2034
J. Phys. Chem. 1989, 93, 2034-2038
with a minimal H content; this is consistent with the weak intensity observed in the v(CH) and 6(CH) spectral regions (Figures 3 and 5). The fact that, at temperatures above 350 K, a weak v(CH) feature is observed in the characteristic absorbance range for olefinic groups (Figure 3 e,f) suggests that polymerization may have occurred in the adlayer; this is also consistent with the increase in absorbance in the v(C=C) spectral region at T 2 400 K (not shown). A summary of the basic stages of C4H6(a) adsorption, rearrangement, and decomposition is shown in Figure 6 for the Rh/A1203 substrate.
V. Conclusions On the basis of the spectroscopic data the following conclusions regarding strctural rearrangements in the 1,3-butadiene adsorbed layer on Rh/A1203 can be made: (1) A configurational rearrangement involving the end = C H 2 functional groups is observed in the multilyer structure of C4H6
on Rh/A1203 at T 2 120 K. This is a purely physical effect in the multilayer. (2) The skeletal C atoms rehybridize at T > 200 K from their initial sp2 state to the sp3 state. A certain population of the parent molecule is structurally transformed to the di-u (MCH2MCHCH=CH2) structure that coexists with a stable “metallocycle pentane” complex. (3) In accordance with the general reactivity scheme for olefins on group VI11 transition metals, the conjugated diene C4Hs also produces ethylidyne by symmetric C - C bond scission at T 2 300 K. It is postulated that C-C bond scission occurs via a C4 metallocycle surface complex.
Acknowledgment. We acknowledge, with thanks, support of this work by the DOE-Division of Basic Energy Sciences. We thank Professor W. H. Weinerg for helpful discussions. P.B. is grateful for the supported educational leave from Alcoa. Registry No. Rh, 7440-16-6; 1,3-butadiene, 106-99-0.
Fourier Transform Infrared External Reflection Study of Molecular Orientation in Spontaneously Adsorbed Layers on Low-Absorption Substrates J. A. Mielczarski* and R. H. Yoon Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received: June 8, 1988)
An infrared reflection absorption spectroscopy (IRAS) technique has been developed to study the structure of spontaneously adsorbed layers of ethyl xanthate (C2HSOCS2-ion) on a semiconductor,cuprous sulfide (chalcocite), from aqueous solutions. Owing to the optical properties of the substrate, positive as well as negative absorption bands are observed in the recorded spectra of the same sample, depending on the angle of incidence and the polarization of the incident radiation. The spectroscopic study was performed on mono- and multilayer coverages of ethyl xanthate. Generally good agreement has been obtained between experimental and theoretically calculated absorbance values for a model of the system investigated. The results show that the spectroscopic data obtained with the IRAS method make it possible to determine both the chemical nature and the structure of an adsorbed anisotropic layer at mono- and multilayer coverages on low-absorption substrates. The orientation of the individual molecular groups of adsorbed ethyl xanthate on cuprous sulfide and the chemical nature of the adsorbed species have been determined. The measurement conditions chosen for the IRAS studies of the adsorption layer on low-absorption substrates are also discussed.
Introduction The structure, formation, and properties of organic films have been extensively investigated by applying infrared reflection absorption spectroscopy (IRAS).1-9 The infrared technique is an excellent method of probing a surface because it minimizes the amount of destruction of the surface layer as compared to a number of other surface analysis methods used today. Investigations of the surface layer using the IRAS method can also be performed in (1) Swalen, J. D.; Rabolt, J. F. Fourier Transform Infrared Spectroscopy; Academic Press: New York, 1985; Vol. 4, Chapter 7, p 283. (2) Golden, W. G. Fourier Transform Infrared Spectroscopy; Academic Press: New York, 1985; Vol. 4, Chapter 8, p 345. (3) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (4) Nuzzo, R. G.; Fusco, F.A,; Allara, D. L. J. Am. Chem. Soc. 1987,109, 2358. (5) Liedberg, B.; Ivarsson, B.; Hegg, P.-0.; Lundstrom, I. J . Colloid Znterface Sci. 1986, 114, 386. (6) Liedberg, B.; Carlsson, C.; Lundstrom, I. J. Colloid Interface Sci. 1987, 120, 64. (7) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, M. Longmuir 1987, 3, 932. (8) Mielczarski, J.; Leppinen, J. Surf.Sei. 1987, 187, 526. (9) Debe, M. K. Prog. Surf. Sei. 1987, 24, 1. (10) Foley, J. K.; Pons, S. Anal. Chem. 1985, 57, 945A.
