible range. The indigoids are a class ofsymmetric compounds which show only those vibrationsin the infrared spectrum which were unsymmetrical to thecenter of symmetry, thus submitting only limited informa-tion about the molecular structure. For the determinationof the typical structure of these molecules the symmetricvibrations, only active in the Raman spectrum, were nec-essary. Figure 7 shows the Raman and the IR spectrum ofindigo, recorded with the same spectrometer [13, 14].

A typical new field of application is the food industry.Raman spectra of food supply more relevant informationthan the NIR absorption spectroscopy, they may be em-ployed for quality control, and the detection of preservingagents [15, 16]. Figure 8 shows the Raman spectra of ed-ible oils, butter and margarine [16]. It demonstrates thatthe concentration of cis- and trans-di-substituted doublebonds can be determined, with the C = O band as internalstandard. The samples can be investigated directly withoutany special preparation. Such analyses can be performedwith fiber optics for controlling production processes.

The successful application to food analysis has stimu-lated NIR FT Raman spectoscopy in medical diagnostics.This is a very delicate problem, since spectra of living tis-sues have to be recorded with high S/N in a limited timewithout the danger of denaturation. The first results arevery promising [17, 18].

A large number of successful applications of NIR FTRaman spectroscopy in many fields have been already re-ported [4, 5, 19–21] or may easily be organized withoutspecial effort. Of special importance are the applicationsof Raman microscopy (1-, 2- or even 3-dimensional) toproblems of geology, archaeology, restauration of worksof art [22–30]. A list of promising applications follows:Quality assurance, quality control and production control in

• Food industry• Pharmaceutical industry• Mineral oil industry• Chemical industry (Polymers, Pigments etc.)

Routine analytics in

• Biochemistry• Inorganic and organic chemistry

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Fig.6a–h Raman spectra of some polycyclic aromatic hydrocar-bons (the larger ones of the EPA list [11, 12], recorded with a sam-ple of 20 mg in a melting point capillary, Laser power 150 mW,scan time 3 min, resolution 4 cm–1

a

c

b

d

e f

g hFig. 6 a–h

• Semi- and superconductor industry• Development of new materials• Environmental analyses• Crystallography, Geology, Gemmology

New aspects can be anticipated in

• Medical diagnostics• On line and real time production control.

Typical new examples are: gas analyses [31, 32], highpressure microscopy [33], transition metal carbonyls [34],

chemical vapor deposition [35], latex systems [36], tex-tiles [37], cell walls [38], skin, hair [39], the 5200 year oldskin of “Ötzi”, the “Iceman” [40].

3 Outlook for the future

The introduction of diode array detectors and, especially,of so-called charge coupled devices, CCDs, has a largeimpact on the development of Raman spectroscopy. How-ever they allow only the recording of Raman lines up to1050 nm. Detector arrays for Raman spectroscopy withexcitation at 1064 nm, which really would eliminate fluo-rescene, are under development. However, at present pro-totypes of such detectors are very expensive and of lowquality. When such array detectors of sufficient qualitywill be available at a reasonable price, the application ofNIR compact spectrometers for production and qualitycontrol may be anticipated. Such instruments use an arrayof detector elements in parallel. All together provide themulti channel advantage, meaning that in principle manyspectrometers are working simultaneously, with each chan-nel having the same properties as a single grating spec-trometer. Therefore each channel can measure continuously,allowing a real time control. These spectrometers do notuse movable parts, they are robust and best suited for pro-duction and product control. Their disadvantage is thatthey cannot easily change the spectral resolution or thewavelength range. They can be used to record the wholespectrum with low resolution or only limited ranges of thespectrum with high resolution.

Interferometers are, as already reported, superior tograting spectrometers in several respects. They use onedetector for the whole spectral range. Thus, the noise pro-duced at one detector element is distributed over thewhole spectrum, enhancing the signal/noise-ratio (S/N)considerably. This is called the multiplex advantage [41].Therefore, interferometers permit the use of the Jacqui-not- and the multiplex advantage. Since the spectral reso-lution can be easily changed and they record the wholespectrum, they are much more suitable than dispersivespectrometers for flexible application to different analyti-cal problems and scientific applications.

However, the multiplex advantage is large in a rangewhere the detector noise predominates – the IR range. Forthe visible and NIR range where the noise of the detectoris lower, the multiplex advantage decreases. For somevery few applications one has to deal even with a multi-plex DISadvantage. Its reason is the fact that every flux ofphotons follows a Poisson distribution: the fluctuation ofa quantum flux is proportional to the square root of itsmagnitude. In a spectrum where all bands have an inten-sity of about the same order of magnitude there is no mea-surable disadvantage. However, when there are verystrong bands of the solvents or a strong background and avery weak band has to be measured with a maximal S/N,the multiplex disadvantage has to be taken into account.The fluctuation of the strong bands or the background isrecorded as noise in the interferogram. By the Fourier

238

Fig.7 Infrared and Raman spectrum of indigo, recorded with thesame spectrometer (Bruker IFS 66 with Raman module FRA 106);recording conditions: IR: 0.5 mg, in a pellet of 200 mg KBr, reso-lution 4 cm–1, Raman: 3 mg in a melting point capillary, Laserpower 180 mW, scan time 4 min, resolution 4 cm–1 [13, 14]

Fig.8 Raman spectra of safflower, sunflower and olive oil, sun-flower margarine and butter, normalized to the intensity of the C = O band, laser power 300 mW, recording time 20 min, resolu-tion 4 cm–1 [15, 16]

transformation into the real spectrum this noise is distrib-uted over the whole spectrum. Therefore, under these con-ditions only the spectrum near the weak spectral lineshould be analyzed by the spectrometer in order to maxi-mize its S/N. This means that an additional filter with aband pass at the line under investigation has to be intro-duced. This may be preferably been done by using a tun-able filter. Thus, even after development of suitable NIRarray detectors, interferometers will continue to be suc-cessfully used – however then parallel to compact spec-trometers for special controlling and monitoring prob-lems. In conclusion, NIR Raman spectroscopy has goodchances to outmatch infrared spectroscopy to be the work-horse of vibrational spectroscopy.

Acknowledgements Financial help by the Fonds der ChemischenIndustrie and the Deutsche Forschungsgemeinschaft is gratefullyacknowledged. It is a pleasure to thank the engaged young scien-tists of the Essen group for their contributions: Bernd Dippel, AxelEpding, Sonja Fendel, Helge Gottwald, Dr. Stefan Keller, JuttaLanger, Emil Lentz, Thomas Löchte, Jörn Lübben, Hans-UlrichMenzebach, Regina Schulte, Ellen Tatsch and Lutz Zolondz. HeinzSprünken and Elke Manzel supplied technical help. Dr. JürgenSawatzki and Dr. Arno Simon of Bruker Analytische Meßtechnik,Karlsruhe improved instruments, measuring techniques and thismanuscript.

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