FT-IR Emission Spectroscopy and Its Applications

J A N O S MINK* and GABOR KERESZTURY Institute o[ Isotopes, Hungarian Academy o[ Sciences, H-1525 Budapest, P.O. Box 77, Hungary, and Department of Analytical Chemistry, University o[ Veszprem, H-8201 Veszprem, P.O. Box 158, Hungary (J.M.); and Central Research Institute for Chemistry, Hungarian Academy of Sciences, 1-I-1525 Budapest, P.O. Box 17, Hungary (G.K.)

The practical aspects of infrared emission spectroscopy are reviewed briefly, with emphasis on recent developments and on special problems encountered in measurements of adsorbed species on opaque subst ra tes . A few examples are given for cases when the species of interest give rise to inverted emission bands on the background of substrate emission. The feasibility of recording IR emission spectra of adsorbates on metal pow- ders is demonstrated.

Index Headings : Emiss ion, IR; Adsorbed species; Catalysts ; IR emission studies of catalysts .

I N T R O D U C T I O N

The availability of very stable and fast FT-IR spec- trometers equipped with highly sensitive, low-tempera- ture detectors has led in the last decade to a revival of interest in IR emission spectroscopy, a spectroscopic technique rarely applied in the past. The increasing in- terest is manifested by a number of recently published papers 1-H ranging from experimental and theoretical reviews 1-5 through various application reports to tech- nical notes. Among the recent methodological develop- ments are, for instance, a commercially available infrared emission accessory with large angle collector optics, ~ its modified version to permit quantitative applications, 7 the use of visible or NIR laser irradiation in order to heat the front surface of bulky samples, s,9 etc. The in- creasing interest in the IR emission technique is clearly indicated by the fact that today most of the instrument manufacturers offer emission ports or adapters and soft- ware support for emission experiments.

Compared to IR transmission work, the emission ex- periment is not yet routine. Experimental difficulties and distorting effects associated with sample thickness, with temperature changes during measurement or between sample and reference, with emission of eventual sub- strate or support material, with self-emission of the in- strument, and with double passing of light through the interferometer have all been described and discussed.

In spite of the cited inconveniences, an increasing number of researchers are exploring the potentials of IR emission spectroscopy nowadays, looking for a method to record spectra of samples not suitable for transmission or reflection techniques. Besides the obvious application of IR emission spectroscopy in astrophysics or in the analysis of natural sources of radiation like stack gases, this technique can be very useful or can even be the only one promising solution to several IR spectroscopic prob- lems to be studied in laboratory conditions. Thus, the capability of FT-IR emission spectroscopy to observe

Received 26 March 1993. * A u t h o r to whom correspondence should be sent .

weak signals offers a new approach for time-resolved study of pulsed laser-induced photochemical reactions, to detect emission spectra of radicals and vibrationally highlyexcited products. 12,13 There are also some types of "conventional" samples that are not amenable to spectral observation:

1. Highly absorbing samples, such as most catalysts or supported metal catalysts in the low-frequency IR region.

2. Samples that are opaque or badly reflecting in the whole IR spectral range, such as porous thin films or metal substrates.

3. Nontransparent black powdered samples, such as highly dispersed metal powders (e.g., Pd-black or Pt- black).

In this paper, after a short review, we would like to demonstrate the potentials of IR emission spectroscopy in the field of catalyst research and its capability for studying the structure of chemisorbed species or moni- toring catalyst surfaces.

THE ROLE OF IRES IN CATALYST RESEARCH

As has been noted by us and by other investigators, supported metal catalysts are opaque to IR radiation at the usually applied thicknesses of self-supporting pellets in the region below 1200 cm 1. But most of the catalyst supports become transparent again in the far-IR region, 14 e.g., SiO2 below 300 cm -1, A1203 below ~240 cm -1, TiO2 below ~170 cm -1, and MgO below ~150 cm -1 (these limits, of course, strongly depend on the thickness of the pellet). This means that the spectral range from about 1200 to 200 cm -1, which contains key information re- garding interactions between the adsorbate and catalyst, is impossible to study in transmission; thus, one should try to use nontransmission techniques. Other vibrational spectroscopic techniques that may immediately come to mind are diffuse reflectance (DR), photoacoustic (PA) detection, Raman spectroscopy, and, finally, IR emission spectrometry (IRES).