Most of the IRAS studies have been done to probe the molecular nature of the surfaces of metals, which strongly reflect the incident beam. In a few studies, however, the IRAS technique has been used to characterize nonmetalllic surfaces.I4J5 In these studies, the surface layer was deposited on glassy carbon by a spin-coating techniqueI4 or on water by using the LangmuirBlodgett method.I5 The lower reflectivity of the low-absorption substrate relative to that of the metal reduces the sensitivity of the technique. Moreover, since the electric field vectors at the interface of the low-absorption substrate are present in three dire~tions,’~J’ the interpretation of the recorded spectra in terms of an oriented surface structure is more complicated than with a metal substrate. The biggest difference between IRAS and transmission spectra (1 1) Bewick, A.; Pons, S. Advances in Infrared and Raman Spectroscopy; Wiley-Hayden: London, 1985; Chapter 1. (12) Korzeniewski, C.; Pons, S. Prog. Anal. At. Spectrosc. 1987, 10, 1. (1 3) Kellner, R.; Neugebauer, M.; Nauer, G.; Neckel, A. Proceedings of the 5th FTS Meeting, Ottawa, Canada, June, 1985. (14) Porter, M. D.; Bright, T. B.; Allara, D. L.; Kuwana, T. Anal. Chem.
1986, 58, 246 1. (15) Dluhy, R.A. J . Phys. Chem. 1986, 90, 1373. (16) Hansen, W. N. Advances in Electrochemistry and Electrochemical Engineering, Wiley Interscience: New York, 1973; Vol. 9, Chapter I . (17) McIntyre, J. D. E. Advances in Electrochemistry and Electrochemical Engineering, Wiley Interscience: New York, 1973; Vol. 9, Chapter 2.
0022-3654/89/2093-2034$01.50/00 1989 American Chemical Society
FTIR Study of C2H50CS2-on a Semiconductor
The Journal of Physical Chemistry, Vol. 93, No: 5, 1989 2035
p
\
n,
k,
s
polarization
-
polarization
ambient a d sorption layer substrate
Figure 1. Electric field vectors in a three-phase system. Parallel (p) (Ellz, &) and perpendicular (s) ( E L y ) components are shown for incident radiation.