According to Van Every and Griffiths, ~5 diffuse reflec- tance FT-IR spectrometry has made limited impact on catalysis research in spite of the fact that DRIFTS has great potential in measuring the spectra of powdered samples. Using an IR reactor cell, these authors 1~ dem- onstrated that the detection limit can be brought down as low as 10 -6 monolayer coverage, but this seems to work only for very strong absorption bands in the region of full transmission of the support material. Our own experience 3 shows that while the CO stretching bands of metal carbonyls adsorbed on silica can be measured with

1446 Volume 47, Number 9, 1993 0003-7028/93/4709-144652.00/0 APPLIED SPECTROSCOPY © 1993 Society for Applied Spectroscopy

.05

p:~ O-

-.05 -

A : I ....

(b) j r __j 40'00 30'00 20'00 lo'oo 6 -15oo

Raman shi f t (ore -1)

FIG. 1. FT-Raman spectra of commonly used catalytic support ma- terials excited with near-IR laser radiation: (a) All03; (b) Si02.

excellent signal-to-noise (S/N) ratio in the vicinity of 2000 cm -1, the region below ~900 cm ~ in the double- beam DRIFTS spectrum (with silica as the reference) contains nothing but excessive noise.

Photoacoustic spectroscopy, the other special method used to measure IR absorption spectra of powdered sam- ples, has been shown by Grifiiths 16 to be about 20 times less sensitive than DRIFTS. Consequently, it is not likely to give any better results in the region of support ab- sorption bands either.

Raman spectroscopy can be more successful in this respect, as evidenced, for example, by two recent, exten- sive reviews. 17as On the one hand, catalytic support ma- terials are poor Raman scatterers and do not interfere with the study of adsorbed species. On the other, they are seldom free from fluorescing impurities, which de- creases the odds for detection of the species of interest. FT-Raman spectroscopy using near-IR excitation has the reputation of being free from problems of sample fluorescence. This, however, does not seem to be the case with the most frequently used support materials; the FT- Raman spectra of alumina and silica shown in Fig. 1 are rather discouraging. The possible reasons for these de- terring results may be (1) small particle size resulting in strong heating of the sample by the YAG laser; (2) high surface activity of the samples leading to the presence of special impurities that fluoresce in the near-IR region. It seems that with specially purified samples this fluo- rescence is not present, but then Si02, for instance, be- comes "transparent" for near-IR laser radiation or, on the other hand, it has such a negligibly small Raman scattering cross section that no interferogram can be de- tected at all ~9 (this may become an advantage when ad- sorbed species are present).

Since none of the above techniques can offer a general and easy solution to the measurement of IR spectra of species adsorbed on catalysts, the capabilities of IR emis- sion spectroscopy should also be explored in this respect.

Emission of Support Materials. In transmittance, the cutoff frequency of self-supporting pellets varies between 1300 and 900 cm -1 depending on the material and thick- ness of the pellet. Below the cutoff frequency, there is no energy; therefore no absorption spectrum is to be measured. In emission, the situation is different.

q) O

(d)

(c)

(b) 0-

100-

O O ¢d

~ 0

.15

(a)

20'00 16'00 '12'oo'8 o'4 o W a v e n u m b e r ( c m - 1)

Fro. 2. IR transmission spectrum of A120~ in CsI pellet (a); and emis- sion spectra of different AltO 3 samples at 393 K (ratioed against black- body): the same CsI pellet (b), a thin film (c), and a self-supporting pellet of pure Al~O~ (d).

In Figs. 2 and 3, emission spectra of catalytic support materials are shown with different sample preparations. For comparison, the transmission spectrum of alumina and silica in a CsI pellet is also presented. As expected, there is a close correspondence between the absorbance/ transmittance and emittance spectra of the CsI pellets. The emittance of the undiluted, thick samples (self-sup- porting pellets) is close to 100% emittance; that is, it approaches the emission level of a blackbody. But instead of zero energy as in absorption, there is plenty of emitted energy reaching the detector, which presents the oppor- tunity to detect adsorbed species.