of the same sample is expected for an anisotropic layer. This paper describes the use of the IRAS method in a study of spontaneously adsorbed ethyl xanthate from aqueous solution onto cuprous sulfide (chalcocite) at mono- and multilayer coverages. Adsorbed xanthate ions change the properties of the surface of chalcocite from hydrophilic to hydrophobic and form the basis for the flotation separation of the mineral from its ore. The results obtained have allowed determination of the chemical nature of the adsorbed species and the orientation of the molecular groups in the adsorption layer, which are found to change with increasing coverage. A comparison of the theoretically calculated absorbance values with experimental results is presented. The measurement conditions chosen for the study of the adsorption layer are also discussed. Experimental Section Preparation of Adsorption Layer. The cuprous sulfide (chalcocite from Messina, Transvaal, South Africa) slabs (12 X 25 mm) were cleaned with emergy paper and polished with 1-, 0.3-, and 0.03-pm alumina (Buehler, Ltd., Evanston, IL). The samples were washed first in an ultrasonic bath in ethyl alcohol, then with an HC1 solution with a pH of 2 in order to remove the preadsorbed products, and finally with water. Next, the sample was immediately immersed into 100 mL of a 9.0 X le5M solution of potassium ethyl xanthate with a pH of 6.0 f 0.2 for a period of 6 min or 18 h. After adsorption, the samples were removed from solution, washed, and immediately placed into the FTIR spectrometer to record the spectra. The surfaces of the samples were hydrophobic and did not require drying before spectroscopic study. The ethyl xanthate was synthesized from CS2,KOH, and ethyl alcohol and then purified by recrystallization from acetone and ether. Measurement. The infrared spectra were obtained on a Perkin-Elmer Model 1710 FTIR spectrometer with an MCT detector using an external reflection attachment (Spectra Tech, Inc.) with one reflection. A wire grid polarizer (Harrick Scientific Corp.) was placed before the sample and provided polarization selection. The spectrometer was purged with nitrogen gas to minimize the amount of water vapor and carbon dioxide present. The spectra were taken at 4-cm-' resolution by co-adding 100 scans in the 4000-5OO-cm-' region. All of the spectra presented were smoothed by using a routine computer program. The unit of intensity was defined as -log ( R / R o ) ,where Ro and R are the reflectivities of the freshly exposed cuprous sulfide before and after exposure to an ethyl xanthate solution, respectively. The transmission spectra of cuprous ethyl xanthate and cuprous sulfide in KBr pellets were prepared and measured in the standard way. Calculation of Reflection Absorbance. Figure 1 represents a model in which an electromagnetic wave is interacting with the three-phase system investigated in this work. The optical properties of each phase are characterized by the complex refractive index, ti, = nj + ikj, where nj is the real refractive index and k, is the absorption constant. The adsorption layer has a thickness,
4.
-
L
C \ U
U
C C c
I
1200
1000
V
1200
1000
c
Figure 2. IRAS spectra of the spontaneously adsorbed monolayer of cuprous ethyl xanthate on cuprous sulfide, recorded at various angles of incidence with p- and s-polarization: treatment time, 6 min; initial concentration, 9.8 X M; pH 6.0.
The theoretical calculation of the intensity of an absorption band in the reflection method is based on interface value solutions of Maxwell's equations for an isotropic three-phase system, as described in several reference^.'^-^^ In the present paper, the calculation was made for the following data: phase 1 (air), nl = 1.O and kl = 0. The optical constants of phase 2 (adsorption layer) are n2 = 1.4 and k2 = 1.2, 0.35, and 1.27 at 1200, 1050, and 1036 cm-I, respectively. The k2 values were assumed on the basis of the recorded transmission spectra of KBr pellets of precipitated cuprous ethyl xanthate. The cuprous ethyl xanthate was found (in an in situ ATR study) to be the only adsorption product of ethyl xanthate on cuprous sulfide.26 The n2 value was assumed on the basis of the optical constants reported2' for a similar compound, lead ethyl xanthate. Knowing that ethyl xanthate molecules are well-oriented in the first layer, as was found in the XPS study22and in this work, and observing the results of diffraction analysis of several ethyl xanthate salts,23* one can assume the thickness of the adsorbed monolayer to be 8 X 1O-Io m. The optical constants of cuprous sulfide (phase 3) are found to be n3 = 6.27 and k3 = 0.19 at 1000 cm-I. These results are calculated on the basis of the transmission spectra of KBr pellets of cuprous sulfide and measurements of the ratio of the reflectivities of p and s-polarization at different angles of incidence. A similar value for the absorption constant of cuprous sulfide, k3 = 0.1, can also (18) Hansen, W. N. J. Opt. SOC.Am. 1968, 58, 380. (19) Hansen, W. N. Symp. Faraday SOC.1970, 4, 27. (20) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978,11, 1215. (21) Nowak, P. Ph.D. Thesis, Silesian Technical University, Poland, 1979. (22) Mielczarski, J.; Suoninen, E. Surf. Interface Anal. 1984, 6, 34. (23) Hagihara, H.; Yamashita, S . Acta Crystallogr. 1966, 21, 350. (24) Ray, A.; Sathyanarayana, D. N.; Prasad, G. D.; Patel, C. C. Spectrochim. Acta 1973, 29A, 1579.