In the case of adsorbed species on metal oxides or oxide-supported dispersed metal catalysts, the main dif- ficulty is caused by the fact that the emission of the adsorbent or support material can be orders of magni- tude greater than that of the adsorbed species of interest. As was pointed out by Griffiths, 2° in this case the spectral features of the adsorbate may not appear on top of the broader and stronger emission bands of the substrate because, if the sample is in thermal equilibrium, it is compensated for by the absorption of some part of sub- strate emission at the same frequencies. Primet eta/ . 21,22

APPLIED SPECTROSCOPY 1447

claim that the adsorbate bands can be detected in emis- sion if the sample is thin enough (about 0.1 mg/cm2). In this way, the metal-adsorbate mode could be detected in our laboratory 23 in the 600-400 cm -1 region in the case of CO chemisorbed on Pt/AI203. Contrary to this ap- proach, the use of much thicker samples (15 mg/cm 2 or 0.5 mm) has been suggested, 24,25 in which case the ad- sorbate bands can appear as very weak bands either in emission or in absorption, 24 depending on the actual tem- perature distribution within the sample, whereas the lat- ter seems to be sensitive to the pressure inside the cell.

Chalmers and Mackenzie 2b cited several occasions when inverted or negative bands, sometimes referred to as "re- duced emission," were observed in the emission spectra of opaque samples. This phenomenon was attributed ei- ther to self-absorption of cooler outer layers of the sample 2° or to increased front-surface reflection in the vicinity of strong bands. Rytter 2s showed that both mech- anisms can be justified by theoretical calculations and argued that both kinds of distortions can be eliminated by employing a thick sample of the same material for the reference instead of the blackbody.

In the case of adsorption studies, however, the ap- pearance of inverted (negative) bands may be most wel- come, since if they belong to the adsorbed species, they may provide the only chance to detect the spectral fea- tures of interest. Examples of this phenomenon will be shown later.

An interesting recent development is the use of NIR s and visible lasers 9 for local heating of the front surface of the sample. These pioneering works in laser-induced thermal emission (LITE) spectroscopy noted the reduc- tion of background emission and the possibility of study- ing thicker samples as advantages, in comparison to the requirements of conventional thermal emission, al- though much remains to be explored in this field (e.g., how to control and measure the sample temperature).

Self-Emission of the Interferometer and Significance of the Detector Temperature. Following from the basic prin- ciples of thermal emission, the band intensities can be increased by increasing the temperature of the sample. This approach, however, is counterindicated in cases when the adsorbate decomposes at high temperatures. Also, the equilibrium of adsorption-desorption processes is shifted towards desorption at higher temperatures (150- 200°C). To suppress desorption, one must keep the sam- ple under pressure, which may lead to the appearance of a vapor absorption band superimposed on the emission spectrum. Thus, an optimum sample temperature is to be found at which desorption is not yet significant but the level of emission is already sufficiently high. With the use of a room-temperature detector, this consider- ation leads to sample temperatures of 80 to 150°C. (For studies of catalyst systems under realistic processing con- ditions, higher temperatures and high pressures would be required. 2a)

The use of low-temperature MCT detectors (in order to decrease the sample temperature, e.g., to room tem- perature) may necessitate correction for self-emission of the instrument, since emission of the room-temperature parts of the spectrometer will also appear in the mea- sured spectrum. For elimination of this background Kember e t at . 27 have suggested the use of a four-mea-

0

ioo

i0

0

0

(a)

20'00 16'00'12'00

ivy

Wavenumber (era -i) FIG. 3. IR transmission (a) and emission spectra of Si02 in a CsI pellet (b), compared to the emission spectra of a thin (c) and thicker layer (d) of pure Si02. The emission spectra were measured at 393 K and ratioed against a blackbody spectrum.

surement method according to which both the sample and the blackbody reference are measured at two dif- ferent temperatures; then the difference in sample emis- sion is divided by the difference in background emission in order to obtain the sample emittance. The use of room- temperature detectors (despite their lower sensitivity and the need to heat or cool the sample) has the advantage that there is no need for background elimination, 2s unless the interferometer is equipped with a heated Ge/KBr beamsplitter, which may give rise to spurious emission lines at 850 and 1125 cm -l.

One must also be careful with room-temperature de- tectors if the purpose of the IR emission measurement is to obtain the energy curve of a blackbody sample to estimate its temperature. Such temperature measure- ments may be based on a computer search among a series of theoretical blackbody radiation curves for the one that fits the measured single-beam emission spectrum best. 4 However, direct comparison of the measured and theo- retical spectra would lead to false results; to solve the problem correctly, we have suggested a procedure in- cluding the following steps: n

1. Correction of the measured single-beam emission

1448 Volume 47, Number 9, 1993

TABLE I. Emission FT-IR spectrum of [Re(CO)3OH]4 supported on A!203 (with 1.5% Re content).

Band position Intensity Band type (cm-1)a (% E) (direction) b Assignments

2038 s 8 e(+) CO sym stretch 1925 vs 10 e(+) ~ CO asym stretch. 1883 s,sh 6 e(+) f 890 vw 0.2 a(-) ~ ReOH deformation 870 w 0.5 a(-) ) 801 w 0.9 a(-) 653 m 1.6 a (-) ReCO bending 545 sh a (-) 525 m 2.7 a (-) ReC stretching 518w 0.9 a(-) 500 m 3.6 a(-) 470 s 5 a(-)

" s = strong; vs = very strong; vw = very weak; w = weak; sh = shoulder; m = medium.

h e = emission (positive band); a = absorption (negative band).

spectrum for ins t rument self-emission and instru- ment response.

2. Correction of the series of theoretical blackbody ra- diation curves for detector tempera ture by subtract- ing from each the curve corresponding to the tem- perature of the detector.

3. Computer search for the best matching corrected curve.

The efficiency of this correction procedure is demon- s t rated in Ref. 11.

The above procedure utilizes the ins t rument response function which can be determined as the ratio of the measured single-beam emission spectrum to the cor- rected theoretical curve corresponding to the same tem- perature. I t is advisable to use a tempera ture tha t is as high as possible for this determination, to obtain the ins t rument response function, R(~), with a good S/N ratio. A reliable ins t rument response function can be extremely useful; it can be utilized to t ransform the mea- sured single-beam emission spectra of any sample to em- i t tance units without the need to remeasure blackbody emission at every new temperature setting.

Gu id e l in e s for Adsorpt ion Studies by I R E S . From the above discussion it can be concluded tha t in chemisorp- t ion studies by IR emission spectroscopy the most con- venient approach is to observe the following guidelines:

1. Have moderate ly high temperatures and some vapor pressure inside the cell.

2. Consider the temperature of the detector and other parts of the ins t rument and apply any corrections needed.

3. Use the four-measurement method (two different temperatures) if there are no other opportunit ies for background elimination or if a suitable reference spec- t rum cannot be measured (e.g., tha t of pure, reduced catalyst under H2 atmosphere).

4. Obtain the emit tance spectrum of the adsorbed spe- cies by dividing the single-beam emission spectrum of the (supported) catalyst with the adsorbed species by tha t of the reduced catalyst under hydrogen at- mosphere (a blackbody reference spectrum is not needed if both spectra are measured at the same tern- perature).

TABLE II. Emission FT-IR spectrum of (CsHs)Fe(CO)~I supported on SiO2 (with 5% Fe content).

Band position Intensity Band type (cm-l) a (%E) (Direction) b Assignments c

2048 vs 88 e (+) CO sym stretch 2008 vs 55 e (+) CO asym stretch 1425 w 3.7 e(+) CC ring stretch (Cp) 1017 vw 1.0 a(-) CH i.p. bend (Cp) 851 vw 2 a(-) CM o.o.p, bend (Cp) 617 w 2.2 a(-) FeCO bending 589 vw 1.1 a(-) 561 w 2.4 a(-) FeCp tilt 550 sh ... a(-)

FeCp stretch 470 vw 1.6 a(-) FeC stretch (CO) 205 w a(+) FeI stretch 135 w a(+) CFeC deform. (CO) 70 vw a (+) skeletal deform.

Note: vs = very strong; w = weak; vw = very weak; sh = shoulder. See remarks of Table I. The bands below 400 cm 1 are taken from transmission measurement.

° Cp = modes belonging to vibrations of the C~H5 ligand. CO = modes belonging to vibrations of the CO ligand.

E x a m p l e s of I R E S of Adsorbed Spe c i e s on M e t a l Ox- ides. The following examples of successful IR emission measurements of difficult samples from our own expe- rience demonst ra te the simultaneous appearance of pos- itive and negative bands in different regions of the spec- tra (after background elimination via ratioing single-beam energy spectra or subtract ing emit tance spectra).