2036 The Journal of Physical Chemistry, Vol. 93, No. 5. 1989
Mielczarski and Yoon
943 0008
'
'c u/
---m
5/
1
/
7
cu2s
/
,',/
Figure 3. Proposed molecular orientation of ethyl xanthate on cuprous
sulfide. TABLE I: Band Assignments of C2H5OCS2CuMolecule in the 1400-900-cm-' Region
band, cm-I
-
(I
1200 1129 1122 1051 1035
direction of transition moment vs chain axis
II 1 II I II i
1010 I References 8, 24, and this work.
assignt" U,(COC) U,(SCS) %(COC) Va(SCS) ua(COC) Va(CC) ua(SCS) U*(SCS)
be deduced from previous studies.25 These particular optical values for the three phases are only approximations of the real values and were chosen to simulate the optical properties of the monolayer of cuprous ethyl xanthate on cuprous sulfide.
Results and Discussion The reflection spectra of the adsorption layer of ethyl xanthate on cuprous sulfide after 6 min of treatment are shown in Figure 2. From previous studies,u26 it can be concluded that under these conditions the coverage of ethyl xanthate is approximately a well-oriented monolayer. For s-polarization, only the negative absorption bands are observed, and their intensities decreased with an increase in the angle of the incident beam. However, with p-polarization, both the positive and the negative absorption bands are recorded and show dependence on the angle of incidence. Negative absorption bands in infrared external reflection specor lowtroscopy were theoretically p r e d i ~ t e d lfor ~ ~ transparent '~ absorption substrates with k3 < 1 and were experimentally observed for waterI5 and silicone20substrates. The different positions and intensities of the absorption bands (Figure 2) are dependent on the polarization and the angle of incidence. For s-polarization, which only has electric field a" ponents parallel to the interface plane (Figure l ) , only those molecular groups that have transition moment components parallel to the substrate surface can interact with the incident radiation and result in observable absorption bands in the recorded spectrum. In the recorded spectra of the monolayer of ethyl xanthate with s-polarization (Figure 2), only those bands that are caused by molecular groups having their transition moment perpendicular to the chain axis of the molecule are observed (Figure 3, Table I). These results indicate that the molecules at near-monolayer coverage have a position close to perpendicular to the surface of cuprous sulfide (Figure 3) and support the adsorption model of ethyl xanthate on cuprous sulfide proposed earlierZ2on the basis of the XPS study. More complex changes are observed in the recorded spectra for p-polarization. At higher angles of incidence (between 70' and 40°),three positive absorption bands at 1197, 1129, and 1051 cm-l are observed (Figure 2), which are caused by the vibration of molecular groups that have transition moments parallel to the (25) Marshall, R.; Mitra, S. S. J . Appf. Opt. 1965, 36, 3882. (26) Mielczarski, J. Ph.D. Thesis, Silesian Technical University, Poland, 1979.
Angle o f Incidence Figure 4. Calculated absorbance for hypothetical monolayer of cuprous ethyl xanthate (n2 = 1.4; k 2 = 1.2 at 1200 cm-I; dz = 8 X m) on cuprous sulfide (n3 = 6.27; k3 = 0.19) (solid lines). Experimental results (note different absorbance scale) at nearly monolayer coverage are shown as circles.
chain axis (Figure 3, Table I). These results again show that the adsorbed molecules are oriented on the surface of the cuprous sulfide in the first layer (Figure 3). These recorded spectra are similar in principle to the IRAS spectra recently reporteds for an adsorbed ethyl xanthate monolayer on metallic copper. Close inspection of both the present and the recent6 results reveals some differences, however. In the case of the cuprous sulfide substrate, an additional negative absorption band at 1035 cm-I is observed. Moreover, the intensity ratios between the bands at 1197 and 1129 cm-I and at 1197 and 1151 cm-' are lower than those recorded for monolayer xanthate adsorbed on copper.6 The reason for these differences will be discussed later. At angles of incidence of nearly 30°, the lowest intensities of absorption bands are observed (Figure 2). However, at a 20' angle of incidence, the absorption bands became distinctly negative and the positions of the bands are similar to those observed for spolarization. In order to explain the changes in the IRAS spectra, a calculation has been carried out for a model of the three-phase system employed in the present work by using the equations described by Hansen.19 The results are given in Figure 4, in which solid lines represent the calculated changes of absorbance ( A ) at 1200 cm-I as a function of the angle of incidence for both s- and p-polarization ( A L ,AI,). The experimental absorbance values are shown as circles although at a different scale from that used for the calculated values. In this figure, only the most characteristic part of the calculated results for the is shown. In general, good agreement between the experimental results and the theoretically predicted values is observed. For s-polarization, only the negative absorption bands are predicted, which is in good agreement with the recorded spectra. The theoretical calculation also predicts that for p-polarization, the absorption band changes from positive to negative at an angle of incidence near 30°. This has been verified in the recorded spectra (Figure 2). Note that there is a difference in magnitude between the calculated and the experimental absorbance values. The absorbance measured at 1200 cm-' is much higher than that calculated. One of the reasons for this is that the calculations were done for an isotropic layer while the experimental adsorption layer displayed an anisotropic behavior. This would result in a 3-fold increase in the observed intensities3 Moreover, the density of a closepacked monolayer may be considerably greater than that of the bulk phase. Also, there may be some difference between the thickness of the real adsorption layer and the monolayer coverage assumed in the calculation. Surface roughness would also contribute to the discrepancy. While the calculations are based on the assumed optical properties of the bulk cuprous ethyl xanthate, the exact values
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2037
FTIR Study of C2HSOCS2-on a Semiconductor for the oriented monolayer are not known. Moreover, the absorption band at 1200 cm-' has two components, i.e., v,(C-O-C) and v,(S-C-S), both of which show a dependency on the angle of incidence, as will be discussed in greater detail later. It may be stated, however, that this is probably the main reason that the experimental absorbance for p-polarization at a 70' angle of incidence deviates more significantly from the theoretically predicted value than at other angles of incidence. Many investigators also reported experimental absorbance values that are several times higher than the calculated values for the isotropic layer on and water.I5 It is interesting to try to understand the physical reason behind the change from positive to negative absorption bands observed for ppolarization (Figure 2). As shown in Table I, the absorption band at 1200 cm-' has been attributed to the asymmetric stretching vibrations (va) of the C-0-C and S-C-S groups. For p-polarization, which has two components, Ell, and Ell,, (Figure l ) , the absorption band is due to both the u,(C-0-C) and the v,(C-S-C) components; hence two absorption components, All, and All,, should be considered. It should also be noted here that the absorption components are proportional to the mean square ~'~ electric field component^.'