T h e f o r m a t i o n of a cube- l ike r h e n i u m complex , [Re(CO)3OH]4 , on the surface of alumina impregnated with Re2(CO)10 was followed by IRES2 The CO stretch- ing modes of the product in the region above 1200 cm -1 free from support emission could be readily observed as strong emission bands (they are labeled e ( + ) in Table I, indicating tha t they are pointing upwards). In the re- gion of strong emission of the alumina suppor t below 1200 cm -1, a number of very weak negative bands [point- ing downwards and hence labeled a ( - ) in Table I] ap- peared when the four-measurement method was applied. Due to the closeness of these frequencies to those of the free tetrameric molecule available in the literature, they could be used to identify the expected reaction product.

Table II contains similar data of the IR emission spec- t rum of (C~H~)Fe(CO)2I adsorbed on silica. 17 Again, the CO stretching modes appear as normal emission bands, while weak inverted bands of the adsorbed species were observed in the spectral region of strong suppor t emis- sion. In this measurement the same sample was used for background after thermal decomposit ion of the adsorbed species (some amount of Fe2Q might have been left over on the support) . The negative bands are approximately 60 times weaker than could be expected from the inten- sity of the CO stretching bands.

In the case of 1,5-cyclooctadienyl-PtCl2 supported on silica, 14 positive bands were observed in the emission spectrum above 1200 cm -1 and also between 400 and 300 cm -1, as well as an order-of-magnitude less intense neg- ative bands between these two regions (see Table III). Here the emission spectra of both the sample and the pure suppor t material (silica) were recorded against the

APPLIED SPECTROSCOPY 1449

TABLE I lL Emission FT-IR spectrum of (COD)PtCI~ supported on S i O 2 (with 5% Pt content).

Band position Intensi ty Band type (cm-~)° (% E) (direction) b Assignments

1479 m 1.5 e (+ ) C=C stretch 1432 m 1.4 e ( + ) 1339 s 2 e (+ ) 1310 s 2.2 e (+ ) 1086 vw 0.2 a ( - ) 1017 vw 0.3 a ( - ) 820 w 0.35 a(-) 780 vw 0.2 a ( - ) 610w 0.35 a ( - ) 579 vw 0.3 a ( - ) 576 vw 0.3 a ( - ) 469 w 0.4 a ( - ) 410 vw 0.2 a ( - ) 384 w 0.35 e (+ ) 340 w 2.25 e (+ ) 315 vs 3 e ( + ) 232 vw a ( + ) 114 vw a ( + ) 70 vw a(+) 50 vw a(+) 26w a ( + ) 10w a(+)

Pt(COD) tilt Pt(COD) tilt Pt(COD) stretch

PtC12 sym stretch PtCI 2 asym stretch

PtC12 wagging PtCI2 twist

" m = medium; s = strong; vw = very weak; w = weak; vs = very strong. " See remarks of Table I. The bands below 250 cm ~ are taken from

transmiss ion measurement .

~9

U~

a . ,4.

W a v e n u m b e r ( c m -1)

FIG. 4. Emission spect rum of carbon monoxide adsorbed on Pt-black contaminated with formaldehyde (top) and tha t of the same Pt sample after desorption of CO and more formaldehyde added (bottom).

blackbody, and the double-beam spectrum of the later was subtracted from that of the former.

The above three systems show marked similarities in the sense that the measurements were done with rela- tively thick samples (~0.1-0.2 mm), so that the emission of the support material in the 1000-400 cm-1 region came close to that of the blackbody (100%). The appearance of inverted bands is not likely to originate from reduced emission due to enhanced surface reflectivity in these samples since they are highly dispersed powders and the adsorbed species are present in monomolecular layers whose thickness is much smaller than the wavelength of the radiation. We believe that the weak negative bands observed are absorption features due to reabsorption of support radiation by the adsorbed species in the outer layers of the sample.

CO Adsorption on Metal Powders. Adsorption on pure metal powders has the significance that it may represent a bridge between highly dispersed, supported metal cat- alysts used in real catalytic processes and metal single crystals accessible to most advanced theoretical studies. In the literature, however, there seem to be no vibrational spectroscopic studies of adsorption on metal powders since this measurement poses great experimental diffi- culties. These samples appear to be totally black, non- transmitting, and nonreflecting, and in addition have a

relatively small specific surface area. Recently we have been able to detect carbon monoxide chemisorbed on Pd-black in the vicinity of 2000 cm -1 in the IR emission spectrum. 29

A thin layer of Pd black (3.7 m 2 / g ) o n a stainless steel heater plate of the emission cell was first treated with p u r i f i e d H 2 gas at 473 K to remove oxygen; then after thorough evacuation, 4 KPa of CO gas was introduced into the cell. Single-beam emission spectra were mea- sured under H2 atmosphere and under CO at 373 K. By ratioing these two single-beam spectra, we obtained a double-beam emission spectrum that contained very weak emission bands hardly exceeding the noise level. The reality of the observed bands was confirmed by 13CO isotope substitution; the observed 45-cm -I shift of the CO stretching band was consistent with the expected isotope shift.

The relevant data obtained are listed in the first two columns of Table IV. For comparison, the corresponding data measured with evaporated Pd film, Pd(100) single crystal face, and supported Pd samples are also given. A comparison of the data shows that the spectral pattern obtained for Pd black is much closer to those of the Pd film and Pd single crystal, while in case of supported Pd samples two strong bands are always present at other frequencies. These data permit us to conclude that the different types of unsupported Pd samples show consid-

TABLE IV. Comparison of CO stretching frequencies detected on different Pd surfaces by different methods?

f

J

Pd black (emission) Pd film Pd(100) Pd/Si02 Pd/A1203

(transm.) (RAIRS) (emission) (emission) Approx. ~2CO laCO 12CO 12CO ~2CO 12CO assignment

2140 w 2105 vw 2120 w 2090 vs 2130 sh Pd ÷-CO 2030 sh 2000 sh 2052 s Pd°-CO lin. 1998 s 1965 s 1995 s 1997 vs 1954 vs 1920 vs Bridged CO

" w = weak; vw = very weak; vs = very strong; sh = shoulder; s = strong; vs = very strong.

1450 Volume 47, Number 9, 1993

e r a b l e s i m i l a r i t y r e g a r d i n g t h e t y p e of b o n d i n g b e t w e e n t h e c h e m i s o r b e d CO a n d t h e m e t a l sur face .

W e have p e r f o r m e d s i m i l a r m e a s u r e m e n t s w i th CO a d s o r b e d on P t b l a c k h a v i n g a g r e a t e r spec i f ic su r f ace a r e a (17 m2/g). A t 120°C a n d u n d e r 30 T o r r CO, we ob- t a i n e d t h e d o u b l e - b e a m e m i t t a n c e s p e c t r u m s h o w n in Fig . 4 ( a f t e r s u b t r a c t i o n of t h e e m i s s i o n of p u r e P t ) . T h r e e b a n d s were o b s e r v e d in t h e r eg ion s t u d i e d (2200-1300 c m -1) w i t h m u c h b e t t e r S / N t h a n in t h e case of P d b lack . T h e s t r o n g e s t b a n d a t 1980 c m -1 can i n d e e d b e l o n g to CO c h e m i s o r b e d on P t , b u t t h e o t h e r two fre- q u e n c i e s a r e too low to be long to t h e C O / P t s y s t e m . T r a c k i n g d o w n p o s s i b l e i m p u r i t i e s , we d i s c o v e r e d t h a t t r a c e s of f o r m a l d e h y d e u s e d as a r e d u c i n g a g e n t d u r i n g t h e p r e p a r a t i o n of P t b l a c k m i g h t have r e m a i n e d on t h e c a t a l y s t , a n d cou ld be on ly p a r t i a l l y r e m o v e d b y r e d u c i n g t h e s a m p l e w i t h H2 a t 240°C. A c o n t r o l e x p e r i m e n t in w h i c h f o r m a l d e h y d e was i n t r o d u c e d to t h e cel l f u r n i s h e d us w i t h t h e ev idence .

A p a r t f r om h a v i n g b e e n ab l e to d e m o n s t r a t e t h a t emis - s ion s p e c t r o s c o p y can be success fu l ly u s e d to s t u d y m e t a l p o w d e r s , we have l e a r n e d t h e l e s son t h a t c a t a l y s t s s h o u l d a lways be p u r i f i e d ve ry ca r e fu l l y be fo re use.

ACKNOWLEDGMENTS

We would like to acknowledge the financial support of the Hungarian National Scientific Research Fund, OTKA (Grant No. 1892) and COST (D5, ERBCIPE CT 92-6104). We also thank Bruker Instruments Inc. for the FT-Raman spectra.

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APPLIED SPECTROSCOPY 1451