^*'^ As was reported p r e v i o ~ s l y , ' ~the mean square electric field component Ell, is higher than the Ell, component for higher angles of incidence; however, for lower angles of incidence, the situation is reversed. Hence, at lower angles of incidence, the absorption band at 1200 cm-I for p-polarization is mainly due to the v,(S-C-S) component and becomes negative (Figure 2). The negative absorption bands observed at an angle of incidence of 20° (Figure 2, p-polarization) have the same position as those observed in the spectra with s-polarization; hence, the bands at 1051 and 1129 cm-I disappear (Figure 2). This also indicates that the All, component for p-polarization predominates at a lower angle of incidence. Close analysis of the ppolarization spectra shows that the negative absorption band at 1035 cm-' is observed in all recorded spectra even for higher angles of incidence, while the theoretical calculation for All,,, (Figure 4) does not predict negative bands. The band at 1035 cm-' is due to the asymmetric stretching vibration of the S-C-S molecular group (Table I); hence, for an oriented monolayer (Figure 3) it is caused by interaction of this group with the Ell, component for p-polarization. The observations discussed above indicate that the Ell, component is responsible for the negative absorption bands observed for p-polarization in all of the recorded reflection spectra (Figure 2). This conclusion also explains the differences mentioned above between the IRAS spectra of an oriented ethyl xanthate monolayer on copper* and cuprous sulfide. Hitherto, the changes in the absorbances for p-polarization, All,,,, were calculated together for both All, and All, compon e n t ~ . ~However, ~ - ~ ~ as shown above, additional structural information about the anisotropic layer can be obtained by considering these two components separately. On the basis of the equations describing external reflection phenomena for a transparent substrate and for both s- and p-polarization (ALy and A11,J)19 and by incorporation of the discussions inherent in some papers,i6,i7J9the theoretical absorbances for the three components A,,, All,, and All, can be calculated as follows:
A,,, =
--16n
[
In 10 ~ A , , ,= - -
][
9]?
cos 0 ~ - C O~ S ~e / n
sin2 e
~
~
nzk2dz
where t3= (fi3z- n , sin2 6')'l2, fi is complex index, fi3 = n3 + ik3, 6' is the angle of incidence, and h is the wavelength in a vacuum. (27) Allara, D.L.; Swalen, J. D.J . Phys. Chem. 1982, 86, 2700.
I 'OoW
-
.OW4
t
I I I -I
I Ill
,004
0
a
-a \
0.wOo
0.000
m 0
d
'
-.0004 -.004
- ,008 Angle o f I n c i d e n c e Figure 5. Calculated absorbance for hypothetical monolayer (d2 = 8 X m) of cuprous ethyl xanthate (n2 = 1.4) on cuprous sulfide (n3 = 6.27; k3 = 0.19) as functions of angle of incidence (solid lines) for optical constants: AlI,:k2= 0.35 at 1051 cm-I; Allxand A,,:k2 = 1.27 at 1035 cm-I. Experimental results: (0)band at 1050 cm-I, p-polarization; (A) band at 1035 cm-I, s-polarization; (+) band at 1035 cm-I, p-polarization.
The absorption band at 1200 cm-' is due to two components of the transition moment, parallel and perpendicular to the interface (Figure 3, Table I). The first component produces the negative component of the band, while the second produces the positive component, as was discussed above for an oriented monolayer. Therefore, the calculation was performed for these bands, which are mainly due to one of the transition moments. Hence, the Ail, component was calculated for the band at 1051 cm-I, assuming nz = 1.4 and k', = 0.35. However, the All, and A,, components for the band at 1035 cm-I were calculated by assuming that nz = 1.4 and k'12 = 1.27. The results of the theoretical calculations are shown in Figure 5. In this figure are also presented experimental data for which a different absorbance scale is necessary, as was the case with Figure 4. In principle, good agreement between the calculated values and the experimental data is observed. This supports the explanation presented above for the IRAS spectra (Figure 2) and shows great possibilities for determining the orientation of individual molecular groups of adsorbed species on low-absorption substrates. The calculated and experimental data for p-polarization, All, and All,, show some discrepancies, which increase with increasing angles of incidence. This is caused by the very close positions of the bands at 1051 and 1035 cm-I. Because the first is positive and the second is negative, a cancellation of the intensities of these bands is observed. Hence, lower intensities of the bands are recorded than are actually occurring. Figure 5 clearly shows that the two components for p-polarization, Ail, and All,, change in a dramatically different way. If the absorption bands in the spectrum are due to both parallel and perpendicular components of the transition moment (for instance, the band at 1200 cm-I of cuprous ethyl xanthate; see Table I) the bands may disappear under certain measurement conditions. Therefore, the study of the adsorption layer should be started at a lower angle of incidence for s-polarization and should be continued for p-polarization with increasing angles of incidence. Knowledge of the All, and All, components individually (Figure 5) allows an explanation, for example, of why similar spectra are observed for p- and s-polarized radiation at an angle of incidence of 20' (Figure 2) for an anisotropic monolayer. Thus, at lower angles of incidence, the surface layer on low-absorption substrates gives the same results independently of the polarization even for an anisotropic surface layer. These same results were recorded in an investigation of organic compounds spread on ~ a t e r . I ~ To ,~* (28) Dluhy, R. A.; Wright, N. A,; Griffits, P. R. Appl. Spectrosc. 1988, 42, 138.
2038
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
p
- polarization
s
- polarization
m
m
0
U \
-a m 0
H
T
1
[,02
0.0°4
A
1200
1000
1200
1000 c
Figure 6. IRAS spectra of the spontaneously adsorbed multilayer of ethyl xanthate on cuprous sulfide recorded at various angles of incidence and two polarizations: adsorption time, 18 h; initial concentration of solution, 9.8 X M; pH, 6.0.
obtain more information concerning the orientation of molecular groups of surface species, it is, therefore, necessary to conduct p-polarization studies at higher angles of incidence than was suggested by D 1 ~ h y . l ~ The IRAS spectra of the multilayer of cuprous ethyl xanthate on cuprous sulfide after 18 h of adsorption are shown in Figure 6 . The observed changes in the intensities of the absorption bands are in general agreement with the predicted values shown in Figure
Mielczarski and Yoon
4. Negative and positive absorption bands are observed for the same angles of incidence and polarizations. However, the absolute intensities of the absorption bands (Figure 6 ) are about 5 times higher than those for monolayer coverage (Figure 2). This agrees with previous findingsz6that under these adsorption conditions, about five monolayers of coverage is expected on the surface of cuprous sulfide. Comparison of the reflection spectra in Figures 6 and 2 shows several differences. In the spectra of the multilayer for s-polarization, an absorption band at 1051 cm-' is observed. Moreover, the intensity ratios for the observed absorption bands in the spectrum are different in the spectra recorded for monolayer (Figure 2) and for multilayer (Figure 6 ) . These findings indicate that adsorbed molecules on top of the monolayer are not welloriented to the plane of the surface. The increase in coverage involves a gradual increasing of the intensity ratio of the bands at 1051 and 1035 cm-' (Figure 6 , s-polarization), which can be assumed to be an index of the orientation of the ethyl xanthate molecules adsorbed on the surface. It should be noted that the distortion of the spectra caused by an increase in the thickness of the adsorption layer reported previously for a glassy carbon s ~ b s t r a t e 'at ~ five-monolayer coverage of cuprous ethyl xanthate on cuprous sulfide is negligible in comparison to monolayer coverage. Conclusion The results presented here show that it is possible to obtain infrared spectra of mono- and multilayers adsorbed on low-absorption substrates (semiconductors). Also, it is possible to determine the orientation of the individual molecular groups and the chemical nature of the adsorbed species by recording the reflection spectra of the adsorption layer at different angles of incidence for both p- and s-polarization. Generally good agreement has been obtained between theoretically calculated and experimental data. The differences between the absolute absorbance values that were calculated and those that were obtained experimentally are the result of the anisotropic character of the adsorption layer and other differences between real systems and the model calculations. Knowledge of the three absorbance components Allz,Alix, and A,, makes it possible to predict the absorbance values for the system under investigation and provides for an easier interpretation of the reflection spectra, especially for the anisotropic adsorption layer. Registry No. Co2S,22205-45-4;C2H50CS2H,151-01-9.
